THE CLINICAL VALUE OF AUDITORY STEADY STATE RESPONSES IN THE AUDIOLOGICAL

THE CLINICAL VALUE OF AUDITORY STEADY STATE RESPONSES IN THE AUDIOLOGICAL
University of Pretoria etd - De Koker, E (2004)
THE CLINICAL VALUE OF AUDITORY STEADY
STATE RESPONSES IN THE AUDIOLOGICAL
ASSESSMENT OF PSEUDOHYPACUSIC
WORKERS WITH NOISE-INDUCED HEARING
LOSS IN THE SOUTH AFRICAN MINING
INDUSTRY
by
ELIZABETH DE KOKER
In partial fulfilment of the requirements for the Degree
DOCTOR PHILOSOPHIAE
in the
Faculty of Humanities,
UNIVERSITY OF PRETORIA,
PRETORIA
May 2004
University of Pretoria etd - De Koker, E (2004)
The real secret of success is enthusiasm.
Walter Chrysler
Enthusiasm releases the drive to carry you over obstacles and adds
significance to all you do.
Norman Vincent Peale
If you can give your son or daughter only one gift, let it be enthusiasm.
Bruce Barton
You can do anything if you have enthusiasm. Enthusiasm is the yeast that
makes your hopes rise to the stars. With it, there is accomplishment. Without
it there are only alibis.
Henry Ford
Success is the ability to go from failure to failure without losing your
enthusiasm.
Winston Churchill
Knowledge is power, but enthusiasm pulls the switch.
Ivern Ball
2 Kor. 1:3
Aan God, die Vader van ons Here Jesus Christus, kom
al die lof toe! Hy is die Vader wat Hom ontferm en die
God wat in elke omstandigheid moed gee
University of Pretoria etd - De Koker, E (2004)
To my parents, Faan and Elsabe Alberts,
my husband, Dr F.D. de Koker, and
my four children, Derek, Elsabe, Stefan and Lize.
University of Pretoria etd - De Koker, E (2004)
ACKNOWLEDGEMENTS
I would like to express my sincere appreciation for the valuable contributions
of the following individuals and organisations:
Prof S.R. Hugo for her guidance, support and motivation;
Dr D. Schmulian for her academic input;
Dr M. Soer for her support and guidance;
Mrs E. Lotheringen for her help with data collection;
Mrs C. Goodchild for her help with the literature survey;
Mrs V. Posthumus for assistance in the compilation of the manuscript;
Dr J. Levin for the statistical analysis of data;
SIMRAC for financial, technical and administrative support;
Dr M. Ross for technical guidance and assistance;
Dr S. Shearer for promoting this programme of work and providing support to
ensure its successful completion;
the management and staff of Gold Fields and Harmony for making their
facilities available and for providing clinical assistance;
Mr R.M. Franz for assistance in a literature search and language and
technical editing, and for loyal support;
HASS (Mr. N. van der Merwe Jnr.) and NS Clinical Technologies (Mr. W. de
Klerk) for supplying auditory steady state equipment for the experimental
phase;
Ms I. Noome for language editing;
Ms T. Steyn for technical editing;
Dr A. Hough & Dr. G. Olivier for professional advice and guidance, and
the de Koker family for their patience and unflagging support.
University of Pretoria etd - De Koker, E (2004)
TABLE OF CONTENTS
Page
TABLES
…………………….……………………….…………………………
viii
FIGURES
……………………………………..…………………………………
x
LIST OF ABBREVIATIONS
SUMMARY
………………………………………………………
xi
…………………………………………….…………………..………
xiii
OPSOMMING ………………………………….………………………………….…
xvi
CHAPTER 1:
1
INTRODUCTION…………………...………………..
1.1
BACKGROUND……………………………………………………………
1
1.2
RATIONALE………………………………….………..…………..……….
2
1.3
PROBLEM STATEMENT…………………………………………………
7
1.4
PROPOSED SOLUTION………………………………..…..……………
8
1.5
PURPOSE OF THE STUDY…………………………………………….
11
1.6
CLARIFICATION OF TERMINOLOGY………………………………..
11
1.6.1
Auditory evoked potential (AEP)……………………………….
11
1.6.2
Auditory steady state response (ASSR)…..………………….
11
1.6.3
Noise-induced hearing loss (NIHL) ……….…………………..
11
1.6.4
Pseudohypacusis………………………………………………..
12
1.7
OUTLINE OF THE STUDY……………………………………………..
12
1.8
SUMMARY………………………………………………………………..
16
CHAPTER 2:
PSEUDOHYPACUSIS AND APPROPRIATE
STRATEGIES TO DETECT AND QUANTIFY
THE CONDITION…………………..………………
17
2.1
INTRODUCTION……………………………………………………………
17
2.2
DEFINITION OF PSEUDOHYPACUSIS…………….……………………
18
2.3
PREVALENCE AND ETIOLOGICAL FACTORS……………………….
18
2.4
AUDIOLOGICAL ASSESSMENT OF PSEUDOHYPACUSIS……..…
22
2.4.1
Reason for referral…………………………………………………
23
2.4.2
Patient behaviour……………………..…………………………..
23
2.4.3
Pure-tone audiometry…………………………………………….
24
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University of Pretoria etd - De Koker, E (2004)
2.4.3.1
Inconsistent thresholds………………………………….
25
2.4.3.2
Different pure-tone threshold determination methods..
26
2.4.3.3
Audiometric configuration……………………………….
26
2.4.3.4
Lack of interaural attenuation………………………….
26
2.4.3.5
Lack of correlation between air- and bone conduction
tests..………………………………..…………………...
27
Speech testing……..…………………………………………….
28
2.4.4.1
Speech reception thresholds………………………….
28
2.4.4.2
Word discrimination tests……………………………...
29
Special tests……………………………………………………..
29
SUMMARY……………………………………………………………..…
30
2.4.4
2.4.5
2.5
CHAPTER 3:
ELECTROPHYSIOLOGICAL TESTS AND THEIR
USE IN THE ASSESSMENT OF
PSEUDOHYPACUSIS……………..………….……
32
3.1
INTRODUCTION……………………………………………………………
32
3.2
ELECTROPHYSIOLOGICAL (EP) TESTS……………..……………….
34
Qualitative electrophysiological tests……………………………
34
3.2.1
3.2.1.1
Immittance………………………………………………..
35
3.2.1.2
Oto-acoustic emissions…………………………………
37
3.2.2
Quantitative electrophysiological tests………………………..
37
3.2.2.1
Introduction………………………………………………...
37
3.2.2.2
Background: the development of the use of AEPs….
38
3.2.2.3
Nomenclature and definitions……………………………
40
3.2.2.4
Problematic issues in the field of AEP…………………
42
3.2.2.5
The use of different potentials in pseudohypacusis….
43
3.2.2.5.1
Early potentials………………..……….………..
44
3.2.2.5.2
ABR…………………………….………………..
45
3.2.2.5.3
Middle latency responses…………...………..
48
3.2.2.5.4
Late latency responses (LLR)…………………
50
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University of Pretoria etd - De Koker, E (2004)
3.3
SUMMARY…………………………..…………………..……………….
CHAPTER 4:
53
AUDITORY STEADY STATE RESPONSES (ASSR)
AND PSEUDOHYPACUSIS ……..………….…..
56
4.1
INTRODUCTION…………………………………………………………
56
4.2
THE DEVELOPMENT OF AUDITORY STEADY STATE
RESPONSES…………………………………………………………….
57
RESEARCH FINDINGS WITH ASSRs…………….………………..…
61
4.3
4.3.1
Types of stimuli…………………………………………….………..
61
4.3.2
Stimulus intensity……………………………………………………
61
4.3.3
Carrier frequency………………………………………………….
62
4.3.4
Modulation frequency……………….……………………………..
64
4.3.5
Dichotic stimulation……………………………………………….
66
4.3.6
Limited clinical validation…………………………….………….
68
4.3.7
Length of procedures……………………………………………
69
4.3.8
Subject-related factors………………………………….………..
70
4.3.9
Applications of ASSR in clinical audiology……….…………..
72
4.3.10
Apparatus……………….………………………………………..
73
4.3.11
Threshold determination technique…………………………….
73
4.3.12
Response generators............................................................
73
4.3.13
Frequency-specificity…………………………..……………….
74
4.3.14
Resistance to state of consciousness……………..………….
74
4.3.15
Absence of gender bias……………………………….………..
75
4.3.16
Accuracy of threshold estimates………………..……………..
75
4.3.17
Detection of thresholds through the severity range………….…
77
4.3.18
Lack of age-related influences…………………………………
78
4.3.19
Threshold detection in the frequency domain………………..
78
4.3.20
Assessment of sound processing………………..…………….
80
SUMMARY………………………………………………………………..
81
4.4
CHAPTER 5:
RESEARCH METHODS……..……………….…..
84
5.1
INTRODUCTION…………………………………………………………
84
5.2
AIMS OF THE RESEARCH…………………………………………….
85
5.2.1
Principal aim………………………………………………………
iii
85
University of Pretoria etd - De Koker, E (2004)
5.2.2
5.2.2.1
5.2.2.2
5.2.2.3
5.2.2.4
Sub aims………………………………………………….….…..
86
To compare ASSR and pure-tone thresholds in a
co-operative population of adult mine workers with
sensory neural hearing loss.......................................
86
To compare the accuracy of multiple-frequency
(dichotic)and single frequency (monotic) ASSR
stimulation methods in estimating pure-tone thresholds in a mine worker population…………………..…
87
To compare different modulation frequencies’
effectiveness in estimating pure-tone thresholds……
87
To determine the effect of sedation on the ASSR
test’s ability to estimate pure-tone thresholds……….
87
To determine if pure-tone threshold estimates can
be obtained in unco-operative mine workers…………
88
.
5.2.2.5
5.3
RESEARCH PLAN…………………………………………………………
88
5.4
ETHICAL CONSIDERATIONS……………………………………………
91
5.4.1
Willing participation……………………………………………….
92
5.4.2
Informed consent…………………………………………………
92
5.4.3
Consent to sedation……………………………………………..
92
5.4.4
Employers’ permission…………………………………………..
93
5.4.5
Ethical clearance…………………………………………………
93
5.5
SUBJECTS……………………………………………………………….
93
5.5.1
Population………………………………………………………..
93
5.5.2
Sampling………………………………………………………….
94
5.5.3
Characteristics of subjects and the procedures followed in
the selection of these subjects…………………………………
95
5.5.3.1
Occupation………………………………………………
95
5.5.3.2
Abnormal hearing with and without a functional
overlay……………………………………………………
95
5.5.3.3
Normal middle ear function……………………………
96
5.5.3.4
Age and gender…………………………………………
97
Description of subjects………………………………………….
97
5.5.4.1
Hearing thresholds – co-operative group…..………..
98
5.5.4.2
Age of co-operative group………………..……………
101
5.5.4.3
Age distribution of pseudohypacusic group……….…
104
5.5.4.4
Years of experience/exposure…………………………
105
5.5.4.5
Experience of pseudohypacusic group………………
107
5.5.4
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University of Pretoria etd - De Koker, E (2004)
5.6
MATERIAL AND APPARATUS.……………………………………….
108
5.6.1
The material and apparatus used for subject selection……..
108
5.6.2
The material and apparatus used for data collection………..
109
5.6.2.1
Pure-tone testing………………………………………..
109
5.6.2.2
MF-ASSR-testing……………………………………….
109
5.6.2.3
Single frequency ASSR testing……….………………
110
5.6.2.4
Data preparation………………………………………..
111
5.7
DATA COLLECTION PROCEDURES…………………………………
111
5.7.1
Pure-tone audiometry……………………………………………
111
5.7.2
MF-ASSR data collection……………………………………….
112
5.7.3
Single frequency data collection…………………………….…
115
5.8
DATA ANALYSIS PROCEDURES…………………………………….
123
5.9
SUMMARY………………………………………………………………..
123
CHAPTER 6:
RESULTS……………………..…………….……..
124
6.1
INTRODUCTION…………………………………………………………
124
6.2
CO-OPERATIVE MINE WORKERS WITH NOISE-INDUCED
HEARING LOSS………………………………….………………………
125
Sub-aim: To compare ASSR and pure-tone thresholds in
a co-operative population of adult mine workers with
sensory neural hearing loss……………….............................
125
To compare the correlation of multiple-frequency (dichotic) and
SF-(monotic) stimulation methods in estimating
pure-tone thresholds in a mine worker population……….......
137
To compare different modulation frequencies’ effectiveness in
estimating pure-tone thresholds……………………………….
141
To determine the effect of sedation on the ASSR test’s
ability to estimate pure-tone thresholds……………………….
143
Summary of findings (Phase 1)………………………………..
146
UNCO OPERATIVE MINE WORKERS (PHASE TWO)……….……
147
To determine whether pure-tone threshold estimates can be
obtained for unco-operative workers………………………….
147
6.2.1
6.2.2
6.2.3
6.2.4
6.2.5
6.3
6.3.1
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University of Pretoria etd - De Koker, E (2004)
6.3.1.1
Introduction………………………………………………
147
6.3.1.2
6.3.1.3
Revision of Phase 1 procedures: Implications for
Phase 2………………………………………………….
Results obtained………………………………………..
147
147
6.3.1.4
Time required for ASSR testing……………………….
150
6.3.1.5
Summary of Phase 2……………………………………
150
6.4
6.5
RESEARCH RESULTS REALISING THE PRINCIPAL AIM OF
THE STUDY……………………………………………………….……..
151
SUMMARY………………………………………………………………….
153
CHAPTER 7:
CONCLUSIONS AND RECOMMENDATIONS..
154
7.1
INTRODUCTION…………………………………………………………
154
7.2
RESEARCH FINDINGS…………………………………………………
156
Conclusions based on the result from co-operative mine
workers with noise-induced hearing loss….………………….
157
Conclusions based on results with pseudohypacusic
mine workers…………………………………………………….
158
CRITICAL EVALUATION OF THE RESEARCH…………………….
160
7.3.1
Reliable alternative to pure-tone methods……………………
160
7.3.2
Threshold estimates across the severity range………………
161
7.3.3
Fewer ASSR thresholds obtained in comparison with puretone thresholds…………………………………………………..
161
The difference between ASSR and pure-tone thresholds
at 500 Hz…………………………………..……………………..
162
7.3.5
Intervals of ten dB intervals in ASSR threshold estimation...
163
7.3.6
The Audera system…,,,…………………………………….…..
163
7.3.7
The SF-Monotic technique……………………………………..
164
7.3.8
The 40 Hz Modulation…………………………………….…….
164
7.3.9
Sedation………………………………………………………….
164
7.3.10
Conclusion of audiological procedures………………………..
165
7.3.11
Pseudohypacusic workers had hearing loss…………….……
165
7.3.12
Pseudohypacusic workers with hearing loss were not
necessarily compensable………………………………………
166
7.3.13
Unfitness of pseudohypacusic workers……………………….
166
7.3.14
ASSR thresholds did not correlate well with previous
screening tests…………………………………………………..
166
7.2.1
7.2.2
7.3
7.3.4
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University of Pretoria etd - De Koker, E (2004)
7.3.15
Prevalence of sudden hearing loss……………………………
167
7.3.16
ASSR – an important contribution to the mining industry.…..
167
7.3.17
Lengthy procedure………………………………………………
168
7.4
LIMITATIONS OF THE RESEARCH.................……………………..
168
7.5
THE STUDY IN CONTEXT………………………………………………
170
BIBLIOGRAPHY………………………………………………………………….
172
APPENDIX A:
Informed Consent……………………………………….
185
APPENDIX B:
Consent-Valium…..…………………………………….
187
APPENDIX C:
Consent Gold Fields……..…………………………..….
190
APPENDIX D:
Consent Harmony……………….………………………
191
APPENDIX E:
Ethics Committee approval…………………………….
192
APPENDIX F:
Simrac approval….………………………………………
193
APPENDIX G:
Case History (Research Questionnaire)……………...
194
APPENDIX H:
Calibration certificate GSI 33…………..……………...
197
APPENDIX I:
Calibration certificate Beltone Acutymp 100………...
198
APPENDIX J:
Calibration certificate Madsen OB 822.……………...
199
APPENDIX K:
Calibration certificate GSI 60……………..…………...
200
APPENDIX L:
Calibration certificate: Sound proof booth, Driefontein,
APPENDIX M:
APPENDIX N:
APPENDIX O:
Carletonville……………………………………………...
201
Calibration certificate: Sound proof booth, Phumlani,
Randfontein……………………………………………...
203
Raw Data: ASSR and pure-tone thresholds
(Phase 1)………………………………………..……....
205
Raw Data: pseudohypacusic group. Pure-tone and
ASSR thresholds.……………………………………....
209
APPENDIX P:
Analysis of available data-pseudohypacusic group...
212
APPENDIX Q:
Costing of ASSR methods in the mining industry…...
214
APPENDIX R:
Proof of language editing: I Noomé…………….……
219
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University of Pretoria etd - De Koker, E (2004)
Tables
Table
Page
2.1
Psychological anomalies found in pseudohypacusic patients………
21
3.1
Disadvantages of AEPs…………………………………………………
54
4.1
Rationale for the selection of ASSR in experimental research with
mine workers………………………………………………………………
82
Research plan: Phases, experimental groups and experimental
parameters……………………..…………………………………………
90
5.2
Hearing thresholds (Decibel)(HL) for the co-operative group.………
98
5.3
Modulation frequencies used by MASTER…..………………………..
115
6.1
Comparison between ASSR and pure-tone thresholds according to
severity of hearing loss………………………………………………….
129
6.2
Comparison of ASSR and pure-tone thresholds by test frequency…
131
6.3
Results from the pure-tone and ASSR testing of left and right ears..
132
6.4
Mean difference between ASSR estimates and pure-tone
thresholds (dB)..………………………………………………………….
133
Average number of frequencies completed using SF- and MF-testing
per subject………………………………………………………………….
137
Time taken for SF- and MF-tests, independent of the number of
frequencies completed…………………………………………………..
138
Time taken for SF- and MF-tests, normalised for the number of
frequencies completed…………………………………………………..
138
Differences in sensitivity between SF- and MF-stimulation
techniques…………………………………………………………………
140
Time taken for 40 Hz and 80-100 Hz tests, independent of number of
frequencies completed……………………………………………………
141
Time taken for SF and MF tests, normalised for the number of
frequencies completed…..………………………………………………..
142
The significance of time comparisons of MF- and SF-techniques with
and without sedation……………………………………………………..
143
Significance of sensitivity differences between sedated and
non-sedated SF-ASSRs………………………………………………….
144
5.1
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
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University of Pretoria etd - De Koker, E (2004)
6.13
6.14
Significance of sensitivity difference between sedated and nonsedated MF-ASSRs……………………….……………………………..
145
Deductions made form the ASSR thresholds obtained in
pseudohypacusic workers………………………………………………
148
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University of Pretoria etd - De Koker, E (2004)
Figures
Figure
Page
5.1
Age distribution of the SF/80 Hz/non-sedated group (n=12)………..
102
5.2
Age distribution of the SF/40 Hz/non-sedated group (n=16)…….….
102
5.3
Age distribution of the MF/80 Hz/non-sedated group (n=20)……..…
103
5.4
Age distribution of the SF/40 Hz/sedated group (n=13)……..………
103
5.5
Age distribution of the MF/80 Hz/sedated group (n=20)…………..…
104
5.6
Age distribution of pseudohypacusic group (n=29).……….…………
104
5.7
Experience exposure: SF/80 Hz/non-sedated group…..………...….
105
5.8
Experience exposure: SF/40 Hz/non-sedated group….………….….
105
5.9
Experience exposure: MF/80 Hz/non-sedated group….………….….
106
5.10
Experience exposure: SF/40 Hz/sedated group……….………..…….
106
5.11
Experience exposure: MF/80Hz/sedated group……….……….…….
107
5.12
Experience exposure: pseudohypacusic group……………………….
107
5.13
Audera electrode placement……………………………………………..
117
5.14
Phase–locked response…………………………………….……………
119
5.15
Random response…………………………………………………………
120
5.16
Example of results rejected due to excess noise…………………….
121
5.17
Plotted results of trials during an ASSR test……………………..……
122
5.18
Estimated audiogram based on the ASSR results……………………
122
6.1
Number of normal pure-tone thresholds for test frequency..….…….
127
6.2
Number of pure-tone thresholds indicative of mild hearing loss per frequency……………………………………………………….…….
127
Number of pure-tone thresholds indicative of moderate hearing loss per frequency………………………………………….………………….
128
6.4
Number of severe pure-tone thresholds - per frequency………..…..
128
6.5
Correlation of pure-tone and ASSR thresholds……………………….
134
6.3
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University of Pretoria etd - De Koker, E (2004)
List of abbreviations
A$
ABR
AEP
AM
AP
AR
ASSR
AIDS
CER
CERA
CNS
COIDA
CF
CM
dB
EAM
EcochG
EEG
ENT
EP
ERP
f
FM
FFT
GB
GGG
HIV
HL
HTL
Hz
IBM
kHz
L
LLR
MASTER
MB
Australian dollar
Auditory brainstem response
auditory evoked potential
amplitude modulated
action potential
acoustic reflex
Auditory steady state response
acquired immune deficiency syndrome
cortical evoked responses
cortical evoked response audiometry
central nervous system
The compensation of Occupational injuries and diseases Act
carrier frequency
cochlear microphonic
decibel
external auditory meatus
electrocochleogram
electro encephalogram
Ear- Nose- and Throat specialist
electrophysiological tests
event related potential
frequency
frequency modulation
fast fourier transform
giga byte
geraas-geïnduseerde gehoorverlies
human immunodeficiency virus
hearing level
hearing threshold level
Hertz
Internationa lbusiness machines
kilohertz
left
late latency response
Multiple auditory steady state response system
mega byte
xi
University of Pretoria etd - De Koker, E (2004)
MF
MF-ASSR
mg
MLR
MM
mg
ms
n
NIHL
OAE
OHC
OMP
OSR
P
PC²
PD
Ps
PT
PTA
LLR
R
RAM
RMA
RSA
SD
SIMRAC
SF
SF-ASSR
SNHL
SP
SPAR
SRT
SSEP
SVR
USB
USA
µV
multiple frequency
Multiple frequency auditory steady state response
milligram
middle latency response
multi-modulation
milligram
millisecond
number
noise-induced hearing loss
oto-acoustic emission
occupational health centre
occupational medical practitioner
ouditiewe standhoudende response
probility value
phase coherence squared
permanent disability
pseudohypacusis
pure-tone
pure-tone average
late latency response
rand (South African currency)
random access memory
Rand Mutual Assurance
Republic of South Africa
standard deviation
Safety in mines research advisory committee
Single frequency
single frequency steady state response
sensory-neural hearing loss
summating potential
sensitivity prediction with the acoustic reflex
speech reception threshold
steady state evoked potential
slow vertical response
universal serial bus
United States of America
microvolt
WCC
Workmen’s Compensation Commissioner
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University of Pretoria etd - De Koker, E (2004)
Title
The
:
clinical
value
of
auditory
steady
state
responses in the audiological assessment of
pseudohypacusic
workers
with
noise-induced
hearing loss in the South African mining industry
Name
:
Elizabeth de Koker
Supervisor
:
Prof S.R. Hugo
Co-supervisor
:
Dr D. Schmulian
Department
:
Communication Pathology, University of Pretoria
Degree
:
Doctor Philosophiae
SUMMARY
Large numbers of South African mine workers incur noise-induced hearing
loss. The prevalence of noise-induced hearing loss is such that its financial
implications for the industry are significant.
This situation is often further
compounded by an exaggeration of their hearing loss by some workers in an
attempt to obtain compensation. Questionable cases must be re-assessed,
increasing the cost of evaluations and the number of unproductive shifts.
The inability to obtain true pure-tone thresholds in unco-operative workers
leads to ineffectiveness in and frustration for audiologists and occupational
health centres because they are not delivering an accountable service to the
mining company and individual workers.
The failure to obtain pure-tone
thresholds may also lead to deserving workers not receiving compensation,
and sudden hearing loss not being diagnosed. Workers unfit for their present
occupations can also be further exposed to noise.
Current audiological procedures can identify instances of exaggerated hearing
loss (pseudohypacusis), but do not quantify the extent of exaggeration.
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University of Pretoria etd - De Koker, E (2004)
Traditional testing techniques require patient co-operation and, hence, are
insufficient to resolve cases where patient co-operation is not forthcoming.
As a result this study was undertaken to determine the value of auditory
steady state responses (ASSRs) as a means of estimating the pure-tone
thresholds of noise-exposed workers. ASSRs need no response from the
patient, and the electrical responses to the presented sound are measured by
means of a real-time statistical analysis of the samples, using a computer,
thereby offering real objectivity.
The following research question was addressed: “What is the clinical value of
ASSRs in the audiological assessment of pseudohypacusic workers with
noise-induced hearing loss?”
An experimental study was conducted, where different protocols and types of
equipment used in the testing of ASSRs were evaluated in a group of mine
workers with noise-induced hearing loss (n=81). The influence of sedation on
the threshold estimation was also evaluated. The proven best protocol was
finally evaluated in a pseudohypacusic group of workers (n=29).
The study indicates that ASSRs are a valid and accurate alternative to puretone testing in populations with noise-induced hearing loss.
The test can
serve as a once-off test procedure for an unco-operative client. The mean
threshold estimates of ASSRs never differed more than 10 dB from the mean
pure-tone thresholds.
The test procedure was accurate throughout the
severity range of hearing loss, and age did not influence the reliability of the
threshold estimates.
Single-frequency techniques were found to be the technique of choice in this
population and it is recommended that the 40 Hz response is employed as a
modulation frequency. Sedation did not have any effect on the length and the
sensitivity of the procedure, and is thus not advocated if co-operation can be
obtained. The length of the procedure is estimated at 60 minutes.
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Finally, this study has contributed to the validation of the technique (previous
research was limited). As a result of this study, the implementation of this
procedure in mines’ audiological centres is advocated since it has been
proven to be of clinical value.
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Titel
Die kliniese waarde van ouditiewe standhoudende
:
response in die oudiologiese evaluasie van
werkers met funksionele en geraas-geïnduseerde
gehoorverlies in die Suid-Afrikaanse mynbedryf
Naam
:
Elizabeth de Koker
Promotor
:
Prof S.R. Hugo
Mede- promotor
:
Dr D. Schmulian
Departement
:
Kommunikasiepatologie, Universiteit van Pretoria
Graad
:
Doctor Philosophiae
OPSOMMING
Mynwerkers doen in die loop van ondergrondse werk geraas-geïnduseerde
gehoorverlies (GGG) op.
Die hoë voorkoms van GGG het finansiële
implikasies vir die mynbedryf. Hierdie finansiële implikasies word vererger
indien werkers hulle gehoorverlies oordryf om meer vergoeding te kry.
Werkers by wie daar nie akkurate gehoordrempels vasgestel kon word nie,
moet herevalueer word en so eskaleer kostes verder.
Dit is verstaanbaar dat 'n onvermoë om drempels by 'n werker te bepaal tot
frustrasie en ʼn gevoel van oneffektiwiteit bydra, wat die oudioloog en ook die
beroepsgesondheidsklinieke direk raak. Die oudioloog is immers daarvoor
verantwoordelik om ʼn toerekenbare diens aan die werkgewer en individuele
werkers te lewer. ’n Onvermoë om gehoordrempels te bepaal het ook tot
gevolg dat werkers met GGG nie die kompensasie wat hulle toekom kry nie
en ’n skielike gehoorverlies kan ongediagnoseerd bly. Werkers met ernstige
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gehoorverlies, word by die gebrek aan akkurate gehoordrempels, verder aan
skadelike geraas blootgestel.
Huidige gehoortoetse kan funksionele gehoorverlies identifiseer, maar kan dit
nie kwantifiseer nie. Lug- en beengeleidingstoetse word internasionaal as die
norm aanvaar, maar vereis samewerking van die pasiënt.
’n Studie is gevolglik onderneem om die waarde van ouditiewe standhoudende response (OSR) in die oudiologiese evaluasie van werkers met
GGG te bepaal en die vraag is spesifiek of die OSR akkurate drempels, in
hierdie volwasse bevolking, kan bepaal. Die veronderstelling is dat ouditief
ontlokte potensiale objektief is en dat geen respons van die pasiënt verwag
word nie. OSR het ook 'n verdere dimensie in objektiwiteit waar die elektriese
response, met statistiese ontleding, deur ’n rekenaar gemeet word.
Die navorsingsvraag wat dus aangespreek word is:
“Wat is die kliniese
waarde van OSR in die oudiologiese evaluasie van mynwerkers met
funksionele komponente tot GGG?“
’n Eksperimentele studie het gevolg waar verskillende toetsprotokolle en
apparatuur gebruik is in die evaluasie van mynwerkers met GGG (n=81). Die
invloed wat sedasie op die drempelbepalings gehad het, is ook evalueer. Die
beste protokol is vervolgens ook in ’n groep mynwerkers (n=29) met
funksionele gehoorverlies getoets.
Die studie het bewys dat OSR ’n geldige en akkurate alternatiewe toets vir
suiwertoonoudiometrie, in ’n volwasse bevolking met GGG is. Die toets kan
as ’n enkeltoets funksioneer indien die pasiënt sawewerking weerhou. Die
gemiddelde drempelskattings van OSR het nooit meer as 10 dB van die
suiwertoondrempels verskil nie. Skattings van gehoordrempels was moontlik
by alle grade en erns van gehoorverlies.
Verder het die ouderdom van
werkers nie ’n invloed op die akkuraatheid van die drempelskattings gehad
nie.
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Daar word aanbeveel dat die enkelfrekwensie-tegniek (monoraal) en spesifiek
die 40 Hz respons gebruik word.
Sedasie het geen invloed op die
akkuraatheid van die drempelskattings en die toetstyd gehad nie en daarom
word sedasie nie aanbeveel as passiewe samewerking van die pasiënt
teenwoordig is nie. Die prosedure het ongeveer 60 minute geneem.
Die huidige studie het verder bygedra tot die beperkte kliniese validasie wat
nog ten opsigte van OSR bestaan. Op grond van hierdie studie word die
implementering van hierdie tegniek in die Suid-Afrikaanse mynwese
aanbeveel, aangesien die kliniese waarde daarvan bewys is.
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CHAPTER 1
INTRODUCTION
AIM
To introduce a study of auditory steady state responses in pseudohypacusic
workers with noise-induced hearing loss in the South African mining
industry, indicating the rationale for the study, the problem statement,
proposed solutions, clarifying terminology and providing an outline of the
study.
1.1 BACKGROUND
Large numbers of South African mine workers incur noise-induced hearing
loss (NIHL), which is recognised as a compensable disease by the
Compensation for Occupational Injuries and Diseases Act, (COIDA), No 130
of 1993. The prevalence of noise-induced hearing loss, is so high that the
financial implications for the industry are significant, threatening the viability of
marginal operations and eroding the profitability of larger companies. The
impact of noise-induced hearing loss on workers’ quality of life and their ability
to earn a living is a matter of even greater concern, as the disease has socioeconomic implications for the entire country and for the Southern African
region as a whole (Franz, 2003).
The financial impact of noise-induced
hearing loss is often compounded by the exaggeration of hearing loss by
some workers, in attempts to obtain compensation (De Koker, 2003).
Audiologists who are consulting in the mining industry are tasked with
quantifying the impact of noise on workers’ hearing, not only for compensation
purposes, but also as a means of determining workers’ fitness for work and
evaluating employers’ hearing conservation programmes (De Koker, 2003).
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The audiological procedures currently used in the South African mining
industry
can
identify∗
instances
of
exaggerated
hearing
loss
(pseudohypacusis), but most audiological procedures cannot quantify the
extent of any exaggeration (Roeser, Valente & Hosford-Dunn, 2000b; Martin,
1994).
This study was therefore undertaken to determine the value of
auditory steady state responses (ASSR) as a means of accurately estimating
the true hearing thresholds of noise-exposed workers, to conclude diagnostic
procedures
and
to
enable
appropriate
recommendations
regarding
rehabilitation and/or compensation to be made.
1.2
RATIONALE
Most adults examined by audiologists for complaints about hearing loss have
genuine disorders of the auditory mechanism. The audiologist must establish
(among other tasks) the type and extent of hearing loss in order to determine
the most appropriate course of action. Some options are rehabilitation (Stach,
1998), re-allocation or, in extreme cases, compensation and/or job termination
(De Koker, 2003).
Unfortunately, some patients are not entirely co-operative during audiological
procedures.
This lack of co-operation can be due to several reasons,
including possible misunderstanding of test procedures or their purpose,
physical or psychological disorders, or the intention deliberately to
misrepresent their hearing thresholds (Martin, 1994). Qiu et al. (1998) are of
the opinion that such a lack of co-operation can be unconscious
(psychogenic) or deliberate (malingering). The term “pseudohypacusis”, (from
“pseudo”, meaning “false”/ or “less-than-true”, and “hypacusis”, meaning
“hearing loss”, is generally applied for cases of malingering (Rintelmann,
Schwan & Blakley, 1991; Roeser, Valente & Hosford-Dunn, 2000).
‫٭‬According to Stach (1998) audiological evaluation involve the determination of both the type and the
extent or degree of hearing loss
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In a review of literature on pseudohypacusis, Rintelmann et al. (1991) have
found that the highest prevalence of the condition of pseudohypacusis has
been noted among adult populations in which reporting hearing loss can result
in monetary compensation. In the South African mining industry, the practice
of paying compensation for noise-induced hearing loss is well documented
(RMA guidelines, 2003; Franz, 2003; Franz & Phillips, 2001). Furthermore, it
is well known that the prevalence of compensable noise-induced hearing loss
is greater in the mining industry than in most other industries, largely because
of the use of noisy machinery in confined and highly reverberant underground
workplaces (Franz & Phillips, 2001). The labour-intensive methods used in
many South African mines, particularly in conventional gold and platinum
operations, where workforces are large, greatly increase the risk of noiseinduced hearing loss (Franz & Phillips, 2001).
Recent experience with noise-induced hearing loss in the mining industry has
shown that between 12 and 14 per cent of claims for all forms of disease and
injury have been for noise-induced hearing loss. These 12 per cent of claims
have accounted for nearly 40 per cent of compensation paid out (Begley,
2002). The above numbers indicate that noise-induced hearing loss is costing
the industry a great deal more than would be expected in view of its
prevalence in comparison to that of other occupational diseases.
Noise-
induced hearing loss claims settled by Rand Mutual Assurance, the
underwriters of compensable risks for nearly 80 per cent of the local mining
workforce, have come to between R76m and R110m since 1998 (Begley,
2003). If it is assumed that claims from the 22 per cent of mine workers who
are otherwise insured (for example, by the Workmen’s Compensation
Commissioner) are proportionally similar, settlements for noise-induced
hearing loss, industry-wide, can be estimated at between R98m and R142m
since 1998. These amounts are undeniably substantial and they still fail to
include the cost of repeat assessments, specialist referrals and transport
arrangements. Nor do they include time off work and lost production. It must
therefore be concluded that noise-induced hearing loss risks impose a
significant financial and human resources burden on individual mining
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operations and on the entire industry. They pose a threat to sustainability and
create a potential for socio-economic impact that a developing country can ill
afford (Franz, 2003).
Recent evidence indicates that thousands of workers (14 per cent of the total
workforce of 300 000) have incurred noise-induced hearing loss in South
African mines and are therefore entitled to compensation (Franz &
Phillips, 2001). However, the experience of audiologists working in the mining
industry suggests that there are significant numbers of claimants who
exaggerate the existing hearing loss that they do experience, probably in an
attempt to qualify for compensation or increase their settlement amounts
(Franz, 2003).
Franz and Phillips (2001) claim that audiologists consulting in mines’
Occupational
Health
Departments
universally
cite
malingering,
or
pseudohypacusis, as the greatest impediment to an assessment of the true
hearing status of patients referred to them. Over a three-month period in
2002, De Koker (2003) found clear indications of pseudohypacusis in 32 per
cent of the 160 cases referred to her for audiological assessment. In these
cases, the diagnosis of pseudohypacusis was based on discrepancies of
more than 15 dB between the thresholds recorded during two pure-tone tests,
in accordance with the criterion proposed by Rintelmann et al. (1991).
Rickards, de Vidi and McMahon (1996) have examined the financial impact of
pseudohypacusis, citing Australian studies that report the incidence of
pseudohypacusis to be between nine and 30 per cent among workers tested
for compensation purposes. The same authors found that individual workers
exaggerated their hearing loss by 12,2 per cent, on average, concluding that
undetected exaggeration of hearing loss can lead to substantial increases in
compensation payouts and, hence, in employers’ costs for insuring their
companies against the risk of noise-induced hearing loss. Rickards and De
Vidi (1995) estimate that overcompensation to an average amount per claim
of A$ 7 357, amounting to A$ 12m per year, is awarded to workers with
exaggerated hearing loss in Australia. The South African mining industry, with
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its much larger workforce and greater pressures on profitability (Franz, 2003)
can ill afford such a waste of financial resources.
Pseudohypacusis also has a further financial impact, in that current
audiological procedures rely on workers’ co-operation to determine hearing
thresholds. Consequently, questionable cases must be re-assessed several
times by the consulting audiologists and Ear-, Nose-, and Throat (ENT)
specialists (RMA guidelines, 2003). Such repeated testing increases the cost
of evaluation and the number of unproductive shifts.
Diagnostic hearing evaluations employ mainly pure-tone air- and bone
techniques, combined with speech discrimination testing, in accordance with
the Workmen’s Compensation Commissioner’s (1995) Internal Instruction
No 168. Although these procedures are regarded internationally as the gold
standard for threshold determination, they require patient co-operation and,
hence, are insufficient to resolve cases where such co-operation is not
forthcoming (De Koker, 2003). The discussion above indicates that there is a
need for reliable means of identifying pseudohypacusis, and of accurately
recording noise-exposed workers’ true hearing thresholds.
Martin (2000, p.594) argues that, in the majority of cases, it is not difficult to
detect pseudohypacusis, but that “the more challenging responsibility of the
audiologist is to determine the patient’s organic thresholds of hearing”.
Several indicators of pseudohypacusis and special qualitative tests have been
developed bearing in mind a pseudohypacusic population (Roeser, Valente &
Hosford-Dunn, 2000; Martin, 1994). Qualitative tests have come and gone,
and some have even become obsolete (Martin, 2000), because of the
necessity for this much sought-after procedure to provide true hearing
thresholds, and not only to identify pseudohypacusis as present.
The introduction of electrophysiological tests is the latest development in
Audiology as a clinical science (Hall, 2000; Roeser et al., 2000b): Immittance
measures developed in the 1970s, auditory brainstem response testing in the
1980s and oto acoustic emissions in the last decade of the 20th century
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(Hall, 2000). These audiological procedures differ from earlier tests primarily
in that no voluntary response indicating “hearing” is required from the patient
(Schmulian, 2002).
Hall (1992) specifically advocates the use of
electrophysiological tests as an objective means of determining auditory
sensitivity. Electrophysiological tests have also been seen as the answer in
difficult-to-test populations (Schmulian, 2002), of which pseudohypacusic
mine workers are one example. In this regard, it is true that: “for measures of
true thresholds our profession has tended to turn to electrophysiological
procedures” (Martin, 2000, p.592).
However, in this regard, one must
remember that electrophysiological tests are not tests of hearing, per se, (∗)
but that they do, fortunately, have the ability to predict auditory thresholds
(Sininger & Cone-Wesson, 1994).
In the South African mining industry, the current prescribed procedure in
cases where reliable thresholds cannot be obtained is to retest the worker
involved after six months (RMA guidelines, 2003). If accurate thresholds are
still not obtained, an auditory brain stem response test (ABR) must be done
(RMA guidelines, 2003).
