IMMITTANCE IN INFANTS 0 – 12 MONTHS: TONE

IMMITTANCE IN INFANTS 0 – 12 MONTHS: TONE
IMMITTANCE IN INFANTS 0 – 12 MONTHS:
MEASUREMENTS USING A 1000 Hz PROBE
TONE
BY
SONIA VAN ROOYEN
SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE DEGREE
MCOMMUNICATION PATHOLOGY
IN THE DEPARTMENT OF COMMUNICATION PATHOLOGY,
FACULTY OF HUMANITIES, UNIVERSITY OF PRETORIA
OCTOBER 2006
Dedicated to my loving parents Who always encouraged me to aim higher…
ACKNOWLEDGEMENTS
Sincere gratitude and appreciation especially to
∼ Dr De Wet Swanepoel for his continued guidance and support
throughout the entire process of this study and for sharing in highlights
along the way…
∼ Dr Maggi Soer for valuable contributions in the final completion of this
script
∼ Rina Owen for statistical support and analysis of the data
∼ Caroline Munro, Advisory Teacher of the Deaf, Nuffield Hearing and
Speech Centre, London, UK for proofreading an earlier version of this
script
∼ My family and friends for their devoted support and prayers
∼ My husband, Danny, for his unconditional love, understanding and
immeasurable support during trialling times
∼ My heavenly father who gave me the strength to complete this project –
“…our sufficiency is of God” (2 Cor 3:5b)
TABLE OF CONTENTS
LIST OF FIGURES
i
LIST OF TABLES
iii
ABSTRACT
v
OPSOMMING
vii
1.
INTRODUCTION, BACKGOUND AND RATIONALE OF STUDY
1
1.1
INTRODUCTION ………………………………………………..
1
1.2
BACKGROUND …………………………………………………
2
1.3
RATIONALE ……………………………………………………..
8
1.4
PROBLEM STATEMENT ………………………………………
10
1.5
DIVISION OF CHAPTERS ……………………………………..
11
1.6
DEFINITION OF TERMS ………………………………………
12
1.7
SUMMARY ………………………………………………………
13
2.
CRITICAL REVIEW OF MIDDLE EAR ASSESSMENT IN
INFANTS
14
2.1
INTRODUCTION ………….…...………………………………..
15
2.2
CURRENT ISSUES IN THE ASSESSMENT OF MIDDLE
EAR FUNCTIONING IN INFANTS ……………………………
2.3
ANATOMICAL
STRUCTURE
AND
16
PHYSIOLOGY
RELEVANT TO THE ASSESSMENT OF THE INFANT
MIDDLE EAR ... ………………...............................................
18
2.3.1 Middle ear structure and function ……………………..
19
2.3.2 Tympanic membrane ……………...……………………
20
2.3.3 Eustachian tube and its relation to middle ear
function …………………………………………………...
2.4
FACTORS
AFFECTING
MIDDLE
EAR
STATUS
21
IN
INFANTS …………………………………………………………
23
2.5
2.6
CRITICAL REVIEW OF MIDDLE EAR ASSESSMENT IN
INFANTS ………………………………………………………...
26
2.5.1 Otoscopy ………………………………………………....
26
2.5.2 Otoacoustic Emissions (OAEs) ………………………..
28
2.5.3 Immittance measurement ………………………………
30
CRITICAL REVIEW OF IMMITTANCE MEASUREMENTS:
PRINCIPLES AND APPLICATION IN INFANTS …………….
32
2.6.1 Fundamentals and principles of acoustic immittance
measurements …………………………………………..
32
2.6.2 The middle ear as a mechanical system ……………..
36
2.6.3 Mass, stiffness and resistance components of the
middle ear ………………………………………………..
36
2.6.4 Clinical application of tympanometry ………………….
40
2.6.4.1
Vector tympanometry ……………………….
40
2.6.4.2
Multi-component tympanometry …………...
42
2.6.4.3
Tympanometric peak pressure (TPP) …….