The electrophysiological test generally used in
pseudohypacusic populations in the South African mining industry has thus
been the ABR.
The ABR test measures far field evoked potentials by means of electrodes on
the scalp of the patient, thereby endeavouring to estimate hearing thresholds.
Electrical activity is measured specifically sub-cortically, and only up to
brainstem level.
ABR tests measure transient responses elicited by brief
acoustic stimuli (Swanepoel, 2001). The most widely used stimulus is a broad
band click, which stimulates a large portion of the basilar membrane, giving an
indication of hearing thresholds in a range between 2 000 and 4 000 Hz
(Schmulian, 2002).
∗
Electrophysiological procedures, and particularly the auditory brain stem response, examine only a
limited portion of the auditory system. The presence of an ABR indicates that synchronous neural firing
is present only up to the level of the midbrain. Thus, the processing of sound is not measured at the
cortical level. Similarly, the absence of an ABR does not prove that peripheral hearing loss exists, since
disorders of the brainstem may obliterate an ABR, even though the peripheral auditory system is
normal. A conventional hearing test relies, in the final instance, on a conscious behavioural response
(Weber, 1994).
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The limitation to being able to obtain threshold information only at 2 000 to
4 000 Hz has led to the development of additional (novel) stimuli, including
tone bursts, filtered clicks and masking techniques, to obtain more frequencyspecific information (Hood, 1998). The key to frequency specificity during
ABR testing lies in the type of signal used (Halliday, 1993). Tone bursts are
used to give low frequency information (Swanepoel, 2001), while masking
techniques have to eliminate the effects of unwanted high frequency energy in
gradual stimulus onset techniques (Weber, 1994). It has been found that pure
tones cannot elicit sufficient neural synchrony for ABR testing (Goldstein &
Aldrich, 1999).
Unfortunately, however, it seems that ABR testing in the mining industry has
not supplied a final and satisfying solution to the increasing phenomenon of
pseudohypacusis amongst mine workers. Weber (1994) and Gorga (1999)
have pointed out the practical fact that the time needed to obtain a single ABR
threshold for each ear exceeds 30 minutes, making full audiograms
impractical.
Furthermore, compensation assessments require frequency-
specific information at five frequencies in each ear, namely at the following
frequencies:
500, 1 000, 2 000, 3 000 and 4 000 Hz (Workmen’s
Compensation Commissioner, 1995). Because of the limitations inherent in
the procedure, it is clear that the ABR cannot be used for compensation
purposes. The high cost of instrumentation and software is a further limitation
(Schmulian, 2002). Finally, the subjective interpretation of ABR results (wave
forms) and the considerable amount of experience needed by clinicians to
perform a successful ABR test make the use of this diagnostic procedure a
less than satisfactory option (Swanepoel, 2001).
1.3
PROBLEM STATEMENT
From the above discussion, it is clear that current audiological procedures
(behavioural and electrophysiological) cannot provide all the specified
thresholds
for
determining
fitness
Compensation Commissioner, 1995).
and
compensability
(Workmen’s
It is necessary to search for some
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solution that can address all the needs related to diagnostic audiology in the
mining industry in South Africa.
Is
there
an
audiological
technique
available
that
can
identify
pseudohypacusis and, more importantly, estimate the true behavioural
thresholds of pseudohypacusic mine workers with noise-induced
hearing loss?
1.4 PROPOSED SOLUTION
From the discussion above, it is clear that the required audiological procedure
for obtaining hearing thresholds for members of this unco-operative
population needs to
•
estimate behavioural thresholds accurately;
•
estimate hearing thresholds at all the required frequencies, namely at
500, 1 000, 2 000, 3 000 and 4 000 Hz;
•
estimate hearing thresholds accurately in workers with abnormal
hearing, in all degrees of abnormal hearing ranging from mild to
profound hearing loss;
•
be independent of the patient’s co-operation;
•
be independent of the clinician’s experience and perception; and
•
be cost-effective.
The above criteria for an audiological solution to pseudohypacusis in the
mining industry suggests that a possible solution may lie in the domain of
auditory evoked potential testing.
A novel auditory evoked potential technique known as auditory steady state
responses (ASSRs) was discovered and developed in Australia at the
University of Melbourne during the 1980s (ERA Systems, 2000).
This
technique addresses the main shortcomings of ABR testing, in that it does not
suffer from the spectral distortion problems associated with short-duration
stimuli (Rance et al., 1995). ASSRs are periodic scalp potentials arising in
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response to regularly varying stimuli, such as amplitude and/or frequency
modulated tones (Rance et al., 1998).
Several authors have found a close correspondence between ASSRs and
pure-tone thresholds (Reneau & Hnatiow, 1975; Rance et al., 1998;
Swanepoel, 2001; Schmulian, 2002). Rance et al. (1995) have developed a
linear regression analysis to translate electrophysiological thresholds into a
conventional audiogram to within 10 dB in 96 per cent of cases.
ASSR is a frequency-specific technique used for the estimation of hearing
status. This technique was considered as a possible solution to the problem
of pseudohypacusis, because all frequencies required for compensation
purposes can be tested (John, Dimitrijevic & Picton, 2002) via the
measurement of auditory evoked potentials (Picton, 2001). Electrical activity
is evoked by frequency-specific tonal stimuli within the standard range of 250
to 8 000 Hz (ERA Systems, 2000). When the stimulus is presented at or
above the hearing threshold, hair cells in the cochlea are activated in the
region that is sensitive to the primary frequency of the tone. ASSRs can thus
be elicited at all the frequencies needed for compensation and fitness for duty
assessments. Lins et al. (1996) have further proven that the configuration of
the hearing loss does not have an influence on the accuracy of ASSR results.
The validity of ASSR thresholds has been more extensively researched in
populations with normal hearing (Schmulian, 2002) than on other populations.
Limited research on other populations also seems to indicate that ASSR
testing is a suitable substitute for pure-tone testing in people with hearing loss
(Schmulian, 2002). Rance et al. (1995) mention the further positive point in
the prediction value of ASSRs, in that ASSRs give more accurate estimates of
hearing thresholds in pathological ears, possibly due to the effect of
recruitment.
Apart from the fact that no response is required from the patient, analysis of
the results of this test requires no visual or subjective evaluation from the
clinician, as computer-based algorithms are applied to the recorded signals
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University of Pretoria etd - De Koker, E (2004)
(Perez-Abalo et al., 2001). The latter feature has been an elusive criterion in
auditory evoked potential testing thus far. If no interpretation is required by a
clinician, true objectivity is possible. Lack of experience among clinicians is
also not longer a problematic factor.
From the above discussion of the features of ASSR testing (which are more
extensively discussed in Chapter 4), it seems possible that this technique
could provide a solution to the problems of an audiological evaluation of
pseudohypacusic mine workers.
The implementation of ASSR testing in audiological assessments of noiseinduced hearing loss cases, and particularly pseudohypacusic cases, offers
the potential benefits of accurate threshold determinations, with significant
cost savings for employers and their insurers, due to the elimination of
overcompensation and unnecessary referrals and retests. Savings through
the elimination of unproductive shifts are also envisaged. Secondary benefits
include greater efficiency at audiological test centres.
The application of
current knowledge and state-of-the-art methods ensure that internationally
accepted best practice is followed in the evaluation of noise-induced hearing
loss.
With the above possible contribution (knowledge) of ASSR in mind, the
following research question can thus be explored: What is the clinical value
of auditory steady state responses in the audiological evaluation of
pseudohypacusic mine workers with noise-induced hearing loss?
Schmulian (2002) has found in a review of the relevant literature that ASSR
has, so far, had limited clinical and research validation. Previous studies have
focused mainly on small experimental groups with normal hearing. This study
could thus enhance current knowledge by using a significantly large
experimental group and by focusing on noise-induced hearing loss (no
previous research in this area could be found).
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1.5 PURPOSE OF THE STUDY
The purpose of this study is to evaluate the value of ASSR testing as an
efficient
and
objective
method
to
estimate
hearing
thresholds
for
compensation purposes, with specific reference to mine workers who display
pseudohypacusis and noise-induced hearing loss.
1.6 CLARIFICATION OF TERMINOLOGY
The terms below are used in this study and must be clarified.
1.6.1 AUDITORY EVOKED POTENTIAL (AEP)
AEPs are very small electrical voltage potentials originating from the nervous
system and recorded from the scalp in response to auditory stimuli (Picton,
1991).
1.6.2 AUDITORY STEADY STATE RESPONSE (ASSR)
An auditory steady state response (ASSR) is an auditory evoked potential
arising in response to regularly varying stimuli, such as sinusoidal amplitudeand/or frequency-modulated tones (Rance et al., 1998).
Although the
acronym SSEP (steady state evoked potential) is probably a more correct
term, Sininger and Cone-Wesson (1994) conclude that ASSR has become the
term of choice.
1.6.3 NOISE-INDUCED HEARING LOSS (NIHL)
Noise-induced hearing loss is a sensory neural hearing loss caused by noise
exposure. A decrease in hearing is typically seen first in the frequency range
from 3 000 to 6 000 Hz. Hearing loss is usually symmetrical (Roeser, et
al., 2000b)
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1.6.4 PSEUDOHYPACUSIS
“Pseudohypacusis” is the generally accepted term used to indicate a hearing
loss greater than can be explained by a disorder in the auditory system.
“Pseudo” indicates falseness and “hypacusis” a less than normal auditory
sensitivity, or hearing loss (Martin, 1994, 2000; Roeser et al., 2000b).
1.7
OUTLINE OF THE STUDY
To address the research question set out in Section 1.4 above, this thesis is
organised as set out below.
•
CHAPTER 1: INTRODUCTION
The problem of the high incidence of noise-induced hearing loss is explained.
Noise-induced hearing loss costs the mining industry millions of Rands in
compensation and its effects are further compounded by the high incidence of
pseudohypacusis.
Some workers exaggerate hearing loss and are unco-
operative during audiological evaluations. This lack of co-operation in the
hope of gaining monetary reward leads to problems in obtaining accurate
assessments of workers’ hearing thresholds, as required to estimate
compensation and make recommendations on fitness for duty.
In
consequence, numerous retests are done and specialist referrals are made in
the process of searching for true hearing thresholds. This further escalates
costs, and leads to frustration and ineffectiveness at audiological test centres.
A new AEP, ASSR testing, is put forward as a possible solution to the problem
of identifying pseudohypacusic mine workers.
This technique has the
potential to address the shortcomings of ABR testing (the test of choice where
patient co-operation is absent up to this point in time).
•
CHAPTER
2:
PSEUDOHYPACUSIS
AND
APPROPRIATE
STRATEGIES TO DETECT AND QUANTIFY THE CONDITION
The phenomenon of pseudohypacusis is discussed to enable the research
question to be addressed. Pseudohypacusis is defined, and the reader is
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familiarised with the acronyms used in the relevant literature. The prevalence
of and etiological factors involved in pseudohypacusis are discussed
concurrently, since prevalence is closely linked to motivating factors.
The role of current audiological procedures in detecting pseudohypacusis in
the unco-operative population of mine workers who hope to gain
compensation is evaluated. Most of the current techniques and tests that
have become obsolete fail to establish true hearing thresholds. Hence the
audiological profession has turned to electrophysiological measures, since no
response is needed from the patient when using these tests.
In the
discussion of pseudohypacusis, the limited knowledge about its prevalence
and the shortcomings of the audiological strategies used in the South African
mining industry are evaluated.
•
CHAPTER 3: ELECTROPHYSIOLOGICAL TESTS AND THEIR USE IN
THE ASSESSMENT OF PSEUDOHYPACUSIS
Electrophysiological tests are dealt with in Chapter 3. The discussion of these
tests is the logical next step since, historically, audiologists have turned to
these tests as a solution to the problem of identifying pseudohypacusic
patients. These tests require no behavioural response from patients, and are
thus seen as objective tests. It is thus clear why audiologists have relied on
these types of tests in dealing with difficult-to-test populations.
Different types of electrophysiological tests are described and evaluated.
Electrical responses to auditory stimuli originating in the central nervous
system
and
in
reflexive
muscular
responses
are
sub-groups
of
electrophysiological tests.
Auditory evoked potentials (AEPs) are discussed in more detail, and attention
is paid to nomenclature and definitions of AEPs.
The history and
development of AEPs also receive attention, as does the problematic
classification of auditory evoked potentials.
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The measurement of AEPs is deliberated with specific reference to the system
requirements for amplification, signal averaging and the stimuli used.
Finally, different auditory evoked potentials currently known are described with
reference to the latency epoch after stimulation.
ABR, the most popular
auditory evoked potential method used in clinical audiology and in the South
African mining industry, receives the most attention.
•
CHAPTER 4: AUDITORY STEADY STATE RESPONSES (ASSRs)
AND PSEUDOHYPACUSIS
ASSR, a new and objective test for hearing threshold estimation, is central to
this literature evaluation. Arguments explaining the rationale for choosing this
particular AEP to feature in the empirical part of the research are supplied.
Different acronyms used for this AEP are listed, and definitions are set out.
After its historical development has been explained, the advantages and
disadvantages of this novel audiological technique are discussed in order to
evaluate it critically as a possible solution to the research question.
No discussion of ASSRs would be complete without an explanation of the
types of stimuli that are used in eliciting the AEP. Stimulus intensity, carrier
frequencies and modulation frequency are also important information in the
stimulation of this evoked potential.
Two different stimulation methods,
namely monotic and dichotic stimulation, are explained.
This chapter sets out the apparatus used, the influence of the subject on the
test and the objective analysis of the results. The chapter concludes with the
response generators of ASSRs and the application of this procedure in clinical
audiology.
This application is very important, since it also influences the
potential application in the difficult-to-test experimental group.
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•
CHAPTER 5: RESEARCH METHODS
The literature reviews of pseudohypacusis (Chapter 2), electrophysiological
tests (Chapter 3) and ASSRs (Chapter 4) provide a scientific basis for the
methodology of the experimental research.
An empirical study tested the clinical value of ASSR tests for a sample of mine
workers with noise-induced hearing loss in selected gold mining companies in
Randfontein and Carletonville in South Africa.
The principal and sub-aims of the experimental research are put forward, after
which the research design is explained.
The group of mine workers with
noise-induced hearing loss tested using ASSR consisted of five subgroups,
for which the effects of sedation, monotic and dichotic stimulation and different
modulation frequencies were compared. The experiment on this first group
(Phase 1) was planned to provide the best ASSR method, which was
subsequently tested in a group of mine workers with pseudohypacusis
(Phase 2), to establish whether ASSR methods can conclude the audiological
test procedures for this group and lead to meaningful recommendations.
Data collection apparatus and procedures are highlighted with reference to
pure-tone testing and multiple-frequency and single frequency ASSR testing.
Finally, the data analysis apparatus and procedures are explained.
•
CHAPTER 6: RESULTS
The value of any diagnostic test depends on its ability to fulfil its intended
purpose (Roeser et al., 2000b). Data obtained in this study was analysed,
organised and presented to demonstrate that ASSR thresholds can fulfil its
intended purpose in the normative group of mine workers with noise-induced
hearing loss, as well as in the pseudohypacusic group.
This study proves that ASSR testing is sensitive enough to estimate
behavioural thresholds in a population of mine workers with abnormal and
noise-induced hearing loss, and that, if workers exaggerate their hearing loss,
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ASSRs can estimate the true thresholds and thus conclude the diagnostic
procedures with the correct recommendations regarding the fitness,
compensability and further handling of the patient.
•
CHAPTER 7: CONCLUSIONS AND RECOMMENDATIONS
Having shown the value of ASSR testing in the experimental research, the
thesis concludes by critically evaluating the research and its limitations,
making recommendations for further research and the implementation of this
procedure in practice.
1.8 SUMMARY
This chapter has described the problems audiologists face in identifying and
quantifying pseudohypacusic noise-induced hearing loss patients in the
South African mining industry.
Conventional tests do not provide the
accurate hearing thresholds required for compensation purposes, especially
when patients are unco-operative or attempting to deceive. Unless it can
measure accurate thresholds, the mining industry stands to suffer monetary
loss, audiologists‘ effectiveness is impaired and cases are rarely concluded.
A study of ASSR testing is proposed as a solution for the shortcomings of
existing audiological procedures.
The research problem has been
formulated, and an outline of the thesis is provided. The second chapter is
intended to explore pseudohypacusis as the reason why this study was
necessary.
It shows why audiology is sometimes an art, rather than a
science when one is working with a pseudohypacusic population (De Koker,
2003).
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CHAPTER 2
PSEUDOHYPACUSIS AND APPROPRIATE
STRATEGIES TO DETECT AND QUANTIFY THE
CONDITION
AIM
To define and describe the phenomenon of pseudohypacusis and to analyse
and evaluate the audiological strategies currently available to audiologists to
detect this phenomenon.
2.1
INTRODUCTION
In Chapter 1, the high incidence of exaggerated hearing test results in the
South African mining industry, as well as the negative impact this
exaggeration potentially has in terms of cost and the effectiveness of
audiological centres, was described. This sets the scene for this study which
attempts to evaluate one possible way to address deliberate exaggeration of
hearing loss on the part of noise-exposed mine workers.
Deliberate and potentially deceptive exaggeration of hearing loss is called
“pseudohypacusis” (Martin, 1994). The phenomenon of pseudohypacusis is
examined in this chapter in terms of its prevalence and causative factors in
order to make it possible to evaluate better possible alternatives to present
audiological methods used in the assessment of workers who present with
this problem. This discussion not only looks at audiological methods able to
detect pseudohypacusis, but specifically addresses the techniques employed
in determining hearing thresholds. In searching for an audiological solution to
pseudohypacusis,
the
first
step
is
pseudohypacusis.
17
to
define
the
phenomenon
of
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2.2
DEFINITION OF PSEUDOHYPACUSIS
Hearing loss greater than that which can be explained solely by a disorder
within the auditory system has been variously described as non-organic
hearing loss, pseudohypacusis/pseudohypocusis, psychogenic hearing loss,
feigned hearing loss, malingering, functional hearing loss, conversion
deafness and simulated hearing loss (Rintelmann et al., 1991; Martin, 1994,
2000; Roeser et al., 2000b).
Rintelmann et al. (1991:381) define
pseudohypacusis, from the point of view that “the patient exhibits a hearing
loss in some fashion but where there is no organic basis readily apparent for
the disorder”. To summarise: pseudohypacusis refers to false or exaggerated
hearing thresholds “measured” due to a lack of co-operation from the patient.
Recent literature suggests that pseudohypacusis is the most widely used term
for this phenomenon at the moment. It is also clear that there is very limited
new research on this phenomenon. Audiological textbooks (Roeser et al.,
2000b; Martin, 2000) summarising research in this field refer mainly to
research done in the 1960s and 1970s.
More recent literature on
pseudohypacusis and especially in the field of noise-induced hearing loss is
limited, but authors such as Rickards and De Vidi (1995), Qiu et al. (1998) and
Rickards et al. (1996) deserve to be mentioned as making some contributions.
The phenomenon of pseudohypacusis can be better understood within the
framework of the prevalence and etiology of this condition, which are
interrelated.
2.3
PREVALENCE AND ETIOLOGICAL FACTORS
The basis of pseudohypacusis, according to Qiu et al. (1998), can be
conscious (malingering) or unconscious (psychogenic).
From a clinical
position, it is clear that it is difficult to determine whether a false threshold is
the result of a conscious or an unconscious motive and it is thus more
appropriate to refer to this phenomenon only in terms of the concept of
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false/less than true (pseudo) hearing loss (hypacusis). It is also important to
remember that feigned hearing thresholds can be superimposed on a true
organic component (Roeser et al., 2000b). In this regard, researchers have
proven that pseudohypacusis is more frequently superimposed on a true
organic component than on normal hearing sensitivity (Qiu et al., 1998).
A number of factors may encourage a person to feign a hearing loss that does
not exist or to exaggerate one that does. The reasons for pseudohypacusis
can be classified as two groups, based on the financial and/or the
psychological gain a patient wishes to obtain (Roeser et al., 2000b). From
this, it can be concluded that the prevalence of pseudohypacusis is highly
variable, depending on the population examined (Rintelmann et al., 1991).
Since the prevalence of pseudohypacusis is so closely linked to potential
causative or motivating factors, these two aspects are dealt with concurrently
in the discussion below.
Rickards et al. (1996) have shown that pseudohypacusis plays a significant
role in noise-induced hearing loss claims and in their financial impact on
employers. They have reviewed studies that have found that the prevalence
of pseudohypacusis varies between nine and 30 per cent of compensation
claims, and they add that 18 per cent of noise-induced hearing loss claimants
in the Australian state of Victoria are referred for evoked response testing,
indicating that true thresholds cannot be established through conventional
methods.
Qiu et al. (1998) estimate the prevalence of pseudohypacusis in the military to
be between 15 and 20 per cent of referrals from the US Veterans’
Administration. It is also interesting that, in a review of the literature, it has
been found that the prevalence of pseudohypacusis is greatest among adult
workers who may qualify for monetary compensation if occupational hearing
loss can be demonstrated (Rintelmann et al., 1991). It also seems that the
phenomenon is increasing. In one study, it was found that service-connected,
non-organic hearing loss had increased from ten per cent to nearly 60 per
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cent in the ten years following World War II (Johnson, Work & McCoy, 1956).
Martin (1994) cites a study that found that 24 per cent of 116 workers applying
for compensation were pseudohypacusic. Martin (1994) points to an increase
in the number of pseudohypacusic cases since the implementation of laws
regulating noise in the workplace in the USA.
The incidence of pseudohypacusis in South Africa has not yet been studied,
but audiologists consulting in the mining industry regard it as significant. De
Koker (2003) has kept records of 160 cases referred for compensation
evaluations during a three-month period in 2002. Of these, 32 per cent were
found to have exaggerated their hearing loss An increased prevalence of
pseudohypacusis has been noted in South Africa since the implementation of
the Workmen’s Compensation Commissioner’s (1995) Instruction 168.
Industry-wide, this could partially account for the dramatic increase in noiseinduced hearing loss claims since 1995 (De Koker, 2003). This instruction
lowered the “fence” for compensation from a 42 dB average hearing loss to
26 dB. This entitled more workers to compensation, resulting in an escalation
of mining industry claims for noise-induced hearing loss from eight per cent of
all claims for disease and injury to the current level of 14 per cent (Begley,
2001). It is possible that workers’ awareness of the potential for monetary
gain from hearing loss has also increased, and that this is apparent from the
behaviour of patients during noise-induced hearing loss evaluations.
The foregoing discussion can perhaps lead to a wrong conclusion that all
pseudohypacusic cases are associated with monetary gain. Psychological
factors also contribute to the prevalence of pseudohypacusis (Martin, 1994;
2000). Some of the most important studies of social and psychological factors
associated with this phenomenon are summarised in Table 2.1 (overleaf).
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TABLE 2.1: PSYCHOLOGICAL ANOMALIES FOUND IN PSEUDOHYPACUSIC PATIENTS
TYPES OF
ANOMALIES
Behavioural anomalies
PSYCHOLOGICAL/SOCIAL
ANOMALIES
•
SOURCE
Avoidance of undesirable Martin, 2000
situations
•
Emotional disturbances
•
Tendency to hypochon- Trier & Levy, 1965
Trier & Levy, 1965
dria
•
Diminished confidence in Martin, 1994
meeting needs of everyday life
Deviant social behaviour
•
High incidence of perso- Haseman, 1991
Gold et al., 1991
nality disorders
•
Lack of adjustment to
hearing loss
Financial status
Gold, Hunsaker &
•
•
Lower
Martin, 1994
socio-economic Trier & Levy, 1965
status
Symptoms impacting
•
on health
Intelligence anomalies
Psychosomatic
com- Gold et al., 1991
plaints
•
Lower levels of verbal Trier & Levy, 1965
intelligence
•
Poor academic achieve- Gold et al., 1991
ment
One can deduce from the above table that it is possible for pseudohypacusic
patients to have deviant emotional/social adaptation and symptoms, but also
that they may fall into a lower socio-economic class, which might explain their
malingering for financial gain. Lower levels of verbal intelligence could also
add to the belief that they will be able to exaggerating a hearing loss.
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The question of why a pseudohypacusic patient claims to have a hearing loss
and not some other type of disability can be raised. Martin (1994) suggests
that a patient may select this disorder in consequence of a previous incident
or circumstance that has focused his attention on hearing, for example, an ear
infection, physical trauma to the ears, tinnitus or noise exposure.
This
suggestion is particularly relevant for the mining industry. It is certainly true
that a high incidence of noise-induced hearing loss is already present in this
population, and this increases the awareness of hearing loss. Most mine
workers are also male, and thus carry the burden of being breadwinners.
Receiving a settlement amount of thousands of rands for a hearing loss is
often their only way of ever accumulating a sizable amount of money
(Geyser, J., 10 March 2003: personal communication).
From the above it is clear that there is some disagreement in the literature
about whether pseudohypacusis is psychogenic, or deliberately and
consciously chosen in the hope of personal gain. Goldstein (1966) suggests
that psychogenic (unintentional), cases of exaggerated hearing loss do not
exist, and that all pseudohypacusis cases are conscious pretences.
The
experience of many audiologists in the mining industry suggests that this is
often the case where compensation is involved (De Koker, 2003).
2.4
AUDIOLOGICAL ASSESSMENT OF PSEUDOHYPACUSIS
In dealing with pseudohypacusic patients, audiologists face a twofold
challenge. The first is the detection of pseudohypacusis, and the second the
determination of true hearing thresholds in such patients (Martin, 2000). The
audiologists’ responsibility goes further, taking into account the need for
rehabilitation: “identification of pseudohypacusis is extremely important not
only to ensure that the patient receives appropriate intervention but also to
avoid potentially harmful intervention.”
(Roeser et al., 2000b:329, own
emphasis).
It is thus clear that appropriate, relevant and practical audiological procedures
are imperative to ensure correct, relevant and professional management of
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pseudohypacusic workers. Current audiological testing methods and existing
clinical knowledge do contribute to the detection of pseudohypacusis, but very
often fail to establish true hearing thresholds.
Some of the audiological
indicators used in the detection of pseudohypacusis are discussed below.
2.4.1 REASON FOR REFERRAL
In many cases, the reason for the referral of the patient in itself suggests the
possibility of pseudohypacusis, for example, when a patient is referred in
order for the audiologist to investigate or evaluate a compensation claim (Qiu
et al., 1998). Martin (2000) points out that referrals from attorneys and the
veterans’ administration should alert audiologists to the possibility of
pseudohypacusis. In practice, often this problem is suggested when a patient
already has an extensive file of previous tests and specialist opinions.
2.4.2 PATIENT BEHAVIOUR
Patient behaviour during the interview and test situation very often aids
audiologists in detecting pseudohypacusis.
Information gathered by a skilled clinician in informal observation of the
patient before and during the taking of the case history is helpful in the
diagnosis of pseudohypacusis (Roeser et al., 2000b).
The patient’s body
language can feign reliance on lip-reading, and he may also ask the
interviewer to repeat questions or instructions. This is not common in people
with true loss of hearing (Martin, 2000).
In the author’s experience,
pseudohypacusic patients very often claim to suffer from symptoms
associated with hearing loss, and tend to exaggerate these symptoms. So for
instance, they answer in the affirmative to all symptoms that the clinician
inquires about.
The above indicators of pseudohypacusis are not always available to all
audiologists in the South African mining industry, since language and
communication barriers can arise, especially because foreign workers are
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employed. In such cases, the interpreters employed in audiological centres
need to be made aware of a possible exaggeration of symptoms, and to
receive training in interviewing skills. These interpreters also need a basic
knowledge of how hearing loss and particularly different degrees of hearing
loss affect communication behaviour.
Discrepancies between audiometric results and a patient’s social functioning
should also alert clinicians to the possibility of pseudohypacusis.
It is
impossible for a patient with profound bilateral hearing loss to respond
appropriately
to
questions
or
instructions
presented
at
conversational level of 50-60 dB, particularly if any attempt to
a
normal
lip-read is
subverted (Martin, 1994).
Extremely slow and deliberate responses, according to Martin (1994), are
indicative of pseudohypacusis, because most people respond immediately to
test signals. The experience of the researcher supports the contention that
audiologists should suspect pseudohypacusis where patients responded
slowly. Finally, Gold et al. (1991) state that exaggerated body movements
and facial expressions (for example, sitting on the edge of the chair and
grimacing as if to suggest extreme concentration) should be regarded as
possible signs of pseudohypacusis.
2.4.3 PURE-TONE AUDIOMETRY
Martin (1994) identifies two types of potential error in the determination of
pure-tone thresholds, namely false-negative and false-positive responses.
Failure to respond at levels above the true threshold constitutes a falsenegative
response,
which
is
the
most
important
characteristic
of
pseudohypacusis.
According to Qiu et al. (1998), it is not difficult for an audiologist to detect
pseudohypacusis using conventional audiological procedures. This may be
true for experienced audiologists but, unfortunately, inexperienced audiolo-
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gists often fail to scrutinise patients’ behaviour and other indicators of
underlying intentions, and hence may not detect the presence of exaggerated
hearing loss. Using current methods, considerable time and effort may be
needed for the evaluation of pseudohypacusic cases if lack of co-operation is
not detected by a clinician. Nevertheless, pure-tone audiometry, as part of the
basic audiological assessment battery, plays a very important role in the
detection of pseudohypacusis. Pure-tone audiometry is also prescribed in the
current South African legislation. Instructions 168 and 171 of the Workmen’s
Compensation Commissioner specify that a response to 500, 1 000, 2 000,
3 000, 6 000 and 8 000 Hz needs to be tested for compensation purposes
(Workmen’s Compensation Commissioner, 1995).
Pure-tone audiometry can assist in the detection of pseudohypacusis in the
following ways set out below.
2.4.3.1
Inconsistent thresholds
Rintelmann et al. (1991) state that the best indicator of pseudohypacusis is
inconsistent tests responses.
Where two threshold determinations for the
same frequency differ by more than 15 dB, the results can be treated as
inconclusive. Repeating the test with an intervening time lapse is intended to
confound any attempt to consistently exaggerate a hearing loss. The current
practice of performing two pure-tone air-conduction tests on the same day but
at different sittings for potential compensation cases allows for an
identification of possible pseudohypacusis before any further testing is done
(Workmen’s Compensation Commissioner, 1995). However, it is important to
remember that this is not an infallible detection method, since Haughton et
al. (1979) have found that subjects with normal hearing asked to feign hearing
loss during three tests over a two-week period were able to duplicate their
feigned loss to within 6 dB, on average. This raises the concern that selfdiscipline and familiarity with the test procedure could enable workers to feign
hearing loss consistently and qualify falsely for compensation or for inflated
settlements.
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2.4.3.2
Different pure-tone threshold determination methods
Rintelmann et al. (1991) recommend a procedure that may well be the most
effective and time-efficient method for detecting pseudohypacusis when using
pure tone audiometry.
They recommend the use of two pure-tone air-
conduction tests using different presentation methods. Patients who attempt
to simulate hearing loss often try to select a level above their true threshold as
a reference for recording consistent above-threshold responses. To counter
this tactic, it is recommended that the first test be presented using the
ascending method, and that the second test use the descending method
(Martin, 1994; Roeser et al., 2000b). When it is applied to pseudohypacusic
patients, this procedure generally results in significant discrepancies between
the two pure-tone tests, thereby identifying the patient as pseudohypacusic.
2.4.3.3
Audiometric configuration
Another indication of pseudohypacusis using pure-tone audiometry as a
method of detection is the shape of the audiometric curve. A flat configuration
is very often an indication of pseudohypacusis (Martin, 1994). So, for
instance, it may be found that all the thresholds in one or both ears are at the
same intensity, therefore presenting a straight line on the audiogram. (This is
uncommon in audiology).
2.4.3.4
Lack of interaural attenuation
Roeser et al. (2000b) are of the opinion that many pseudohypacusic patients
feign unilateral hearing loss.
In the case of a true unilateral hearing loss, a patient reacts to loud sound
presented to the poorer ear, due to the fact that the intensity of the sound
presented to the poorer ear is sufficient to cross to the other (better) ear. This
crossover involves the transmission of sound emanating at the test ear to the
cochlea of the non-test ear (Stach, 1998). In the case of a true unilateral
hearing loss, the patient stops reacting to the sound with the better ear as
soon as the better ear is masked and is thus removed from the test situation.
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A naïve pseudohypacusic patient indicates no hearing in one ear and good
hearing in the other, which is impossible given the preceding discussion of
interaural attenuation (Martin, 1994).
The phenomenon of interaural attenuation is of particular importance with
bone-conduction testing, since the lower limit of interaural attenuation is
essentially 0 dB across frequencies. Thus, regardless of bone conductor
placement, both cochleas will be stimulated equally and simultaneously, and
the better cochlea should thus prompt a response (Stach, 1998).
A
pseudohypacusic patient does not normally respond with the bone conductor
placement on the chosen weaker ear.
2.4.3.5
Lack of correlation between air- and bone-conduction tests
A further indication of pseudohypacusis in pure-tone testing is commonly a
lack of correlation between bone- and air-conduction results.
It is impossible for bone-conduction results correctly to indicate worse hearing
than air-conduction results, since air-conduction results have already given an
indication of the status of the whole hearing mechanism.
It is therefore
impossible for a sub-section of that tested mechanism to be worse than the
whole. A second indication of pseudohypacusis is a false air-bone gap (Qiu et
al., 1998). When an audiologist is presented with an air-bone gap it usually
means that an outer or middle ear problem is impeding the conduction of
sound to the cochlea (Dirks, 1973). Pseudohypacusic patients sometimes
present with a conductive hearing loss that cannot be verified by an otoscopic
examination, medical history or, most importantly, the results of immittance
testing (Qiu et al., 1998; De Koker, 2003).
2.4.4 SPEECH TESTING
In the diagnostic audiological battery available to audiologists, pure-tone tests
are commonly perceived as the gold standard for evaluating the specific
effects of auditory system pathological conditions. However, pure-tone
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measurements provide only limited information about the communication
difficulties a patient may experience, or the site of the lesion (Roeser et al.,
2000b). It is therefore imperative to apply a test battery to cross-check the
pure-tone information. In the case of pseudohypacusic patients, clinicians
usually rely on the ability of speech audiometry to assess the validity of the
pure-tone thresholds.
2.4.4.1
Speech reception thresholds
Discrepancies between the speech reception thresholds (SRT) and the puretone average (PTA) can indicate pseudohypacusis. Gold et al. (1991) regard
a difference of 15 dB between the PTA and SRT (with the PTA as the higher
threshold) as an indication of pseudohypacusis. Roeser et al. (2000b) regard
even an 8 dB difference as significant. In the case of people who respond
truthfully, however, the two parameters generally correspond closely. It is
therefore realistic to assume that any discrepancy, in the absence of a
reasonable explanation for it (for example, the slope of the audiogram or poor
word discrimination), is thus indicative of pseudohypacusis (Martin, 1994).
Apart from the above discrepancy between SRT and PTAs, it has also been
noted that pseudohypacusic patients often respond to spondee words by
repeating only half of the word, for example “dog” for “hotdog” (Gold et al.,
1991). Since SRT constitutes a threshold determination test, it could be the
first step in a patient’s evaluation, followed by pure-tone testing. This
corresponds with the recommendation of Rintelmann et al. (1991) to avoid
supra-threshold
tests
at
the
beginning
of
audiological
procedures.
Furthermore, most South African mine workers are very familiar with puretone air-conduction procedures as a result of annual screening, but few have
had exposure to speech audiometry, and thus discrepancies between PTA
and SRT results can be indicators of pseudohypacusis.
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2.4.4.2
Word discrimination tests
When considering word discrimination test results, Gold, Hunsaker and
Haseman (1991) report pseudohypacusic patients with hundred per cent
discrimination scores at levels equalling, or slightly exceeding, admitted puretone thresholds. This phenomenon should alert a clinician to the possibility of
pseudohypacusis, since hundred per cent discrimination is usually achieved
only at a sensation level of 30 to 40 dB (Stach, 1998).
As is the case with pure-tone testing, some indicators of pseudohypacusis can
also be found in the behaviour of the patient during the test. Roeser et al.
(2000b) report a lack of patient co-operation during word discrimination
testing, stating that patients tend to get all words right once, and then start
missing all words.
2.4.5 SPECIAL TESTS
A review of the literature indicates that several specialised audiometric tests
have been developed to identify pseudohypacusis, including:
•
the Stenger test (Chaiklin & Ventry, 1965);
•
automatic audiometry (Jerger, 1960);
•
delayed auditory feedback (Martin, 1994);
•
the swinging story test (Martin, 1994);
•
pulse-count methods (Ross, 1964) ;
•
the yes-no test (Frank, 1976) ;
•
the Doerfler-Stewart test (Doerfler & Stewart, 1946);
•
the Lombard test (Simonton, 1965);
•
the forced choice procedure (Haughton et al., 1979); and
•
electrodermal audiometry (Gold et al., 1991).
Roeser et al. (2000b) label these tests “historical tests” that are not routinely
used in daily audiological practice. The reasons they are not used can be
sought in their involving long and complicated procedures, requiring special
equipment, and most importantly, them being unable to determine true hearing
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thresholds. In a clinical situation, particularly in the mining industry, where
large numbers of workers impose large caseloads on audiologists (Franz,
2003), there is little to be gained from tests that confirm the presence of
pseudohypacusis without establishing true hearing thresholds. The only one
of these tests that is quantitative in nature is the Stenger test, but this is
primarily useful in the detection of feigned unilateral hearing loss, which is not
common in the mining industry.
Accurate and objective information on hearing thresholds is crucial to the
evaluation of compensation claims and to determining workers’ fitness (RMA
guidelines, 2003). This need, along with the high incidence of pseudohypacusis among noise-exposed workers, has led many audiologists to employ
electrophysiological procedures to estimate true hearing thresholds. Roeser et
al. (2000b) have also focused attention on the fact that electrophysiological
tests are quantitative, unlike the qualitative conventional and historical tests
described in this chapter.
Roeser et al. (2000b) argue that, in cases where true thresholds cannot be
obtained, electrophysiological evaluations are indicated. It is thus necessary
for audiologists to move beyond the identification of pseudohypacusis to the
estimation of true thresholds. All audiological procedures discussed up to this
point have aided only in the detection of pseudohypacusis.
2.5
SUMMARY
Pseudohypacusis, where a patient feigns or exaggerates hearing loss, has
been examined in terms of the very limited information in the existing
literature. Definitions have been offered, and the prevalence and causative/motivating factors have been discussed.
Means of identification of
pseudohypacusis by audiologists have also been highlighted.
Most of the available test procedures failed to assist clinicians in objectively
determining true hearing thresholds, especially within reasonable limits with
regard to time and cost. The researcher is of the opinion that there is little to
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be gained from performing an array of specialised and time-consuming tests
that fail to provide accurate hearing thresholds.
The test of choice for identifying pseudohypacusis remains the pure-tone
audiogram. Hence, the answer in the search for an objective test with which
to estimate pure-tone behavioural thresholds appears to lie in the realm of
electrophysiological tests, which will be the subject of the next chapter.
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CHAPTER 3
ELECTROPHYSIOLOGICAL TESTS AND THEIR USE IN
THE ASSESSMENT OF PSEUDOHYPACUSIS
AIM OF THE CHAPTER
To evaluate the usefulness of the electrophysiological tests available,
specifically auditory evoked potentials, in the audiological evaluation of
pseudohypacusic patients.
The main question addressed is:
what
contribution electrophysiological tests can make to the detection of
pseudohypacusis and the determination of thresholds in the difficult-to-test
population of mine workers.
3.1
INTRODUCTION
“All diagnostic procedures are designed to identify the presence of the
disorder as early as possible. That is, the procedure must accurately identify
those patients with the disorder while clearing those patients without the
disorder” (Roeser et al., 2000b:12). This requirement for audiological test
procedures is met by the tests described in Chapter 2, in that the conventional
tests can identify pseudohypacusis.