42
2.6.5 Classification systems in tympanometry ……………...
43
2.6.6 Multi-frequency tympanometry ………………………...
45
2.6.7 Acoustic reflex testing …………………………………..
46
2.6.8 Conventional tympanometry versus high frequency
3.
tympanometry ……………………………………………
48
2.7
IMPLICATIONS FOR INFANT TESTING …………………….
52
2.8
SUMMARY ………………………………………………………
54
RESEARCH METHODOLOGY
56
3.1
INTRODUCTION ………………………………………………..
56
3.2
HYPOTHESIS …………………………………………………...
57
3.3
AIMS OF RESEARCH ………………………………………….
57
3.3.1 Main aim ………………………………………………….
57
3.3.2 Sub-aims …………………………………………………
58
RESEARCH APPROACH ……………………………………..
58
3.4.1 Research design ………………………………………...
59
3.4
3.5
3.6
ETHICAL CONSIDERATIONS ………………………………...
61
3.5.1 Potential harm to research subjects …………………..
61
3.5.2 Informed consent ………………………………………..
61
3.5.3 Research fieldworkers ………………………………….
62
RESEARCH SUBJECTS ……………………………………….
62
3.6.1 Selection criteria ………………………………………...
63
3.6.2 Inclusion and exclusion criteria for compilation of
norms ……………………………………………………..
64
3.6.3 Subject selection procedures ………………………….
64
3.6.4 Description of subjects ………………………………….
65
3.7
MATERIAL AND APPARATUS ………………………………..
66
3.8
DATA COLLECTION PROCEDURES ………………………..
68
3.9
DATA
PREPARATION
AND
ORGANISATION
PROCEDURES …………………………….……………………
72
3.9.1 Division of case sample into two subgroups based on
4.
middle ear function ……………………………………...
73
3.9.2 Procedures for classification of tympanograms ……...
75
3.10
DATA ANALYSIS ……………………………………………….
79
3.11
SUMMARY ………………………………………………………
79
RESULTS AND DISCUSSION
80
4.1
INTRODUCTION ………………………………………………..
80
4.2
DESCRIPTION OF ADCMITTANCE (Ya) TYMPANOGRAM
SHAPES
AND
CHARACTERISTICS
WITHIN
SUBGROUPS A AND B ………………………………………..
82
4.2.1 Associations between results of OAE-testing and
tympanogram shape ……………………………………
4.2.2 Maximum admittance
pressure
4.3
values
for
and
Ya
tympanometric
tympanograms
84
peak
within
subgroups A and B ……………………………………...
87
4.2.3 Double peaked tympanograms ………………..………
92
SUSCEPTANCE
(Ba)
AND
CONDUCTANCE
(Ga)
TYMPANOGRAM ANALYSIS AND DESCRIPTION ………..
96
4.4
ACOUSTIC REFLEXES USING A HIGH FREQUENCY
PROBE TONE …………………………………………………..
102
4.4.1 Comparison of acoustic reflex measurement results
5.
with OAE and tympanometry results ………………….
103
4.5
HIGH FREQUENCY (1000 Hz) IMMITTANCE NORMS ……
105
4.6
SUMMARY ………………………………………………………
119
CONCLUSIONS AND RECOMMENDATIONS
121
5.1
INTRODUCTION ………………………………………………..
121
5.2
CONCLUSIONS …………………………………………………
122
5.3
IMPLICATIONS OF FINDINGS ………………………………
125
5.4
CRITICAL EVALUATION OF RESEARCH PROJECT ……..
127
5.5
CLINICAL GUIDELINES FOR INTERPRETATION OF
HIGH FREQUENCY IMMITTANCE …………………………..
128
5.6
RECOMMENDATIONS FOR FUTURE RESEARCH ……….
128
5.7
FINAL COMMENTS …………………………………………….