The audiologist’s responsibility goes further: it is not only to identify the
presence of a disorder, but to quantify it, thus to determine frequency-specific
hearing thresholds for all patients assessed, in order to provide guidance for
the rehabilitation process, as well as to facilitate recommendations and
decisions regarding patient referrals (Stach, 1998; Roeser et al., 2000b).
Schmulian (2002) supports this position, commenting that poor and inaccurate
diagnostic procedures result in sub-standard recommendations regarding the
rehabilitation of the disorder.
In the field of medico-legal investigations, there is a further reason for not only
identifying but also quantifying the hearing loss, namely financial loss. Coles
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and Mason (1984:71) clarify the importance of true hearing threshold
estimation as follows: “In medico-legal investigations of all kinds, precautions
have to be taken against falsification of disability by the patient since there is a
clear motivation for him to exaggerate and thereby obtain greater financial
advantage. This is particularly necessary where the disability claimed can
only be fully characterized by including subjective aspects, as in the case of
hearing loss.”
This is certainly of particular relevance for mine workers with noise-induced
hearing loss who present with pseudohypacusis.
A pseudohypacusic
worker’s lack of co-operation confounds efforts to obtain accurate frequencyspecific information, and often leads to large numbers of pending cases.
These workers have to be retested, which increases the cost of audiological
and other specialist assessments. Retesting workers also leads to additional
expenditure (further financial implications), since these workers miss shifts
and the mining company thus loses production. Additional transport costs may
also be involved if workers are referred for further assessments.
The lack of the availability of accurate hearing thresholds results in situations
where compensation is not paid to deserving cases and in overcompensation
of
genuine
noise-induced
hearing
loss
where
hearing
threshold
inconsistencies are not detected. The frustration of audiologists, occupational
health centre staff and the mining industry in general is understandable.
Abramovich (1990), Martin (1994) and Schmulian (2002) state that a lack of
patient co-operation, irrespective of the cause or motivation, necessitates the
use of additional, more objective (and sometimes more costly) procedures,
and that other responses apart from behavioural responses to acoustic
signals should be explored for the estimation of hearing thresholds.
In the assessment of hearing, audiologists have always used a test battery
approach (Hall & Mueller, 1997) to ensure acceptable service delivery to
clients. The various tests available to audiologists are used in conjunction
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with each other and allow for cross checks to confirm results.
With a
pseudohypacusic patient, such efforts generally confirm the presence of
pseudohypacusis without quantifying its extent, in other words, tests fail to
provide frequency-specific hearing thresholds.
The most reliable means of cross checking is provided by test procedures that
require no voluntary response from the patient (Schmulian, 2002). Gorga
(1999) indicates that assessments of pseudohypacusic patients require the
use of test procedures that do not rely on voluntary behavioural responses.
The quest for measures not requiring a behavioural response has led to the
development of electrophysiological tests, which provide an objective
assessment of auditory sensitivity (Hall, 1992).
Rintelmann et al., (1991);
Stach, (1998) and Roeser et al. (2000b) also promote the use of physiological
tests for difficult-to-test populations.
Today, audiologists have a wide range of electrophysiological assessment
tools to select from (Roeser et al., 2000b).
These are examined and
evaluated in this chapter. Particular attention is focused on auditory evoked
potential (AEP) methods, which have been shown to provide estimates of
hearing thresholds.
The objective is to identify and evaluate audiological
solutions and test procedures for the population of mine workers, in which
noise-induced hearing loss is frequent and pseudohypacusis is rife.
3.2
ELECTROPHYSIOLOGICAL (EP) TESTS
3.2.1 QUALITATIVE EP TESTS (USED PRIMARILY FOR DETECTION OF
PSEUDOHYPACUSIS)
Discoveries in the field of audiology (and other related fields, including
neurology and electronics) have recently led to rapid advances in the
development of electrophysiological tests (Ferraro & Durrant, 1994; De Waal,
2000; Roeser et al., 2000b). Audiological assessment techniques no longer
need to be limited to traditional behaviour-based psycho-acoustic tests, now
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that EP tests can help satisfy the need to assess auditory sensitivity at
specific frequencies objectively (Schmulian, 2002).
The EP methods used in the past thirty years have included immittance
testing, acoustic reflex (AR) measurements, oto-acoustic emission (OAE)
tests and auditory evoked potential (AEP) tests.
Of the many electrical
responses to auditory stimuli, most originate in the central nervous system.
Some are generated in the cochlea, while others are reflexive muscular
responses (Glasscock, Jackson & Josey, 1987).
Immittance and OAE measurements are not measures of hearing per se, but
are means of evaluating the status of the auditory system at specific
peripheral levels, although never as an entire system (De Waal, 2000). These
measures do not provide frequency-specific thresholds, but merely confirm
the suspicion of pseudohypacusis, thus serving as a means of cross checking.
3.2.1.1
Immittance
Acoustic immittance measures (tympanometry, static compliance and acoustic
reflex) have been well established as a routine part of audiological evaluation
(Rintelmann et al., 1991). The primary application of acoustic immittance is
the evaluation of organic hearing disorders. It can also be useful in the
detection of pseudohypacusis.
Martin (1994) claims that the sophistication of automated middle ear tests may
discourage pseudohypacusis, and is therefore very valuable in the detection
or prevention of pseudohypacusis.
Clinicians should thus remember to
suggest to the patient that the test is fully automatic and that no response is
needed, thereby removing the temptation to feign a hearing loss.
It is
therefore generally good practice to perform an immittance test first if this test
can be used to avoid pseudohypacusis. This is a valid approach, but goes
against the recommendation of Rintelmann et al. (1991) that supra-threshold
tests should be performed after air- and bone-conduction tests. Experience
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has shown that completing threshold testing before embarking on suprathreshold tests does save time and prevents unco-operative patients from
finding a supra-threshold reference level (Dobie, 2001; De Koker, 2003).
The AR threshold is the most useful measurement in the detection of
pseudohypacusis. In persons who have normal hearing, an acoustic reflex is
usually elicited by means of contralateral stimulation at sensation levels that
range from 70 to 95 dB.
For persons with cochlear lesions, as in mine
workers exposed to noise, the reflex may be obtained between 15 and 60 dB
(Rintelmann et al., 1991). When the difference between the AR threshold and
the voluntary threshold is extremely low (5 dB or less), the pure-tone threshold
must be questioned on the basis of organic pathology (Martin, 1994; 2000).
Claims of a profound unilateral or bilateral hearing loss can be refuted if the
AR is present at normal stimulus levels, but the phenomenon of recruitment
may limit the usefulness of AR measurements in estimating hearing
thresholds, especially in cases of noise-induced hearing loss.
Tympanometry provides an immediate evaluation of the middle ear status.
Present ARs and normal middle ear function are not compatible with
conductive hearing loss (Qiu et al., 1998). If conductive hearing loss is present
with normal middle ear function pseudohypacusis can be expected.
The
reason being mine workers’ unfamiliarity with bone conduction testing.
AR measurements may be useful in estimating actual hearing thresholds by
performing the sensitivity prediction with the acoustic reflex test (SPAR).
Middle ear reflex thresholds for pure tones are compared with those for wideband noise, as well as for filtered low- and high-frequency wide-band noise
(Martin, 1994; 2000). In the researcher’s experience, the high incidence of
absent ARs in this population makes the use of the SPAR test impossible.
Dobie (2001) also claims that the SPAR test has no clinical utility in
pseudohypacusic populations. Some reasons for this, although Dobie (2001)
does not mention them, could be elevated and absent ARs.
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In conclusion as was the case with the behavioural tests described in
Chapter 2 immittance testing predicts and detects pseudohypacusis but is not
quantitative in nature.
3.2.1.2
Oto-acoustic emissions (OAE)
Small changes in the biomechanical function of the cochlea can be monitored
by measuring OAEs, which are generated within the cochlea by active nonlinear processes involving the outer hair cells (Kvaerner et al., 1996).
It is impossible for a patient with compensable hearing loss to have normal
OAEs, and OAE testing is thus advocated as a quick and objective means of
confirming hearing status in suspected cases of pseudohypacusis (Qiu et al.,
1998). A patient with normal OAEs should have normal hearing thresholds.
Unfortunately, the usefulness of OAE testing is limited in cases of noiseexposed patients, as such individuals often exhibit abnormal or absent OAEs
with normal hearing as a result of pre-symptomatic cochlear damage (Hall,
2000; De Koker et al., 2003). So far, it has also been difficult to correlate
OAEs and behavioural thresholds (Hall, 2000). OAEs are another qualitative
assessment tool which is useful in the detection of pseudohypacusis.
3.2.2 QUANTITATIVE ELECTROPHYSIOLOGICAL TESTS (ESTIMATION
OF HEARING THRESHOLDS IN PSEUDOHYPACUSIS)
3.2.2.1
Introduction
Despite the considerable interest that has been generated by all the
conventional tests described in Chapter 2 and the electrophysiological tests of
immittance and oto-acoustic emissions, as the foregoing discussion has
indicated, none have provided accurate hearing thresholds in the case of
pseudohypacusic mine workers. The problem faced when compensation is
involved is that the audiologist must obtain ten hearing thresholds (South
African legislation) that are accurate enough to be duplicated in a second test.
The focus is thus on quantitative data.
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Accordingly, attention needs to be paid to auditory evoked potential methods
as the most useful and effective electrophysiological measure of auditory
system function (Rance et al., 1998) with due consideration to both the
peripheral and central auditory systems. Hood (1998) emphasises that EP
tests are not tests of hearing, but tests of synchronous neural function and the
ability of the central nervous system (CNS) to respond to external stimuli in a
synchronous manner.
Nevertheless, numerous authors have shown how
closely thresholds from AEP testing correspond with behavioural thresholds
(Reneau & Hnatiow, 1975; Rance et al., 1998; Barrs et al., 1994). This fact,
combined with the statement of Abramovich (1990) that the verification of
hearing loss and the validation of the pure-tone audiogram is important in
dealing with compensation claims, supports the necessity of evaluating AEP
tests within the framework of this study.
Hyde et al. (1986) argue even more strongly that, if AEPs are accepted as the
ultimate arbiter in medico-legal evaluations, the rationale for interposing
confirmatory tests (detection) between a suspicion of and the quantification of
pseudohypacusis is suspect.
3.2.2.2
Background: the development of the use of AEPs
AEP procedures are not really a “new” technique. Glasscock et al. (1987)
trace the origins of auditory brainstem response (ABR) testing to animal
experiments in the nineteenth century, citing Caton, who reported electrical
activity in the brain of a rabbit in 1875.
They also mentioned other
researchers who investigated electrical activity in the brains of other animals
between 1883 and 1891.
Not only the technique but also the apparatus used to evoke and record the
electrical responses has developed over the years.
Pravdich-Neminsky
photographed an apparatus to record animal electro encephalograms (EEGs)
using a string galvanometer (Glasscock et al., 1987).
During the 1930s,
oscilloscope images were bright enough and electrical amplifiers stable
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enough to allow neurophysiologists to concentrate on experimental work
rather than on equipment problems (Abramovich, 1990).
Berger first observed spontaneous electrical activity of the type now routinely
recorded during EEGs in 1929 (Abramovich, 1990; Ferraro & Durrant, 1994).
In searching for electrical activity in the inner ear, Wever and Bray (1930)
recorded potentials in response to auditory stimuli from the round window of a
cat. These potentials have since been termed cochlear microphonic or CM
(Abramovich, 1990).
The main problem facing early researchers was the difficulty of measuring
very small potentials in isolation from other electrical activity within the brain.
Particular difficulty was experienced when the stimuli were of low intensity, as
EEG voltage was much greater than the voltage of the evoked potential
(Reneau & Hnatiow, 1975). The development of averaging computers has
facilitated more accurate analysis of small bio-electrical signals (Abramovich,
1990). Glasscock et al. (1987) note that Davis acquired a digital computer in
1961, after which he began using it in his electrophysiological experiments.
The ABR, currently the most popular AEP used in clinical contexts, was first
described and defined by Jewett and Williston in the early 1970s (Glasscock
et al., 1987).
In 1963, the New York Academy of Arts and Sciences sponsored a
symposium of investigators of averaged potentials (including visual,
somatosensory, auditory, myogenic and neurogenic), followed by the founding
of the International Electrical Response Audiometry study group in 1968
(Abramovich, 1990).
Much of the research in the field of AEPs tries to correlate the electrical
responses with auditory behavioural thresholds. Reneau and Hnatiow (1975)
cite difficulties in relating physiological thresholds (such as evoked responses)
to behavioural response thresholds.
It was believed that behavioural
responses are binary measures in which a subject decides between “yes” or
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“no”, while physiological thresholds are graded, or quantitative, and that
graded measures are mathematically different from binary ones.
It was
concluded that these two types of tests can be expected to yield different
results. Nevertheless, as a result of subsequent advances in electronics, and
a far greater understanding of brain function, there has been a move in the
field of AEPs, supported in this study, to relate behavioural and physiological
thresholds.
The enthusiasm for auditory evoked potentials in the 1970s resulted in this
type of testing, being incorporated in test batteries for unco-operative patients
such as small children (Martin, 1994). It is thus logical that the use of this
quantitative procedure was also extended to cases of pseudohypacusis
(Roeser et al., 2000b).
As early as 1990, the use of auditory evoked potentials was recommended in
the assessment of pseudohypacusic patients by Abramovich (1990), who also
cites the use of slow vertical responses, auditory brain stem response, middle
latency responses and transtympanic electrocochleograms as possible
auditory evoked potentials to be used with pseudohypacusic patients. Today,
a mere decade, later is it predicted that in future, AEPs will become even
more prominent in the field of Audiology (Roeser, Buckley & Sichney, 2000).
3.2.2.3
Nomenclature and definitions
Picton and Scherg (1990) argue that one of the most important clinical
applications of AEPs is their use in objectively evaluating the hearing of
patients who are unable to respond during conventional testing. However, in
order to evaluate this application, it is important first to define auditory evoked
potentials and to highlight controversial issues.
Stach (1998:293) describes the measurement of AEPs as follows:
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The brain processes information by sending small electrical impulses
from one nerve to another. This electrical activity can be recorded by
placing sensing electrodes on the scalp and measuring the ongoing
changes in electrical potentials throughout the brain. This technique is
called electroencephalography, or EEG, and is the basis for recording
evoked potentials. The passive monitoring of EEG activity reveals the
brain in a constant state of activity; electrical potentials of various
frequencies and amplitudes are measured continually.
If a sound is
introduced to the ear, the brain’s response to that sound is just another of
a vast number of electrical potentials that occur at that instant of time.
Evoked potential measurement techniques are designed to extract these
tiny signals from the ongoing electrical activity.
This described electrical activity can be spontaneous or event-related (Picton,
2001). Responses that are time-linked to some event or stimulus are called
event-related potentials (ERPs), and can be responses to a sensory stimulus
(such as a visible flash or a sound), a mental event, or the interruption, delay
or omission of a stimulus (Picton, 2001).
Auditory evoked potentials (AEP) are a type of ERP in which the stimulus is a
sound, and the response takes the form of very small electrical potentials
originating in the nervous system and recorded at the scalp (Picton, 2001).
AEPs originate from structures such as the auditory cortex, the auditory
brainstem and the auditory cranial nerve (VIII or 8th).
These electrical
potentials are very small: 2 to 10 micro volts for cortical AEPs, and less than
one microvolt for deeper brainstem structures (Picton, 2001).
The measurement of these potentials in response to auditory stimuli has
become a valuable diagnostic tool (Stach, 1998) but is still an evolving field in
which there are problematic issues that need to be resolved.
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3.2.2.4
Problematic issues in the field of AEP
It should be noted that the terms “evoked potential” and “evoked response”
are used interchangeably in the literature (Hood, 1998). The term “response”
is derived from the procedure of pure-tone audiometry in which a stimulus is
presented and a response (motor action) is subsequently recorded. In AEP
testing, a response is not recorded, but a potential is measured. Furthermore,
electrical activity is elicited by a signal, and not a stimulus (Goldstein &
Aldrich, 1999).
The term “stimulus” implies perception, but in tests of auditory brain stem
response and auditory steady state response, electrical activity is measured
sub cortically and only up to brainstem level.
It should therefore be
remembered that the terms “stimulus” and “signal” are interchangeable, as are
“potential” and “response” (Schmulian, 2002).
The field of evoked potentials has been burdened with inconsistencies in
terminology and definitions and its classification system has lacked uniformity
and clarity (Ferraro & Durrant, 1994; Schmulian, 2002). Schmulian (2002)
attributes this lack of clarity to the presence of specialists from the different
fields of audiology, neurology and otolaryngology who all work in the field of
evoked potentials. Classifications of AEPs in the literature can be divided into
those based on anatomical generators, on the type of potential, on the types
of stimuli used, on the location of recording electrodes and on latency
characteristics (the time between stimulus onset and response) (Schmulian,
2002).
The most common classification of AEPs is based on their time domain
(Goldstein & Aldrich, 1999), in which the time between the stimulus and the
response is termed the “latency epoch”. Ferraro and Durrant (1994) mention
that, although this classification system is the easiest to apply, the
classification of latency is not standardised. A familiar method is to classify
response latency as short, middle or late latency responses, depending on the
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time between the presentation of the stimulus and the responses’ becoming
evident as an AEP. Short latency is referred to as “fast” by Glasscock et al.
(1987), and as “early” by Abramovich (1990), while “late” latency responses
are also referred to as “slow”. These types of inconsistency create confusion.
Because
latency
varies
according
to
stimulus
intensity,
temporal
characteristics and pathology, it has been found that authors attribute different
latency epochs to different AEPs.
So, for example, according to Picton
(2001), the ABR is seen 1.5 to 15 milliseconds (ms) after the stimulus, which
contradicts Abramovich (1990), who states that an auditory brain stem
response (ABR) is seen within the first 10 ms after the stimulus. Different
nomenclatures are also used to identify major peaks for AEPs, for example
Roman and Arabic numerals are used for ABR waves, and “No” or “SN10” are
used to identify the slow negative wave appearing in the ABR after 10 ms.
3.2.2.5
The use of different potentials in pseudohypacusis
The use of auditory evoked potentials in the estimation of hearing in patients
that cannot or will not co-operate during behavioural tests has been
advocated by numerous authors (Abramovich, 1990; Mc Pherson & Starr,
1993; Stach, 1998). Schmulian (2002) expresses a stronger opinion, saying
that AEP testing is the only procedure in the audiologists’ test battery that can
quantify the hearing sensitivity of unco-operative patients.
If an audiologist has to rely on a single test in a battery (due to an uncooperative patient), AEP testing needs to meet the following requirements
(Roeser et al., 2000b):
•
The test should diagnose the nature of the hearing loss (conductive or
sensory neural).
•
The degree of hearing loss (from normal hearing to profound hearing
loss) has to be established.
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•
The configuration of the hearing loss (across a range from 250 to
8 000 Hz) is important clinical information and must be determined.
•
Frequency-specific hearing thresholds need to be estimated for both
ears.
The above requirements are used in the discussion below to evaluate the use
of different auditory evoked potentials in pseudohypacusic patients.
3.2.2.5.1
Early potentials
The first three AEPs identified (cochlear microphonic (CM); action potential
(AP) and summating potential (SP) are very early-stage potentials seen during
the first 5 ms after stimulation with a sound (Stach, 1998). Responses to
sound originate in the cochlea and the distal portion of the auditory nerve.
They are also grouped together in clinical use as the electrocochleogram
(EcochG). Tone burst and click stimuli are used to elicit responses, and two
different electrode placements for near-field measurements are possible,
namely
•
transtympanic placement, where an electrode is invasively placed
through the tympanic membrane onto the promontory of the temporal
bone; and
•
the external auditory meatus (EAM) near the tympanic membrane
(Abramovich, 1990).
The value of the EcochG lies in its usefulness for assessing the hearing of
young children who are difficult to control in clinical situations, and in the fact
that these potentials are not altered by anaesthesia. The EcochG provides
information on inner ear function (Abramovich, 1990) in conditions such as
tinnitus, Meniere’s disease and sudden hearing loss (Halliday, 1993).
Its
disadvantages are that low frequency function is almost impossible to assess,
and the surgical procedures required for transtympanic placement make the
EcochG invasive (Abramovich, 1990).
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The use of electrocochleography in pseudohypacusic populations (Qiu et al.,
1998; Rintelmann et al., 1991; Abramovich, 1990) has been reported.
Rintelmann and his co-authors opine that EcochG does not measure the
ability to hear. The invasive nature of the surgical procedures for the EcochG
and the resultant need for an Ear-, Nose- and Throat (ENT) specialist
(Schmulian, 2002), together with the ability of the test to measure only the
most peripheral functions of the auditory system limit its clinical use to a small
number of highly specialised diagnostic centres (Abramovich, 1990; Stach,
1998).
It can be concluded that pseudohypacusic patients are not adequately
evaluated by early potential testing, as it fails to include all of the frequencies
required for compensation assessments, and the invasiveness of the
procedure is unacceptable for Occupational Health applications.
3.2.2.5.2
ABR
ABR is a big misnomer in the field of AEPs (Schmulian, 2002). Since the ABR
is the most widely used AEP (Hood, 1995), all AEPs have come to being
perceived as ABRs, irrespective of the latency epoch and the equipment used
(Goldstein & Aldrich, 1999).
Ferraro and Durrant (1994) have found ten
different names for ABRs in a literature review, including “brainstem auditory
evoked potential”, “brainstem auditory evoked response”, and “auditory
brainstem evoked response”, to list but a few.
In ABR testing, electrical potentials are generated by the VIII (8th) cranial
nerve and neural centres within the brainstem (Stach, 1998). The ABR uses
far-field potentials recorded at the scalp (vertex), and comprises five or more
waves generated in the auditory pathway up to the level of the inferior
colliculus.
The procedure is firmly established in clinical practice for
estimating audiometric thresholds and for neurological/neuro-otological
diagnosis (Abramovich, 1990). In South Africa, ABR has for many years been
the test of choice among the available AEP procedures, particularly for
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difficult-to-test patients for whom the configuration and severity of hearing loss
have to be determined. The waves are robust and easily recorded, and are
unaffected by the patient’s state of consciousness (the patient can even be
asleep or sedated).
ABR potentials are minute, rarely reaching amplitudes greater than 1 micro
volt (µV), and thus it requires a great deal of averaging to distinguish
potentials from background noise and other artefacts (Arnold, 2000).
Furthermore, ABR tests rely on transient responses elicited by brief acoustic
stimuli (Arnold, 2000), as the more abrupt the stimulus, the more clearly
defined the ABR. The most widely used stimulus is a broadband click,
because of its rapid onset (100 µsec) and broad frequency content, which
stimulates a large portion of the basilar membrane to give a reasonable
indication of hearing thresholds between 2 000 and 4 000 Hz. However, due
to the structural and mechanical properties of the cochlea, ABR can only
predict auditory sensitivity in the upper part of this range to within 5 to 20 dB
of behavioural thresholds (Rance et al., 1998). This limitation has led to the
development of other stimuli, including tone bursts, filtered clicks and various
masking techniques to provide more precise information on narrower
frequency ranges (Hood, 1998).
According to Swanepoel (2001), tone bursts are the stimulus of choice where
low frequency threshold information is required.
Tone bursts are more
frequency-specific than clicks, and their gradual stimulus onset ensures good
frequency specificity (Weber, 1994). Unfortunately, the resulting stimulus does
not elicit a clear ABR and, therefore, an abrupt stimulus onset is necessary to
improve the quality of the response. However, this introduces high-frequency
energy into the test stimulus, necessitating the use of masking techniques to
eliminate the effects of unwanted high frequency energy.
Stapells et al.
(1990) have obtained good agreement between ABR and behavioural
thresholds by using tone burst stimuli embedded in notched noise.
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Unfortunately, the time needed to obtain a single ABR threshold for each ear
exceeds 30 minutes, making a full audiogram impractical (Weber, 1994). With
children the test often lasts for as long as the child sleeps and, even with
adults, a long test is tiring and undesirable (Swanepoel, 2001).
At the
moment, the best method for determining hearing loss configuration is to
present first a low-frequency tone burst and then a click ABR. This procedure
is an attempt to shorten the procedure, but should still allow the audiologist to
get an idea of the configuration of the hearing loss.
An advantage of ABR is that the latencies of the various waves are quite
stable within and among different patients (Abramovich, 1990). In addition,
time intervals between peaks are prolonged by auditory disorders central to
the cochlea, making ABR useful in differentiating cochlear and retrocochlear
pathology (Weber, 1994).
A disadvantage is that the interpretation of wave forms is subjective (Martin,
1994), and the interpretation of tone bursts requires considerable expertise
and experience (Abramovich, 1990; Swanepoel, 2001). The ABR is also timeconsuming, and the maximum stimulus level for clicks and tone bursts is
restricted, resulting in a risk that the audiologist may fail to identify residual
hearing at high loudness levels. Furthermore, the high cost of instrumentation
and software are added negative considerations (Schmulian, 2002). Qiu et al.
(1998) point out the further disadvantage that involuntary responses are
generated only by sub-cortical structures and, hence, can never provide a
measure of true hearing thresholds. These authors also criticise the great
technical demands with regard to stimulus, filter settings, recording methods
and response interpretation with bone-conduction ABRs.
This limits the
clinical application of the technique.
In a study by Barrs et al. (1994), it was found that an ABR was a useful
procedure in the threshold confirmation needed in cases of noise-induced
hearing loss, but that middle latency responses were more useful than the
ABR because of the ABRs tendency to overestimate hearing loss in down-
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sloping audiograms.
Middle latency responses were also more frequency
specific which is important in the case of noise-induced hearing loss
evaluations.
From the preceding discussion, it is apparent that the ABR has up to now
been the most widely used electrophysiological procedure, and is the only
electrophysiological procedure prescribed in South Africa for the formal
assessment of pseudohypacusic patients (RMA guidelines, 2003). Despite
the limitations discussed above, frequency-specific threshold determinations
are possible, but only through a long and expensive process requiring a great
deal of skill and experience on the part of the audiologist. These are two
important limitations that hinder the consistent use of ABRs in hearing
assessment in the mining industry.
3.2.2.5.3
Middle latency responses
It is generally accepted that there are two main reasons for the use of auditory
electrophysiological tests, namely the need to make inferences regarding
hearing thresholds and the need to obtain information regarding the functional
and structural integrity of the auditory pathway’s neural components (Kraus,
Kileny & McGee, 1994). The purpose of this section is to provide a basis for
understanding the principles and applications of middle latency response
(MLR) testing and to evaluate the contribution of MLRs in meeting the above
two aims.
An MLR is a series of waveforms occurring 10 to 80 ms after the onset of an
auditory stimulus (Kraus et al., 1994).
Here, again, contradictory
classifications abound in the literature. Abramovich (1990) classifies MLR as
having a latency of 8 to 50 ms, while Picton (2001) and Glasscock et al.
(1987) set latency at 25 to 50 and 12 to 50 ms respectively.
Within the
continuum of components comprising scalp-recorded AEPs, MLRs follow
ABRs and precede late latency responses (LLRs), while evoked potentials No,
Po, Na, Pa, Nb and Pb are classified as MLRs ( Kraus et al., 1994).
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According to Kraus et al. (1994), Geisler and his co-workers were the first
investigators to describe MLRs (in 1958). MLRs are measured at the scalp,
using an electrode montage identical to that used for ABR recordings, and
MLR generators include many brain structures central to the midbrain, as well
as structures outside the primary auditory pathway, such as the auditory
thalamocortical pathway, the reticular formation and the multi-sensory
divisions of the thalamus (Kraus et al., 1994).
MLR is used clinically for electrophysiological determination of hearing
thresholds at lower frequencies, for the assessment of cochlear implants and
auditory pathway function, and for the localisation of auditory pathway lesions.
They are also used intra-operatively (McPherson & Ballachanda, 2000). It is
thus clear that MLR has many applications in audiology, but unfortunately, the
disadvantages of MLRs overshadow the advantages.
The most important limitations include:
•
the inconsistency of responses as specifically observed in the
paediatric population (Kraus et al., 1994);
•
highly
specialised
equipment
requirements
(Schmulian,
2002),
including EEG for example;
•
the need for the patient to be awake, co-operative and alert (Hood,
1995). Ferraro and Durant (1994) state that sensitivity to the patient’s
state of consciousness limits the acceptance of MLR techniques;
Thorton et al. (1984) show that MLRs are distorted and delayed during
sedation, and those potentials are poorly detected in stage IV sleep;
•
a
perception
that
MLRs
are
not
considered
a
mainstream
electrophysiological test (Mc Pherson & Ballachanda, 2000), and are
regarded as difficult to record (Abramovich, 1990), causing a lack of
facilities where these procedures could be tested; and
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•
reports that MLR potentials can be contaminated by muscle potentials
from the neck or peri-auricular region (McPherson & Ballachanda,
2000).
The question that needs to be answered is whether MLRs can be used as a
technique to identify pseudohypacusic mine workers and quantify their
hearing loss.
Abramovich, (1990) advocates the use of MLRs in
pseudohypacusic patients. He is of the opinion that a stimulation rate of 40
per second instead of the usual 10 per second can cause a superimposition of
the peaks of MLRs and an augmentation of the response. He specifies that
MLRs are to be used in this population when slow vertical response (SVR)
measurement conditions are poor. Barrs et al. (1994) mention the possibility
of using MLRs to detect work–related noise-induced hearing loss, stating that
MLRs are more effective in threshold estimation than ABRs, as a result of the
steepness of the audiometric curve in noise-induced hearing loss. Barrs et al.
(1994) also advocate the use of MLRs to verify low frequency thresholds.
McPherson and Ballachanda (2000) argue that the biggest problem in testing
and verifying these MLRs is the fact that these evoked potentials are not
considered to be mainstream electrophysiological tests in audiology practice.
Hence, equipment and facilities are not readily available.
3.2.2.5.4
Late latency responses (LLR)
As indicated previously, confusing nomenclature also exists regarding the
potentials evoked at later latencies. These potentials are described as “slow”
(Halliday, 1993), while Stach (1998) favours the term “late latency response”
(LLR).
“Slow vertical response” (Abranovich, 1990) and “cortically evoked
responses” (Rickards et al., 1996) are other nomenclature in the existing
literature. The confusing nomenclature stated above is further compounded
by a lack in uniformity in the latency epochs of LLRs. Ferraro and Durrant
(1994) define LLRs as potentials manifesting 50 to 800 ms after the stimulus,
while Glasscock et al. (1987) and Picton (1991) relate latencies in this subclass to 250 to 600 and 50 to 200 ms, respectively.
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These potentials have been found to be greatly affected by subject state
(Abramovich, 1990; Stach, 1998), and the potentials are best recorded when
the patient is awake and attending carefully to the sounds presented. It is
thus understandable that these methods are only used in adult, difficult-to-test
populations. Stach (1998) mentions that LLRs are robust and easily recorded
in adults and that the response can estimate auditory sensitivity independently
of behavioural response.
As is the case with other potentials, the late latency response generators are
still unknown. Halliday (1993) attributes the P3 or P300 AEP to widespread
activity of the frontal cortex involving the parietal association areas.
An important disadvantage of LLRs is the fact that the procedure is timeconsuming.
Abramovich (1990) estimates the time requirement for four
thresholds in two ears at 60 minutes.
With
regard
to
the
application
of
late
latency
responses
to
the
pseudohypacusic population, it is worth noting that several authors have
promoted LLRs as a medico-legal test (Halliday, 1993; Rickards et al., 1996;
Rickards & De Vidi, 1995; Abramovich, 1990; Dobie, 2001; McCandless &
Lentz, 1968; Hyde et al., 1986; Coles & Mason, 1984).
As early as 1968 McCandless and Lentz tested LLRs on adults with
pseudohypacusis using pure-tone stimuli with a 700 msec duration. They
found a very good correlation between the electrophysiological and
behavioural thresholds (5dB).
Abramovich (1990) claims that SVR testing is the test of choice for assessing
non-organic hearing loss. He argues that SVRs most closely approximates
the results of conventional frequency-specific audiometry (within 10 dB), and
that SVR is insensitive to neurological dysfunction. Pseudohypacusic patients
are instructed to pay attention, and those who deliberately exaggerate their
level of attention due to anxiety can be clearly identified. The stimulus used is
a 100 ms tone burst with rise and fall times of 10 ms.
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Coles and Mason (1984) used a 50 to 300 ms latency epoch and have proven
that these latency responses have by far the greatest value for verifying puretone thresholds in adult patients, in comparison to brainstem and cochlear
potentials. The tonal signals that these authors used had a duration of 200
ms and a rise and fall time of 10 ms. A specific advantage of LLRs mentioned
by these researchers is the frequency specificity at low frequencies where
non-organic overlay is maximal.
They also argue for the use of LLRs in
medico-legal investigations because of the non-invasive nature of the
procedure and because the procedure tests up to a much higher dB level
than, for instance, the ABR.
Hence, there is a less likelihood of a non-
peripheral disorder causing a discrepancy between the AEP and the
behavioural threshold.
Hyde et al. (1986) have expressed the opinion that the verification of puretone audiometry is a long-standing problem in Departments of Veterans‘
administration, compensation assessment for noise-induced hearing loss and
medico-legal evaluation. These authors have found a correlation between the
slow vertical response and behavioural thresholds of within 10 dB. The stimuli
used are tone bursts with 10 ms rise and fall times, and a 40 ms plateau.
Despite a very good threshold estimation ability, and although by 1986 the
procedure had been used in the Mount Sinai hospital (Toronto), for a decade,
the authors emphasise the following disadvantages of using SVRs:
•
testers in a clinical setting need to be experienced and carefully trained
audiologists whose performance is monitored ( it is clear that the skill
requirement is very high);
•
the test procedure is demanding and the skill requirements are often
underestimated;
•
testers need to have an adequate caseload to maintain their skill;
•
all clinical interpretation is subjective and on-line;
•
slow vertex response audiometry is problematic in 5 per cent of cases
due to high levels of rhythmic activity:
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•
the time exceeds 1.5 hours in 95 per cent of cases (Hyde et al., 1986);
•
from the above it is clear there is still a limited acceptance of the
technique even in North America (Hyde et al., 1986; Dobie, 2001).
Picton (2001) indicates that the British Columbia Workers Compensation
Board has used LLRs, and Rickards et al. (1996) state that cortical evoked
response audiometry (CERA) has been used to assess noise-induced hearing
loss in the Australian state of Victoria for the past 15 years, with 18 per cent of
all noise-induced hearing loss cases referred for CERA.
This seems to
indicate some positive experience with AEP procedures.
CERA thresholds have been found to be within 10 dB of pure-tone thresholds,
but, again, the procedure has failed to gain wide acceptance. Rickards et al.
(1996) indicate that reliance on subjective interpretations of tracings, and the
high levels of skill and training required of testers have hampered acceptance
of CERA as a routine test for pseudohypacusis.
As far as can be determined, late latency responses have not so far been
used in South Africa for the assessment of noise-induced hearing loss or the
evaluation of compensation claims. Although it is clear that, as in any clinical
population, no single AEP method is always the best (Hyde et al., 1986), the
main reason for searching for a better method is a lack of objectivity in
deciding whether the evoked potential is present.
3.3
SUMMARY
This chapter has discussed electrophysiological tests, particularly AEPs, as a
possible
means
of
determining
accurate
hearing
thresholds
for
pseudohypacusic mine workers. Nomenclature, selected definitions and the
historic evolution of AEPs have also been discussed, and the value of various
AEP methods for estimating hearing thresholds have been examined.
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A summary of the disadvantages of currently used AEPs based on the above
discussion, is set out in Table 3.1 below.
r
r
r
r
r
r
r
r
r
r
r
r
r
r
MLRs
r
r
LLRs
r
r
r
r
r
r
r
Although LLRs have been used internationally in medico-legal evaluations, an
even better solution is still sought for. A recent development in the field of
AEPs is auditory steady state responses (ASSRs), which is discussed
comprehensively in the next chapter.
Lins et al. (1995) have found that
results obtained from ASSR testing can be presented as an audiogram,
thereby providing information about the extent, nature and configuration of
hearing loss. Most importantly, it has been reported that ASSR provides true
objectivity, as thresholds are not determined subjectively, through a clinician’s
interpretation of wave forms, as is the case with ABR and LLRs, but are rather
calculated objectively by a computer (ERA systems Pty. Ltd, 2000).
The latter crucial benefit motivated this researcher to investigate this type of
AEP as a possible means for testing pseudohypacusic patients, particularly
those with noise-induced hearing loss in the South African mining industry.
54
skilled tester
r
ABR
invasive
not frequency
specific
r
r
sub cortical
subjective
interpretation
r
r
state
dependant
r
r
influenced by
neural
dysfunction
r
ECochG
age sensitive
r
time
consuming
expensive
Special
equipment
Type of AEP
TABLE 3.1: DISADVANTAGES OF AEPs
r
r
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CHAPTER 4
AUDITORY STEADY STATE RESPONSES (ASSR) AND
PSEUDOHYPACUSIS
AIM
To critically evaluate and describe a specific, auditory evoked potential, the
auditory steady state response, as a frequency-specific threshold estimation
procedure for use in certain difficult-to-test-populations. A motivation for the
use particularly in pseudohypacusic populations with suspected noiseinduced hearing loss is also given.
4.1
INTRODUCTION
In seeking a truly objective hearing threshold estimation technique for difficultto-test populations, the emphasis worldwide has been on auditory evoked
potentials. Hence, this was the main focus in the previous chapter.
The ultimate goal of an objective threshold estimation technique is to establish
an audiogram in a frequency-specific manner without any need for voluntary
responses from the subject (Picton, 1991; Aoyagi et al., 1994). One aspect of
objectivity that is not addressed in this criterion is that of the clinician’s
perception, experience and skill in detecting the appropriate wave form during
AEP tests. This suggests that subjectivity persists in the decision of whether
or not an evoked potential is present.
Rance et al. (1995) point out that ASSRs can be detected automatically,
excluding the subjective evaluation, through real-time statistical analysis of
samples from the response phase using a digital computer. This statement
needs to be qualified somewhat, in that real-time statistical analysis has to be
directed by research in that an appropriate clinical test set-up, noise floor
determinants, number of averages and sweeps need to be standardised
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(especially to make comparisons between research studies more meaningful).
Provided that this final component of objectivity is addressed, it is possible to
use electrophysiological measures to assess patients who cannot or will not
co-operate with conventional hearing test procedures (Sininger & ConeWesson, 2002).
Auditory steady state responses are discussed in this present chapter as a
possible means to determine frequency-specific hearing thresholds estimates
for pseudohypacusic patients, without any need for the subjective detection of
responses on the part of a clinician.
The discussion below defines and
contextualises ASSRs. The stimulus parameters used to elicit responses are
addressed. The chapter concludes with the limitations and advantages of this
technique with specific reference to its application to pseudohypacusic
workers. This theoretical study of ASSRs has formed the basis for a research
programme (see Chapter 5) to evaluate their clinical value in a population of
South African mine workers with noise-induced hearing loss and possible
pseudohypacusis.
4.2
THE DEVELOPMENT OF AUDITORY STEADY STATE
RESPONSES
Auditory steady state responses and steady state evoked potentials (SSEPs)
are the two most frequently used labels found in a survey of relevant literature
to describe this “new” type of AEP. Other, less frequently used, terms are
“steady state fields” (Pantev et al., 1996), “frequency following response”
(Kuwada et al., 1986) and “envelope following response” (Dolphin & Mountain,
1993).
Although there are some differences in their applications, the
definitions of these terms boil down to more or less the same concept. The
term ASSR and SSEP are commonly used interchangeably, but, Sininger and
Cone-Wesson (2002) have concluded that ASSR has become the term of
choice in recent years.
This assessment can, however, not be accepted
without a critical analysis of the uses and implications of the term ASSR as
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the name for a new auditory evoked potential. Such an analysis is provided
below.
Critics of the term “response” argue that in conventional audiometry, this term
is applied to instances where the patient reacts to a stimulus that is presented
in the form of a sound. Schmulian (2002) also questions the use of the term
“response” in relation to evoked potential methods, since electrical waves are
measured without any regard to a conscious or voluntary response on the part
of the subject (Goldstein & Aldrich, 1999). Notwithstanding this discrepancy, it
seems that the use of the term ASSR has gained wide acceptance and it is
therefore used in the rest of this study.
In a clinical context, the term
“response” would certainly be acceptable, as protocols are designed and
recorded to establish a response, for example, at the threshold level.
The AEP technique known as ASSR was discovered and developed at the
University of Melbourne during the 1980s (ERA Systems Pty Ltd, 2000). This
clinical test system was preceded by research on human steady-state evoked
potentials in the visual field (Picton et al., 2003). Galambos, Makerg and
Talmachoff’s (1981) research provided the main impetus for extensive
research into auditory steady state responses (Picton et al., 2003). Rance et.
al. (1995) and Rance et al., (1998) indicate that ASSRs address the main
shortcomings of ABR testing, in that ASSR is an alternative frequency-specific
approach which does not suffer the spectral distortion problems associated
with short-duration stimuli.
ASSRs are periodic scalp potentials arising in
response to regularly varying stimuli, such as a sinusoidal amplitude- and/or
frequency-modulated tones (Rance et. al., 1998).
ASSRs could be conceptualised as follows:
Imagine the waveform for an evoked response which is displayed as a
waveform in the time domain. Imagine the waveform for an evoked
response if two tone burst stimuli were presented within an averaging
epoch. Each tone burst would be expected to produce a response, and
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so the response waveform would be repeated twice, within the averaged
epoch. Now imagine a 200 ms train of 2-1-2 cycle tone bursts, say at
1000 Hz, with an inter stimulus interval between each burst of 20 ms.
Imagine that the signal-averaging epoch is also 200 ms in duration. One
thousand 200-ms trains are presented and the response to each train is
averaged. There are 10 responses in the time-averaged waveform for
the 200-ms sample. Since the recorded response is periodic it can be
analysed using frequency domain methods. To summarise: steady state
responses are recorded when stimuli are presented periodically and they
demonstrate how the brain reacts to a stimulus (Picton et al., 2003)
From this description it can be seen that ASSRs are evoked by stimuli in the
form of rapidly changing auditory signals, presented at such a high rate as to
cause overlapping of responses. This yields what is effectively a steady state
response to a sustained sound or continuous stimuli, as opposed to a
transient response to changing auditory stimuli (Stapells, et al., 1984).
ASSR techniques also use various protocols to evaluate the presence of a
response. Transient responses like ABRs are usually described in terms of
the latencies and the amplitude of specific waves. Latency can be explained
as the time interval between the stimulus onset and the peak of a waveform.
In the case of an ABR, the latency of wave I is for instance, 1,6 ms after
stimulus onset (Hood, 1998). ASSRs by contrast, are not measured in the
time domain (between the stimulus and the response), but in the frequency
domain.
Lins et al. (1996) explain that the compound electrical activity
recordings contain the spectral component for the rate of modulation at which
the tone is presented. Thus the stimulus drives the response to reflect the
same amplitude and frequency modulation with which the stimulus was
presented (Picton et al., 2003).
Human steady state responses were initially studied in the field of visual
modality (Stapells et al., 1984; Picton & Scherg, 1990). A description of the
auditory steady state response by Galambos et al. (1981) reawakened
interest in the phenomenon and its possible use in objective threshold
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estimation (Picton et al., 1987). It was shown that when stimuli are presented
at a rate of 40 per second, the middle latency responses have an amplitude
some two to three times greater than when stimuli are presented at the
conventional rate of 10 per second. (Stapells et al., 1984). Unfortunately, the
40 Hz response has proved to be unreliable for young infants, so clinicians
turned to stimulation rates of 80 to 100 Hz, as they are less affected by sleep,
maturation and sedation (Rance et al., 1995; Herdman & Stapells, 2001;
John, Dimitrijevic & Picton, 2002).
A recent ASSR development is the multiple-frequency technique, where
several carrier frequencies are presented to both ears simultaneously (Lins &
Picton, 1995; John, Dimitrijevic & Picton, 2001b).
The purpose of this
procedure is to shorten test time, which is a critical requirement in clinical
practice, particularly in the case of difficult-to-test patients and infants, who
often do not remain asleep long enough for the test to be completed.
In recent years, the stimuli used in ASSR testing have also been manipulated.
Initially, the pure-tone was only amplitude modulated (John & Picton, 2000;
Cohen, Rickards & Clark, 1991), but later developments showed that tones
modulated in terms of both frequency and amplitude (mixed modulation) give
improved threshold estimates (Dimitrijevic et al., 2001).
From the above it is clear that the ASSR technique has virtually exploded in
the last five years within the AEP context. The initial findings were promising,
but limited due to maturational and wakefulness effects, it was relegated to
more of a research endeavour (Schmulian, 2002). Thus far, ASSRs have
been tested mainly on normal hearing subjects and on very small samples.
Difficult-to-test populations examined have included mainly babies and young
children (Sininger & Cone-Wesson, 1994; Savio et al., 2001; Aoyagi et al.,
1996; Rance et al. 1998). No studies on adult pseudohypacusic populations
could be found.
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The fact that the technique has been used in babies (always a difficult-to-test
population) and since “automated response detection” brings an extra
dimension of objectivity to the evaluation of difficult-to-test populations
motivated an attempt to evaluate this technique for use in an adult
pseudohypacusic population.
Relevant testing parameters and previous research findings related to ASSRs
were evaluated in Section 4.3 to obtain guidelines for an experimental design.
4.3
RESEARCH FINDINGS WITH ASSRs
4.3.1 TYPES OF STIMULI
One of the key differences between ASSR techniques and other AEP
methods are in the stimuli used, as discussed below.
Rob et al. (2000) list the various stimuli used in ASSR testing as click trains,
trains of short tone-bursts and modulated tones. Modulated tones are the
most widely used stimuli for eliciting steady state responses, because tones
are continuous and, hence, are not affected by the spectral distortion
problems associated with brief tone bursts or clicks (Rance et al., 1995). As
has been demonstrated in the previous chapter, tone bursts and clicks have
been used in ABR testing with pseudohypacusic patients, but these stimuli
have not been frequency-specific enough. In medico-legal evaluations (such
as mine workers with noise-induced hearing loss) the availability of frequencyspecific threshold estimates at all the legally specified frequencies are of the
utmost importance and thus the use of tones with longer rise and fall times is
promising with regard to achieving frequency-specificity with pseudohypacusic
adults.
4.3.2 STIMULUS INTENSITY
The speed at which thresholds can be determined with this technique
depends in part on the amplitude of the ASSR, as the response must be
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distinguished from background noise. The greater the response’s amplitude,
the more rapid detection is. Nevertheless, research has shown that ASSRs
can be recorded at low sensation levels (Dobie & Wilson, 1998). Rance et al.
(1995) have found that ASSRs could be recorded at low sensation levels
even with patients who are sleeping or sedated, provided that the modulation
frequency is greater than 70 Hz.
Schmulian (2002) has also discussed the influence of intensity on multiple
frequency (MF)-ASSR techniques, saying that at low-to-moderate intensity
levels, the responses elicited with different carrier frequencies (CFs) show
little overlap, provided CFs are one octave apart to ensure that effects on the
basilar membrane occur at different locations. At higher intensities, the basal
end of the cochlea tends to dominate, causing significant overlap to occurhence, frequency-specific responses are more difficult to detect.
Low intensity MF-stimulation is particularly important in a population of mine
workers, since noise-induced hearing loss is usually a sloping hearing loss
with thresholds at 500 and 1000 Hz, near normal levels (Dobie, 2001).
4.3.3 CARRIER FREQUENCY
The effects of carrier frequency are quite different for stimuli modulated at
rates of 40 to 80 Hz (Picton et al., 2003). The 40 Hz responses significantly
decrease in amplitude with increasing carrier frequency (Galambos et al.,
1981). For the 80 to 100 Hz responses, the amplitude is larger for the middle
frequencies (1000 to 2000 Hz) than for either higher or lower frequencies
(Picton et al., 2003). Some of this effect at 80 Hz MF-techniques might be
due to the fact that the stimuli at different CFs are presented at the same
sound pressure level (normal hearing thresholds are found at lower
frequencies).
It has been proven that the higher the carrier frequency and the greater the
hearing loss, the better the correlation between ASSR and pure-tone
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thresholds is (Sininger & Cone-Wesson, 1994; Rance et al., 1995). This fact
could be due to recruitment when monotic procedures are used.
John and Picton (2000) found that the latency of human ASSRs to amplitude
modulated (AM) tones changes significantly and consistently with the carrier
frequency in a MF-stimulation procedure. Latency periods are shorter for
higher frequencies (for example, latency reduced from 6,0 to 5,5 ms when the
CF was increased from 500 to 6 000 Hz). Such changes in the latency period
appear to result from two cochlear processes: the filter build-up time of the
hair cell transduction process and the transport time for acoustic energy to
reach the responsive region of the basilar membrane, which is at the apex of
the cochlea for low-frequency stimuli.
Schmulian (2002) explains that the lower amplitude of responses observed
when a low CF is used is due to the fact that the activation pattern on the
basilar membrane extends over a greater area than is the case with higher
carrier frequencies. This causes a “jitter”, which could attenuate the amplitude
of the response. The intrinsic jitter at 500 Hz has also been attributed to
neural asynchrony (Lins et al., 1996). Other researchers have also discussed
diminished responses at 500 Hz (John & Picton, 2000; Perez-Abalo et al.,
2001; Lins et al., 1996; Aoyagi et al., 1994). One explanation attributed this
lower amplitude of responses at lower CFs to a possible effect of ambient
noise on stimuli at these frequencies (Lins et al., 1996).
In the evaluation of this technique in a population of mine workers, it is
important to note that 500 Hz is a frequency that must be tested by law (RMA
guidelines, 2003) and thus it is important that accurate threshold estimates
should be obtained at 500 Hz. One way of addressing the problems that
various researchers have experienced in testing at 500 Hz is to limit the
masking effect of ambient noise, in other words, to test in a sound-proof booth
(Herdman & Stapells, 2001). In the clinical situation this should not imply any
extra cost, since an acoustic booth is already used for conventional
audiometry.
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4.3.4 MODULATION FREQUENCY
With AEP methods, such as tone burst ABR, stimuli can evoke a response,
but the latency, amplitude and threshold of the ABR are all affected by the
stimulus level, rise-time and rate of presentation.
Conventional signal
averaging is used to detect the response, which is displayed as a wave form
in the time-domain (Sininger & Cone-Wesson, 1994). This wave form needs
to be identified by the clinician.
By contrast, ASSRs are periodic and can therefore be analysed by means of
frequency domain methods. The spectrum of the response shows a major
component at the rate at which the tone or stimulus is repeated or modulated
and at the second harmonic of that frequency. It is thus clear that a response
follows the same modulation rate as the stimulus and therefore the response
detection is much more objective.
It should be noted that with high
modulation frequencies (for example, 100 Hz), each modulation has a 10 ms
duration, with a 5 ms sinusoidally ramped rise-fall time and no plateau. The
spectrum of the response peaks at the modulation frequency, thus
determining the response’s amplitude and phase characteristics, with no
contamination of the response spectrum by the stimulus (Sininger & ConeWesson, 1994; John et al., 2002).
Not only is the frequency of the stimulus modulated, but the CF amplitude
modulation introduces a replicable stimulus parameter, allowing a reliable
estimation of hearing thresholds across the normal audiological test range,
based on research on a wide range of modulation frequencies (4 to 450 Hz)
(Cohen et al., 1991). The success of amplitude modulation can be attributed
to spectral power being present only at the CF and at two side bands (John et
al., 2002).
This fact that it is possible to estimate behavioural thresholds
across the audiological test range opens up the possibility that the degree and
nature of hearing loss can now be determined in difficult-to-test populations. In
fact, it has made this research possible.
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Galambos et al. (1981) has described the initially popular modulation rate of
40 Hz, for which large and defined response amplitudes have been observed.
One disadvantage of using the 40 Hz response is that at lower modulation
frequencies such as 40Hz, responses have proven to be problematic, in that
threshold estimation is affected by state of consciousness and sleep
(Herdman & Stapells, 2001; Maiste & Picton, 1989; Lins et. al., 1995),
maturation (Lins et al., 1995) and anaesthesia (Plourde & Picton, 1990). As
the modulation frequency is reduced, the principal site of evoked potential
responses is likely to move up the auditory pathway, thereby increasing the
latency period. Such effects were to be expected, given the sensitivity of
response generators in the auditory cortical and lemniscal brainstem to a
person’s state of consciousness.
Nevertheless, some researchers have
proven that the 40 Hz response is a very effective means of threshold
estimation, including John and Picton (2000), who maintain that 40 and 80 Hz
are the most suitable modulation frequencies for threshold estimation.
Unfortunately, the 40 Hz response is not reliable in young infants and children,
due to maturation effects and the effect of state of consciousness, as
mentioned above.
Dobie and Wilson (1998) state that ASSRs for adult patients are best
recorded at low intensities in the alert/awake state, based on reduced 40 Hz
responses among sleeping or sedated adults. They conclude that the 40 Hz
response at low intensity levels is optimal for both alert and sedated adults. In
sedated subjects, the reduced background noise made responses more
detectable.
Due to the above difficulties with the 40 Hz response, a greater interest in the
use of high repetition rates arose after it was found that they increased the
amplitude of responses (Rickards & Clark, 1984). Modulation rates of 75 to
110 Hz were seen to be the most suitable for threshold estimation (Cohen et
al., 1991; Lins & Picton, 1995; Lins et. al., 1996). Lins et al. (1996) have
demonstrated that modulation rates of 75 to 110 Hz can be used to estimate
pure-tone thresholds to within 10 to 20 dB in sleeping babies and in normal
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and hearing-impaired adults. Lins and Picton (1995) have reported that a
modulation frequency of 80 Hz gives response latencies that are similar
during sleep and wakefulness. A rate of 80 Hz has also been regarded as an
effective modulation frequency for sedated adults (Dobie & Wilson, 1998).
Higher modulation rates (770 Hz) have also proven to be effective in
estimating hearing thresholds when they are used at low intensities (Clark et
al., 1991). In terms of the available equipment, the 40 Hz and 80 to 110 Hz
are the most popular modulation frequencies at this stage.
The focus of the preceding discussion is amplitude modulation. However,
Cohen et al. (1991) have found that frequency- and amplitude-modulated
tones (AM/FM) yield larger response amplitudes that amplitude modulated
tones alone, because additional processing channels are associated with
frequency modulation and AM/FM tones excite a larger portion of the basilar
membrane. This combined amplitude and frequency modulation is also called
multiple modulation (MM) (Schmulian, 2002), and produces tones that sound
similar to the warble tone used in paediatric audiology.
John and Picton
(2000) have found that responses elicited using both amplitude and frequency
modulation reaches significance at twice the speed of tones that are only
amplitude modulated.
Since different modulation frequencies have been shown to be successful in
different populations, one can conclude that it is important to evaluate both a
lower (40 Hz) and a higher modulation frequency (80 to 110 Hz) in an
untested pseudohypacusic adult population, and to use mixed modulation in
an experimental design, since it has already been proven to be more accurate
in threshold estimation than amplitude or frequency modulation alone.
4.3.5 DICHOTIC STIMULATION
The above discussion of ASSR stimulus parameters has focused on monotic
stimulus presentation, in which each frequency is assessed separately for
each ear.
Monotic presentation techniques were developed for hearing
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assessments in cochlear implant programmes, because dichotic presentation
limits the separation of responses at high intensities, which are quickly
reached during evaluations of cochlear implant candidates with limited
residual hearing (Rickards et al., 1994).
An optimised variant of ASSRs called multiple simultaneous amplitude
modulation has been described by Lins and Picton (1995).
Distinct
modulation rates (separated by more than one octave) are used for eight
carrier tones (four per ear), and the modulated tones are combined to produce
an acoustic stimulus capable of simultaneously activating different regions of
the cochlea (Perez-Abalo et al., 2001). Herdman and Stapells (2001) have
found that MF-ASSR testing of both ears produces responses comparable to
the use of only one carrier frequency or four carrier frequencies to a single
ear. It is claimed that the technique can predict eight thresholds in the time it
takes to observe one single threshold (Lins et al., 1996; Perez-Abalo et al.,
2001).
The MF-ASSR technique is also a variant of the 75 to 110 Hz ASSR that
Perez-Abalo et al. (2001) have found to be reliable in predicting behavioural
thresholds, with 80,9 per cent of ASSR and behavioural thresholds within
20 dB of each other. Similar results were reported by Herdman and Stapells
(2001) with 87 per cent of ASSR and behavioural thresholds within 20 dB of
each other.
There is an urgent need for techniques that will enable audiologists to
determine behavioural thresholds in a time-efficient manner. An ASSR test
time of 164 minutes for eight separately determined frequencies and a
corresponding time of 83 minutes for multiple dichotic ASSR testing have
been reported (Herdman & Stapells, 2001). Although 83 minutes is shorter,
this is still impractical for clinical applications, especially for difficult-to-test
patients. This is true even for a test time of 21 minutes, as reported for
normal hearing subjects (Perez-Abalo et al., 2001).
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Swanepoel (2001) maintains that MF-ASSR techniques show great promise
as a threshold estimation technique for patients of all ages, but, clinical
validation is limited (see Section 4.3.6). It has thus been postulated that the
technique cannot be considered for clinical use until additional studies have
optimised parameters (John et al., 2001b). Furthermore, Schmulian (2002)
has pointed out that studies thus far have only used normal adults, well infants
and a very limited (small) number of hearing-impaired subjects.
An exciting topic for future study is indicated by John et al. (1998) , who point
out that everyday sounds contain multiple frequencies and, that therefore, the
results of MF-ASSR methods may be more representative of actual hearing
than those of tests using discrete stimuli.
Finally, the mere fact that simultaneous testing of eight frequencies is possible
is an important advantage in a difficult-to-test population and in an industry
(mining) that produces very high case loads.
This is another (important)
motivation for validating the technique in a mining environment.
4.3.6 LIMITED CLINICAL VALIDATION
In 2001, Swanepoel commented that ASSRs had not been studied very
extensively. This is still the case, as no literature could be obtained pertaining
to ASSRs and to noise-induced hearing loss and pseudohypacusis, which
constitute the focus of the present study. When experimental testing began in
September 2002, only one ASSR system was available at the University of
Pretoria.
As indicated before, clinical applications of ASSRs are in their
infancy , and relevant research findings are limited (Schmulian, 2002).
The above debate will be illuminated further because the clinical validation of
MF-ASSR is particularly limited for hearing-impaired subjects (Perez-Abalo et
al., 2001).
Schmulian (2002) quotes six MF-ASSR studies in which no
findings are reported regarding the possible impact of ASSR on an impaired
auditory system.
The present author would add that ASSR research is
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characterised thus far by very small experimental groups. Johnson and Brown
(2001) used only ten subjects, and Valdez et al. (1997) used only 16. The
limited clinical validation and research is a confounding factor to the present
research, since there are no similar studies available to which results can be
compared to. In that sense then, this study is exploratory in nature.
4.3.7 LENGTH OF PROCEDURES
One disadvantage of AEP techniques, mentioned before, is the length of test
procedures. ABR, the most popular AEP method, also presents this limitation
in evaluating difficult-to-test patients (Stach, 1998). John et al. (2001a+b)
report that, particularly with children, the examiner must obtain as much
information as possible in the shortest possible time.
A positive factor is that continuing research has led to newer developments
that reduce the time required for threshold determinations. The amplitude of
the response limits the speed of threshold determination, as responses must
be distinguished from background noise, indicating that it would be
advantageous to increase response amplitude (John et al., 2002).
Techniques that have already increased the speed of determination include
the following:
•
the use of multiple modulated (amplitude and frequency) stimuli for
more rapid determination of thresholds than with simple amplitude
modulation or frequency modulation of stimuli (John et al., 2001b);
•
amplitude modulation of stimuli using exponential envelopes can
reduce the average test time by up to 21 minutes (Perez-Abalo et al.,
2001). This was achieved by increasing ASSR amplitude and latency,
to reduce the time needed for responses to become significant (John et
al., 2002);
•
evaluation of responses to several (eight) simultaneously presented
amplitude-modulated (at different rates) stimuli (Lins & Picton , 1995)
can reduce test time by allowing eight frequencies to be assessed
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simultaneously.
(This is in contrast with the more time-consuming
separate assessment of individual frequencies in single carrier
frequency tests (Herdman & Stapells, 2001; Perez-Abalo et al., 2001).
However, John et al. (2001b) postulate that MF-ASSR testing is not yet
suitable for clinical applications, saying that more trials are needed to
optimise stimulus and recording parameters before this procedure can
be validated); and
•
the use of analysis algorithms to automatically conclude stimulation
and sampling once a predetermined probability value (for example P<0,
3) is achieved, thereby minimising test time for any given trial (ERA
systems Pty Ltd, 2000).
Lengthy testing time can be seen as a negative factor when testing
pseudohypacusic mine workers with ASSRs, since the mining industry
produces very large case loads. A further negative influence of testing time is
the impossibility of evaluating different test protocols with the same subject
(De Koker, 2003).
4.3.8 SUBJECT-RELATED FACTORS
In recording AEPs and ASSRs, it is important to consider that a subject may
induce inaccurate recordings by interfering with procedures or the test
environment (Aoyagi et al., 1994; Schmulian, 2002).
Body movement,
tenseness and an inability to follow instructions or remain still create
excessive background noise and have a negative effect on the quality of data
collected (Sininger & Cone-Wesson, 1994).
The same authors have
recommended that the clinician optimises the amplitude of the response and
minimises background noise to ensure quality recordings:
•
a correct placement of electrodes improves recordings;
•
adequate epoch duration is important;
•
a suitable filter bandwidth should be selected;
•
minimal electrical noise should be present;
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•
a sufficient number of sweeps is needed to yield reliable averages;
and
•
accommodation for the patient’s age and state of consciousness
should be made.
Because factors such as filter bandwidth, epoch duration and the number of
sweeps averaged are controlled by computer software using algorithms
developed during research, the clinicians main concern should be to control
artefacts and background noise. Clinician’s should also be aware of the need
for a quiet test environment during ASSR threshold estimates, according to
Herdman & Stapells (2001), who have found that the accuracy of threshold
estimates improved by 5 to 10 dB when tests were conducted in an
acoustically treated test booth.
Subjects should be relaxed to minimise
artefacts (John & Picton, 2000), and the head should be positioned for a
relaxed posture to reduce peri-auricular and muscle potentials (Halliday,
1993).
Dobie and Wilson (1998) recommend that patients be tested in a
supine position, and in a darkened room.
Sedation is sometimes administered to ensure low noise levels, but this
practice has medico-legal and ethical implications.
Furthermore, patients
must give informed consent before such a procedure is performed and
medical support must be available.
The latter aspect has financial
implications. This statement paints a negative picture but, on the positive
side, John and Picton (2000) observe that it is possible that, as researchers’
experience with ASSR methods increased, inter-subject variance may
diminish.
Since there are no previous data available on the adult difficult-to-test
population of pseudohypacusic mine workers, it is important to verify if
sedation will influence the accuracy of threshold estimates and to control the
factors that have already been proven to reduce the quality of threshold
estimation. Lack of co-operation and tenseness has led to routine sedation of
pseudohypacusic mine workers during ABR testing (De Koker, 2003).
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Sedation might thus be needed if pseudohypacusic patients withhold cooperation.
4.3.9 APPLICATIONS OF ASSR IN CLINICAL AUDIOLOGY
Various applications for ASSR testing have been proposed in the literature:
•
probing the ongoing state of a subject during operations (Sininger &
Cone-Wesson, 1994);
•
neonatal screening (Rickards et al., 1994);
•
neuro-otological diagnosis of retro-cochlear abnormalities (Sininger &
Cone-Wesson, 1994);
•
as
an
electrophysiological
technique
analogous
to
speech
discrimination tests (Picton et al. (1987) state that the ability to
discriminate changes in a sound’s frequency and intensity is essential
to auditory perception, and Dimitrijevic et al. (2001) have followed the
same line of thought in proposing ASSRs as an objective test for
supra-threshold hearing); and
•
estimating pure-tone behavioural thresholds (clearly the most
important clinical application for ASSRs , particularly in difficult-to-test
patients).
Pseudohypacusic patients certainly fall into the difficult-to-test category, and
discussions of AEP and ASSR testing in the last two chapters raises the
question whether ASSR testing is an accurate, feasible and time-efficient
way to evaluate pseudohypacusic mine workers with noise-induced
hearing loss, or, more to the point, whether ASSR-based threshold
estimates for this group (who are difficult-to-test and have true sensoryneural hearing loss) are accurate enough to finalise compensation and
fitness-for-work assessments.
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4.3.10
APPARATUS
MF-ASSR methods use the same recording montage as ABR tests. Rance et
al. (1995) advise the use of silver-silver chloride disk electrodes on the
forehead and earlobe/mastoid, with a third electrode on the contra-lateral
mastoid or cheek to serve as an earth. ASSR test systems and software
require
a
personal
computer
running
Windows,
as
well
as
an
electroencephalogram amplifier. Earphones are inserted in addition to the
electrodes.
The fact that the same electrode montage is used as for the ABR enables the
clinician to perform an ABR, when needed, as well.
4.3.11
THRESHOLD DETERMINATION TECHNIQUE
Attention has been drawn to the fact that different threshold-seeking
procedures may account for differences between ASSR and behavioural
thresholds, where 10 dB steps have mainly been used in AEP procedures and
5 dB steps in behavioural testing.
A concern in experimental work is the lengthy procedure involved for all AEPs.
Is it practicable to test at 5 dB intervals when using ASSR-methods when a
clinician has a large case load as is typical in the mining industry?
4.3.12
RESPONSE GENERATORS
There has been very little research on neural generators of ASSR as a
function of the modulation rate (Sininger & Cone-Wesson, 1994). The physiological interpretation of scalp-recorded ASSR latencies remains difficult. The
main problem is that responses may be derived from more than one generator
in the auditory pathway (John & Picton, 2000). Sininger and Cone-Wesson
(1994) cite studies of ASSR neural generators in relation to modulation rate,
which found that the VIII cranial nerve, cochlear nucleus, inferior colliculus
and primary auditory cortex are all responsive to amplitude and frequency
modulated signals.
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The literature clearly indicates that, for the purpose of threshold estimation,
the presence or absence of an ASSR is mainly determined by the integrity of
the cochlea and the VIII cranial nerve (Dimitrijevic et al., 2001). The cochlea
is the area of concern in noise-induced hearing loss, at it is thus relevant to
use this technique on a population with noise-induced hearing loss.
4.3.13
FREQUENCY-SPECIFICITY
As for the clinical determination of hearing thresholds, AEP threshold
estimates should be provided for each ear at frequencies corresponding with
the range of human speech communication (Sininger & Cone-Wesson, 1994).
The reason for this is that once a person develops a hearing loss, a clinician
needs to characterise its degree, type and configuration. Relevant frequencyspecific information enables a clinician to apply appropriate amplification and,
in the mining sector, to evaluate compensability and fitness for work. In South
Africa, compensation assessments must consider hearing at 500, 1 000,
2 000, 3 000 and 4 000 Hz (Workmen’s Compensation Commissioner, 1995).
According to ERA Systems Pty Ltd (2000) and John and Picton (2000),
ASSRs can be elicited in the frequency range between 250 and 8 000 Hz,
thereby meeting the need for specificity across the range of frequencies for
conventional audiometry and satisfying legal requirements.
The excellent frequency specificity of ASSRs is based on the frequency
content of an amplitude-modulated stimulus that is concentrated where there
is no spectral splatter (Lins et al., 1996). Rance et al. (1995) and Lins et
al. (1996) have shown that the configuration of hearing loss does not influence
the accuracy of ASSR results.
4.3.14
RESISTANCE TO STATE OF CONSCIOUSNESS
A clinician must be aware of factors like the patient’s state of consciousness,
which can affect the quality of AEP measurements. ABR testing has proven
to be effective, particularly for infants, since it is not affected by the infant’s
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state of consciousness or sleep, in contrast to the 40 Hz responses, which are
considerably affected by sleep and sedation (Cohen et al., 1991).
It is of the utmost importance that the testing procedures used for difficult-totest patients are not affected by sleep or sedation, as such cases are
characterised by a lack of co-operation.
Testing under sedation often
becomes a necessity. Cohen et al. (1991) and Rance et al. (1995) have
found that ASSR techniques give reliable results for sleeping adults and
children, while Hood (1998) also concludes that ASSRs evoked by tones with
a modulation rate of 75 to 110 Hz are not significantly affected by sleep or
sedation.
4.3.15
ABSENCE OF GENDER BIAS
During ASSR research, no evidence of gender bias has been found (Stapells
et al., 1984). This is not only an important clinical characteristic of a specific
research technique, but it is also of specific importance in the present study,
since mine workers are traditionally male and thus it is highly unlikely that a
comparison between male and females in this population would be possible.
Results of research using male mine workers can therefore quite possible be
generalised to females as well.
4.3.16
ACCURACY OF THRESHOLD ESTIMATES
The main problem clinicians have with pseudohypacusic patients is great
difficulty in obtaining the accurate, reliable and objective hearing thresholds
which are imperative to meaningful assessments. This problem can possibly
be overcome by using ASSRs, but clinicians must take into account that
ASSR thresholds are not hearing thresholds per se, but physiological
thresholds used to predict auditory thresholds (Sininger & Cone-Wesson,
1994).
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Furthermore, it is important to acknowledge that, when one compares puretone and physiological thresholds, pure-tone thresholds are influenced by
factors such as:
•
instructions given to patients;
•
the size of the dB step or increment used in tests;
•
the earphone fit;
•
background noise in the test environment; or
•
the threshold determination criterion used by the audiologist, for
example, a 50 per cent or a 75 per cent detection rate (Sininger &
Cone-Wesson, 1994).
The above issues are not relevant to ASSRs. Electrophysiological thresholds,
by contrast, are detected when they are distinct from random neural and
muscle potentials, and from random airborne activity.
Any factors that
influence the amplitude of the response or the amplitude of the noise affect
detection. Nevertheless, several researchers have found a high correlation
between ASSR and pure-tone thresholds.
Lins et al. (1996) have found ASSR thresholds to be approximately 10 dB
higher than conventional pure-tone hearing thresholds among adults with
normal hearing. They have also found that threshold estimation in a group of
infants was slightly worse than reported by Rickards et al. (1994), who found
differences of 41, 24 and 35 dB hearing level at frequencies of 500, 1 500 Hz
and 4 000 Hz respectively, among well babies. Lins et al. (1996) have tested
adolescents with quantified hearing losses, and have found that ASSR
measures provide reliable frequency specific information for this population.
Due to the excellent correlation found between behavioural and ASSR
thresholds (an overall coefficient of 0,97 for all the frequencies tested) (Rance
et al., 1995), a linear regression analysis has been developed to translate
electrophysiological thresholds into a conventional audiogram.
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Use of the
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regression line enables predictions of behavioural thresholds across a range
of carrier frequencies to within 10 dB in 96 per cent of cases.
The accuracy of the estimation of behavioural thresholds by ASSRs is the one
very important factor that will decide whether this technique will be acceptable
in medico-legal investigations in general and in the mining industry in
particular.
4.3.17
DETECTION OF THRESHOLDS THROUGH THE SEVERITY
RANGE
The validity of ASSR thresholds in normal hearing populations has so far been
the most extensively researched. Rickards et al. (1994), Swanepoel (2001)
Schmulian (2002), Rance et al. (1995) and Lins et al. (1996) have studied the
threshold estimation accuracy of ASSRs in normal hearing people, and they
all conclude that ASSR is a suitable procedure for this application.
Although it has not been as extensively studied (Schmulian, 2002), threshold
estimation in people with hearing loss, has also shown ASSR testing to be a
suitable substitute for pure-tone testing. Lins et al. (1996) found the prediction
of pure-tone thresholds from ASSR thresholds to be in the order of r = 82, with
differences averaging between 9 and 14 dB. Rance et al. (1998) have tested
infants and children who were candidates for cochlear implants to assess the
ASSRs ability to predict severe hearing loss and establish the presence of
residual hearing. ASSR thresholds were within 20 dB of pure-tone thresholds
for 99 per cent of these cases, and within 10 dB for 82 per cent of them.
It can therefore be concluded that ASSR methods of threshold estimation are
suited for normal and impaired hearing cases, but that estimates of hearing
thresholds are better in pathological ears, due to the effects of recruitment
(Rance et al., 1995). This again motivates the drive to test this method in a
mine worker population that is known to have a high incidence of hearing loss.
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4.3.18
LACK OF AGE-RELATED INFLUENCES
The use of ASSRs has been studied for a wide range of age groups, including
neonates, children, adolescents and adults. In all these groups, it has been
found that ASSR testing provides a reliable and objective measure of hearing
thresholds. Stapells et al. (1984), Sininger & Cone-Wesson (1994) and Rance
et al. (1995) have found no age effects during ASSR testing.
It has also been proven that ASSRs are appropriate for screening neonates
during the first four days after birth (Rickards et al., 1994). Savio et al. (2001)
have shown that ASSR techniques are valid, but they are the only researchers
who have demonstrated changes in threshold amplitude and detectability
during the first year of life. They have found that thresholds at 4 000 Hz
decrease by 14 dB between birth and 12 months of age, and that such
changes occurred more slowly for ASSR thresholds at lower frequencies.
Age effects are not relevant to this study, since the difficult-to-test population
are all adults.
4.3.19
THRESHOLD DETECTION IN THE FREQUENCY DOMAIN
As stated previously, a critical requirement that has to be met by AEP testing
is an objective detection of responses. Although no voluntary responses are
needed from the patient (Lins et al., 1995), it is preferable that clinicians also
play no role in determining or assessing the presence of a response.
When an ASSR stimulus is presented at or above a threshold, hair cells in the
cochlea are activated in a locus corresponding with the carrier frequency. An
analysis of the response in the cochlea and subsequent parts of the auditory
pathway requires no visual detection of wave forms, nor any measurement of
peak latency or amplitude.
ASSRs are detected by applying computer
algorithms to the recorded elctroencephologram. The algorithms analyse the
magnitude and phase of the electrical activity corresponding with the
modulation frequency. Lins and Picton (1995) explain that the complex wave
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forms in the time-domain are transformed to the frequency-domain by means
of Fast-Fourier processing. In the frequency-domain, the analysis is done
using spectral analysis techniques.
ERA Systems Pty Ltd (2000), the manufacturers of the Audera ASSR system,
state that 64 samples are analysed in each trial, which comprises a tone of a
specific frequency-amplitude combination, for example, 1000 Hz at a 30 dB
hearing level.
In each electroencephalogram sample, the magnitude and
phase of the electrical activity corresponding with the modulation frequency
are quantified and shown as a vector in a polar plot. The vector’s length
represents amplitude, and its angle reflects the phase or time delay between
tone modulation and the brain’s response (ERA Systems Pty Ltd, 2000).
When vectors are clustered, this indicates a phase-locked brain response; in
other words, the electroencephalogram samples are synchronised with the
tone modulation frequency, which can only occur if the ear and brain have
responded to a sound. Vectors distributed randomly around the polar plot
indicate a lack of phase relationship between the electroencephologram and
tone modulation (no response).
Statistical analyses are done in real-time as samples are collected, and the
analysis algorithms (Sininger & Cone-Wesson, 1994) halt stimulation and data
sampling when certain probability values have been obtained, for example,
p (probability value)<0, 3.
The statistical analysis of vector phases uses a measure known as phase
coherence squared (PC²), calculated as each new vector is obtained for an
electroencephalogram sample.
The resulting PC² values can range from
0 to 1, with values approaching 0 indicating low phase coherence between the
sample and tone, and those approaching 1 indicating high phase coherence.
The PC² value is evaluated using statistical tables of circular variance to
obtain a probability value, “p”. This level of significance is thus determined by
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a statistical test and gives an indication of whether a response is present. A
probability value of p<0,03 sets the false positive rate for ASSR detection at
3 per cent (there is a less than 3 per cent chance that results are due to noise
alone). A trial contaminated by excessive noise is automatically terminated,
labelled as such and excluded from further evaluations. The lowest level at
which
a
phase-locked
electrophysiological
response
threshold,
which
is
is
obtained
used
is
to
taken
estimate
as
the
pure-tone
behavioural thresholds by means of an algorithm based on the research of
Rance et al. (1995) (see Figures 5.14 to 5.18).
Picton et al. (2001) has found that detection protocols based on both phase
and amplitude (the f-test and the phase-weighted t-test) are more effective
than those using phase alone (phase coherence and phase-weighted
coherence) (Stapells et al., 1984; Aoyagi et al., 1994). The f-test evaluates
whether a response to the stimulus differs from noise in the recording at
adjacent frequencies (Lins et al., 1996; Perez-Abalo et al., 2001), and the T2
statistic determines whether a response is replicable across a number of
averaged responses (Valdez et al., 1997; Picton et al., 1987). Lins et al.,
(1996) have found the f-test to be slightly more effective than the T2 test.
Picton et al. (2001) have found that using both the phase and the amplitude
data in detection protocols identified more ASSRs than phase data alone.
The above detection of responses and thus threshold estimation objectively
done by means of computer algorithms is the most important reason for
evaluating this technique in an adult population with pseudohypacusis, since
this objectivity has been lacking in traditional AEP testing.
4.3.20
ASSESSMENT OF SOUND PROCESSING
ASSR testing has created the possibility of evaluating sound processing by
means of binaural stimulation, rather than traditional monaural stimulation.
Multi-sensory processing and interactions between the visual and auditory
systems have not yet been researched (Schmulian, 2002), but a possibility
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may exist that one could use ASSRs in the evaluation of reading difficulties
where auditory and visual processing abnormalities coincide. The possible
advantages of evaluating a patient’s hearing using this technique would be the
fact that binaural multiple-frequency stimulation can approximate human
hearing to a much greater degree than monaural pure-tone testing does.
4.4
SUMMARY
In this chapter, auditory steady state responses have been defined and put
into a historical perspective.
The relevant testing parameters have been
discussed with reference to their importance for a pseudohypacusic adult
population. Advantages and disadvantages of this AEP have been evaluated
in order to decide on the possibility of using this method as a threshold
estimation technique in adults with noise-induced hearing loss.
A summary of the current research findings related to the rationale for the
clinical and research use of ASSRs is set out in Table 4.1 below.
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TABLE 4.1:
RATIONALE FOR THE SELECTION OF ASSR
EXPERIMENTAL RESEARCH WITH MINE WORKERS
ADVANTAGE OF ASSRs
IN
REFERENCES
Objective threshold estimation
Sininger and Cone-Wesson (1994)
ERA Systems Pty Ltd (2000)
Rance et al. (1995)
Frequency-specificity
Sininger and Cone-Wesson (1994)
ERA Systems Pty Ltd (2000)
John and Picton (2000)
Lins et al. (1996)
Rance et al. (1995)
Resistance to state of consciousness
Cohen et al. (1991)
Rance et al. (1995)
Hood (1998)
Absence of gender bias
Stapells et al. (1984)
No amplitude deterioration with pathology
Schmulian (2002)
Correlation with behavioural thresholds
Sininger and Cone-Wesson (1994)
Lins et al. (1994)
Rance et al. (1995)
Response generators: cochlea and VIII nerve
Dimitrijevic et al. (2001)
Application in threshold estimation
Rance et al. (1995)
Rickards et al. (1994)
Age unimportant
Stapells et al. (1984)
Rance et al. (1995)
Rickards et al. (1994)
Tonal stimuli
Rob et al. (2000)
Rance et al. (1995)
Stimulation of eight simultaneous
Perez-Abalo et al. (2001)
frequencies
Herdman and Stapells (2001)
Accurate throughout severity range
Rickards et al. (1994)
Rance et al. (1995)
Lins et al. (1996)
Rance et al. (1998)
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The above theoretical advantages indicated in Table 4.1 motivated the
application of ASSRs in an empirical clinical study as is discussed in
Chapter 5.