129
5.8
SUMMARY ………………………………………………………
129
REFERENCE LIST
130
APPENDICES
141
Appendix A: DATA SHEET ……………………….…………………………... 141
Appendix B: ETHICAL CLEARANCE ………………………….….…………
148
LIST OF FIGURES
Figure 2.1
Graphic illustration of impedance (Z) vs. admittance (Y)
33
tympanograms ………………………………………………….
34
Figure 2.2
Diagram of electroacoustic immittance instrumentation …..
38
Figure 2.3
Influence of frequency on admittance vector system ………
44
Figure 2.4
Vanhuyse classification model ……………………………….
Figure 3.1
Research methods representing the research design of the
60
current study ……………………………………………………
65
Figure 3.2
Age distribution of infants (n = 510) ………………………….
69
Figure 3.3
Three stage data collection procedure……………………….
72
Figure 3.4
Procedures performed (n = 1020 ears) ……………………...
74
Figure 3.5
OAE results acclaimed for division of groups ………………
77
Figure 3.6
Flowchart of research process ……………………………….
Figure 3.7
Illustration of research process according to main and sub-
78
aims …………………………….……………………………….
83
Figure 4.1
Results of admittance (Ya) tympanometry (n = 936) …….…
Figure 4.2
Relationships between OAE and Tympanometry results
85
(n = 936) …………………………………………………….…..
Figure 4.3
Independent relationships between OAE results and peak
86
or flat tympanograms (n = 936) ……………………………....
i
Figure 4.4
Distribution of maximum Ya-admittance values for ears with
89
OAE pass and refer results (n = 934) ………………………..
Figure 4.5
Distribution of tympanic peak pressure for ears with OAE
90
pass and refer results (n = 917) ……………………………...
Figure 4.6
Gender relation of ears displaying double peaked
92
tympanograms (n = 41) ……………………………………….
Figure 4.7
OAE test results of ears that displayed double peaked
93
tympanograms (n = 42) ……………………………………….
95
Figure 4.8
Examples of double peaked tympanograms ………………..
Figure 4.9
Random examples of tympanograms recorded during this
study at age groups of 1, 2, 6, 10, 14, 20, 36, 40 and 44
98
weeks ……………………………………………………………
.
Figure 4.10
1000 Hz acoustic reflex results using a 1000 Hz probe
102
tone (n = 914) …………………………………………………..
103
Figure 4.11
Distribution of acoustic reflex compared to OAE results…...
Figure 4.12
Relationship of tympanometry to acoustic reflex results
104
(n = 882) ………………………………………………………...
Figure 4.13
Distribution of admittance results for Ya tympanograms
108
(n = 809) ………………………………………………………...
Figure 4.14
Mean values for Ya tympanometric variables across age
groups: (A) peak admittance in mmho, (B) tympanometric
116
peak pressure in daPa ………………………………………...
117
Figure 4.15
Peak admittance and pressure norms ………………………
ii
LIST OF TABLES
Table 1.1
Outline and description of chapters…………………………..
11
Table 1.2
Terminology list…………………………………………………
12
Table 2.1
Normative Tympanometric Values from 1kHz
Tympanograms by Margolis et al., (2003:385) ……………..
Table 2.2
Normative data for 1000 Hz tympanometry by Kei et al.,
(2003:25) ………………………………………………………..
Table 2.3
50
51
Comparison of methods for middle ear assessment in
infants …………………………………………………………...
53
Table 3.1
OAE stimulus parameters (DPOAE 2) ………………………
68
Table 4.1
P-values for differences between left and right ears ……….
81
Table 4.2
1000 Hz Admittance (Ya), Susceptance (Ba) and
Conductance (Ga) tympanometry norms (Total sample) .….
Table 4.3
107
Comparison between results obtained from the current
study and results by Margolis et al., (2003:386) ……………
109
Table 4.4
1000 Hz tympanometry norms (Female ears) ……………… 110
Table 4.5
1000 Hz tympanometry norms (Male ears) …………………
Table 4.6
Comparison between peak admittance values in Ya
tympanograms in male and female ears …………………….
Table 4.7
110
111
Norms for 1000 Hz admittance tympanometry across four
age groups ……………………………………………………...
115
iii
Table 4.8
1000 Hz probe tone acoustic reflex norms (n = 727) ………
118
Table 5.1
Conclusions according to sub-aims ………………………….
122
Table 5.2
Strengths and limitations of the current study ………………
127
iv
ABSTRACT
TITLE:
Immittance in infant 0 – 12 months: Measurements using
a 1000 Hz probe tone
NAME:
Sonia van Rooyen
SUPERVISOR:
Dr DCD Swanepoel
CO-SUPERVISOR: Dr M Soer
DEPARTMENT:
Communication Pathology, University of Pretoria
DEGREE:
M Communication Pathology
Rapid implementation of universal newborn hearing screening programs has
exposed a need for a reliable test of middle ear function for timely identification
of middle ear pathology and for differentiation between true sensorineural and
conductive hearing losses. Use of higher probe tone frequencies for the
assessment of immitance measures have proven to be more reliable and
accurate in identifying MEE in infants. However a lack of classificationguidelines and age specific normative data exists. This study investigated the
characteristics and normative values of high frequency tympanometric and
acoustic reflex results for infants (n = 936 ears). Participants were 510 infants
(262 male, 248 female) aged 0 – 12 months (mean age = 12.8 weeks) recruited
from primary health care and immunization clinics in a South African
community. A three-part procedure was performed on each test ear: 1) OAEs
were recorded and pass results served as control variable for normal middle ear
functioning; 2) 1000 Hz probe tone admittance, susceptance and conductance
tympanograms were recorded and analysed in terms of shape, tympanometric
peak pressure and maximum (peak) admittance; 3) 1000 Hz probe tone
acoustic reflexes, measured with a 1000 Hz ipsilateral stimulus, were recorded
and thresholds determined. Significant associations were observed between
tympanogram shape, and OAE pass or fail results. 93% of ears with an OAE
pass result displayed peaked tympanograms, while 79% of ears with absent
OAE’s displayed flat tympanograms. Single peaked tympanograms were
recorded in 782 ears (84%), double peaked tympanograms in 41 (4%) ears and
flat sloping tympanograms in 112 (12%) ears. Admittance (Ya) tympanograms
for the total sample displayed a mean admittance value of 2.9 mmho, with a
standard deviation of 1.1 mmho. The 90th percent range was determined at 1.5
mmho (5th percentile) to 4.9 mmho (95th percentile). Mean tympanometric peak
pressure in Ya tympanograms was 0.1 daPa, with a standard deviation of 61
daPa. The 90th percent range was -110 daPa to 90 daPa for the 5th and 95th
percentiles respectively. Gender specific norms indicated a higher admittance
for male ears. Age specific norms indicate a gradual increase in admittance
indicating the need for age specific normative classification systems. Ipsilateral
1000 Hz stimuli acoustic reflex measurement proved successful with a 1000 Hz
v
probe tone and present reflexes were recorded in 84% of ears tested.
Significant association between acoustic reflex presence, OAE pass and
peaked tympanogram results were observed. The normative tympanometric
values derived from the cohort may serve as a guide for identification of middle
ear effusion in neonates. High frequency tympanometry in combination with
acoustic reflexes proves a useful measure for verifying middle ear functioning in
young infants.