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CHAPTER 5
RESEARCH METHODS
AIM
This chapter’s aim is to describe and justify the methodology followed in the
empirical research of the study. The end goal is to answer the research
question: What is the clinical value of ASSRs in the audiological evaluation
of pseudohypacusic mine workers with noise-induced hearing loss?
5.1
INTRODUCTION
The research question put forward in Chapter 1 centres around the clinical value
of auditory steady state response methods in audiological assessments of
pseudohypacusic mine workers with noise-induced hearing loss.
In the South African mining industry a large number of workers (between 68 and
80 per cent) are exposed at equivalent levels of noise exceeding 85 dB (Franz &
Phillips, 2001). The high incidence of noise-induced hearing loss, combined with
workers’ awareness of noise-induced hearing loss compensation, creates a
situation in which workers commonly exaggerate symptoms of their hearing loss
for compensation purposes.
Conventional assessment methods available to
audiologists currently fail to provide accurate and reliable hearing thresholds in
such cases, delaying the conclusion of some claims and, in all likelihood,
resulting in overcompensation of others.
Promising alternative methods to address the current situation include auditory
tests utilizing evoked potentials (AEPs: Chapter 3) and more specifically auditory
steady state response testing (ASSRs: Chapter 4). The need to be met is for a
once-off test, capable of concluding diagnostic procedures, for pseudohypacusic
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workers by estimating accurate hearing thresholds for compensation claims and
“fitness-for-duty” assessments.
According to most of the literature reviewed,
ASSR testing provides an accurate means of predicting pure-tone hearing
thresholds without any need for the patient to respond to the sound - thus
providing a possible solution to the research question.
In a survey of the literature it has become clear that primarily two modulation
frequencies have been used in research with ASSRs, that is 40 Hz (Stapells et
al., 1984) and 80 to 110 Hz (Lins & Picton, 1995). There are also currently two
stimulation methods namely monotic (Rickards et al., 1994) and dichotic (PerezAbalo et al., 2001).
These presentation variations need to be taken into
consideration when planning empirical research in this field.
The fact that
auditory evoked potentials are affected by the state of consciousness of the
patient (Dobie & Wilson, 1998) is another important aspect to incorporate in the
research design especially in situations were the co-operation or lack of cooperation of the patient is a factor that can influence the assessment outcome. It
is thus clear that empirical research designed to answer the stated research
question will of necessity be complex and involved.
5.2
AIMS OF THE RESEARCH
The aims of the present research are detailed in the sections below:
5.2.1 PRINCIPAL AIM
The principal aim of the study was to determine the clinical value of ASSR
methods in the hearing assessment of pseudohypacusic mine workers
presenting with noise-induced hearing loss.
Roeser et al. (2000b) drew attention to the fact that the effectiveness of an
audiological test needs to be evaluated. The same authors stated that tests
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could be evaluated to decide on validity, reliability, sensitivity and specificity. An
audiological test’s value lies in its ability to perform as intended. In order to
determine the value of ASSR tests the norm for “clinical value” was the threshold
estimation ability of this procedure. Could ASSR tests accurately estimate puretone thresholds in a pseudohypacusic population in order to conclude diagnostic
procedures and thus facilitate in correct and meaningful recommendations
regarding rehabilitation?
Apart from the clinical efficiency in estimating thresholds, the cost- and time
efficiency of ASSR methods will also aid in decisions related to the ultimate value
of the specific method.
5.2.2 SUB AIMS
The principal aim of the study, to decide on the threshold estimation ability of
ASSRs in a pseudohypacusic population, can only be attempted if ASSRs have
been validated in an adult mine worker population with noise-induced hearing
loss. Since this procedure has not been validated in this population the sub-aims
are:
5.2.2.1
To compare ASSR and pure-tone thresholds in a co-operative
population of adult mine workers with sensory neural hearing
loss
The clinical value of ASSR techniques, in other words the ability to estimate puretone thresholds, has to be investigated for co-operative noise-exposed mine
workers and specifically those with identified noise-induced hearing loss. The
pure-tone and ASSR threshold estimates of all the subjects need to be compared
in order to evaluate the effectiveness of ASSRs in estimating pure-tone
thresholds. All the frequencies specified in legislation for the mining industry
should be tested, namely, 500, 1 000, 2 000, 3 000 and 4 000 Hz (RMA
guidelines, 2003) and in both ears.
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5.2.2.2
To compare the accuracy of multiple-frequency (dichotic) and
single frequency (monotic) ASSR stimulation methods in
estimating pure-tone thresholds in a mine worker population
The effectiveness of multiple-frequency (MF) ASSR and single frequency ASSR
methods for threshold estimates should be compared in order to make
recommendations regarding the most effective method possible.
The reason
being that time saving is an important factor in an industry with large case loads.
The ASSR threshold estimates for both these stimulation methods are compared
to pure-tone thresholds. Comparing the testing time of both stimulation methods
will also be an indication of the stimulation method of choice.
5.2.2.3
To compare different modulation frequencies’ effectiveness in
estimating pure-tone thresholds
Modulation frequencies of 40 and 80 to 110 Hz are usually used in ASSR testing.
Threshold estimates obtained when using the different modulation frequencies
are compared to pure-tone thresholds. A decision regarding the most accurate
and time effective modulation frequency for carrier frequencies in ASSR testing
of adults with impaired hearing is then possible.
5.2.2.4
To determine the effect of sedation on the ASSR test’s ability to
estimate pure-tone thresholds
In order to evaluate the effect of sedation on the threshold estimates obtained
with ASSR tests, the threshold estimates’ accuracy with and without sedation
needs to be compared. The testing time with and without sedation will aid in the
above decision.
The reason why a study of the effect of sedation is needed is that the 40 Hz
response will be used in the experimental phase.
There are contradictory
research results with regard to the effect of sedation on the 40 Hz response. The
dramatic effect of sleep and state of consciousness on the 40 Hz response has
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been cited by Galambos et al. (1981). Dobie and Wilson (1998) in comparison
could find no real negative influence of sedation on the 40 Hz response of adults.
See Section 4.3.14.
5.2.2.5
To determine if pure-tone threshold estimates can be obtained in
unco-operative mine workers
In a clinical situation pseudohypacusic patients do not co-operate and accurate
hearing thresholds cannot be obtained. ASSR methods were used in a group of
unco-operative mine workers to investigate if thresholds could be obtained.
ASSR thresholds were compared to pseudohypacusic pure-tone thresholds and
the information gained from the ASSR thresholds were analyzed in order to
obtain clinical information.
5.3
RESEARCH PLAN
The discussion below focuses on the research design as the strategic framework
for action that serves as a bridge between the research question and the
execution of the research (Dane, 1990).
An empirical study was conducted. Mouton (2001) describes an empirical study
as the use of primary and numerical data with high control. Sources of data used
in this study were physical measurements: in this case auditory thresholds. An
experimental research method was also selected for this study (Leedy, 1997). In
experimental research, the researcher attempts to maintain control over all the
factors that may affect the result of an experiment (Key, 1997). The strength of
an experimental design lies in its ability to infer causality and test causal
relationships.
One limitation of an experimental design that needs to be
addressed is the fact that small sample sizes make generalisability risky (Mouton,
2001).
The research was also quantitative.
Berg (1998) explains that quantitative
research has to provide rigorous, reliable and verifiably large aggregates of data,
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and that quantitative research can be regarded as a formal and systematic
process. In this study, the experimental research process was pursued by using
a quasi-experimental design, as described by De Vos (2002).
The main
disadvantage of this method is the lack of a control group – the difficulty with
including a control group in this study or doing different ASSR procedures on the
same group was the lengthiness of these procedures. To prevent bias from
creeping in, it was therefore important to ensure a random allocation of subjects
to different sub-groups.
In order to answer the research question and to meet the research aims set out
in Section 5.2 (the clinical value of ASSR testing in a population of
pseudohypacusic mine workers with noise-induced hearing loss), a multi-group
design was followed (De Vos, 2002): six experimental groups were organised
and utilised in two research phases. Groups 1,1 to 1,5 were mine workers (cooperative) with proven noise-induced hearing loss and Group 2 were non-cooperative mine workers with suspected pseudohypacusis. The research plan is
logically set out in Table 5.1.
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TABLE 5.1: RESEARCH PLAN: PHASES, EXPERIMENTAL GROUPS AND
EXPERIMENTAL PARAMETERS
Research
phase
Experimental
groups
Instrument
Modulation
frequency
(Hz)
Monotic/dichotic
Stimulation
Sedation
Phase 1
1,1
Audera
80-110
Monotic
No
1,2
Audera
40
Monotic
No
80-110
Dichotic
No
40
Monotic
Yes
80-110
Dichotic
Yes
40
Monotic
No
1,3
1,4
1,5
Phase 2
2
MASTER
Biologic
Audera
MASTER
Biologic
Audera
Number of
subjects
12
(subject 1-12)
16
(subject 13-28)
20
(subject 29-48)
13
(subject 49-61)
20
(subject 62-81)
NIHL
NIHL
NIHL
(subject 82-119)
pacusis
The selection and grouping of the 81 subjects for Phase 1 (co-operative subjects
with noise-induced hearing loss) in the different groups listed (Table 5.1) enabled
the following comparisons:
All 81 subjects’ pure-tone and ASSR thresholds (Groups 1.1, 1.2, 1.3, 1.4
and 1.5) could be compared to decide whether ASSR thresholds can
estimate pure-tone thresholds accurately.
ASSR-thresholds’ accuracy, obtained with an 80 to 110 Hz stimulation rate
(1.1, 1.3 and 1.5) (Rickards & Clark, 1984) could be compared to the
90
NIHL
Pseudohy-
compare the different methods’ ability to estimate pure-tone thresholds.
•
NIHL
29
Different ASSR test procedures were used on the different groups in order to
•
Type of
hearing
loss
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ASSR thresholds’ accuracy obtained using a 40 Hz stimulation rate
(Groups 1.2 and 1.4) (Rance et al., 1995).
•
A comparison of the prediction value of ASSR thresholds was possible
when multiple frequency and single frequency ASSR procedures were
followed (Groups 1.1, 1.2, 1.4 vs 1.3 and 1.5) (Perez-Abalo et al., 2001
and Rance et al., 1995).
•
Lastly, a comparison between the ASSR and pure-tone thresholds was
possible between sedated and non-sedated subjects (Groups 1.1, 1.2 and
1.3 versus Groups 1.4 and 1.5).
The testing of subjects in Phase 1 was used to determine the most effective test
equipment, stimulation rate and stimulation method (multiple- or single
frequency), as well as the effect of sedation, thereby establishing a protocol of
choice for a population with noise-induced hearing loss.
•
The last experimental group was a group of mine workers (29) with known
noise-exposure but who were not co-operating and for whom thus no
pure-tone thresholds were available (Phase 2).
The goal was to
determine whether ASSR thresholds can be obtained for unco-operative
subjects. The questions to be answered were whether thresholds can be
obtained at all the needed frequencies for unco-operative patients and in
what space of time this can be done.
A total of 110 subjects participated in the study.
5.4
ETHICAL CONSIDERATIONS
Ethical concerns need to be taken into account in order for research to be
conducted in a manner which is fair to all the participants and employers.
Furthermore research ethics, according to Neuman (1997), define what is
legitimate and moral during research.
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following ethical aspects were taken into account: willing participation, informed
consent, permission for the use of sedation, employers’ permission and ethical
clearance. These aspects are discussed in more detail below.
5.4.1 WILLING PARTICIPATION
Subjects were assured that if they chose not to participate in the study, they
would not be disadvantaged in any way. Workers who did not wish to participate
were routed back for a continuation of standard medical surveillance procedures.
Subjects were not coerced or manipulated into volunteering, in line with the
principles set out by Berg (1998). Subjects were also able to withdraw from the
research whenever they chose to do so, in accordance with the ideas of Strydom
(1998).
5.4.2 INFORMED CONSENT
Informed consent was obtained in writing from each subject (see Appendix A for
the form used). Obtaining such consent implies that the worker was informed
about the goal of the investigation and the procedures followed.
The
presentation of accurate and complete information was emphasised, so that
subjects fully comprehended the investigation, in accordance with suggestions by
De Vos (2002). The subjects' comprehension of the procedure was aided by
providing a trained African languages translator. Voluntary participation was the
goal and subjects were assured of anonymous participation.
5.4.3 CONSENT TO SEDATION
Apart from the informed consent obtained as stated in Section 5.4.2 (above),
subjects who would be sedated were supplied with a patient information sheet on
the effect of the medication (see Appendix B). Additional consent (see Appendix
B) for this participation was also obtained in writing with the help of a translator.
The subjects who gave consent were then referred to an Ear-, Nose- and Throat
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specialist or occupational medical practitioner (OMP) who perused the subjects’
medical history and prescribed the sedation. The sedation of the subjects took
place at Occupational Health Centres (OHCs) where an OMP was on duty. After
their participation, the subjects were transported back to their hostels. They only
returned to work the following day.
5.4.4 EMPLOYERS’ PERMISSION
Permission to involve their employees was obtained from the mining companies
whose workers participated (Gold Fields - see Appendix C, and Harmony - see
Appendix D).
5.4.5 ETHICAL CLEARANCE
Ethical clearance was obtained from the University of Pretoria’s Ethics
Committee (Faculty of Humanities) and the Research Committee of the
Department of Communication Pathology (see Appendix E).
5.5
SUBJECTS
5.5.1 POPULATION
The population of this study, in other words, all the individuals who possessed
the specific characteristics that represent the measurements of interest in the
study as described by De Vos (2002) were South African mine workers with
noise-induced hearing loss (Phase 1) and pseudohypacusic South African mine
workers (Phase 2). A population of mine workers was selected from workers
undergoing their annual Certificate of Fitness assessments at their mines’
Occupational Health Centres in the Randfontein and Carletonville areas. All the
subjects worked underground and, hence, had been exposed to hazardous noise
(Franz & Phillips, 2001).
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5.5.2 SAMPLING
Results from a study can only be generalised if the sample tested is seen to be
representative of the population. A sample is, in other words, a small portion of
the total set of persons that comprise the subject of the study (De Vos, 2002).
The reason for sampling is feasibility, since it is impossible to include all the
possible members of a population of this nature.
Non-probability quota sampling (Neuman, 1997; De Vos, 2002) was used in this
study, in other words, in the selection of an underground mine worker in the
predetermined group.
Any subjects who happened to undergo medical
surveillance at the OHC and who had noise-induced hearing loss and worked
underground were included in the sample. All potential subjects complying with
the selection criteria were selected, within the time constraints imposed by the
length of a working day and the lengthy test procedures. A three-month period
was allowed for the experimental research, from September to November 2002.
The objective was to conduct experimental testing on the same day as medical
surveillance procedures, to prevent interference with normal production at the
mines. It was not always possible to achieve this, particularly with subjects who
had been sedated, since the occupational medical practitioner had to peruse the
worker’s medical history and prescribe the sedation.
A total of 81 male subjects (162 ears) between the ages of 23 and 60 were
selected for the first phase of the research and 29 (58 ears) were selected for the
pseudohypacusic group. The sample size was verified by a statistician of the
Medical Research Council (Pretoria).
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5.5.3 CHARACTERISTICS OF SUBJECTS AND THE PROCEDURES
FOLLOWED IN THE SELECTION OF THESE SUBJECTS
5.5.3.1
Occupation
Subjects had to be mine workers (in a gold mine) allocated to underground duties
and therefore exposed to hazardous noise. Noise exposure was important since
the study aimed to evaluate the effectiveness of ASSR techniques in subjects
with noise-induced hearing loss. Occupational Health Centre staff verified that
these workers did indeed work underground.
5.5.3.2
Abnormal hearing with and without a functional overlay
As mentioned previously, the population under scrutiny was one of mine workers
with proven noise-induced hearing loss.
The subjects had to have sensory
neural hearing loss (no persons with mixed and conductive hearing losses were
selected) and proven noise exposure of more than five years (Begley, 2003). In
order to confirm exposure to hazardous noise and exclude other possible causes
of sensory-neural hearing loss (for example, ototoxic drugs, ear infection and
head injury), a case history (see Appendix G) was compiled and recorded by a
trained African languages translator.
Based on the aims of the study, it is clear that the subjects in the study had to
have hearing loss. Subjects (without a functional overlay) were required to have
a pure-tone average exceeding 25 dB (500, 1 000, 2 000, 3 000 Hz) thereby
qualifying them for consideration for noise-induced hearing loss compensation.
This criterion was derived from the legislation implemented in the South African
mining industry at the time when the experimental research was done, namely
the Workmen’s Compensation Commissioner’s (WCC) internal instruction 168,
1995.
Hearing loss is also commonly defined in the literature as hearing
thresholds worse than 25 dB (Northern & Downs, 1991). The initial selection was
done on the basis of the results of the screening hearing test done during
medical surveillance. Pure-tone air- and bone conduction audiograms performed
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by audiologists in a controlled environment on the same subjects served as a
confirmation of the screening thresholds.
A group of pseudohypacusic workers (functional overlay) was also evaluated. By
definition their true hearing status was unknown since they exaggerate their true
hearing thresholds (Martin, 2000). Two pure-tone audiograms were performed at
500, 1 000, 2 000, 3 000 and 4 000 Hz, to enable threshold comparisons for the
purpose of identifying pseudohypacusis.
A difference of 15 dB or more
(Rintelmann et al., 1991) at all the frequencies and in both ears was regarded as
an indication of pseudohypacusis. The two audiograms recorded used different
threshold determining techniques, namely the ascending (first procedure) and
descending methods (Rintelmann et al., 1991).
The two audiograms were
performed by the same audiologist in the same environment during two
consecutive test sessions. This group of workers also had to have normal middle
ear function to exclude conductive hearing loss and proven noise exposure to fit
into the category of mine workers with sensory neural hearing loss.
5.5.3.3
Normal middle ear function
Normal middle ear function was a prerequisite for subject selection. The findings
of Hood (1995) and Hall and Mueller (1997) have indicated that middle ear
pathology affects ASSR amplitude. To exclude cases of middle ear pathology
and conductive impairment, subjects had to have normal middle ear function.
Furthermore, normal middle ear function was also included as a criterion since a
population of people with noise-induced hearing loss was the focus of the study.
Middle ear function was assessed by means of tympanometry. The following
selection criteria (indicating normal middle ear functioning) were applied to the
tympanometry results:
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•
ear canal volume: 0,5-1,5 cc;
•
compliance: 0,3-1,6 cc (Stach, 1998);
•
Type A tympanograms – Northern and Downs (1991) define Type A
tympanograms as adequate compliance and normal middle ear pressure
at the point of maximal compliance. Normal middle ear pressure was
taken as –50 mm to +50 mm H2O.
Normal middle ear function was further verified by otoscopy. Otoscopic
examinations were performed on both ears for each subject, to identify any
middle ear/tympanic membrane pathology or obstruction of the external auditory
meatus that could affect the conduction of sound, as proposed by Stach (1998).
Finally, an air-bone gap of 10 dB indicating possible middle ear abnormalities
excluded some subjects (Roeser et al., 2000b).
5.5.3.4
Age and gender
All the subjects were male. This was not a prerequisite of the study but arose
from the fact that the vast majority of mine workers in South Africa are
traditionally male. Stapells et al. (1984) have proven that there is an absence of
gender bias with ASSR testing and thus the results will be applicable to both
sexes. Because Hood (1998) has shown that electrophysiological tests show no
age effects between 10 and 60 years, it was required that the age of all subjects
be within this range. This requirement was easily met, since the working age of
mine workers is between 18 and 60 years. The age information was obtained
from patient files and the case history information (see Appendix G).
5.5.4 DESCRIPTION OF SUBJECTS
The subjects who eventually participated in this study and their characteristics
are described in the following tables and figures.
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5.5.4.1
Hearing thresholds - co-operative group
Table 5.2 supplies hearing thresholds in decibels at all the frequencies required
for the subjects with noise-induced hearing loss without a functional overlay.
TABLE 5.2: HEARING THRESHOLDS
OPERATIVE GROUP
(DECIBEL)(HL)
PURE-TONE THRESHOLDS FOR LEFT EAR
FOR
THE
CO-
PURE-TONE THRESHOLDS FOR RIGHT EAR
500Hz
1000Hz
2000Hz
3000Hz
4000Hz
500Hz
1000Hz
2000Hz
3000Hz
4000Hz
Subject
60
80
60
55
65
40
70
60
55
70
1
20
45
55
50
50
20
45
40
40
45
2
35
45
50
35
30
30
40
40
35
30
3
10
50
55
55
60
10
45
50
60
50
4
15
25
45
55
65
20
20
25
50
60
5
5
5
40
65
80
5
10
35
70
70
6
15
40
50
75
75
25
30
30
50
55
7
15
25
30
35
35
15
30
35
30
30
8
30
45
45
50
50
25
40
40
45
50
9
20
20
20
30
35
15
25
20
30
30
10
30
45
35
40
45
30
40
45
35
45
11
35
40
35
35
40
30
30
20
40
45
12
10
15
50
55
55
10
15
50
65
50
13
40
45
40
70
75
30
45
35
45
55
14
20
45
70
80
70
10
35
60
60
55
15
25
35
45
50
50
20
40
45
50
55
16
15
35
40
45
40
5
25
40
45
45
17
25
40
35
30
40
25
40
25
35
35
18
10
30
45
35
35
5
35
45
35
35
19
15
35
45
45
40
10
20
35
40
40
20
15
45
35
25
40
20
50
55
55
50
21
40
50
55
65
70
25
50
50
60
65
22
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PURE-TONE THRESHOLDS FOR LEFT EAR
PURE-TONE THRESHOLDS FOR RIGHT EAR
500Hz
1000Hz
2000Hz
3000Hz
4000Hz
500Hz
1000Hz
2000Hz
3000Hz
4000Hz
Subject
10
30
45
50
50
30
25
40
45
65
23
50
50
40
15
25
50
50
40
20
15
24
20
25
45
35
40
5
15
25
25
30
25
30
45
50
55
55
30
50
50
60
60
26
20
35
35
40
45
20
35
45
40
45
27
35
40
35
35
40
30
30
20
40
45
28
45
55
55
50
50
35
60
55
55
60
29
10
30
40
50
40
15
15
35
45
30
30
40
40
35
25
25
45
55
50
50
50
31
20
30
35
35
25
15
30
40
20
25
32
10
30
50
55
65
10
20
50
50
65
33
5
15
50
45
45
5
20
35
40
55
34
30
40
35
30
25
30
25
35
30
40
35
10
45
60
45
50
5
45
60
55
60
36
15
25
55
60
80
5
5
35
50
80
37
30
40
60
65
65
20
25
30
45
55
38
20
25
30
55
65
15
25
35
60
90
39
30
40
50
50
55
25
30
50
50
50
40
30
45
40
50
35
30
35
30
30
45
41
15
25
45
45
50
20
30
45
55
55
42
15
20
50
45
45
10
25
30
60
65
43
45
65
65
70
75
45
55
55
65
75
44
0
30
50
60
55
0
20
30
65
50
45
10
45
45
50
50
10
35
45
55
40
46
25
20
35
55
60
20
50
50
45
55
47
30
45
40
50
40
30
45
40
35
35
48
5
10
30
40
45
10
15
40
40
45
49
30
55
60
60
60
35
50
50
60
65
50
15
20
35
40
40
15
30
35
45
50
51
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PURE-TONE THRESHOLDS FOR LEFT EAR
PURE-TONE THRESHOLD FOR RIGHT EAR
500Hz
1000Hz
2000Hz
3000Hz
4000Hz
500Hz
1000Hz
2000Hz
3000Hz
4000Hz
Subject
10
20
35
55
50
15
20
35
45
55
52
20
50
50
50
55
30
50
45
45
55
53
15
65
75
70
65
30
65
75
75
80
54
5
20
30
35
40
20
30
50
55
50
55
15
40
45
55
50
15
45
55
50
50
56
25
45
55
60
65
25
35
55
65
65
57
5
15
15
85
85
15
20
10
75
85
58
30
35
30
50
60
30
45
45
55
50
59
10
30
45
45
45
10
30
40
50
40
60
20
40
45
50
40
5
15
35
40
30
61
20
40
60
60
55
10
30
55
55
40
62
35
40
55
55
75
25
40
35
40
65
63
20
40
45
55
50
10
35
40
45
55
64
15
50
50
45
50
20
45
50
45
55
65
15
25
75
75
75
10
25
55
45
45
66
20
55
50
60
60
25
50
55
60
60
67
15
40
45
40
40
25
40
45
45
50
68
25
45
45
50
35
30
45
45
50
45
69
0
25
40
50
60
5
20
35
50
65
70
15
20
40
30
25
15
25
45
40
40
71
30
40
40
40
50
25
35
55
65
60
72
25
40
35
35
40
30
40
40
35
40
73
30
40
50
50
60
25
30
50
55
65
74
20
45
55
50
50
20
45
45
45
55
75
15
40
50
50
50
20
40
45
40
50
76
30
30
35
55
50
20
50
60
55
65
77
20
40
45
30
30
20
45
35
35
20
78
30
40
45
40
45
30
40
45
40
45
79
10
35
55
45
35
10
25
40
40
45
80
30
60
50
50
45
35
60
55
50
50
81
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To summarise the information in the above table: it can be seen that the subjects
had hearing thresholds representing different degrees of hearing loss, ranging
from mild (26-40dB), moderate (41-65dB) to severe (66-95dB). The numbers of
hearing thresholds per frequency in the different severity ranges were the
following:
•
Mild hearing thresholds – 251.
•
Thresholds indicating moderate hearing loss – 346.
•
Thresholds indicating severe hearing loss – 32.
•
Due to the sloping nature of sensory-neural hearing loss, 181 normal
thresholds (0-25dB) were also obtained, mainly in the 500 Hz area.
5.5.4.2
Age of co-operative group
The subjects were 81 male mine workers with noise-induced hearing loss
between the ages of 23 and 60 years. Figures 5.1 to 5.5 represent the age
distributions of mine workers with noise-induced hearing loss across five-year
age intervals. The subjects were randomly assigned to different groups to study
the influence of different ASSR-equipment and techniques on the comparison of
the ASSR and pure-tone thresholds. The structure for this was already indicated
in Table 5.1.
In addition, in Figures 5.7 to 5.12, the participants’ years of noise exposure in the
mining industry and their age is indicated for the different research groups and
the different experimental phases.
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Age In Yea35-40
Number Of
41-45
46-50
51-55
Subjects
1 Number Of
7
0
8
7
6
5
4
3
2
1
0
56-60
4
0
Number Of
Subjects
35-40
41-45
46-50
51-55
56-60
Age In Ye ars
FIGURE 5.1:
AGE DISTRIBUTION OF THE SF/80 HZ/NON-SEDATED
GROUP (n=12): MEAN AGE 45,8 YEARS
Age In Yea35-40
Number Of
41-45
46-50
51-55
Subjects
1 Number Of
6
6
7
6
5
4
3
2
1
0
56-60
2
1
Number Of
Subjects
35-40
41-45
46-50
51-55
56-60
Age in Ye ars
FIGURE 5.2:
AGE DISTRIBUTION OF THE SF/40 HZ/NON-SEDATED
GROUP (n=16): MEAN AGE 47,5 YEARS
.
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Age In Yea35-40
Number Of
41-45
46-50
51-55
Subjects
4 Number Of
5
3
56-60
5
3
6
5
4
Number Of
Subjects
3
2
1
0
35-40
41-45
46-50
51-55
56-60
Age in Ye ars
FIGURE 5.3:
AGE DISTRIBUTION OF THE MF/80 HZ/NON-SEDATED
GROUP (n=20): MEAN AGE 46,38 YEARS
Age In Yea35-40
Number Of
41-45
46-50
51-55
Subjects
1 Number Of
4
6
7
6
5
4
3
2
1
0
56-60
2
0
Number Of
Subjects
35-40
41-45
46-50
51-55
56-60
Age in ye ars
FIGURE 5.4:
AGE DISTRIBUTION OF THE SF/40 HZ/SEDATED GROUP
(n=13): MEAN AGE 47,3 YEARS
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Age In Yea35-40
Number Of
41-45
46-50
51-55
1 Number Of
2 Subjects
4
56-60
5
8
10
8
6
Number Of
Subjects
4
2
0
35-40 41-45 46-50 51-55 56-60
Age In Ye ars
FIGURE 5.5:
5.5.4.3
AGE DISTRIBUTION OF THE MF/80 HZ/SEDATED GROUP
(n=20): MEAN AGE 52 YEARS
Age distribution of pseudohypacusic group
The group of mine workers with pseudohypacusis consisted of 29 subjects. In
Figure 5.6, the age distribution of this group is shown.
Age In Yea20-25
Number Of
26-30
1
31-35
1
36-40
41-45
46-50
Number
Of Subjects
3
7
10
51-55
4
56-60
2
1
12
10
8
6
4
2
0
20-25
26-30
31-35
36-40
41-45
Age in Years
46-50
51-55 56-60
Number Of Subjects
FIGURE 5.6: AGE DISTRIBUTION OF PSEUDOHYPACUSIC GROUP (n=29):
MEAN AGE 41,86
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5.5.4.4
Years of experience/exposure
Figures 5.7 to 5.11 represent the experience and hence period of exposure for
various sub-groups within the experimental groups.
Experience
5-10
Number of Subjects
11-15
16-20
Number
of Subjects
1
1
21-25
0
26-30
7
8
7
6
5
4
3
2
1
0
31-35
6
1
Number of Subjects
5-10
11-15
16-20
21-25
26-30
31-35
Years Experience/exposure
FIGURE 5.7: EXPERIENCE/EXPOSURE: SF/80 HZ/NON-SEDATED GROUP
Experience
5-10
Number of Subjects
11-15
16-20
Number
of Subjects
1
1
21-25
0
8
7
6
5
4
3
2
1
0
26-30
7
31-35
6
1
Number of Subjects
5-10
11-15
16-20
21-25
26-30
31-35
Years Experience/exposure
FIGURE 5.8: EXPERIENCE/EXPOSURE: SF/40 HZ/NON-SEDATED GROUP
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Experience
5-10
Number of Subjects
11-15
1
16-20
21-25
Number
of Subjects
1
1
26-30
6
31-35
36-40
5
40-45
4
7
6
5
4
3
2
1
0
0
2
Number of Subjects
5-10
11-15
16-20
21-25
26-30
31-35
36-40
40-45
Years experience/exposure
FIGURE 5.9: EXPERIENCE/EXPOSURE: MF/80 HZ/NON-SEDATED GROUP
Experience
5-10
Number of Subjects
1
11-15
16-20
21-25
Number0 of Subjects
1
26-30
4
31-35
4
36-40
1
5
4
3
Number of Subjects
2
1
0
5-10
11-15
16-20
21-25
26-30
31-35
36-40
Years experience/exposure
FIGURE 5.10:
EXPERIENCE/EXPOSURE: SF/40 HZ/SEDATED GROUP
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Experience
5-10
Number of Subjects
2
11-15
16-20
21-25
Number
1 of Subjects
0
26-30
4
31-35
36-40
4
7
6
5
4
3
2
1
0
3
6
Number of Subjects
5-10
11-15
16-20
21-25
26-30
31-35
36-40
Years experience/exposure
FIGURE 5.11:
5.5.4.5
EXPERIENCE/EXPOSURE: MF/80 HZ/SEDATED GROUP
Experience of pseudohypacusic group
The group of mine workers with pseudohypacusis consisted of 29 subjects,
distributed by experience, as can be seen in Figure 5.12 below.
Experience
5-10
Number of Subjects
11-15
16-20
Number
of Subjects
4
5
21-25
2
14
12
10
8
6
4
2
0
26-30
12
31-35
1
4
Number of Subjects
5-10
11-15
16-20
21-25
26-30
31-35
Years experience\exposure
FIGURE 5.12:
EXPERIENCE/EXPOSURE: PSEUDOHYPACUSIC GROUP
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5.6
MATERIAL AND APPARATUS
The apparatus used in this study to obtain the research questions was the
following:
5.6.1 THE MATERIAL AND APPARATUS USED FOR SUBJECT SELECTION
•
Otoscopic examinations were performed using a Heine Mini 2000
otoscope.
•
Tympanometry was performed with a GSI 33 middle ear analyser and a
Beltone 2000 immittance tester, both of which were calibrated
(certificates in Appendices H and I) according to IEC 1027 regulations.
•
Pure-tone audiometry (air- and bone-conduction) was performed by
audiologists using a Madsen OB 822 and GSI 60 diagnostic
audiometers. This equipment was calibrated in accordance with SABS
0154-1 & 2 for pure-tone audiometers (the calibration certificates are
appended as Appendices J and K).
•
Diagnostic audiometry was performed in acoustically-treated test
enclosures, calibrated in accordance with SABS 0182-1998 (the
background noise certificates are appended as Appendices L and M).
•
The patient files of the mine workers were perused at the Occupational
Health Centres. The files were used to verify the participants’ number of
years of exposure and the use of ototoxic drugs and to obtain previous
screening audiograms.
•
A Case History questionnaire (see Appendix G) was used to exclude all
other possible causes of sensory neural hearing loss and record all
experimental procedures.
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5.6.2 THE MATERIAL AND APPARATUS USED FOR DATA COLLECTION
5.6.2.1
Pure-tone testing
Pure-tone thresholds were obtained using the same calibrated audiometers and
acoustic enclosures as detailed in the preceding section, that is a Madsen OB
822 and a GSI 60. The calibration certificates are appended as Appendices J
and K.
5.6.2.2
MF-ASSR testing
MF-ASSR responses were recorded using a multiple auditory steady state
response system (MASTER), a Windows-based test and a data acquisition
system developed by the Bio-logic Systems Corporation (2002). The MASTER
system includes both software and hardware and is run using a personal
computer. Bio-Logic’s Navigator Pro TM unit performed the necessary analogueto-digital and digital-to-analogue conversions, including the production of the
stimulus
output
to
earphone
inserts
and
electroencephalogram input from the electrodes.
the
gathering
of
the
The Navigator Pro was
connected to the computer’s serial port in order for the RS-232 communication
protocol to be used. The computer hardware specifications were the following:
COMPUTER SYSTEM:
•
an IBM-compatible 166 MHz Pentium computer
•
64 MB of RAM
•
a 150 MB hard disk
•
a Windows-compatible mouse
•
a Windows 98 Operating system
•
a 1.44 Mb 3,5” floppy disk drive
The installation and operation of the MASTER system requires a minimum of
20 MB free space on the hard drive (Bio-logic Systems Corporation, 2002).
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PRINTING DEVICE:
•
a Windows 98-compatible printer (Hewlett Packard DeskJet 840C).
OTHER HARDWARE:
•
a Navigator Pro TM EP unit and accessories.
Disposable ear probe tips were supplied by Bio-logic Systems Corporation. The
electrodes were latex-free and made of hypoallergenic material.
ASSR measurements were obtained in a calibrated environment, for which
calibration certificates are supplied in Appendices L and M.
5.6.2.3
Single frequency ASSR testing
SF-ASSR data were collected using a GSI Audera system, manufactured by
Grason-Stadler. The Audera system comprises:
•
a notebook computer system with a Pentium II 200 MHz processor,
256 MB of RAM, a 5 GB hard disk, a 1,4 MB 3,5” diskette drive and
pointing device (mouse/touch pad), running Windows XP;
•
a USB connector;
•
Audera software;
•
an Audera unit;
•
an Audera amplifier;
•
electrodes; and
•
GSI tip-50 inserts earphones with disposable ear tips.
Two Audera systems were used, a Beta prototype and a commercial production
unit, because Grason-Stadler’s South African agent (HASS) lent the equipment
to the researcher and it was not possible to keep it on loan for the entire three-
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month experimentation period. The Beta unit was a single channel instrument,
requiring the researcher to switch channels after testing each ear.
5.6.2.4
Data preparation
Data preparation was performed using Excel for Windows 1998 (Levin, 2003).
5.7
DATA COLLECTION PROCEDURES
Three sets of data were collected from each subject in the co-operative noiseinduced hearing loss group (Phase 1), namely a pure-tone air-conduction test
(500, 1 000, 2 000, 3 000, 4 000 Hz), ASSR threshold estimates at the same
frequencies and the test duration for each ASSR procedure. Data from each
subject were collected on the same day, whenever possible starting with puretone testing (which also served as a subject selection procedure). Audiologists
performed data collection procedures either at the Phumlani Occupational Health
Centre in Randfontein, or at the Driefontein Occupational Health Centre in
Carletonville. Recording was done in calibrated sound environments.
For the pseudohypacusic group of 29 subjects (Phase 2), four sets of data were
obtained. These included two pure-tone air-conduction threshold tests at 500,
1 000, 2 000, 3 000, 4 000 Hz (ascending technique) followed by thresholds
obtained at the same frequency, but using a descending method, SF-ASSR
threshold estimates at the same frequencies, using a 40 Hz modulation rate and,
lastly, the time required for testing.
5.7.1 PURE-TONE AUDIOMETRY
Pure-tone audiometry was performed during subject selection and data collection
procedures, at 500, 1 000, 2 000, 3 000, 4 000 Hz, in line with Instructions 168
and 171 (Workmens’ Compensation Commissioner, 1995 and 2000).
These
frequencies were selected since they are used for evaluations of fitness and
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compensability. These thresholds were also used as a basis for comparison with
ASSR thresholds. The pure-tone average from the audiogram was required to
be in excess of 25 dB, to confirm potentially compensable abnormal hearing. In
the pseudohypacusic group, the two pure-tone tests confirmed pseudohypacusis
when they demonstrated a discrepancy of 15 dB or more between the two tests.
Pure-tone audiometry was performed using descending steps of 10 dB and
ascending steps of 5 dB, with a 50 per cent positive response at the same level
taken as the threshold, in accordance with the criteria of Stach (1998).
Thresholds were determined first for the left and then for the right ear and were
recorded on audiograms attached to the Case History questionnaire form (see
Appendix G).
5.7.2 MF-ASSR DATA COLLECTION
Two groups of subjects were tested using a dichotic MF-ASSR technique, one
without sedation and the other with sedation, to obtain threshold estimates at
500, 1 000, 2 000, 3 000, 4 000 Hz. Multiple amplitude moduated tones were
selected with the carrier frequencies modulated between 80 and 110 Hz. It is
important to note that a 40 Hz modulation is not available in multiple frequency
test systems. Carrier frequencies were spaced at least one octave apart in line
with suggestions by Perez-Abalo et al. (2001), and four frequencies were
evaluated (dichotic) for each ear.
Previous studies have indicated that a
modulation rate of 80 to 110 Hz is appropriate for adults and that there are no
significant differences between results using single- and multiple-frequency
techniques (Lins & Picton, 1995). Time efficiency could also be evaluated in this
way, since the design of the experiment left options for comparing the time
required for using single- and multiple-frequency techniques.
The carrier
frequency, starting intensity and the size of the decrements (5 or 10 dB steps)
were selected by the researcher, after which the computer directed the test
procedure.
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In the sedated group, 10 mg of Valium was administered after informed consent
and medical clearance had been obtained. A medical doctor was present on the
same premises to supply medical back-up or assistance if it were to be needed,
and testing commenced one hour after the medication had been administered, to
allow time for the medication to be absorbed. Subjects were transported back to
their hostels and only reported for work the following day.
An electrode skin-preparation swab coated with Nuprep (an abrasive paste) was
used to clean the areas where electrodes were to be affixed. Once the electrode
sites had been cleaned, the skin was dried with a gauze pad to remove any
residue, and disposable self-adhesive snap electrodes supplied by Biologic were
affixed to the skin. Electrode impedance was immediately confirmed to be below
five kilo-ohms, with no differences greater than two kilo-ohm between electrodes
(Bio-logic Systems Corporation, 2002).
The electrodes were placed as followed:
•
on the mastoid process – test ear
•
on the mastoid process – (reference) contra-lateral to the test ear
•
high on the forehead as recommended by Bio-logic Systems Corporation
(2002)
Earphone probes were then inserted, using an appropriately sized disposable ear
tip in accordance with the size of the subject’s ear canal.
The ear tip was
securely coupled to the probe and fully inserted into the ear canal, to ensure
proper stimulus delivery. In addition, correct cable connections were confirmed
to prevent any juxtaposition of results for the right and left ears.
The test parameters used during this multiple frequency ASSR procedure were
the default values as determined by the software supplied by Bio-logic System
Corporation (2002).
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The subject was asked to lie still, to relax or sleep and to keep his eyes closed.
A pillow was provided for support to prevent any myogenic noise from impacting
on the data collection when modulation frequencies between 80 and 100 Hz were
used (Bio-logic Systems Corporation, 2002). Testing was performed in a soundproof booth and the air conditioning in adjacent rooms was switched off, as were
all telephones and cell phones. In addition, the door to the adjacent test room
was closed, and visual distractions were minimised by switching off lights in the
booth and the adjacent room. Before testing commenced, electrode impedance
was re-confirmed. The audiologist was positioned in an adjacent room and had
visual contact with the subject through a window in the test booth.
To ensure safety, power to the system was never switched on or off while a
subject was connected to the system. Threshold determination occurred within a
hearing level range of -20 to 120 dB, and the software warned the researcher
when very high intensities were selected.
The software recorded the test data, providing for an exact measurement of the
time taken for each subject.
Electrophysiological thresholds were eventually
determined from the responses obtained, based on a requirement for a less than
5 per cent chance that the subject’s response might be attributable to chance (fratio statistics at a 0,05 level of confidence). The electrophysiological thresholds
were eventually converted to pure-tone thresholds by subtracting 10 dB to predict
a conventional audiogram, Guidelines on estimating a pure-tone thresholds were
requested by the researcher in a personal communication with Bio-logic (Biologic, Systems Corporation, 2002).
•
Carrier frequencies
The default protocols were selected in order to obtain thresholds (four per
ear) at 500, 1 000, 2 000, 3 000, 4 000 Hz. Default protocols prevented
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University of Pretoria etd - De Koker, E (2004)
testing at all the frequencies required in a single stimulation sequence,
thereby requiring more than one set of stimulus presentations.
•
Modulation frequencies
The modulation frequencies used were as indicated in Table 5.3
Table 5.3:
MODULATION FREQUENCIES USED BY MASTER
CF
500 Hz
1 000Hz
2 000Hz
3 000Hz
4 000Hz
Modulation
86.914Hz
89.844Hz
91.797Hz
83.008Hz
94.727Hz
frequency
The amplitude modulation percentage of the carrier frequency was set at 100 per
cent and the frequency modulation percentage was set at 10 per cent (per side).
•
Number of sweeps
The number of sweeps the MASTER runs per subject and per test threshold
was set to 32 sweeps per test in accordance with the recommended protocol
(Bio-logic Systems Corporation, 2002).
•
Epochs per sweep
The number of epochs collected per sweep before the fast fourier transform
(FFT) was performed was set at 16. Data transmitted to the FFT represented
an averaged response from the subject, obtained from a running sum of all
the sweeps that were recorded, divided by the number of sweeps collected.
5.7.3 SINGLE FREQUENCY DATA COLLECTION
Single frequency data collection procedures using the GSI Audera (GrasonStadler) were applied to a group of sedated mine workers with noise-induced
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University of Pretoria etd - De Koker, E (2004)
hearing loss, as well as to two groups of subjects with noise-induced hearing loss
who were not sedated. This allowed comparisons to be made to determine the
most advantageous “state of consciousness” during ASSR testing. The two nonsedated groups were compared by using different stimulation rates (40Hz and
80Hz).
Both of these rates had previously been found to provide reliable
estimates of pure-tone thresholds during previous research.
Thresholds were required for 500, 1 000, 2 000, 3 000, 4 000 Hz, to allow
comparisons of the single frequency ASSR, multiple frequency-ASSR and puretone thresholds.
ASSR thresholds were obtained using both ascending and
descending threshold-seeking procedures, starting at a hearing level of 40 dB, as
with behavioural testing, and increments and decrements of 10 dB were used to
limit the testing time. Single frequency ASSR tests were performed immediately
after pure-tone testing, to ensure that all the procedures were completed on the
same day. For sedated subjects, one hour was allowed for the absorption of the
10 mg of Valium, with the same provisions for consent and medical support met
as for multiple frequency testing (again same-day testing was not always
possible).
Electrodes were placed according to Grason-Stadler’s specifications, as follows:
•
the Audera Beta version: on the left and right ear lobes and high on
forehead
•
the Audera Commercial version: on the left and right ear lobes, high on
forehead and low on forehead (the extra electrode allows clinicians to
perform ABR testing as well).
Figure 5.13 illustrates the electrode placement for the Audera system.
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University of Pretoria etd - De Koker, E (2004)
FIGURE 5.13:
AUDERA ELECTRODE PLACEMENT (HASS:
AFRICA)
SOUTHERN
The same skin preparation procedures were used as for MF-ASSR tests before
affixing re-usable electrodes (supplied by Grason-Stadler) with conductive gel
(Elefix) and electrode tape. An electrode impedance of five kilo-ohms or lower
was confirmed, and earphone inserts of an appropriate size were selected and
fitted snugly into the external auditory meatus. After each test, the electrodes
were removed and thoroughly cleaned in soapy water with a soft brush.
Instructions to the subjects and management of the test environment were similar
to those for multiple frequency testing, in that subjects were asked to lie down,
relax or sleep and to keep their eyes closed.
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confirmed once the subject was lying down, and the audiologist was positioned in
an adjacent room. Environmental noise was controlled in the same way as for
multiple frequency tests.
The testing and data collection parameters were the following:
•
Carrier frequencies
Carrier frequencies of 500, 1 000, 2 000, 3 000 and 4 000 Hz were used to allow
comparisons between single frequency and multiple frequency estimated
thresholds. With the Audera commercial version, it was possible to test all the
above frequencies whereas with the Beta version the test software made no
provision for the testing of 3000 Hz.
•
Modulation frequencies
Two modulation frequencies were compared, namely, 40 Hz (awake) and 80 Hz
(asleep).
•
FM and AM modulation
The modulation rates used were the default values of 10 per cent for frequency
modulation and 100 per cent for amplitude modulation.
•
Number of samples
A total of 64 samples were taken per carrier frequency and hearing level set, for
example, 1 000 Hz at 30 dB.
The number of samples was specified by
algorithms provided by the manufacturer.
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•
Statistical measures
For each electroencephalogram sample, the magnitude and phase of the
electrical activity corresponding with the frequency of the tone modulation were
quantified. Magnitude and phase information was shown as a vector in a polar
plot, with the vector length corresponding with the magnitude and vector angle
reflecting the phase or time delay between the tone modulation and the brain’s
response. Figure 5.14 illustrates a polar plot for a case where both the ear and
the brain responded to a tone. The vectors in the plot are clustered, indicating a
“phase-locked” brain response.
FIGURE 5.14:
PHASE-LOCKED RESPONSE
Figure 5.15 shows the vectors obtained when the tone was presented at an
inaudible level. Vector length varies and, most importantly, vectors are randomly
distributed around the plot, indicating that there is no phase relationship between
the electrical response and the tone modulation, in other words, no response.
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FIGURE 5.15:
RANDOM RESPONSE
The identification of responses such as those illustrated in the preceding two
figures as phase-locked or random was based on statistical analyses
performed in real-time while samples were being recorded, and not on
subjective visual assessments. A probability value of p<0,03 set the falsepositive threshold for the single frequency technique at 3 per cent, and any
trial contaminated with excessive noise was automatically terminated and
labelled accordingly, as shown in Figure 5.16.
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FIGURE 5.16:
EXAMPLE OF RESULTS REJECTED DUE TO EXCESS
NOISE
The results of all trials were plotted on a graph (Figure 5.17) with phaselocked results marked by an upward arrow to indicate that the ASSR
threshold
was
better
than
the
corresponding
pure-tone
threshold.
Conversely, “random” or no-response results were marked with a downward
arrow to indicate a lack of response. Thresholds were taken as the lowest
level at which a “phase-locked” response was obtained for a given frequency.
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FIGURE 5.17:
PLOTTED RESULTS OF TRIALS DURING AN ASSR TEST
Pure-tone thresholds were estimated on the basis of an algorithm developed
from the research findings of Rance et al. (1995), as illustrated in the example in
Figure 5.18, where the estimated pure-tone thresholds are presented objectively
and without the clinician’s input.
FIGURE 5.18:
ESTIMATED AUDIOGRAM BASED ON THE ASSR
RESULTS
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Estimated pure-tone audiograms such as that shown in the preceding figure were
compared with multiple frequency ASSR and conventional pure-tone thresholds.
5.8
DATA ANALYSIS PROCEDURES
A Microsoft Excel (2000) spreadsheet was used to collate data, which were then
analysed using statistical measures developed by the Medical Research Council
(Levin, 2002). Data analysis seeks to identify patterns in the data, in accordance
with criteria determined by the test protocol used.
This involves examining,
sorting, categorising, evaluating, comparing, synthesising, contemplating and
reviewing the data (Neuman, 1997). The following statistical procedures were
applied:
•
the sample t-test is a test that is used to compare different populations’
means; and
•
the two way analysis of variance is used for the analysis for experiments
involving several independent variables (Wackerly, Mendenhall &
Schaeffer, 1996).
5.9
SUMMARY
This chapter has described the research methods used in the acquisition of data
in this study to determine the clinical value of ASSR methods in the audiological
assessment
of
pseudohypacusis.
mine
workers
with
sensory
neural
hearing
loss
and
The experimental design was discussed, after which the
criteria and procedures for subject selection were detailed. The equipment used
in the subject selection, data collection and data analysis were subsequently
considered, after which data collection and analysis procedures were listed.
The next chapter presents the data obtained from the use of these methods.
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CHAPTER 6
RESULTS
AIM
To present, discuss and interpret the results of the study and to evaluate
these against the framework of the body of knowledge set out in the
literature review.
6.1
INTRODUCTION
Roeser et al. (2000b) stated that the value of any diagnostic test depends on
its ability to fulfil its intended purpose. The principal aim of the present study
was to assess the clinical value of ASSR methods in the audiological
evaluation of pseudohypacusic mine workers, particularly those with noiseinduced hearing loss.
Accurate estimations of hearing thresholds for the
purposes of assessing compensability and fitness for work was the norm for
deciding the clinical value of ASSRs.
The present study differs from previous work on ASSRs, in that it considered
subjects with abnormal hearing, namely those with noise-induced hearing
loss, a very specific form of sensory neural hearing loss (SNHL). Various
protocols and instruments were compared in order, to identify the most
appropriate and practicable procedure for assessing pseudohypacusic mine
workers with noise-induced hearing loss. Because such individuals are often
inclined to withhold co-operation during test procedures, the use of sedation in
such testing was also evaluated. Another important criterion for evaluating
the practicability of possible assessment procedures was the time required for
testing, along with the overall cost of implementation of a procedure for the
industry. This chapter is structured using the sub-aims of the study. These
the sections are presented individually as they were in Chapter 5 (Sections
5.2.2.1 to 5.2.2.5).
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The results are described and summarised using tables and figures.
Consequently the results are discussed. Finally the findings are interpreted
as suggested by Mouton (2001).
6.2
CO-OPERATIVE MINE WORKERS WITH NOISE-INDUCED
HEARING LOSS (PHASE ONE)
6.2.1 SUB-AIM: TO COMPARE ASSR AND PURE-TONE THRESHOLDS
IN A CO-OPERATIVE POPULATION OF ADULT MINE WORKERS
WITH SENSORY NEURAL HEARING LOSS
In the assessment of the results the ASSR thresholds are compared to the
relevant pure-tone thresholds in order to determine whether ASSR thresholds
can predict pure-tone thresholds accurately. The norm used in this case was
a 0 to 10 dB difference between any two threshold tests, which in clinical
practice is generally seen as an acceptable inter-test difference (RMA
guidelines, 2003).
In order to realise the aim it is thus necessary to determine what the difference
is between pure-tone and ASSR thresholds for every individual subject as well
as the mean difference in a whole experimental group. The significance of any
differences was determined using statistical procedures (the sample t-test and
two way analysis of co-variance).
All the subjects were required to have potentially compensable hearing loss
and, thus, a binaural pure-tone average exceeding 25 dB over the range of
500, 1 000, 2 000 and 3 000 Hz (Workmen’s Compensation Commissioner,
1995). As has already been mentioned the abnormal pure-tone thresholds
obtained from subjects varied from mild through to severe hearing thresholds
(see Chapter 5, p98). Because noise-induced hearing loss is sensory neural
in nature, the subjects’ hearing was most severely affected at the higher
frequencies and, thus, some subjects had normal hearing at the lower
frequencies. This finding was not anticipated but it was eventually included in
the data thus providing a base line of normal hearing thresholds with which
the ASSR thresholds could be evaluated as a starting point. A breakdown of
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the pure-tone thresholds obtained for the 81 subjects has been given in order
to enable some understanding of the nature and extent of noise-induced
hearing loss and to indicate in what severity range of hearing loss the ASSR
procedures were used (see Section 5.5.4.1).
The clinical value of ASSR thresholds was evaluated using the norm of a 10
dB inter-test variance, which is seen as acceptable in the mining industry
(RMA guidelines, 2003).
All the pure-tone thresholds obtained for the 81
subjects were compared to the ASSR thresholds obtained for the same
subjects for both ears and at all the frequencies tested. The overall mean
pure-tone threshold obtained for the frequencies tested in the group of 81 cooperative subjects was also compared to the overall mean ASSR threshold.
To gain further insight into the clinical value of ASSR thresholds an analysis
was also done on how much ASSR thresholds differed from the pure-tone
thresholds (for example, between 0 to 10 dB; 10 to 20 dB etc.).
Although the participating workers, had been selected because they have
noise-induced hearing loss, it was found that the pure-tone thresholds
obtained varied throughout the severity range from normal to severe hearing
thresholds. The following figures (6.1 – 6.4) give an indication of the number
of thresholds per frequency obtained in the normal, mild hearing loss,
moderate hearing loss and severe hearing loss categories.
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Number of Thresholds
120
100
80
60
40
20
0
500 Hz
1000 Hz
2000 Hz
3000 Hz
4000 Hz
Frequency
FIGURE 6.1
NUMBER OF NORMAL (≤25 dB) PURE-TONE THRESHOLDS FOR TEST FREQUENCY (n=181).
Number of thresholds
70
60
50
40
Series1
30
20
10
0
500 Hz
1000 Hz
2000 Hz
3000 Hz
4000 Hz
Frequency
FIGURE 6.2
NUMBER OF PURE-TONE THRESHOLDS INDICATIVE OF
MILD HEARING LOSS - PER FREQUENCY, (n=251)
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Number of thresholds
120
100
80
Series1
60
40
20
0
500 Hz
1000 Hz
2000 Hz
3000 Hz
4000 Hz
Frequency
FIGURE 6.3: NUMBER OF PURE-TONE THRESHOLDS INDICATIVE OF
MODERATE HEARING LOSS- PER FREQUENCY (n=345)
Number of thresholds
18
16
14
12
10
Series1
8
6
4
2
0
500 Hz
1000 Hz
2000 Hz
3000 Hz
4000 Hz
Frequency
FIGURE 6.4
NUMBER OF SEVERE PURE-TONE THRESHOLDS- PER
FREQUENCY (n=33)
As can be seen from Figure 6.1 the ASSR technique was unintentionally,
tested in normal hearing thresholds (n=181). The known sloping character of
noise-induced hearing loss makes the finding of the majority of normal
thresholds in the 500 and 1000 Hz area an expected result. Mild hearing
thresholds (n=251) (Figure 6.2) were obtained in subjects in all tested
frequencies despite significant years of noise exposure (see Figures 5.7 to
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5.11).
Thresholds obtained in the moderate range were the highest in
numbers (n=345) as seen in Figure 6.3. Significant moderate thresholds were
obtained at 1 000, 2 000, 3 000, and 4000 Hz. It can be deducted that noiseinduced hearing loss as seen in mine workers in the majority of cases
presents as a moderate sensory neural hearing loss.
Only a few (n=33)
thresholds were obtained in the severe range as can be seen in Figure 6.4.
From the above figures it can be concluded that the value of ASSR thresholds
can be evaluated throughout the severity range of hearing loss, varying from
mild to severe hearing loss. In Table 6.1 below the mean differences between
ASSR- and pure-tone thresholds are highlighted.
TABLE 6.1:
FREQUENCY
COMPARISONS BETWEEN ASSR AND PURE-TONE
THRESHOLDS ACCORDING TO SEVERITY OF HEARING
LOSS
EAR
Hz
NORMAL
MILD
MODERATE
SEVERE
HEARING
HEARING
HEARING
HEARING
LOSS
LOSS
LOSS
26-40 dB
41-65 dB
66+ dB
0-25 dB
Mean
SD
Mean
SD
500
Left
4,22 13,93
3,75 10,41
1 000
Left
8,53 11,01
2,59
2 000
Left
4 000
Left
10,0 12,91
1,92
500
Right
1 000
Mean
SD
Mean
P
VALUE
SD
5
0
-
-
0,99
7,63
0,53
7,80
-
-
0,02
8,54 11,75
-0,97
8,18
-2,5
3,54
0,00
6,30
2,5
6,96
4
9,62
0,28
9,79 15,74
3,57 10,82
2,5
3,53
-
-
0,33
Right
9,75 10,57
2,71
7,52
0,65
7,28
-5
0
0,00
2 000
Right
10 22,91
4,62
9,37
2,35
7,71
10
0
0,43
4 000
Right
0,42
7,53
1,84
7,66
5 21,21
0,85
-
3,33
-
5,77
From the above table it can be deducted that there is evidence that the
sensitivity of ASSR estimates does depend on the category of hearing loss
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(L, 1 000 Hz; L, 2 000 Hz; R, 1 000 Hz). The overall differences between
ASSR- and pure-tone thresholds are greatest at normal hearing.
These
findings support that of numerous other researchers (Rance et al., 1995;
Sininger & Cone-Wesson, 1994; John & Picton, 2000 and Schmulian, 2002)
that ASSR thresholds favour pathological ears.
This finding has been
explained due to the phenomenon of recruitment.
If all the pure-tone and ASSR thresholds of all the subjects in Phase 1 of the
study (co-operative workers) were compared, it resulted in 810 pure-tone
thresholds (81 subjects x 2 ears x 5 frequencies) that were compared with 542
ASSR threshold estimates (see Appendix N). The discrepancy in numbers
was due to the fact that the Audera Beta (prototype) instrument failed to make
provision for testing at 3 000 Hz, affecting 24 readings (12 subjects x 2
thresholds = 24), and that the Biologic instrument only had the capacity to
determine eight thresholds at once, making it necessary to test one frequency
separately, thereby extending what was already a lengthy procedure.
The specific comparison between pure-tone and ASSR thresholds will be set
out in the following Tables 6.2 to 6.4 and in Figure 6.5.
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TABLE 6.2
TEST
FREQUENCY
(HZ)
COMPARISON OF ASSR AND PURE-TONE THRESHOLDS
BY TEST FREQUENCY
THRESHOLD
ESTIMATION
TECHNIQUE
MEAN
THRESHOLD/SD
(dB)
DIFFERENCE IN
THRESHOLDS
(dB)
Left ear
500 Hz
ASSR
PT
1 000 Hz
ASSR
PT
2 000 Hz
ASSR
PT
3 000 Hz
ASSR
PT
4 000 Hz
ASSR
PT
24,8/15,5
n=64
21,38/11,70
n=81
39,8/12,5
n=66
37,06/13,14
n=81
48,1/11,2
n=62
45/10,61
n=81
54,5/14,6
n=20
48,50/12,60
n=81
52,96/15,59
n=55
50,13/14,16
n=81
3,42
2,74
3,1
6
2,83
Right ear
500 Hz
ASSR
PT
1 000 Hz
ASSR
PT
2 000 Hz
ASSR
PT
3 000 Hz
ASSR
PT
4 000 Hz
ASSR
PT
27,81/16,0
n=65
20,43/10,65
n=81
40/12,19
n=69
35,19/13,39
n=81
47,27/12,15
n=65
42,19/10,96
n=81
48/12,40
n=20
47,43/11,52
n=81
50,73/14,95
n=56
51,06/14,20
n=81
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7,38
4,81
5,08
0,57
0,33
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Table 6.2 gives the mean pure-tone and ASSR thresholds for all the subjects
(Phase 1: n=81).
As mentioned before, the same number pure-tone and
ASSR thresholds were not obtained.
The differences between the mean
ASSR and pure-tone thresholds vary from 0,33 to 7,38 dB (Table 6.2), which
was well within the 10 dB variation that was taken to be an acceptable
difference between two audiometric tests. The biggest difference was found
in the right ear at 500 Hz.
TABLE 6.3:
RESULTS FROM THE PURE-TONE AND ASSR TESTING
OF LEFT AND RIGHT EARS
FIVE FREQUENCY
MEAN FOR GIVEN
EAR
THRESHOLD
ESTIMATION
TECHNIQUE
ASSR n=70
Left ear
PT
n=81
ASSR
n=77
MEAN
THRESHOLD (dB)
SD
41,73
9,31
40,41
8,21
42,18
9,65
39,26
8,14
42,40
8,91
39,84
7,52
Right ear
PT
n=81
ASSR
Overall mean for both
n=78
ears
PT
n=81
Table 6.3 compares thresholds from the ASSR (all procedures) and pure-tone
testing for all the frequencies combined for the left and right ears respectively.
It is again clear that the mean differences of the ASSR and pure-tone tests
corresponded to within 10 dBs.
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TABLE 6.4:
MEAN DIFFERENCE BETWEEN ASSR ESTIMATES AND
PURE-TONE THRESHOLDS (dB) PER FREQUENCY
TESTED
FREQUENCY (Hz)
DIFFERENCE (dB)
DIFFERENCE (dB)
Ear:
Left
Right
500 Hz
4,14
8,20
1 000 Hz
3,53
3,97
2 000 Hz
2,66
3,75
3 000 Hz
4,75
-1,25
4 000 Hz
3,06
1,72
All frequencies
1,69
2,50
Table 6.4 indicates the differences between the mean thresholds from the
ASSR- and pure-tone testing. On average all the ASSR and pure-tone
thresholds obtained only differed 1,69 dB in the left ear and 2,50 in the right
ear.
Another way to obtain an idea of the clinical value of ASSR tests, (the ability
to predict pure-tone thresholds) is set out in Figure 6.5. The number of puretone and ASSR thresholds that corresponded within a range of 10 dB, 15 dB,
20, dB and 25 dB is illustrated in the following Figure 6.5.
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Number of ASSR and pure tone
thresholds within a dB range
University of Pretoria etd - De Koker, E (2004)
450
400
350
300
250
200
150
100
50
0
10dB
15dB
20dB
25dB
Comparison of ASSR and pure tone thresholds
FIGURE 6.5: CORRELATION OF PURE-TONE AND ASSR THRESHOLDS
In the overwhelming majority (78, 41 per cent) of thresholds obtained, the
ASSR and pure tone thresholds correlated within the 10 dB range as needed.
From the preceding three tables, it is apparent that the ASSR thresholds and
pure-tone thresholds correlated within the acceptable 10 dB range
(Workmen’s Compensation Commissioner, 1995 and 2000), thus making
ASSR testing a clinically acceptable measure to predict pure-tone thresholds.
The largest difference of 8,2 dB occurred at 500 Hz for right ears, which
corresponds with findings by other authors (John et al., 2001; Lins et al.,
1996; Schmulian, 2002; Herdman & Stapells, 2001). Rance et al. (1993) have
described larger response amplitudes for higher carrier frequencies.
This
reduced ability to estimate lower frequency thresholds accurately has been
explained as a result of an intrinsic jitter, where the activation pattern along
the basilar membrane covers a larger area for lower frequency stimuli or lower
carrier frequencies (Schmulian, 2002). Lins et al. (1996) also refer to the
masking effect of background noise on 500 Hz steady-state stimuli.
This
explanation is possibly not relevant to the present study, since testing was
done in an acoustically treated booth, as was also the case with the pure-tone
testing.
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The same authors also postulate that stimuli at 500 Hz may be masked by
higher frequency signals during MF-ASSR testing. This could have affected
the present study, since sensory neural hearing loss made it necessary to use
high-intensity stimuli at the higher test frequencies. Another explanation is
that the ASSR thresholds for the lower test frequencies, particularly 500 Hz,
were the closest to normal hearing (172 normal thresholds). Many studies
have indicated that ASSRs tend to favour abnormal hearing – that is a closer
correlation with abnormal pure-tone thresholds as a result of recruitment.
(John & Picton, 2000; Schmulian, 2002) (See Table 6.1).
To reduce test time, the present study used 10 dB intervals during thresholdseeking procedures for both the MF- and SF-ASSR tests, in accordance with
accepted practice for auditory evoked potential methods (Picton et al., 2003).
SF- and MF- techniques allow the use of 5 dB steps to provide greater
accuracy than that achieved in the present study, but it is important to note
that the mean differences between the pure-tone and ASSR thresholds
obtained here were smaller than those obtained in many previous studies (3034 dB: Swanepoel, 2001; 8-18 dB: Lins & Picton, 1995; 28-34 dB: Aoyagi et
al., 1994). One explanation for the smaller mean differences in the present
study is that ASSR instrumentation and algorithms have improved in recent
years. During the course of this study the Audera equipment was upgraded
from the Beta to the commercial version. John et al. (2001) have also noted
better response detection with the introduction of mixed modulation methods,
which were used in the present study.
This research strove to use an objective procedure- thus to avoid any
influence by the clinician on the determination of thresholds.
The only
variables that could be manipulated by the clinician during MF-testing were
the number of sweeps and the extent of averaging. Swanepoel (2001) and
Schmulian (2002) have both noted the current lack of standards for the latter
parameter, and have stated that more averaging is needed for stimuli with
intensities near the threshold level. The Audera system, unlike the Biologic
system, uses built-in algorithms to control the number of samples, thereby
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eliminating any possibility that the clinician could influence this test parameter.
Clearly, testing for clinical purposes should employ standardised sampling
and averaging methods that are uniformly controlled by algorithms in the test
system.
To summarise: the data indicate that ASSR thresholds can predict pure-tone
thresholds to within 10 dB in more than 70 per cent of the cases and that the
mean ASSR and pure-tone thresholds of 81 subjects correlated to within 1,69
and 2,50 dB for the respective ears (see Table 6.4).
The previously limited clinical validation of ASSR testing has been extended
by the present study’s demonstration of ASSR thresholds that were well within
10 dB of pure-tone thresholds, for a large population of subjects with noiseinduced hearing loss (sensory neural in nature) across the entire severity
range.
As previously mentioned, 536 ASSR thresholds were obtained in comparison
to 810 threshold results for pure-tone testing, due to shortcomings in both the
Audera Beta and Biologic systems.
South African compensation assess-
ments require 10 thresholds (RMA guidelines, 2003) but the Biologic system
can only determine eight thresholds in a single test run.
Subject-related
factors such as noise from body movement and myogenic noise were also
found to influence the difference in thresholds obtained as mentioned above.
Influences such as movement, fidgeting, coughing and sneezing accounted
for some of the shortfall in ASSR thresholds, as was found in previous studies
(Aoyagi et al., 1994) where test procedures were also lengthened by such
interventions. ASSR tests were performed with the clinician in an adjacent
room and, although visual contact was possible through the booth’s window,
the booth and test room were both darkened, limiting the audiologist’s
awareness of coughing, sneezing and movement by the subject. The system
identified any substantial occurrence of noise artefacts, but the audiologist
had no direct control over this potential source of error.
This raises the
possibility that the clinician’s presence in the same room could have limited
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subject movement and fidgeting, as well as any deliberate disregarding of
instructions on the part of unco-operative subjects.
The preceding presentation and discussion of results indicates that ASSR
testing is a reliable and accurate method for objectively estimating frequencyspecific hearing thresholds and that it can be successfully applied as an
alternative to pure-tone testing for adults with noise-induced hearing loss.
The present results have also confirmed previous findings that ASSR methods
are not influenced by the age of the subject (Picton, 1991) and that ASSR
thresholds are more accurate in pathological ears (Schmulian, 2002).
In order to analyse the clinical usefulness of ASSR testing further, various test
protocols, including the effect of sedation, are considered in the sections
below.
6.2.2 TO COMPARE THE CORRELATION OF MULTIPLE-FREQUENCY
(DICHOTIC) AND SF-(MONOTIC)-ASSR STIMULATION METHODS
IN ESTIMATING PURE-TONE THRESHOLDS IN A MINE WORKER
POPULATION
Single-frequency (monotic) stimulus tests were performed on 41 subjects
using the Audera system (single-frequency).
Multiple-frequency (dichotic)
stimulus testing of 40 subjects was done using the Biologic Master, which
provided for simultaneous stimulation at four test frequencies in each ear.
Table 6.5 indicates the average number of test frequencies at which a
threshold was determined using each technique.
TABLE 6.5:
AVERAGE NUMBER OF FREQUENCIES COMPLETED
USING SF-AND MF- TESTING PER SUBJECT
STIMULATION TECHNIQUE
NUMBER OF
FREQUENCIES
Single-frequency (Audera)
6
Multiple-frequency (Biologic)
7,4
137
PAIRED “t” AND
“p” VALUE
t =-2,39
p>0,0193
University of Pretoria etd - De Koker, E (2004)
As can be seen from the preceding table, the Biologic completed more
threshold estimates (7,4 versus 6), possibly due to its ability to complete eight
frequencies simultaneously. In addition the Audera Beta prototype (singlefrequency method) also did not provide for testing of 3 000 Hz which placed
this test procedure at a disadvantage.
The difference in the number of
threshold estimates obtained was statistically significant with a p-value of
0, 01.
Table 6.6 indicates the average time taken for the two stimulation techniques,
independent of the number of thresholds obtained, while Table 6.7 shows the
time taken normalised for the number of thresholds obtained.
TABLE 6.6:
TIME TAKEN FOR SF- AND MF-TESTS, INDEPENDENT OF
THE NUMBER OF FREQUENCIES COMPLETED
TIME
(MINUTES)
PAIRED “t” AND
“p”VALUES
SF (Audera)
50,44
t= -7,19
MF (Biologic)
85,4
p=0,00
STIMULATION TECHNIQUE
TABLE 6.7:
TIME TAKEN FOR SF- AND MF -TESTS, NORMALISED
FOR THE NUMBER OF FREQUENCIES COMPLETED
TIME
(MINUTES)
PAIRED “t” AND
“p” VALUES
SF (Audera)
51,56
t=-6,56
MF (Biologic)
84,18
p=0,000
STIMULATION TECHNIQUE
The two preceding tables show that the stimulation technique used (monotic
SF- or dichotic MF) is a highly significant factor (p=0, 00 in Table 6.6 and 6.7),
with the SF- technique being the more time-efficient. This finding contradicts
previous findings (Perez-Abalo et al., 2001).
Several researchers have
suggested that it would take the same time to test eight different frequencies
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using the MF-technique as for a single frequency using the SF-method. One
possible explanation for this apparent anomaly is that most previous studies
looked at subjects with normal hearing, implying that threshold-seeking
procedures would start at 40 dB, after which only two or three descending
steps would be required. For subjects with hearing loss, a multi-frequency
technique would start at 40 dB and, after obtaining no response, stimuli would
then be presented at higher intensities thereby lengthening the test
procedure.
It must also be considered that the SF-technique employs the 40 Hz
response, which is more robust in adults than in children. The use of higher
stimulation rates, as with the Biologic system, is specifically intended to
address the 40 Hz response’s sensitivity to infants’ maturation and state of
consciousness, which was not a concern in the present context.
Furthermore, there are discrepancies in previously reported test times for MFprocedures. Herdman and Stapells (2001) have reported an average time of
83 minutes, three times longer than the 21 minutes reported by Perez-Abalo
et al. (2001), while Swanepoel (2001) has reported test times between 15 and
31 minutes. It is also relevant to note that Perez-Abalo et al. (2001) and
Swanepoel (2001) both tested normal hearing subjects.
Herdman and
Stapells (2001) used 5 dB increments to determine thresholds which could
explain the longer test time.
Testing during the present study took an average of 84,18 minutes but there
are no standards governing the number of sweeps and averages obtained,
and it would therefore be invalid to compare the present test times directly
with those reported previously. Stimulation at a low intensity increases the
number of averages required and, thus, the recording time, indicating a need
for internationally accepted standards for averaging methods and algorithm
specifications, particularly for clinical applications. Although the SF-technique
used in the present study eliminated any influence by the audiologist on
averaging, the MF-technique allowed the number of sweeps and averages to
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be selected, indicating that the need for objectivity was better met by SFASSR testing.
A further disadvantage of the MF-stimulation technique for individuals with
sensory neural hearing loss is that this condition is progressively more severe
at higher frequencies, which means that some subjects could have normal
hearing at the low frequencies despite severe or even profound hearing loss
at higher test frequencies.
This made it impossible to select a uniform
intensity protocol for the 500 to 4000 Hz range. A level of 100 dB, while
possibly suitable for higher frequencies, would have been dangerously loud at
a frequency of 500 or 1 000 Hz, making it necessary to use the MF- technique
in what was essentially a SF-mode, by first testing at 1 000, 2 000 and
4 000 Hz, and then testing at 500 Hz separately. This partially accounts for
the longer times required for the MF-testing.
Table 6.8 indicates the differences in prediction value of the pure-tone
thresholds between the SF- and MF- techniques and the levels of significance
of the data.
TABLE 6.8:
DIFFERENCES IN SENSITIVITY BETWEEN THE SF- AND
MF-STIMULATION TECHNIQUES
MEAN DIFFERENCE BETWEEN ASSR AND
PT THRESHOLDS
STIMULATION TECHNIQUE
500 Hz
500 Hz
1 000 Hz
Left
Right
Right
SF
7,69
8,39
6,13
MF
11,71
16,66
8,92
-1,85
p≥0,0694
-3,34
0,0014
-1,83
p≥0,072
t-test
From the table it can be seen that SF-testing yielded more accurate estimates
of the thresholds than the MF-methods, particularly at the low frequencies.
The SF-technique’s higher sensitivity may be attributable to the high
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stimulation levels required, as mentioned previously. Lins and Picton (1995)
found no significant differences in the response amplitude between the MFand SF-methods, provided intensity was at low-to-moderate levels. John and
Picton (2000) also caution against the dangers of high-intensity stimulation.
From the above discussion it can be deducted that, although the SF-method
completed fewer threshold estimates in comparison to the MF-method, that
the SF-procedure was more accurate in determining thresholds and that it
took less time to obtain a threshold. This last finding appears to contradict
what one would intuitively expect namely that it would less time to obtain eight
thresholds tested simultaneously.
6.2.3 TO COMPARE DIFFERENT MODULATION FREQUENCIES’
EFFECTIVENESS IN ESTIMATING PURE-TONE THRESHOLDS
28 subjects were tested using a 40 Hz stimulation rate (the Audera-awake
protocol), while 52 subjects underwent testing with the higher rate of 80 to 110
Hz (the Audera asleep protocol and Biologic MASTER). The results are set
out in Table 6.9 and 6.10.
TABLE 6.9:
TIME TAKEN FOR 40 HZ AND 80-110 HZ TESTS,
INDEPENDENT
OF
NUMBER
OF
FREQUENCIES
COMPLETED
TIME
(MINUTES)
PAIRED “t” AND
“p”VALUES
40 Hz (Audera)
50,44
t= -7,19
80-110 Hz (Biologic)
85,4
p=0,00
STIMULATION TECHNIQUE
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TABLE 6.10: THE TIME TAKEN FOR SF- AND MF-TESTS, NORMALISED
FOR THE NUMBER OF FREQUENCIES COMPLETED
TIME
(MINUTES)
PAIRED “t” AND
“p” VALUES
40 Hz (Audera)
51.56
t=-6,56
80-110 Hz (Biologic)
84.18
p=0,000
STIMULATION TECHNIQUE
From these tables it can be seen that the average testing time (normalised for
the number of frequencies evaluated) was 33 minutes longer using the 80 to
110 Hz stimulation rate than with a rate of 40 Hz, but there was statistical
evidence in only one frequency that the SF-method was more accurate in
determining pure-tone thresholds (500 Hz, see Table 6.8; p=0.001) (The
single-frequency technique used a 40 Hz stimulation rate and the multiplefrequency method a 80 to 110 Hz rate, therefore there is referred to Table
6.8).
Stapells et al. 1984 have found the amplitude of auditory evoked potential
responses to be two to three times greater with a 40 Hz stimulation rate than
with a 10 Hz rate while Dobie and Wilson (1998) have also found 40 Hz to be
the stimulation rate of choice for alert or sedated adults. The stimulation rate
of 40 Hz was also favoured in the present research. Another research team
that came to the same conclusion was Rickards and De Vidi (1995) who found
the 40 Hz rate to be more suitable for use in adults.
These researchers
explain the finding by stating that the 40 Hz response did not require
compensation or allowance for maturational effects. Other researchers have
investigated the use of other stimulation rates to overcome the effect of
wakefulness on the 40 Hz response (Herdman & Stapells, 2001; Lins et al.,
1995).
Difficult-to-test populations mainly consists of young children and
infants, which may help to explain the move towards higher stimulation rates
that are less affected by sleep, sedation and maturation (in these
populations).
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6.2.4 TO DETERMINE THE EFFECT OF SEDATION ON THE ASSR
TEST’S ABILITY TO ESTIMATE PURE-TONE THRESHOLDS
28 non-sedated subjects were tested using the SF-method without sedation,
while 13 were tested by the same method while sedated. For the MF-ASSR
tests, 20 subjects were sedated and an equal number were not, to determine
the effect of this factor on the sensitivity and test time. The significance of the
differences between the two methods is set out in Table 6.11 (raw data is
seen in Appendix N).
TABLE 6.11: THE SIGNIFICANCE OF TIME COMPARISONS OF MF- AND
SF-TECHNIQUES WITH AND WITHOUT SEDATION
T-TEST
P VALUE
SF
1,86
0,19
MF
2,18
0,15
TECHNIQUE
From the above table it is clear that time comparisons between both SF- and
MF-testing yielded no significant difference between the test times for sedated
and non-sedated subjects (p=0,19 and 0,15)(Table 6.11).
In order to evaluate the effect of sedation further, comparisons were also
made between the accuracy of the threshold estimates with and without
sedation.
Results are set out in Table 6.12 and 6.13 below.