Key words: acoustic reflex, admittance, conductance, high frequency probe
tone, immittance, middle ear effusion, neonatal hearing screening, peak
admittance, susceptance, tympanometric peak pressure, tympanometry
vi
OPSOMMING
TITEL:
Immittansie in kinders 0 – 12 maande: Metings met ‘n
1000 Hz meettoon
NAAM:
Sonia van Rooyen
PROMOTOR:
Dr DCD Swanepoel
MEDE-PROMOTOR: Dr M Soer
DEPARTEMENT:
Kommunikasie Patologie, Universiteit van Pretoria
GRAAD:
M Kommunikasie Patologie
Die implementering van universele neonatale gehoortoetsing het ‘n behoefte
onthul aan ‘n betroubare toets vir middeloor funksionering vir die tydige
identifikasie van middeloor patologie, en om sensories-neurale en konduktiewe
gehoorverliese te kan onderskei. Die gebruik van ‘n hoër frekwensie meettoon
is as meer betroubaar en akkuraat bewys. ‘n Gebrek aan klassifikasie-riglyne
en ouderdom-spesifiek normatiewe waardes bestaan egter tans. Hierdie studie
het die eienskappe en normatiewe waardes van hoë frekwensie timpanometrie
en akoestiese refleks resultate vir kinders (n = 936 ore) wat OAE toetsing
geslaag het, beskryf. Deelnemers was 510 kinders (262 manlik, 248 vroulik)
tussen die ouderdom van 0 – 12 maande (gemiddeld = 12.8 weke) wat gewerf
is vanaf 6-week primêre gesondheidsorg en immuniserings klinieke in ‘n SuidAfrikaanse gemeenskap.
‘n Drievoudige prosedure is op elk van die
deelnemers se ore uitgevoer: 1) OAE toetsing is uitgevoer en ‘n slaag resultaat
het gedien as kontrole vir normale middeloorfunksionering; 2) 1000 Hz
meettoon admittansie-, suskeptansie- en konduktansie timpanogramme is
opgeneem en geanaliseer na aanleiding van vorm, piek timpanometriese druk
en maksimum (piek) admittansie; 3) 1000 Hz meettoon akoestiese refleks
toetsing is uitgevoer met ‘n 1000 Hz ipsilaterale stimulus en drempels is
vasgestel. Betekenisvolle assosiasies is waargeneem tussen timpanogram
vorm en OAE slaag of verwys resultate. 93% van ore met OAE slaag resultate
het timpanogramme met duidelike pieke getoon, terwyl 79% wat ore met OAE
verwys resultate plat timpanogramme vertoon het. Enkel-piek timpanogramme
is in 782 (84%) van ore waargeneem, dubbel-piek timpangramme in 41 (4%)
van ore en plat timpanogramme in 112 (12%) ore.
Admittansie (Ya)
timpanogramme vir die totale groep het ‘n gemiddelde admittansie waarde van
2.9 mmho, met ‘n standaard afwyking van 1.1 mmho getoon. Die 90 % omvang
was waargeneem tussen 1.5 mmho (5de persentiel) en 4.9 mmho (95ste
persentiel). Gemiddelde timpanometriese piek druk in Ya timpanogramme was
0.1 daPa, met ‘n standard afwyking van 61 daPa. Die 90 % omvang was -110
daPa tot 90 daPa vir die 5de en 95ste persentiel afsonderlik. Geslag-spesifieke
norme het ‘n hoër gemiddelde admittansie waarde vir manlike ore getoon.
Ouderdom-spesifieke norme dui op ‘n geleidelike verhoging in admittansie wat
vii
die behoefte aan ouderdom-spesifieke normatiewe klassifikasie-sisteme
bevestig. Ipsilaterale 1000 Hz stimulus akoestiese refleks metings het getoon
dat reflekse suksesvol waargeneem word wanneer ‘n 1000 Hz meettoon
aangewend word.
Reflekse is waargeneem in 84% van die toetsore.
Betekenisvolle assosiasies tussen die teenwoordigheid van ‘n akoestiese
refleks, OAE slaag resultaat en duidelik gepiekde timpanogramme is aangedui.
Die normatie waardes kan dien as ‘n riglyn vir die identifkasie van middeloor
effusie in kinders. Hoë frekwensie timpanometrie tesame met akoestiese
reflekse blyk ‘n waardevolle metode vir evaluering van middeloor funksionering
in jong kinders te wees.