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TABLE 6.12: SIGNIFICANCE OF SENSITIVITY DIFFERENCES BETWEEN
SEDATED AND NON-SEDATED SF-ASSRs
t-TEST
FREQUENCY (HZ)
p-VALUE
LEFT EARS
500
0,4956
0,6251
1 000
-0,9221
0,3660
2 000
-1,0345
0,3132
4 000
0,7614
0,4553
500
0,1028
0,9190
1 000
1,1867
0,2475
2 000
-0,2813
0,7811
4 000
-0,6505
0,5221
RIGHT EARS
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TABLE 6.13:
SIGNIFICANCE
OF
SENSITIVITY
DIFFERENCES
BETWEEN SEDATED AND NON-SEDATED MF-ASSRs
t-TEST
FREQUENCY (HZ)
p-VALUE
LEFT EARS
500
1,1208
0,2698
1 000
1,3545
0,1840
2 000
0,1524
0,8798
4 000
0,8331
0,4118
500
0,8687
0,3911
1 000
1,9412
0,0603
2 000
0,9459
0,3509
4 000
0,9461
0,3535
RIGHT EARS
The same lack of significant differences was found if attention was focused on
the threshold estimation accuracy in SF- and MF-techniques when they were
compared in terms of the sensitivity with and without sedation. The preceding
tables indicate no significant effect from sedation on the sensitivity or test time
for SF- and MF-testing (all the p values were higher than 0, 05) and, hence,
there is no reason to sedate adults, provided they co-operate and limit their
movement during the test procedures. Other researchers have found that
sedation significantly diminishes the amplitude of the 40 Hz response (Lins et
al., 1995) but this research was done on children.
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6.2.5 SUMMARY OF FINDINGS (PHASE 1)
•
ASSR threshold estimates were found to be sufficiently accurate to
predict pure-tone thresholds;
•
ASSR thresholds and pure-tone thresholds correlated to within 10 dB;
•
ASSR thresholds prediction value was the poorest at 500 Hz;
•
ASSR thresholds favoured abnormal hearing;
•
10 dB decrements, as a threshold estimation technique, was sufficient
to predict pure-tone thresholds accurately;
•
ASSR methods were objective;
•
ASSR methods were accurate in an adult population with sensory
neural (noise-induced hearing loss);
•
subject related factors such as movement, coughing and fidgeting
influenced the quality of ASSR recordings;
•
the fact that the audiologist is seated in an adjacent room during
testing, makes it difficult to observe patient behaviour and thus
precluded control over potential sources of error;
•
SF- and MF- methods were not significantly different in their accuracy
to estimate pure-tone thresholds but the SF-method were more time
efficient;
•
there were no significant effect from sedation on the sensitivity or test
time of all ASSRs; and
•
thus there is no motivation to use sedation if a patient co-operates.
With these results obtained the experimental research could thus now be
focused on unco-operative subjects
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6.3
UNCO-OPERATIVE MINE WORKERS (PHASE TWO)
6.3.1 TO DETERMINE WHETHER PURE-TONE THRESHOLD ESTIMATES
CAN BE OBTAINED FOR UNCO-OPERATIVE WORKERS
6.3.1.1
Introduction
After proving that ASSR methods could accurately estimate pure-tone
thresholds in an adult mine worker population with noise-induced hearing loss
and after the most efficient modulation frequency and stimulation technique
had been decided on, the experimental research could be advanced to the
final phase, in which the clinical value of these methods could be tested in an
unco-operative sample of mine workers.
6.3.1.2
Revision of Phase 1 procedures: Implications for Phase 2
The 29 subjects in the unco-operative group (Phase 2) were tested using
ASSR methods, in particularly the SF-technique with a modulation rate of 40
Hz. Although the findings in the first phase with co-operative subjects had
indicated that sedation did not improve sensitivity or reduce test times for cooperative subjects (as reflected in Tables 6.12 and 6.13), common experience
with pseudohypacusic workers, who may be motivated by the prospects of
noise-induced hearing loss compensation, led to a decision to use sedation
for the unco-operative group. A second variation in the procedures from those
used for the phase one subjects was the use of a single room for both the
subject and the audiologist, to allow control over body movement and other
sources of noise from subjects.
6.3.1.3
Results obtained
The results of the pseudohypacusic groups’ diagnostic- and ASSR test are set
out in Appendix O. The ASSR and pure-tone thresholds of the 29 subjects
differed on average from each other by 61, 08 dB. This is in contrast to the
less than 10 dB difference with co-operative subjects. The ASSR results have
conclusively proven that the pseudohypacusic group’s diagnosis was
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accurate. This diagnosis was made where there was a discrepancy larger
than 15 dB between the same frequency’s thresholds during two tests.
Table 6.13 indicates the deductions that were made from the results of the
pseudohypacusic group.
TABLE 6.14:
DEDUCTIONS MADE FROM THE ASSR THRESHOLDS
OBTAINED IN PSEUDOHYPACUSIC WORKERS
PERCENTAGE
OF CASES
CONCLUDED
ABNORMAL
HEARING
COMPENSABLE
LOSS
(>25 dB PTA)
(RMA
guidelines)
48%
96,5%
82,8%
UNFIT
POOR
SUDDEN
CORRELATION HEARING
WITH PRELOSS
VIOUS TESTS
20,7%
48,3%
Of the 29 pseudohypacusic subjects, 96,5 per cent could be successfully
diagnosed and the cases could be concluded on the basis of the ASSR
results (Table 6.14 and Appendix O). In only one case of the 29 (subject 2,
Appendix O) did ASSR testing fail to estimate hearing thresholds, and this
was in one ear only, due to excessive electrical activity that was unrelated to
the subject’s hearing. These results provide overwhelming support for the use
of ASSR testing as a valid method to determine hearing thresholds for
pseudohypacusic mine workers with noise-induced hearing loss.
It was also found that 10, 3 per cent of the left ears and 17,2 per cent of the
right ears of the pseudohypacusic subjects tested had normal hearing (Table
6.14 shows abnormal hearing of 82,2 per cent). (See Appendix O as well).
This is an important and logical finding when it is taken into consideration that
as mine workers these subjects had been exposed to hazardous noise for
considerable periods (with a mean of 20 years). Audiologists assessing such
patients must be aware of the strong likelihood that pseudohypacusic
individuals will be hearing-impaired, and failure to conclude a diagnosis may
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have moral as well as health and safety implications in such cases. Although
82, 8 per cent of the subjects had abnormal hearing, only 48,3 per cent were
compensable according to South African standards (see Table 6.14 and
Appendix O), indicating that the determination of all the thresholds necessary
(through the use of ASSRs) makes differential diagnosis possible, such as in
cases of unilateral hearing loss which is not attributable to noise exposure.
Of the pseudohypacusic subjects, 20,7 per cent were found to be unfit for their
present duties (Table 6.14 and Appendix O), based on current guidelines
(Geyser, 2003). If audiologists fail to adequately assess worker fitness, as
can easily occur with conventional screening and diagnostic procedures, the
employer and workers are subject to greater safety risks, and there is likely to
be a negative impact on productivity.
In this respect, accurate once-off
threshold estimation using ASSR methods would be beneficial.
Less than half (48,3 per cent) (Appendix O) of the ASSR thresholds correlated
well with previous screening results, which is cause for some concern. In
dealing with pseudohypacusic patients, audiologists are compelled to make
recommendations based largely on previous screening results where this is
the only source of additional information. The present finding indicates that
previous screening results may be an unreliable indicator of hearing status for
more than half of pseudohypacusic workers, possibly because workers have
been manipulating their test results over several years. However, a more
worrying possibility is that of a sudden deterioration in hearing, that may be
present which will invariably progress to compensable levels. In examining
subjects’ previous screening results, it was found that 31,0 per cent (Appendix
O) showed signs of sudden deterioration not attributable to noise exposure
and warranting further medical investigation. (This was possible by studying
previous screening results).
6.3.1.4
Time required for ASSR testing
After an average time of 8,1 minutes for skin cleaning/preparation and the
placement of electrodes, an average of 49.86 minutes was required for the
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ASSR recordings in the pseudohypacusic group (Appendix O).
This
compares very well with the 51.56 minutes (Table 6.7) required to obtain 10
thresholds (5 test frequencies per ear) in the co-operative group (Phase 1).
This indicates that one hour would be needed for each ASSR test.
This
makes it a lengthy procedure in comparison to conventional methods, but it
provides more essential information.
In comparing these test times with those for co-operative subjects (Phase 1) it
does not appear that the use of a single room for the audiologist and the
subjects (as opposed to a separate test booth in Phase 1) made any
appreciable difference to the test time. It is also possible that the testing time
was very similar due to the fact that the audiologists’ presence inhibited
negative behaviour from pseudohypacusic subjects.
Nevertheless, it is
recommended that a single room be used, to discourage deliberate movement
and other sources of noise from unco-operative patients.
6.3.1.5
Summary of Phase 2
•
ASSR testing confirmed the diagnosis of pseudohypacusis;
•
in 96,5 per cent of cases with pseudohypacusis could diagnostic
procedures be completed;
•
82,8 per cent of pseudohypacusic subjects had abnormal hearing;
•
48 per cent of abnormal cases were compensable;
•
20,7 per cent of cases were unfit for their current duties;
•
in 48,3 per cent of pseudohypacusic cases did ASSR thresholds show
poor correlation with previous screening tests;
•
there were evidence of sudden hearing deterioration in 31 per cent of
pseudohypacusic cases;
•
ASSR testing makes differential diagnosis possible;
•
previous screening results were not a good indicator of present hearing
status
•
the time needed for ASSR testing in Phase 2 was very similar to the
time required for the co-operative group;
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•
an hour is sufficient time for ASSR testing if skin preparation
procedures is also taken into consideration; and
•
it is recommended that a single room set up is followed when testing
pseudohypacusic workers.
6.4
RESEARCH RESULTS REALISING THE PRINCIPAL AIM OF THE
STUDY
The principal aim of the present research was to determine whether ASSR
testing could successfully conclude audiological assessment procedures for
pseudohypacusic mine workers. The question that had to be addressed was:
Is there clinical value in using this AEP technique with mine workers
with noise-induced hearing loss and more specifically these with
pseudohypacusis?
The inability of conventional procedures to provide accurate thresholds for
difficult-to-test individuals who are often unco-operative, commonly leads to a
repetition of screening and diagnostic procedures and referral to an Ear-,
Nose- and Throat specialist in an effort to resolve possible compensation
cases. Very often, ABR testing is recommended. This test provides limited
threshold information in the 2000 to 4000 Hz frequency range, but otherwise
only confirms the presence of pseudohypacusis without determining the
thresholds needed for a compensation claim or for fitness-for-work
evaluations.
In some instances this leaves deserving claims unresolved,
while in others it results in overcompensation due to deliberately exaggerated
hearing loss.
Through the current study it has been conclusively proven that ASSR
methods have sufficient clinical value in a mine worker population with
sensory neural hearing loss (noise-induced hearing loss). Even in a sample
of unco-operative workers this auditory evoked potential managed to assist in
concluding the diagnostic procedures. The clinical value lies in the fact that it
is an accurate and reliable alternative to pure-tone methods for determining
thresholds in adult mine workers. It can furthermore serve as a single test in a
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battery if co-operation is withheld. The time requirements are certainly
reasonable in the field of AEPs.
Further clinical value is also derived from the fact that the use of ASSRs in a
pseudohypacusic population with noise-induced could conclude audiological
procedures and lead to correct recommendations re compensability,
differential diagnosis and amplification. Roeser et al. (2000b) alert to the fact
that the identification of pseudohypacusis is extremely important to ensure
that the patient receives appropriate intervention but also to avoid harmful
intervention.
The fact that the overwhelming majority of pseudohypacusic
workers had hearing loss shows the danger of only rescheduling
pseudohypacusic workers for annual testing if thresholds could not be
obtained.
An important finding that should be considered by audiologists is the fact that
previous screening tests were not a good indicator to use as a basis for
recommendations if hearing thresholds cannot be obtained. Very often this is
all an audiologist has if a patient withholds co-operation.
Much clinical value is derived from the fact that ASSRs are an objective
procedure.
results.
The audiologist as well as the patient does not influence the
Definitely an important finding in a population that is traditionally
unco-operative.
Roeser et al. (2000b:12) define the effectiveness of audiological tests as
follows:
All diagnostic procedures, whether for the auditory system or any
other system, are designed to identify the presence of a disorder as
early as possible. When indicated, diagnostic procedures can also
help to identify the cause or nature of the disorder. The value of a
diagnostic test depends on the ability to perform as intended. That
is, the procedure must accurately identify those patients with the
disorder while clearing those patients without the disorder.
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In summary ASSRs have performed as intended.
6.5
SUMMARY
The results of the experimental research were presented in Tables,
Figures and Appendices. The results were discussed and correlated
with the current literature in the field of ASSRs and pseudohypacusis
and conclusions were finally drawn.
The clinical value of ASSR testing in mine workers with noise-induced
hearing loss and pseudohypacusis have thoroughly been researched,
tested and evaluated and found to be a reliable alternative to puretone testing.
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CHAPTER 7
CONCLUSIONS AND RECOMMENDATIONS
AIM
To integrate the findings of the study and to critically evaluate the results of
this study against the theoretical framework supplied in Chapters 1 to 4.
The value of this study is discussed in terms of the application of auditory
steady state response testing in the mining industry in general and in the
testing of workers with pseudohypacusis in particular.
7.1
INTRODUCTION
Mine workers who have noise-induced hearing loss and believe that they may
qualify for financial compensation may be unco-operative patients. This lack of
co-operation during audiological assessments leads to an inability on the side of
audiologists to establish reliable pure-tone hearing thresholds.
If feigning of
hearing loss or the exaggeration of an existing hearing loss is not identified by
the clinician it is logical to conclude that the financial impact on employers
originating from fallacious claims and overcompensation is considerable. If on
the other hand the pseudohypacusis is identified but not quantified, the number
of pending cases is likely to escalate, thus impeding efforts to finalise genuine
claims for noise-induced hearing loss. It has been the experience in the clinical
situation that in many instances follow-up assessment procedures also fail to
provide the accurate hearing thresholds needed to finalise a claim, putting the
clinicians in a position of having to make debatable recommendations with regard
to rehabilitation, fitness for work and compensation.
One or all of the following complications can follow from the clinical problem of
pseudohypacusis in the mining industry (De Koker, 2003):
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•
frustration is experienced on the part of audiologists and occupational
health personnel. There develops a mistrust on the part of workers, with
retesting and counselling failing to make a difference in eliciting their cooperation (Franz, 2003);
•
escalating costs for audiological assessments, often without a successful
diagnosis other than confirmation of pseudohypacusis (De Koker, 2003);
•
greater number of specialist referrals is generated due to the failure of
current audiological procedures to finalise cases, including many that have
been referred previously and remain inconclusive due to a lack of patient
co-operation (Geyser, 2003);
•
claims from workers who genuinely deserve to be compensated, but that
are not settled due to the absence of reliable hearing thresholds;
•
the compensation of workers with normal hearing;
•
further exposure of workers with severe hearing loss who should have
been declared unfit for work in noisy areas, to the detriment of their
remaining hearing and quality of life, as well as to their safety and that of
their fellow workers (De Koker, 2003);
•
the overlooking or misdiagnosis of cases of a sudden onset of hearing loss
and ear pathology due to the audiologist’s inability to obtain hearing
thresholds in unco-operative clients; and
•
the impossibility to make a differential diagnosis without reliable pure-tone
thresholds (Roeser et al., 2000b).
It is clear that above stated problem of pseudohypacusic mine workers
exaggerating and feigning hearing loss makes the audiological assessment of
these workers problematic as can be seen in the above description. This is
however a responsibility that cannot be disregarded as Roeser et al. (2000b)
said. “Today audiologists are the primary health care professional involved in the
identification, prevention, and evaluation of auditory and related disorders”. The
responsibility therefore also lies with the audiologist to find a solution to the time
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consuming and costly problem. An audiologist needs to find a solution in the
interest of audiology as a profession, the individual worker, the insurance
companies and mine management.
A possible solution to the problem of pseudohypacusis was addressed in this
research project by studying pseudohypacusis as a phenomenon and by
researching the previous audiological solutions in difficult-to-test populations. A
literature review of auditory evoked potential procedures, the most common
solution in unco-operative patients, has lead to the possibility that a new auditory
evoked potential could bring a solution to the described problem. ASSRs, an
objective evoked potential was researched and experimented with.
The question asked was: Is there an audiological technique available that
cannot only identify pseudohypacusis but, more importantly, estimate the
true behavioural thresholds of pseudohypacusic mine workers with noiseinduced hearing loss? In the definition (and realisation) of the aim of the study
there was emphasis placed on what the clinical value of ASSR testing would be
in the described population.
Clinical value was defined as the accuracy in predicting or estimating thresholds
and the time-effectiveness in assessing noise-induced hearing loss in
pseudohypacusic workers. The results of the empirical parts of this study have
been presented in the previous chapter. However it is necessary to conclude the
study by interpreting, evaluating and summarising the findings.
Offering
recommendations for further research logically emerge from such an evaluation.
This final chapter concludes by suggesting the way forward.
7.2
RESEARCH FINDINGS
The findings of the present study, considering all the sub-aims (Phase 1) where
different ASSR procedures were evaluated, can be summarised as follows:
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7.2.1 CONCLUSIONS BASED ON THE RESULTS FROM CO-OPERATIVE
MINE WORKERS WITH NOISE-INDUCED HEARING LOSS
The most important result was that all procedures were within 10 dB of the puretone threshold. From this it can be concluded that:
•
ASSRs (all procedures) offer an accurate alternative to behavioural
methods for determining/estimating pure-tone thresholds for adult mine
workers with noise-induced hearing loss, a type of sensory neural hearing
loss (ASSR and pure-tone thresholds within 10 dB from each other);
•
ASSRs offer accurate threshold estimates across the range from normal to
severe sensory neural hearing loss however ASSRs were found to favour
pathological ears;
•
The biggest difference between pure-tone and ASSR thresholds were
found at lower frequencies as is the case in other research (500 Hz: 8,20
dB in the right ears) (Lins et al., 1996 and Schmulian, 2002);
•
10 dB intervals (decrements and increments) used in ASSR threshold
estimation did supply accurate estimates of pure-tone thresholds. It can
be concluded that it will thus not be necessary to lengthen an already long
procedure by using 5 dB intervals;
•
The Audera testing system (SF- and monotic) enabled an objective test
procedure that could not be manipulated by the clinician or the subject;
•
The SF-monotic technique was found to be more time-efficient than the
MF- method (51,56 minutes versus 84,14 minutes), and also yielded more
accurate threshold estimates at 500 Hz. (right-ear SF=8,39 (mean):
MF=16,66 (mean); p=0,0014);
•
In comparing the modulation rates it was found that the 80 to 110 Hz was
33 minutes longer than the average test time for the 40 Hz procedure. It
seems to be clear that the 40 Hz rate is more time-efficient when applied
to adults with sensory neural hearing loss;
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•
Sedation did not improve the sensitivity or reduce it, nor reduce the test
time for the SF- or MF-methods and thus is there no motivation for using
sedation if passive co-operation can be obtained from the patient;
•
ASSR results were, as was found in other research, not influenced by the
age of the subjects (Stapells et al., 1984; Rane et al., 1995);
•
Subject related factors such as movement, coughing and fidgeting seem
to influence the quality of the ASSR recordings as found by Aoyagi et al.,
(1994) as well. This behaviour needs to be prevented, especially if there
is an intention on the side of the patient to confound results. A single
room set-up, where the audiologist is seated in the testing room with the
patient, offers a solution to this possibility.
7.2.2 CONCLUSIONS BASED ON RESULTS WITH PSEUDOHYPACUSIC
MINE WORKERS
These findings for the co-operative group of 81 subjects with noise-induced
hearing loss indicated that the principal aim of the study could be addressed
next, in other words to determine the clinical value of ASSR methods for
evaluating pseudohypacusic mine workers with noise-induced hearing loss:
•
ASSR tests confirmed the diagnosis of pseudohypacusis;
•
It has been found that the use of ASSRs in a pseudohypacusic population
with noise-induced hearing loss could conclude audiological procedures
(in 96,5 per cent of cases) and lead to correct recommendations with
regard to compensability, differential diagnosis and amplification;
•
The overwhelming majority (82, 8 per cent) of pseudohypacusic workers
had some hearing loss. It is thus important for audiologists not to keep on
re-scheduling workers with hearing loss without correct recommendations
with regard to specialist referrals and rehabilitation;
•
The majority (82,8 per cent) of the pseudohypacusic patients had hearing
loss but only 48,3 per cent were compensable. From this observation it is
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clear that ASSR testing made differential diagnosis possible and it is thus
incorrect to assume that all workers with hearing loss are compensation
candidates due to the fact that they work in noise;
•
The use of ASSRs made it clear that 20,7 per cent of the workers in the
pseudohypacusic sample were unfit for their work.
The inability to
diagnose the organic component in pseudohypacusis thus does cause
workers to work in conditions detrimental to their health and safety;
•
Less than half (48,3 per cent) of the ASSR thresholds correlated well with
the results of previous screening tests. This is a worrying finding since
very often previous results are all that an audiologist has to base
recommendations on. A possible explanation of this phenomenon is that
pseudohypacusic behaviour might have been present for a number of
years but more serious that there might have been a sudden deterioration
of hearing which leads to the following finding that;
•
Disturbingly 30,1 per cent of the pseudohypacusic subjects have
experienced a sudden deterioration in hearing if one studies their previous
thresholds. Due to the possible serious nature of sudden hearing loss
audiologists are obliged to make the diagnosis;
•
The ASSR procedure has proven to be lengthy in comparison to
conventional testing (approximately 60 minutes including preparation time,
compared with the 17 minutes typically required for pure-tone audiometry,
an otoscopic examination and immittance measurements).
It is
nevertheless advantageous if it is considered that the 17 minutes is not
standard in the case of pseudohypacusis and how many times
pseudohypacusic workers need repetitive testing;
To summarise:
the findings indicate scientific support for the use of ASSR
methods as a more reliable alternative to pure-tone testing of adults with noiseinduced hearing loss, and that ASSRs can serve as a once-off procedure to
conclude the diagnosis of pseudohypacusis and make the correct handling of the
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case possible.
ASSR results met the requirements for accurate threshold
estimates at all the frequencies required for compensation and fitness-for-work
evaluations.
7.3
CRITICAL EVALUATION OF THE RESEARCH
7.3.1 RELIABLE ALTERNATIVE TO PURE-TONE METHODS
The norm adopted in this study of a reliable difference between two different
hearing tests of 10 dB (RMA guidelines, 2003) is audiometrically acceptable.
The threshold seeking procedure of 10 dB threshold bracketing (Picton et al.,
2003) used in this study can probably be improved when one uses 5 dB intervals
which is possible with the equipment used in this study. Nevertheless the study
has shown that ASSRs can be used as an alternative to pure-tone testing in
adults with hearing loss.
Furthermore, the reliability of the use of ASSR thresholds is enhanced by a
positive critique on this study namely the number of subjects used (in Phase 1 a
sample of 81 was used). Picton et al. (2003) summarised studies (done up to
2003) that used the 40 Hz response (11 studies) and indicated that the number of
subjects used in these studies varied between six and 40 per study.
It can be seen as a limitation of the present study that no indication of inter-test
repeatability was provided, because the need to avoid interference with
production schedules precluded any repetition of the lengthy testing procedures
by a second clinician. It was also not possible to compare the same subjects’
performance on different ASSR protocols due to the same time constraint.
These limitations may well have influenced the results, or more probably the
generalisation of the results.
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7.3.2 THRESHOLD ESTIMATES ACROSS THE SEVERITY RANGE
ASSR testing proved to be accurate in estimating hearing thresholds that ranged
from normal to profound. Picton et al. (2003) cite 22 studies done with ASSR
methods. Only six of these studies used a sample of primarily hearing impaired
subjects. The present study thus adds to the limited research on ASSR testing
applied to hearing impaired subjects. Furthermore, since the subjects had noiseinduced hearing loss, a specific type of sensory neural hearing loss, it is possible
that these results could be generalised to adults with sensory neural hearing loss
derived from other types of etiology.
It was also concluded that the lack of algorithms for sloping sensory neural
hearing loss in the multiple dichotic stimulation was a negative factor that needs
to be addressed.
7.3.3 FEWER ASSR THRESHOLDS OBTAINED IN COMPARISON WITH
PURE-TONE THRESHOLDS
The influence of patient-generated noise on electrophysiological techniques is an
ongoing clinical concern (Abramovich, 1990; Ferraro & Durrant, 1994). Artefacts
from high levels of background EEG activity can lengthen ASSR procedures and
there is a lack of standardisation for the testing environment, patient instructions
and permissible level and number of artefacts. These should be specified on the
basis of current knowledge and they should be published, to enable inter-study
comparisons and a further refinement of ASSR techniques. In the case of an
unco-operative client it is recommend that the clinician be placed in the same
room as the subject to prevent deliberate movement and coughing. Sedation
might also still be needed in a subject who refuses to co-operate.
Apart from the electrophysiological noises influencing threshold estimation, one
needs to ask whether limitations of the equipment used also impeded the study.
Multiple-frequency methods would not be practical in the mining industry, since
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only eight out of the ten thresholds required can be covered in a single test run.
That means that a MF-technique must be repeated to get all the required
thresholds and thereby a lengthy procedure is prolonged even further.
Finally, in the planning of the research it was deemed important to select
equipment that can test all the frequencies specified by current South African
legislation. The Audera equipment that was available during the initial phases of
the experimental research could not test 3 000 Hz which is problematic since this
frequency is legally required (Workmen’s Compensation, 1995).
It is thus invisaged that as equipment improve in reaction to further research that
ASSR method will gain acceptance into standard audiological procedures. In the
words of Roeser et al. (2000b:p10):
Auditory evoked responses have been used in diagnostic
audiology for more than 3 decades, and as more knowledge is
being made available in this area, it is clear that auditory
electrophysiological measures will become an even more
prominent tool in audiology in future.
7.3.4 THE DIFFERENCE BETWEEN ASSR AND PURE-TONE THRESHOLDS
AT 500 HZ
The ASSR test has, as was the case with other researchers, proven to be less
accurate at 500 Hz (Rickards et al., 1994).
It is important for clinicians to
remember this fact when testing patients and to be aware that using 5 dB
increments and decrements in testing might improve the threshold estimates’
accuracy.
Clinicians should also do everything in their power to limit background noise
since background noise can influence threshold estimates at 500 Hz. The most
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important requirement is certainly to test subjects in a calibrated sound proof
environment.
7.3.5 INTERVALS OF TEN dB IN ASSR THRESHOLD ESTIMATION
In the above paragraph it was mentioned that 5 dB increments could improve the
accuracy of ASSR techniques. It must nevertheless be remembered that 10 dB
threshold estimation techniques were proven to supply threshold estimates that
were within the required 10 dB variance.
This finding gives a clinician the
freedom to assess a clinical situation and to choose the decrements accordingly
depending on the time constraints and the patient’s needs.
7.3.6 THE AUDERA SYSTEM
The fact that the Audera system (Biologic Systems Corporation, 2002) had a test
procedure in place where the number of sweeps and averages were controlled
by computer algorithms made the use of this equipment more objective (neither
the audiologist nor the patient do decide on the results). It is very important that
ASSR testing should be an objective procedure, particularly where clinicians may
be inexperienced and in working with a population where thresholds are used to
calculate the compensation to be paid out. It is understandable that the more
objective and accurate these thresholds are, the less chance there is of
distributing the available monetary resources incorrectly. This objectivity is also a
very important finding in a population that is traditionally unco-operative.
A problem with the Biologic system was that the clinician could select how many
averages and sweeps to use (which is obviously important in research settings).
In this specified population these parameters should be held constant at the
researched best parameters in order to ensure objectivity and to be able to
compare different research endeavours.
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7.3.7 THE SF-MONOTIC TECHNIQUE
The conclusion is that SF-ASSR procedures proved to be the method of choice
for pseudohypacusic patients with noise-induced hearing loss, due to the
robustness of the 40 Hz response and the fact that the procedure eliminated any
need to expose subjects to high-intensity stimulation at the low test frequencies.
In conjunction with this, it was also found that the SF-testing also provided for
manual control of the stimulus intensity, allowing the intensity level to be adjusted
where a response could not be obtained, as with conventional pure-tone tests.
The SF-technique could thus prevent high intensity stimulation in high
frequencies from influencing the thresholds at other frequencies. This was not
possible with the MF-procedure (Picton, et al., 2003).
7.3.8 THE 40 Hz MODULATION
Since most of the members of difficult-to-test populations are usually infants,
there has there been a tendency for researchers to move away from the 40 Hz
response (Rance et al., 1995; Herdman & Stapells, 2001; John et al., 2002).
However, the present research seems to indicate that this stimulation rate was
still the best to use in an adult population and thus confirms the opinion of
Galambos et al. (1981) and Dobie and Wilson (1998) in this regard.
7.3.9 SEDATION
The fact that sedation did not improve or have a negative effect on the sensitivity
or reduce the test time of ASSR methods leads to the conclusion that there is
limited justification using it when passive co-operation can be obtained from a
patient. This was a welcome result, in view of the ethical and medical constraints
with regard to the use of sedation.
On the other hand particularly in the case of pseudohypacusic mine workers from
whom passive co-operation cannot be obtained, it is reassuring that sedation has
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been seen not to affect the ASSR thresholds and time negatively and thus
sedation can still be used if electrophysiological noise is found to affect the
results.
7.3.10 CONCLUSION OF AUDIOLOGICAL PROCEDURES
It has previously been shown that there is a very high incidence of
pseudohypacusis in the South African mining industry (De Koker, 2003). The
lack of cooperation from workers leads to a high case load of unresolved cases.
The fact that these cases could have been diagnosed and completed with the
use of ASSR methods leads one to conclude that it would be unethical not to use
this tool available to audiologists to resolve pending cases. The audiologist, as a
professional, is obliged to give an accountable service.
The one unconcluded case in the present study (one ear) was due to
electrophysiological noise and therefore the limiting of factors influencing the data
is an ongoing clinical and research concern.
7.3.11 PSEUDOHYPACUSIC WORKERS HAD HEARING LOSS
In the past it was common practice that pseudohypacusic workers would be
counselled and re-tested at a later date. The fact that such a high percentage of
pseudohypacusic workers tested had a true basic hearing loss emphasises the
need to raise the awareness with audiologists that it is not acceptable to
reschedule a population with pathology for numerous tests over many years
without diagnosing the degree and cause of the hearing loss (Roeser et al.,
2000b) and without making the correct recommendations with regard to
amplification, fitness and compensability (De Koker, 2003).
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7.3.12 PSEUDOHYPACUSIC WORKERS WITH HEARING LOSS WERE NOT
NECESSARILY COMPENSABLE
In the mining industry workers are mainly referred since their hearing loss is
potentially compensable. This leads to a specific focus from the side of the
audiologists and other health workers in this sector that is problematic and should
change.
This study has proven that less than half of the pseudohypacusic
workers with hearing loss were compensable. However one can only conclude
that ethical audiologists should remember their role in differential diagnosis
(Roeser et al., 2000b) and that this population, like any other population, also
suffers from other types of hearing loss (not only noise-induced).
Certainly
pseudohypacusic workers do form part of the client base of audiologists and
deserves the best the profession of audiology can offer.
7.3.13 UNFITNESS OF PSEUDOHYPACUSIC WORKERS
In the past the fact that pseudohypacusic cases stayed pending led to the
possibility that workers unfit for duty could pose a risk to fellow workers due to
their unability to hear danger signals. An unfortunate worker with serious noiseinduced hearing loss could also have been exposed indefinitely due to the fact
that accurate hearing thresholds were outstanding.
This possibility can be
eliminated by the use of ASSR techniques.
7.3.14 ASSR THRESHOLDS DID NOT CORRELATE WELL WITH PREVIOUS
SCREENING TESTS
In the past the only tools the audiologist had to resolve a long-standing pending
case was to do an ABR test or to study previous screening tests. If an ABR
could not be done due to monetary constraints, the previous screening tests were
the only guideline to base recommendations on.
This study has shown the
danger of rescheduling pseudohypacusic workers for annual testing if thresholds
could not be obtained. It has also been proven that previous screening tests
were not a good indicator to use as a basis for recommendations if hearing
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thresholds were outstanding. It is thus concluded that every effort should be
made to resolve and diagnose pending cases.
7.3.15 PREVALENCE OF SUDDEN HEARING LOSS
A worrying finding was that there was evidence that 30,1 per cent of the
pseudohypacusic subjects have experienced a sudden deterioration in hearing.
This deterioration would not have been diagnosed without determining or
estimating the true pure-tone thresholds.
Again the importance of resolving
outstanding cases is highlighted with this finding.
7.3.16 ASSR-AN IMPORTANT CONTRIBUTION TO THE MINING INDUSTRY
ASSR methods are more costly than conventional pure-tone tests, but ASSRs
can save the mining industry a lot in terms of cost of lost production, transport,
referrals and overcompensation. The well-being of the individual worker and his
co-workers are promoted in that deserving compensation cases are diagnosed,
sudden deterioration in hearing is identified and a worker is notified when he is
not fit to work in a noisy environment any longer. ASSR tests can also limit the
financial impact of overcompensation and unresolved claims.
Based on the theoretical and empirical results of this study an additional situation
analysis pertaining to the financial implications of ASSR methods was executed.
The specific details are set out in Appendix Q, but the most important results are:
•
It is impossible to know how much pseudohypacusis has cost the industry
(Begley, 2003) but it can be unequivocally stated that pseudohypacusis
has got financial implications due to lost production and shifts, transport
costs, specialist referrals and overcompensation. It is thought to be
substantial;
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•
On information received it was found that an ASSR system can at present
cost as much as R154 000,00 (HASS, December, 2003). ASSR systems
are thus more expensive than conventional audiometrical equipment;
•
However,
when
comparing
costs
of
overcompensation,
transport
arrangements, specialist referrals, numerous hearing tests and lost
production it is clear that ASSR testing will save the industry.
7.3.17 LENGTHY PROCEDURE
The procedure has proven to be a lengthy one in comparison to conventional
testing (approximately 60 minutes including preparation time, compared with the
17 minutes typically required for pure-tone audiometry, otoscopic examination
and immittance measurements). In the field of auditory evoked potentials it is
nevertheless an acceptable time frame.
The length of the procedure is a
negative point to consider with the general high case loads found in the mining
industry.
It must nevertheless be remembered that numerous testing of
pseudohypacusic workers with conventional tests are also time consuming.
7.4
LIMITATIONS OF THE RESEARCH
The following limitations were experienced in the current research and it is
recommended that these issues be addressed in forthcoming research:
•
The MF-ASSR (dichotic) procedure had no algorithms (at this point in
time) to compensate for the greater hearing thresholds at higher
frequencies typical of sensory neural hearing loss. This lead to the fact
that the lower frequencies’ thresholds were influenced by the high intensity
high frequency stimulation.
•
Standards are required for the number of sweeps and averages needed to
ensure accuracy. In a clinical situation the extent of averaging should be
determined by appropriately formulated algorithms and not by the clinician,
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to ensure objectivity.
The lack of standardisation makes it difficult to
compare studies. In the present study, the amount of averaging was
controlled in the SF-method but was left to the clinician’s discretion in the
MF- procedure.
•
In selecting experimental subjects with noise-induced hearing loss, it was
stated that subjects had sensory neural hearing loss. The “neural” aspect
was not investigated to determine, for example any influence of retrocochlear damage. It has recently been proven that patients with Human
Immunodeficiency Virus (HIV) do experience retro-cochlear deterioration
(Chandrasekhar et al., 2000). As this condition is currently an African
pandemic it is reasonable to recommend that a clinical study be done
using ASSR methods in combination with click ABR testing, to allow
differential diagnoses which are not possible with ASSRs alone.
•
The present study makes no mention of the possible influence of the HIV
on the study results. HIV, the causative agent of Acquired Immunodeficiency Syndrome (AIDS), is associated with the development of
opportunistic infections and central nervous system disorders known to
induce hearing impairment. In addition, a large percentage of patients in
the mining industry are also treated with various combinations of ototoxic
drugs for the treatment of tuberculosis and HIV-related manifestations.
This points to a need for an investigation of the contribution of HIV to
hearing problems among mine workers and how a differential diagnosis of
multi-factor hearing loss can be made.
Since all evoked responses,
including ABRs, are highly dependent on the temporal synchronisation of
neural activity, it is reasonable to expect alterations in ABR and ASSRs
among patients with varying degrees of HIV infection. The preceding point
raises the question of the extent to which noise-induced hearing loss
compensation is affected by audiological changes due to HIV infection or
its complications.
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•
The present study did not evaluate the accuracy of late cortical evoked
responses (CER) in estimating hearing thresholds for pseudohypacusic
patients. This procedure has been used in Australia for more than 20
years (Rickards and De Vidi, 1995) but at the time of the research,
equipment was not readily available in South Africa.
Apart from the
unavailability of the equipment did the skill and knowledge required of the
clinician in executing CER methods discourage the researcher. However
future investigations could well be directed at evaluating this method of
audiological assessment.
7.5
THE STUDY IN CONTEXT
In conclusion, ASSR testing offers an objective and accurate means of
determining hearing thresholds for pseudohypacusic mine workers.
ASSR
testing also offers the option of the use of complex stimuli for threshold
estimation, thereby stimulating the auditory system in a manner that is more
representative of the way in which the hearing sense functions, in comparison to,
for instance pure-tone clicks and tone bursts (Picton et al., 2003).
With this study a contribution was made to the field of Audiology in that limited
clinical validation of ASSR methods has been extended. This procedure had not
previously been tested on mine workers with noise-induced hearing loss and no
other study could be found where ASSRs had been used in a pseudohypacusic
population. The present research has also made a contribution to the scientific
body of knowledge in the South African mining industry and has contributed to
the setting of international standards in audiological assessments in the industry.
Current research can now be implemented in the industry and a contribution has
been made to a best practice procedure for evaluating noise-induced hearing
loss.
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“As expected, in the 50 years the profession of audiology has been in existence
the scope of practice has grown. This metamorphosis has occurred gradually as
a result of emerging clinical, technological, and scientific developments, which
are now commonplace in our modern world.
Whereas only 25 years ago
audiologists were primarily performing behavioural tests of auditory function,
today the typical audiologist has a wide range of electrophysiological assessment
tool to select from” (Roeser et al., 2000b: p1).
The audiologist should use these tools in the interest of the patient, the client and
the profession and in a wider sense also in the interest of South Africa as a
whole.
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APPENDIX A
INFORMED CONSENT
Informed consent Simhealth 02 07 01: Background information and request for
consent of workers asked to participate
This form is to be administered to selected workers before their participation in
audiometric and ASSR testing by the audiologists.
Read the following to each prospective subject, pausing to answer any questions:
This mine has agreed to help the SIMRAC research team investigate how
certain hearing tests might be helpful in identifying noise-induced hearing loss.
Information from the study will be used to decide if changes can be made to
normal testing procedures that will allow better identification of hearing losses
caused by noise, in order to improve workers’ health and safety. The study
has been approved by the Union, because all of the workers who agree to
participate will remain unanimous, and the results will be used to help protect
workers from noise.
If you agree to participate, we will ask you some questions about you, any
problems that you might have experienced with your hearing, your job and the
noise in places on the mine where you work.
Your hearing will then be tested in the normal way, after which some special
tests will be used to check your hearing. Comparisons will be made between
results from the normal tests and from the special tests, to find out if the
special tests would be better for identifying and describing hearing loss
caused by noise.
The experiment is not meant to check your hearing, but to find out the best way
of testing ears. Accordingly, the tests and the results will have no effect on
your job, and will have nothing to do with compensation. Your test results will
be kept confidential, and only you and the research team will be able to look at
them. The results will be used to find out if the new tests are helpful in the
correct description and identification of noise-induced hearing loss.
185
University of Pretoria etd - De Koker, E (2004)
We will explain to you the way each test is done, we will show you the results
and we will explain what they mean. Some of the tests will be done more than
once, to double-check on the results.
We will keep your name and any information you tell us in strict confidence,
and not tell the mine or the managers anything about you or your test results.
Your participation in the study is voluntary. If you do not want to take part, it
will not affect your job in any way. If you do decide to take part, this will also
not affect your job in any way, but will be helpful to all workers who are
working in the noise. We ask that you decide for yourself whether you want to
participate, and if you have some questions that need to be answered before
you decide, please ask them.
Will you help us with this research? (YES or NO)
If NO, ask the next worker. If YES, ask worker to sign or make a mark in the
space below to indicate that he has been given the information and
understands it. Then record the other details.
…………………………………………………………………………………………………
I have been told about the study and have been given the chance to ask
questions about it and about my participation. I also understand that if I have
any questions at any time, they will be answered, and that if I am not satisfied
with the answers I can withdraw from the study.
Name:……………………………….. Company number: ……………………….
Date:………………………..
186
University of Pretoria etd - De Koker, E (2004)
APPENDIX B
CONSENT-VALIUM
Informed consent Simhealth 02 07 01: Background information and request for
consent of workers asked to participate.
This form is to be administered to selected workers before their participation in
audiometric and ASSR testing by the audiologists.
Read the following to each prospective subject, pausing to answer any questions:
This mine has agreed to help the SIMRAC research team investigate how
certain hearing tests might be helpful in identifying noise-induced hearing loss.
Information from the study will be used to decide if changes can be made to
normal testing procedures that will allow better identification of hearing losses
caused by noise, in order to improve workers’ health and safety. The study
has been approved by the Union, because all of the workers who agree to
participate will remain unanimous, and the results will be used to help protect
workers from noise.
If you agree to participate, we will ask you some questions about you, any
problems that you might have experienced with your hearing, your job and the
noise in places on the mine where you work.
Your hearing will then be tested in the normal way, after which some special
tests will be used to check your hearing. Comparisons will be made between
results from the normal tests and from the special tests, to find out if the
special tests would be better for identifying and describing hearing loss
caused by noise.
The experiment is not meant to check your hearing, but to find out the best way
of testing ears. Accordingly, the tests and the results will have no effect on
your job, and will have nothing to do with compensation. Your test results will
be kept confidential, and only you and the research team will be able to look at
them. The results will be used to find out if the new tests are helpful in the
correct description and identification of noise-induced hearing loss.
187
University of Pretoria etd - De Koker, E (2004)
We will explain to you the way each test is done, we will show you the results
and we will explain what they mean. Some of the tests will be done more than
once, to double-check on the results.
We will keep your name and any information you tell us in strict confidence,
and not tell the mine or the managers anything about you or your test results.
Your participation in the study is voluntary. If you do not want to take part, it
will not affect your job in any way. If you do decide to take part, this will also
not affect your job in any way, but will be helpful to all workers who are
working in the noise. We ask that you decide for yourself whether you want to
participate, and if you have some questions that need to be answered before
you decide, please ask them.
I agree to taking medicine (10mg of Valium) to help me relax
during the test
Will you help us with this research? (YES or NO)
If NO, ask the next worker. If YES, ask worker to sign or make a mark in the
space below to indicate that he has been given the information and
understands it. Then record the other details.
…………………………………………………………………………………………………
I have been told about the study and have been given the chance to ask
questions about it and about my participation. I also understand that if I have
any questions at any time, they will be answered, and that if I am not satisfied
with the answers I can withdraw from the study.
Name:……………………………….. Company number: ………………………….
Date:……………………….
188
University of Pretoria etd - De Koker, E (2004)
Patient information sheet
ASSR tests: VALIUM
1. Thank you for agreeing to participate in this study.
2. The medicine that you have agreed to take will make you feel
sleepy/drowsy. There is a bed available where you can lie down.
3. During the test you will also lie down and be able to sleep/rest. The test
will take an hour.
4. After completion of the test you will be transported back to your hostel
5. Please refrain from driving a car. Remain at the hostel for the duration of
today. Do sleep or rest.
6. You are not required to work today and will receive a shift.
189
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
APPENDIX G
CASE HISTORY (RESEARCH QUESTIONNAIRE)
ASSR
Research questionnaire
Ind no.
:
____________________________________________
Study no.
:
__________________________________________________
Date
:
__________________________________________________
Date of birth
:
__________________________________________________
Mine
:
__________________________________________________
Audiologist
:
__________________________________________________
1.
Otoscopy
Landmarks
L
Cone of light
_______
Tympanic membrane
R
L
R
Cerumen
L
R
Occluding
________
Minimal
________
Excessive
________
None
________
External canal
L
R
Normal
_______
Normal
________
Dull
_______
Red/Swollen
________
Perforated
_______
Foreign body
________
Scarring
_______
Growth
________
Drainage
________
Blood
________
Collapsed
________
194
University of Pretoria etd - De Koker, E (2004)
2.
Immittance measurements
L
3.
R
Compliance
_______
Volume
_______
Pressure
_______
Reflex ipsilateral 1000 Hz
_______
Specific ear and medical history
Head injuries:
Blow to head, accidents
Ear operations:
___________________________________________________________________
Injury to ears:
Blood draining from ear
___________________________________________________________________
Barotrauma:
-
Medical history
-
Air from ear when blowing nose
___________________________________________________________________
Middle ear pathology:
Ear infections
-
Pain
-
Discharge
___________________________________________________________________
Ototoxic drugs:
-
TB
-
Malaria
-
Intensive care
___________________________________________________________________
Job history:
-
Years underground
-
Job description
195
University of Pretoria etd - De Koker, E (2004)
4.
Diagnostic audiogram:
Attach copy of diagnostic audiogram
5.
ASSR estimated audiogram
Attach copy of printout
Time taken to complete ASSR test_____________________________________
Type ASSR test ___________________________________________________
6.
Comparison of pure-tone and ASSR threshold
Left ear
Right ear
KHz
KHz
Description
Date
.5
KHz
1
ASSR threshold
Pure tone-threshold
196
2
3
4
.5
1
2
3
4
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
University of Pretoria etd - De Koker, E (2004)
APPENDIX N
RAW DATA: ASSR AND PURE TONE THRESHOLDS
(Phase 1) (n=81)
Data
Simhealth
02 07 01
Machine
Audera 1
Test
Protocol
Sedation
Asleep
no
Asleep
no
Asleep
no
Asleep
no
Asleep
no
ASSR
Asleep
no
PT
Asleep
PT threshold
ASSR
ditto
PT threshold
ASSR
ditto
PT
ASSR
ditto
PT
ASSR
ditto
PT
ASSR
ditti
ditto
LEFT EAR
PT
ASSR
"
Pt
no
Asleep
ASSR
"
PT
no
Asleep
ASSR
"
PT
no
Asleep
ASSR
"
no
PT
ASSR
"
Asleep
no
Pt
ASSR
Machine
Audera 1
Test
ditto
ASSR
ASSR
ASSR
ASSR
ASSR
60
80
60
55
65
40
.
.
.
.
.
.
20
45
55
50
50
20
.
.
.
.
.
15
35
45
50
35
30
30
.
.
.
.
.
.
10
50
55
55
60
10
.
.
.
.
.
55
1000
Hz
2000
Hz
3000
Hz
4000
Hz
70
60
55
65
60
.
45
40
40
45
2
35
35
.
50
2
40
40
35
30
3
30
.
.
45
3
45
50
60
50
4
50
65
.
60
4
Subject
Time
70
1
min.
.
1
40
15
25
45
55
65
20
20
25
50
60
5
5
20
45
.
75
.
.
.
.
.
5
5
5
40
65
80
5
10
35
70
70
6
.
20
55
.
80
.
.
.
.
.
6
15
40
50
75
75
25
30
30
50
55
7
.
.
.
.
.
.
.
.
.
.
7
15
25
30
35
35
15
30
35
30
30
8
20
.
45
.
.
.
.
.
.
.
8
30
45
45
50
50
25
40
40
45
50
9
.
.
.
.
.
40
40
50
.
50
9
20
20
20
30
35
15
25
20
30
30
10
.
.
.
.
.
.
.
.
.
.
10
30
45
35
40
45
30
40
45
35
45
11
42
12
40
52
18
10
38
23
46
.
.
.
.
.
.
50
.
.
.
11
35
40
35
35
40
30
30
20
40
45
12
.
.
.
55
12
.
4000
Hz
500
Hz
4000
Hz
Subject
Time
1000
Hz
10
15
50
55
55
10
15
50
65
50
13
Awake
no
10
25
60
.
50
10
20
60
.
60
13
40
45
40
70
75
30
45
35
45
55
14
Awake
no
.
.
.
.
.
45
50
45
.
70
14
20
45
70
80
70
10
35
60
60
55
15
Awake
no
.
60
.
.
60
20
60
80
.
70
15
25
35
45
50
50
20
40
45
50
55
16
15
30
45
.
50
15
45
50
.
55
16
15
35
40
45
40
5
25
40
45
45
17
17
70
18
66
19
51
Awake
no
Awake
no
Awake
no
Awake
no
.
.
LEFT
EAR
2000 3000
Hz
Hz
.
.
RIGHT
EAR
1000 2000 3000
Hz
Hz
Hz
.
500
Hz
PT
"
500
Hz
Sedation
PT
"
4000
Hz
Protocol
PT
"
3000
Hz
.
PT
"
2000
Hz
.
PT
ditto
1000
Hz
no
PT
ASSR
500
Hz
Asleep
PT
ASSR
RIGHT EAR
5
35
45
.
40
5
35
45
.
40
25
40
35
30
40
25
40
25
35
35
0
40
20
.
30
0
25
15
.
30
10
30
45
35
35
5
35
45
35
35
.
.
.
.
.
0
35
55
.
25
205
48
59
60
68
University of Pretoria etd - De Koker, E (2004)
Machine
Test
Protocol
Sedation
PT
"
ASSR
Awake
no
PT
"
ASSR
Awake
no
PT
"
ASSR
Awake
no
PT
"
ASSR
Awake
no
PT
"
ASSR
Awake
no
PT
"
ASSR
Awake
no
PT
"
ASSR
ASSR
ASSR
Machine
Test
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
52
22
75
23
35
24
75
25
48
26
63
27
81
55
28
32
4000
Hz
Subject
Time
60
29
.
45
15
35
45
.
40
35
25
40
20
50
55
55
50
10
40
45
.
55
20
55
60
.
60
40
50
55
65
70
25
50
50
60
65
35
60
60
.
.
.
.
.
.
.
10
30
45
50
50
30
25
40
45
65
0
35
50
.
55
20
30
50
.
.
50
50
40
15
25
50
50
40
20
15
55
60
.
.
30
50
60
.
.
15
20
25
45
35
40
5
15
25
25
30
15
30
50
.
45
0
20
30
.
25
30
45
50
55
55
30
50
50
60
60
50
50
.
60
20
55
55
.
60
20
35
45
40
45
30
40
60
.
45
35
40
35
35
40
30
30
20
40
Awake
no
.
.
.
.
.
Protocol
Sedation
500
Hz
1000
Hz
4000
Hz
500
Hz
45
55
55
50
35
50
50
60
.
.
10
30
40
50
40
35
40
40
50
40
40
35
25
45
40
40
45
20
30
35
35
25
40
40
.
10
30
50
55
Master
no
ditto
no
ditto
no
ditto
no
ditto
no
ditto
no
ditto
no
ditto
no
.
.
LEFT
EAR
2000 3000
Hz
Hz
50
.
.
RIGHT
EAR
1000 2000 3000
Hz
Hz
Hz
55
45
60
55
45
.
60
.
.
15
15
35
45
30
40
40
40
.
.
.
.
25
45
55
50
50
50
31
.
.
60
35
45
.
25
15
30
40
20
25
20
40
35
30
.
35
65
10
20
50
50
65
20
.
.
.
.
25
40
.
.
65
5
15
50
45
45
5
20
35
40
55
.
15
.
.
55
.
15
35
.
55
30
40
35
30
25
30
25
35
30
40
20
25
45
45
50
20
.
45
35
30
10
45
60
45
50
5
45
60
55
60
35
50
50
45
50
30
30
55
50
70
15
25
55
60
80
5
5
35
50
80
30
65
.
85
30
30
60
.
70
ditto
no
30
30
40
60
65
65
20
25
30
45
55
ditto
no
40
45
55
.
.
40
.
30
.
.
20
25
30
55
65
15
25
35
60
90
ditto
no
.
25
40
.
.
20
40
30
.
.
30
40
50
50
55
25
30
50
50
50
ditto
no
40
45
45
50
55
50
25
45
30
65
30
45
40
50
35
30
35
30
30
45
ditto
no
40
35
45
60
30
40
40
35
40
55
15
25
45
45
50
20
30
45
55
55
ditto
no
40
45
45
60
55
40
35
55
50
.
PT
"
21
50
45
45
PT
"
67
45
60
PT
"
20
0
15
.
PT
"
Time
40
40
PT
"
Subject
40
35
PT
"
35
55
PT
"
20
35
PT
"
10
45
PT
"
40
20
PT
"
45
30
PT
"
45
4000
Hz
no
PT
ditto
35
500
Hz
Awake
PT
ditto
15
4000
Hz
30
PT
Biologic
1000
Hz
no
PT
"
500
Hz
RIGHT
EAR
1000 2000 3000
Hz
Hz
Hz
Awake
PT
"
LEFT
EAR
2000 3000
Hz
Hz
206
87
30
86
77
32
120
33
87
34
80
35
45
36
91
37
120
38
114
39
106
40
90
41
60
42
70
University of Pretoria etd - De Koker, E (2004)
Machine
Test
Protocol
Sedation
500
Hz
15
20
ditto
no
10
20
45
65
65
70
75
45
ditto
no
50
65
65
.
70
50
0
30
50
60
55
0
50
30
.
.
70
10
45
45
50
50
10
45
50
50
25
20
35
55
30
60
60
.
30
45
40
50
PT
"
ASSR
PT
"
ASSR
PT
"
ASSR
ditto
no
PT
"
ASSR
ditto
no
PT
"
ASSR
ditto
no
PT
"
ASSR
Machine
Audera 2
Test
ASSR
ditto
ASSR
ditto
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
Machine
Biologic
Test
ASSR
ASSR
PT
.
55
55
65
75
45
60
.
.
20
30
65
50
55
45
40
.
.
10
35
45
55
40
45
5
25
50
40
45
60
20
50
50
45
55
70
.
60
60
60
.
40
30
45
40
35
35
50
50
45
4000
Hz
500
Hz
75
44
40
45
75
46
65
47
50
35
48
68
4000
Hz
Subject
Time
40
45
10
15
40
40
45
50
40
0
20
45
45
50
30
55
60
60
60
35
50
50
60
65
awake
yes
35
55
60
60
55
30
55
55
50
60
15
20
35
40
40
15
30
35
45
50
awake
yes
30
40
55
.
.
.
.
.
.
.
awake
yes
awake
yes
15
65
75
70
65
30
65
awake
yes
10
65
70
70
75
30
65
5
20
30
35
40
20
30
awake
yes
awake
yes
awake
yes
awake
yes
awake
yes
awake
yes
25
55
RIGHT
EAR
1000 2000 3000
Hz
Hz
Hz
43
30
45
51
15
52
.
53
85
54
60
55
45
56
60
57
58
58
62
59
50
60
55
30
61
70
4000
Hz
Subject
Time
62
130
63
99
64
80
10
20
35
55
50
15
20
35
45
55
0
30
50
60
50
15
15
50
40
50
20
50
50
50
55
30
50
45
45
55
.
50
.
.
.
20
45
45
40
45
75
75
80
85
80
.
50
55
50
0
20
40
40
40
35
40
50
45
50
15
40
45
55
50
15
45
55
50
50
15
55
55
50
50
15
50
65
60
60
25
45
55
60
65
25
35
55
65
65
30
55
55
60
60
15
35
55
55
55
5
15
15
85
85
15
20
10
75
85
10
30
.
100
105
20
30
45
.
105
30
35
30
50
60
30
45
45
55
50
35
55
65
65
65
40
55
55
55
55
10
30
45
45
45
10
30
40
50
40
15
40
55
50
45
0
35
55
60
45
20
40
45
50
40
5
15
35
40
30
40
0
15
4000
Hz
500
Hz
yes
0
45
Protocol
Sedation
500
Hz
1000
Hz
20
40
60
60
55
10
30
55
55
40
Master
yes
30
40
60
.
60
25
50
.
.
.
35
40
55
55
75
25
40
35
40
65
Master
yes
30
30
30
.
75
10
45
30
.
55
20
40
45
55
50
10
35
40
45
55
30
50
40
.
55
30
40
50
.
60
15
50
50
45
50
20
45
50
45
55
Master
yes
40
25
LEFT
EAR
2000 3000
Hz
Hz
207
40
25
RIGHT
EAR
1000 2000 3000
Hz
Hz
Hz
49
50
awake
PT
"
65
.
40
PT
ditto
60
10
PT
ASSR
30
40
15
PT
"
25
35
5
PT
"
20
0
PT
"
10
.
Time
yes
PT
"
45
Subject
awake
PT
"
4000
Hz
1000
Hz
PT
"
3000
Hz
500
Hz
PT
ASSR
2000
Hz
Sedation
PT
ditto
45
55
1000
Hz
Protocol
PT
ditto
50
60
55
.
LEFT
EAR
2000 3000
Hz
Hz
500
Hz
60
PT
ASSR
4000
Hz
55
PT
"
3000
Hz
no
PT
ditto
2000
Hz
ditto
PT
ASSR
1000
Hz
University of Pretoria etd - De Koker, E (2004)
Machine
"
Test
ASSR
Protocol
Sedation
500
Hz
1000
Hz
Master
yes
45
45
45
15
25
75
.
75
.
PT
"
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
ASSR
65
85
45
.
30
50
.
.
.
66
129
67
85
68
90
69
80
70
85
71
80
72
90
73
85
74
114
75
80
76
70
77
66
78
70
79
145
80
72
81
75
60
25
50
55
60
60
30
50
50
.
50
15
40
45
40
40
25
40
45
45
50
Master
yes
30
40
40
.
.
50
50
40
.
50
25
45
45
50
35
30
45
45
50
45
Master
yes
35
45
45
.
35
.
45
50
.
45
0
25
40
50
60
5
20
35
50
65
Master
yes
10
30
50
.
75
20
30
50
.
75
15
20
40
30
25
15
25
45
40
40
Master
yes
10
20
20
.
40
20
30
30
.
40
30
40
40
40
50
25
35
55
65
60
Master
yes
30
40
40
.
60
50
40
60
.
60
25
40
35
35
40
30
40
40
35
40
Master
yes
30
35
30
.
40
35
40
35
.
50
30
40
50
50
60
25
30
50
55
65
40
40
50
.
50
40
40
45
.
50
20
45
55
50
50
20
45
45
45
55
40
50
60
.
.
50
50
60
.
.
15
40
50
50
50
20
40
45
40
50
10
40
40
.
40
10
40
45
.
50
30
30
35
55
50
20
50
60
55
65
10
20
40
.
50
40
40
60
.
60
20
40
45
30
30
20
45
35
35
20
25
45
25
.
35
20
50
25
.
20
30
40
45
40
45
30
40
45
40
45
40
40
30
.
45
40
40
40
.
40
10
35
55
45
35
10
25
40
40
45
25
35
55
.
.
25
15
45
.
.
30
60
50
50
45
35
60
55
50
50
40
50
50
.
50
50
50
50
.
60
Master
yes
Master
yes
Master
yes
Master
yes
Master
yes
Master
yes
Master
yes
PT
"
55
60
PT
"
.
45
.
PT
"
55
60
PT
"
45
25
50
PT
"
45
10
40
PT
"
45
75
55
PT
"
45
40
PT
"
.
75
20
PT
"
time
20
PT
"
Subject
yes
PT
"
4000
Hz
Master
PT
"
500
Hz
20
PT
"
4000
Hz
yes
PT
"
RIGHT
EAR
1000 2000 3000
Hz
Hz
Hz
Master
PT
"
LEFT
EAR
2000 3000
Hz
Hz
Master
yes
208
University of Pretoria etd - De Koker, E (2004)
APPENDIX O
RAW DATA: PSEUDOHYPACUSIC GROUP
PURE TONE AND ASSR THRESHOLDS (n=29)
Machine
Audera
Test
PT 1
Protocol Sedation
awake
no
PT2
Audera
Biologic
50
70
65
85
.
.
.
.
.
.
.
.
30
45
45
.
.
ASSR
10
15
25
.
20
0
30
15
.
30
PT1
awake
no
.
.
.
.
.
.
.
.
.
.
95
95
90
95
90
95
95
95
95
95
Diagnostic
.
105
110
110
.
.
110
110
.
.
ASSR
.
.
.
.
.
65
80
65
.
70
.
.
.
.
.
.
.
.
.
.
PT 1
awake
no
PT 2
55
60
80
90
95
55
60
55
55
60
Diagnostic
65
65
80
95
90
50
50
50
50
60
ASSR
35
60
65
.
95
30
50
50
.
60
PT 1
Master
yes
PT 1
awake
yes
.
.
.
.
.
.
.
.
.
.
70
75
90
90
95
90
95
95
95
95
.
.
110
.
.
105
110
.
.
.
25
30
30
.
40
15
30
40
.
40
25
20
40
20
40
30
10
20
10
25
PT 2
40
55
55
50
55
5
10
20
10
35
Diagnostic
100
100
100
.
.
100
85
100
.
.
30
35
50
60
55
15
10
25
40
30
10
30
50
55
45
25
30
65
60
55
PT 1
awake
yes
.
.
.
.
.
.
.
.
.
.
110
110
110
90
100
110
110
110
.
.
0
30
50
50
45
0
30
55
35
60
25
10
35
40
45
25
20
40
40
35
PT 2
40
40
60
60
90
40
50
60
60
100
Diagnostic
80
75
90
100
105
110
105
105 110
110
ASSR
0
30
40
45
50
10
20
70
70
30
25
20
10
60
80
70
55
45
55
70
0
0
PT 1
PT 1
awake
awake
yes
yes
PT 2
80
80
100
0
0
0
0
Diagnostic
70
80
80
90
110
105
110
110 110
110
ASSR
PT 1
awake
yes
55
55
55
100
100
55
85
110 105
110
25
20
40
30
50
45
60
25
55
45
100
PT 2
50
90
95
95
95
90
95
95
90
85
70
95
.
.
110
110
110
.
.
ASSR
0
10
20
15
50
30
25
50
50
80
25
35
25
10
25
20
30
30
20
20
awake
yes
PT 2
45
60
70
60
70
60
70
70
65
70
Diagnostic
90
90
95
105
110
105
100
100 110
110
.
.
.
.
.
15
35
25
35
30
ASSR
PT 1
awake
yes
15
5
15
20
15
15
15
15
25
15
PT 2
50
60
65
85
90
40
20
40
25
30
Diagnostic
70
70
80
.
.
45
60
.
.
.
ASSR
50
70
90
100
120
15
25
20
.
.
209
prep
test
1
8
60
2
8
62
3
.
45
4
.
55
5
7
32
6
.
75
7
.
45
8
.
53
9
.
46
10
.
62
11
.
73
0
Diagnostic
PT 1
no.
85
.
ASSR
Audera
90
.
PT 2
Audera
95
Subject Time Time
.
Diagnostic
Audera
90
Hz
1000 2000 3000 4000
.
ASSR
Audera
75
500
60
ASSR
Audera
75
Right
2000 3000 4000
.
Diagnostic
Auders
1000
40
PT 2
Audera
Hz
500
Diagnostic
PT 2
Audera
Left
University of Pretoria etd - De Koker, E (2004)
Machine
Audera
Audera
Audera
Audera
Audera
Test
PT 1
Protocol Sedation
awake
yes
Audera
Audera
20
30
5
Subject Time Time
0
0
20
5
20
35
35
10
10
25
10
95
110
95
90
90
110 110
55
ASSR
65
75
70
80
80
15
25
55
70
70
PT 1
15
20
10
10
30
10
15
15
20
30
PT 2
50
70
65
65
85
45
60
65
70
80
Diagnostic
95
100
100
110
.
95
110
110
.
.
ASSR
10
40
55
70
80
15
35
75
75
80
5
45
50
60
55
10
40
45
60
55
PT 1
awake
awake
yes
yes
PT 2
25
55
55
65
70
20
55
55
70
60
Diagnostic
110
110
110
110
110
110
110
110 110
110
ASSR
30
60
65
65
60
40
50
60
80
PT 1
5
5
0
10
5
5
5
5
5
10
PT 2
20
30
25
55
60
30
20
15
50
60
Diagnostic
100
85
110
110
110
110
110
110 110
110
ASSR
10
25
40
45
.
0
30
20
35
.
10
5
5
5
15
10
0
15
5
0
PT 1
awake
awake
yes
75
yes
PT 2
25
70
75
70
70
40
65
75
70
75
Diagnostic
85
90
105
105
110
95
100
105 105
105
105
90
110
110
110
0
60
55
45
55
20
25
20
30
30
25
25
30
15
25
PT 1
awake
yes
PT 2
20
25
35
20
20
70
75
80
80
75
Diagnostic
90
85
110
110
110
110
110
110 110
110
15
35
50
45
30
60
60
55
35
.
55
45
30
25
40
45
30
35
20
40
PT 2
85
75
70
70
70
70
70
75
70
80
Diagnostic
100
105
110
100
100
100
105
110 110
110
ASSR
40
35
50
35
55
30
25
35
45
80
15
25
55
65
55
40
25
50
60
60
PT 1
PT 1
awake
awake
yes
yes
PT 2
10
25
55
65
60
15
25
50
60
55
Diagnostic
100
100
100
.
.
100
95
95
90
110
40
35
60
60
55
15
30
60
60
55
5
0
0
10
5
0
0
0
5
5
PT 1
awake
yes
PT 2
20
15
10
20
20
70
90
90
90
95
Diagnostic
85
95
100
110
110
95
110
110 110
110
PT 1
awake
yes
10
10
25
30
30
5
10
20
20
30
20
30
35
50
45
25
30
40
45
45
70
PT 2
45
45
50
60
65
55
55
60
70
Diagnostic
70
65
85
.
.
75
85
90
.
.
ASSR
0
35
50
60
55
40
55
85
95
110
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
110
110
110
110
110
110
110
110 110
110
40
60
70
70
80
40
60
70
70
80
5
10
5
10
10
0
5
5
0
5
PT 1
awake
yes
ASSR
PT 1
awake
yes
90
95
95
95
95
95
95
95
95
95
Diagnostic
105
105
105
110
110
110
110
110 110
110
0
25
20
35
20
0
5
20
35
30
30
PT 1
awake
yes
35
20
35
20
15
20
20
30
10
PT 2
25
45
40
40
40
85
70
95
95
85
Diagnostic
75
65
70
70
80
105
100
110 110
110
ASSR
0
30
65
65
55
110
100
110 110
110
210
prep
test
12
.
77
13
.
73
14
8
45
15
.
42
16
.
40
17
10
64
18
.
51
19
7
30
20
10
45
21
8
50
22
10
26
23
10
48
24
10
35
.
PT 2
ASSR
no.
5
55
Diagnostic
Audera
15
Hz
1000 2000 3000 4000
80
PT 2
Audera
55
500
70
ASSR
Audera
55
Right
2000 3000 4000
80
ASSR
Audera
1000
PT 2
ASSR
Audera
Hz
500
Diagnostic
ASSR
Audera
Left
University of Pretoria etd - De Koker, E (2004)
Machine
Audera
Audera
Test
PT 1
Protocol Sedation
Audera
500
Hz
Subject Time Time
1000 2000 3000 4000
20
20
30
40
0
25
5
10
5
40
35
65
45
25
40
25
45
30
Diagnostic
70
60
95
95
100
80
90
90
100
110
ASSR
30
40
60
60
55
30
50
50
70
70
25
25
20
20
20
35
30
40
35
35
yes
PT 2
10
15
10
5
20
60
60
70
80
80
Diagnostic
100
110
110
110
110
100
100
110 110
110
PT 1
awake
yes
0
20
25
45
45
0
10
40
45
45
25
40
20
35
35
50
55
25
50
45
85
PT 2
80
90
90
85
85
70
85
80
95
Diagnostic
90
85
90
.
.
100
100
95
.
.
ASSR
60
90
85
75
70
40
70
60
75
85
20
30
45
50
50
5
30
45
50
50
PT 1
awake
yes
PT 2
25
45
60
60
60
25
50
60
65
65
Diagnostic
100
100
110
110
110
105
110
110 110
110
ASSR
Audera
Right
2000 3000 4000
0
ASSR
Audera
1000
35
awake
yes
Hz
500
PT 2
PT 1
awake
Left
PT 1
awake
yes
5
50
50
60
45
0
50
60
60
50
20
40
40
30
35
15
35
35
50
50
PT 2
55
65
70
70
85
60
65
70
75
85
Diagnostic
95
95
95
110
110
110
110
110 110
110
ASSR
0
35
25
60
50
30
50
60
50
211
40
no.
prep
test
25
9
37
26
7
55
27
5
55
28
5
32
29
7
33
University of Pretoria etd - De Koker, E (2004)
APPENDIX P
ANALYSIS OF AVAILABLE DATAPSEUDOHYPACUSIC GROUP
KEY:
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
Pseudohypacusis proofed left
Pseudohypacusis proofed right
Normal hearing left ear
Normal hearing right ear
Abnormal exaggerated hearing left
Abnormal exaggerated hearing right
Case managed successfully
Compensable
within compensable range
Fit
Correlates with previous test left
Correlates with previous test right
Sudden deterioration left
Sudden deterioration right
Referred by Occupational Health centre/ENT
Subject
no
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
1
yes
yes
yes
yes
no
no
yes
no
no
yes
no
no
no
no
OHC
2
no
yes
.
no
.
yes
no
.
yes
no
no
no
.
.
OHC
3
yes
yes
no
no
yes
yes
yes
yes
yes
yes
no
yes
.
no
OHC
4
yes
yes
no
no
yes
yes
yes
yes
yes
yes
no
no
.
.
OHC
5
yes
yes
no
no
yes
yes
yes
no
no
yes
yes
yes
no
no
ENT
6
yes
yes
no
no
yes
yes
yes
yes
yes
yes
yes
yes
no
no
OHC
7
yes
yes
no
no
yes
yes
yes
yes
yes
yes
yes
no
no
no
OHC
8
yes
yes
no
no
yes
yes
yes
yes
no
no
no
no
yes
yes
ENT
9
yes
yes
no
no
yes
yes
yes
no
no
yes
no
no
no
no
OHC
10
yes
yes
.
no
.
yes
yes
yes
yes
yes
.
yes
no
no
OHC
11
yes
yes
no
yes
yes
no
yes
no
no
yes
no
yes
yes
no
ENT
12
yes
yes
no
no
yes
yes
yes
no
yes
yes
no
no
yes
yes
ENT
13
yes
yes
no
no
yes
yes
yes
no
yes
yes
yes
yes
yes
yes
ENT
212
University of Pretoria etd - De Koker, E (2004)
Subject
no
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
14
yes
yes
no
no
yes
yes
yes
yes
yes
yes
yes
yes
no
no
OHC
15
yes
yes
no
yes
yes
no
yes
no
no
yes
yes
no
no
no
OHC
16
yes
yes
no
no
yes
yes
yes
no
yes
no
no
no
yes
yes
OHC
17
yes
yes
no
no
yes
yes
yes
no
yes
yes
yes
no
no
yes
ENT
18
yes
yes
no
no
yes
yes
yes
yes
yes
yes
no
no
no
no
OHC
19
yes
yes
no
no
yes
yes
yes
yes
yes
yes
yes
yes
no
no
ENT
20
yes
yes
yes
yes
no
no
yes
no
no
yes
yes
yes
no
no
ENT
21
yes
yes
no
no
yes
yes
yes
no
yes
yes
yes
no
no
yes
OHC
22
yes
yes
no
no
yes
yes
yes
yes
yes
no
.
.
.
.
OHC
23
yes
yes
yes
yes
no
no
yes
no
no
yes
yes
yes
no
no
OHC
24
yes
no
no
no
yes
yes
yes
yes
no
no
no
no
yes
yes
OHC
25
yes
yes
no
no
yes
yes
yes
no
yes
yes
yes
no
yes
yes
OHC
26
yes
yes
no
no
yes
yes
yes
no
no
yes
no
yes
yes
no
ENT
27
yes
yes
no
no
yes
yes
yes
no
yes
no
yes
no
yes
yes
OHC
28
yes
yes
no
no
yes
yes
yes
yes
yes
yes
yes
yes
no
no
OHC
29
yes
yes
no
no
yes
yes
yes
yes
yes
yes
no
no
no
no
OHC
213
University of Pretoria etd - De Koker, E (2004)
APPENDIX Q
COSTING OF ASSR METHODS IN THE MINING
INDUSTRY
”I feel that it would be difficult, if not impossible, to derive an accurate formula for
estimating the financial impact of malingering (pseudohypacusis) in respect of noiseinduced hearing loss in the mining industry.” (Begley, 2003)
Complicating factors that lead to this difficulty include the following:
•
Production teams consist of 16 to 18 workers. If one worker is absent, the job
still continues, making it difficult to quantify any production loss due to one
individual’s absence.
•
Groups of workers are transported to hospitals and clinics on a daily basis,
and one or two additional cases per day may not have a significant impact.
•
It is impossible to say how much overcompensation occurs or has occurred,
as no objective measure or indicator has ever been put in place. Insurers
contend that two separate diagnostic audiograms and assessment by the
Occupational Health or Medical Practitioner, along with a review of each case
by the insurer’s claims assessors should minimise false claims (Begley,
2003).
The present author and other audiologists consulting to the industry have noted an
escalation
of
apparently
erroneous
compensation
or
overcompensation
of
pseudohypacusic individuals, particularly since the implementation of WCC
Instruction 168 in 1995.
Haugton et al. (1979) found that subjects were able to
consistently feign or exaggerate hearing loss within 6 dB (nine per cent), well within
the 10 db of variance needed to refute a compensation claim. In addition, Rickards
and De Vidi (1995) found that individuals who had been compensated had
exaggerated their hearing loss by 12, 2 per cent.
Taking into account the preceding points, the potential cost of pseudohypacusis has
been analysed considering the following components:
•
lost production;
•
lost shifts;
214
University of Pretoria etd - De Koker, E (2004)
•
transport costs;
•
specialist referrals;
•
overcompensation;
1.
LOST PRODUCTION
Lost production can be estimated is as follows (Geyser, 2003):
A 30-metre panel worked by a team of 16 workers carries a production cost of
R 79 000 per day, indicating that a single worker’s absence for one day amounts to
R 4 937,50 in lost production. Admittedly, a drill operator’s absence would have a
more direct impact on production, but there are very few instances of stope teams
being over-complemented and, hence, the overall average is calculated across the
entire team.
2.
LOST SHIFTS
A rock drill operator, normally classified as Category 4, earns an average monthly
wage of R 2 260 per month, or R 113 per day.
3.
TRANSPORT COSTS
Transporting workers to Occupational Health Centres, hospitals and clinics costs
R 70 000 per month for a single region in one mining group (Geyser, 2003). The
average number of workers transported each month is 584, implying a cost of
R 120,68 per worker.
4.
SPECIALIST REFERRALS
Various scenarios are possible in cases of pseudohypacusis, as follows:
4.1
Second referral for audiology
A worker may be referred for re-evaluation by the audiologist where thresholds have
not been obtained. The cost can be calculated as follows:
215
University of Pretoria etd - De Koker, E (2004)
Lost shift
R 113,00
Lost production
R4 937,50
Transport
R 120,68
Audiology:
Consultation
R
82,30
Air-conduction audiometry
R
37,20
Bone-conduction audiometry
R
37,20
Tympanometry
R
37,20
Acoustic reflexes
R
37,20
Cost of audiology:
R 231,10
Total cost of audiologist referral:
R5 402,28
4.2
ENT referral
If the audiologist’s second attempt to determine thresholds is unsuccessful, the
worker is often referred to an ENT specialist.
Lost shift
R 113,00
Lost production
R4 937,50
Transport
R 120,68
Consultation
R 113,40
The ENT will be unable to finalise the diagnosis without a reliable audiogram, and it
may be necessary to repeat audiological procedures.
Air-conduction audiometry
R 37,20
Bone-conduction audiometry
R
37,20
Tympanometry
R
37,20
Acoustic reflexes
R
37,20
Cost of audiology:
R 231,10
Total cost of ENT referral:
R5 515,68
4.3
ABR testing
If the ENT is still unable to make a final diagnosis and determine hearing thresholds,
an ABR may be requested.
Lost shift
R 113,00
Lost production
R4 937,50
Transport:
R 120,68
ABR testing
R 503,36
Revisit ENT
R 113,40
Total cost of ABR assessment:
R5 787,94
These costs indicate that without considering the effect of any overcompensation, the
cost of assessing a pseudohypacusic worker can amount to between R 5 402,28 and
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R 16 705,90. After all these costs have been incurred, it often happens that puretone thresholds have still not been determined across the frequency range and thus
the case remains unresolved.
A total of 2 526 diagnostic evaluations were performed for employees in one region
of a single mining group during the past financial year (Geyser, 2003). If only 10 per
cent of these involved pseudohypacusis (a very conservative estimate), it implies that
253 workers cost the employer R 1,367M in unnecessary diagnostic evaluations,
assuming that each one required only one day off work and that no ABR testing or
ENT referrals were involved.
In this light, the R 154 000 cost for an ASSR test system (HASS, December 2003)
would be recovered in a matter of months, and the instrument would not need
replacement for at least five years.
In addition, ASSR testing would enable the
diagnosis and evaluation of noise-induced hearing loss cases to be finalised more
quickly, serving the interests of both the employer and deserving workers.
4.4
Overcompensation
The literature indicates that between 9 and 33 per cent of workers who face the
prospect of claiming compensation exaggerate their hearing losses. Haughton et al.
(1979) shown that it is possible to consistently exaggerate a hearing loss within
six dB (nine per cent), which should be compared with the 10 dB of variance needed
to refute a test as unreliable. It is quite possible for an audiologist to overlook this
amount of exaggeration.
The average compensation settlement for noise-induced hearing loss among 228
workers at one regional operation of a single mining group was approximately
R 12 000 during the past financial year (Geyser, 2003). If only 10 per cent of these
claimants exaggerated their hearing loss by 6 dB (a discrepancy which would be
taken as a reliable reading), this would amount to a total overcompensation of
R 184 000 (R 8 000 per worker x 23 workers). This is based on the following:
A worker with earnings of R 4000 per month (including salary, overtime, holiday
allowance and housing) is compensated by an amount of R 12 000 for a permanent
disability (PD) of 6 per cent.
This amount is based on
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Earnings multiplied by percentage of PD, multiplied by 15 and divided by 30, i.e.
R 4 000 x 6 x 15 ÷ 30 = R 12 000.
If this worker has exaggerated his hearing loss by 9 per cent, his percentage PD
would have risen to 10 per cent, with the following effect:
R 4 000 x 10 x 15 ÷ 30 = R 20 000, i.e. an overcompensation of R 8 000.
This is a simplistic way of evaluating the possible financial impact of
overcompensation, since claimants earn different salaries, and have varying levels of
hearing loss and, hence, percentages permanent disability.
Nevertheless, this
exercise demonstrates that the use of truly objective methods for assessing noiseinduced hearing loss in pseudohypacusic workers would yield considerable cost
savings.
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APPENDIX R
PROOF OF LANGUAGE EDITING:
I NOOMÉ
14 May 2004
TO WHOM IT MAY CONCERN
This is to certify that I have language edited the whole thesis by Elize de
Koker on hard copy on the understanding that she would make the language
changes required on the electronic version. The last three chapters were
edited electronically, using the ‘track changes’ facility in MS WORD to enable
her to accept or reject changes and respond to editorial queries.
Yours faithfully
Idette Noomé (Mrs)
(MA English) (UP)
Enquiries: 012 333 5456 (H) / 012 420-2421 (W)
219
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