Sleutelterme: admittansie, akoestiese refleks, hoë frekwensie meettoon,
immittansie, konduktansie, middeloor effusie, neonatale gehoortoetsing, piek
admittansie, suskeptansie, timpanometriese piek druk, timpanometrie
viii
1. INTRODUCTION, BACKGROUND AND RATIONALE
The aim of this chapter is to present the effects of hearing
loss as rationale to hearing screening and the need to
develop
standardized
procedures
and
criteria
for
differentiating middle ear effusion from sensorineural hearing
loss in early infancy.
A breakdown of all the chapters
included will also be provided.
1.1
INTRODUCTION
“With the rapid implementation of universal newborn hearing
screening (UNHS) programs, there is a need for a test of
middle-ear function to distinguish sensori-neural hearing loss
from middle ear pathology.” (Margolis, Bass-Ringdahl, Hanks,
Holte & Zapala, 2003:384)
Hearing loss is often described as an invisible condition as it offers no obvious
indications and is often asymptomatic.
It may merely be overlooked or be
mistaken for developmental delay or attention deficit disorder (Hogan, Stratford
& Moore, 1997:350; Roush, 2001:3). Permanent hearing loss in infants almost
always affects the development of speech and language and may leave them
with deficits in the ability to communicate with an effective language system
(Roush, 2001:3; Rossetti, 1996:35; Mauk & White, 1995:6).
Furthermore,
eighty percent of a child’s ability to develop speech, language and related
cognitive skills is established by the time the child is thirty-six months of age,
with hearing being vitally important for the healthy development of such skills.
Therefore hearing loss can result in impaired academic achievement and socialemotional development (Luterman 1999:41; JCIH, 2000:798,800; Northern &
Downs, 2002:81).
The degree of hearing loss, age of identification and
intervention and each individual’s unique characteristics interact to determine
1
the consequences of a hearing loss.
In general, the greater the degree of
hearing loss, the greater the implication for oral language development (Roush,
2001:18; Luterman, 1999:107).
The negative consequences of severe bilateral sensorineural hearing loss, have
long been recognized, but it has only been in recent years that the damaging
consequences of mild bilateral and unilateral sensorineural hearing loss, or
conductive hearing loss have been realised (Mauk & White, 1995:6). Persistent
conductive hearing loss, often caused by the presence of middle ear effusion, is
common in infants. Untreated chronic middle ear effusion, although rare, can
result in serious medical complications including cholesteatoma, meningitis, and
sensorineural hearing loss (Roush, 2000:18). Conductive hearing loss
secondary to middle ear effusion early in life, may impact on the development of
auditory processing skills, and understanding of speech in the presence of
competing noise.
The relationship between middle ear effusion and
communication skills has been well documented (Northern & Downs 2002:66).
It is also recognized that persistent or recurrent middle ear effusion has
potentially detrimental long-term consequences, especially for children already
experiencing communicative disorders related to learning disabilities or
developmental delays (Roush, 2001:19). Bearing this in mind, early detection,
diagnosis, and habilitation of hearing loss, both sensorineural and conductive, is
crucial to forestall delays in speech, language and general development (JCIH,
2000:798; Koivunen, Uhari, Laitakari, Alho and Luotonen, 2000:216; Luterman,
1999:35).
1.2
BACKGROUND
Research has shown that children who are identified with hearing loss, and
given appropriate intervention before six months of age, maintain language
development consistent with their cognitive abilities (Yoshinaga-Itano, 2001).
The positive outcomes of early identification and management of hearing loss in
infants, has lead to the development of Early Hearing Detection and
Intervention Programs (EHDI) (JCIH, 2000:799; Northern & Downs, 2002:259).
2
In the USA, according to the Joint Committee on Infant Hearing (JCIH,
2000:798), all infant’s hearing should be screened using objective, physiologic
measures in order to identify those with congenital or neonatal onset hearing
loss and such procedures should be in progress with identification of hearing
loss by three months of age and intervention initiated by six months of age.
Newborn hearing screening protocols typically include the use of Otoacoustic
Emmission (OAE) and Automated Auditory Brainstem Response (AABR)
procedures to detect hearing loss (Kei, Allison-Levick, Dockray, Harrys,
Kirkegard, Wong, Maurer, Hegarty, Young and Tudehope, 2003:21; Mencher et
al. 2001:4; Roush, 2001:50; JCIH, 2000:802).
OAEs (low-intensity sounds
emitted by the ear that can be detected by a sensitive microphone placed in the
ear canal) can be recorded reliably in response to auditory stimuli (Roush,
2001:49). The AABR reflect electrical activity of the VIIIth (auditory) nerve and
brainstem, in response to auditory stimuli (Roush, 2001:60; Martin & Clark,
2000:165).
ABR
screenings
are
usually
performed
using
automated
instruments, reporting a pass or refer result after comparing acquired responses
to normal ABR responses stored in computer memory (Roush, 2001:61).
However, successful recording of OAEs and AABR responses not only require
a healthy cochlea and intact auditory nerve, but also necessitates normal or
near normal middle ear functioning (Roush, 2001:50; Fowler and Shanks,
2002:202; Koivunen et al., 2000:212; Sutton, Gleadle and Rowe, 1996:10;
Thornton, Kimm, Kennedy and Cafarelli-Dees, 1993:319).
The recording of
OAEs from neonates and infants can also be influenced and affected by
external and middle ear pathology leading to ‘false positive’ test results
(Thornton et al., 1993:322).
False positive test results refer to ears with normal underlying hearing but who
do not have sufficiently strong OAE and / or ABR responses at the time of the
newborn hearing screening test (Keefe, Zhao, Neely, Gorga and Vohr,
2003:389, Roush, 2000:19). OAE and ABR results are known to be sensitive to
outer ear canal obstruction and middle ear effusion, causing temporary
3
conductive dysfunction. Transient middle ear dysfunction can cause high falsepositive test outcomes in the presence of normal cochlear function (Keefe et al.,
2003:389; Doyle, Burggraaff, Fujikawa, Kim and MacArthur, 1997:598; Sutton et
al., 1996:9; Thornton et al., 1993:319).
It is clear therefore that the results of OAE and auditory brainstem response
testing depend, to a large extent, on the condition of the middle ear. Abnormal
middle ear functioning can be a major reason for failure of neonatal hearing
screening (Sutton et al., 1996:9). As universal screening for hearing in infants
develops, it becomes increasingly important to distinguish and separate middleear pathology from pathology of the cochlea and brainstem to ensure
appropriate follow-up procedures are initiated (Fowler & Shanks, 2002:202). As
transient middle ear dysfunction may be more prevalent in neonatal and infant
ears than cochlear or sensori-neural hearing losses, there is a critical need for a
better understanding of middle ear functioning in neonates and infants (Keefe,
Folsom, Gorga, Vohr, Bulen & Norton, 2000:443).
A key contributing factor on the condition of the middle ear and in turn on the
outcome of OAE and ABR test procedures is the presence of middle ear
effusion (Yeo, Park, Park & Suh, 2002:798; El-Refaie, Parker & Bomford,
1996:3; Wada, Ohyama, Kobayashi, Koike and Noguchi, 1995:162; Thornton et
al., 1993:319; Trine, Hirsch & Margolis, 1993:401). MEE is characterized by
fluid in the middle ear without evidence of infection. A number of synonyms are
used to describe this condition, including serous otitis, secretory otitis,
nonsuppurative otitis, fluid ear and middle ear effusion (MEE) (Northern &
Downs, 2002:75). As OAEs are transmitted from the cochlea to the ear canal
via the middle ear, the transmission properties of the middle ear directly
influence the OAE characteristics (Yeo et al., 2002:794). Middle ear effusion
(MEE) has a very high prevalence, affecting nearly all children at least once
during the early months or years of life (Northern & Downs, 2002:65; Roush,
2001:11; Hogan et al., 1997:350).
In contrast, sensorineural hearing loss
occurs far less frequently with an incidence of approximately three per 1000 for
all newborn infants (Roush, 2001:20).
4
MEE creates a mild to moderate conductive hearing loss that averages 20 dB,
but can be as great as 50 dB (Hogan et al., 1997:350; Northern & Downs,
2002:66).
The fluctuating hearing loss caused by MEE, which is often
recurrent, can lead to the inability to hear short unstressed words and low
intensity speech sounds, distortion of sound and inconsistent auditory reception
(Northern & Downs, 2002:66). Unlike acute episodes of middle ear disease,
which are readily apparent from pain and other overt symptoms, non-infected
middle ear fluid and hearing loss are often overlooked without appropriate
screening procedures (Roush, 2001:14, Hogan et al., 1997:350).
In the clinical setting MEE is generally identified and diagnosed by means of an
array of examinations and electrophysiological test-procedures. These include
otoscopic examination, pneumatic otoscopy, acoustic immittance measures,
and Otoacoustic emissions (Casselbrant, Gravel, Margolis & Marchisio,
2002:95; Roush, 2001:35; Koivunen et al., 2000:212).
Acoustic immittance
measures, specifically tympanometry, have proved over the years to be the best
single audiologic procedure for assessing middle ear functioning (Hall &
Mueller, 1997:177). Added advantages of acoustic immittance measures when
acoustic reflexes are also recorded include the possibility for differential
diagnosis between middle ear, cochlea, eighth nerve and lower auditory
brainstem pathology (Hall & Mueller, 1997:177).
Acoustic immittance
measurements have evolved from a specialty procedure, to a fundamental and
routine part of the audiological test battery (Wiley & Fowler, 1997:1).
Its
contribution to clinical diagnosis has become well accepted as an objective tool
for diagnosing middle ear pathologies (Fowler & Shanks, 2002:175; Petrak,
2002:1; Palmu, Puhakka, Rahko & Takala, 1999:178). The high sensitivity of
acoustic immittance measures in screening for middle ear disorders forms the
basic rationale for the use of such measures in screening protocols (Roush,
2001:66; Wiley & Fowler, 1997:1).
Acoustic immittance refers to acoustic admittance or acoustic impedance of the
middle ear structures, or both (Wiley & Stoppenbach, 2002:161; Wiley & Fowler,
1997:1). Acoustic impedance and admittance are reciprocal terms. Admittance
5
is the ease with which sound energy flows through the acoustic middle ear
system, whereas impedance refers to the opposition to the flow of energy
(Wiley & Stoppenbach, 2002:161; Hall & Mueller, 1997:178; Wiley & Fowler,
1997:22). An acoustic transmission system, such as the human ear, that offers
high acoustic admittance to the flow of sound has low acoustic impedance.
Although both terms have been used to describe acoustic measures of middle
ear function, current commercially available acoustic immittance instruments
typically provide measures of admittance (Wiley & Stoppenbach, 2002:161).
Clinical admittance measures can be broadly separated into two general areas:
tympanometry and acoustic reflex measures (Wiley & Stoppenbach, 2002:168).
Tympanometry and recording stapedius reflexes for acoustic signals are the two
procedures that form the primary set of acoustic immittance measurements
used in most audiology clinics (Wiley & Fowler, 1997:1).
As previously
mentioned, acoustic immittance measures are sensitive to middle ear
pathologies and require no behavioral response on the part of the patient.
Thus, the measures can be obtained in clinical patients for whom behavioral
response techniques are not always feasible, such as very young children.
Tympanometry involves measures of the acoustic admittance in a hermetically
sealed ear canal as air pressure is varied above (+) and below (-) atmospheric
level in the ear canal (Wiley & Fowler, 1997:2). In tympanometry the mobility of
the tympanic membrane is measured while the membrane is exposed to an
acoustic energy, commonly referred to as the probe tone (Gruber, 2002:1; GSITympStar Manual, 2002). In the ear, the tympanic membrane is mechanically
coupled with the middle ear ossicles to the oval window; the interface between
the middle and inner ear. It is this entire system (membrane, middle ear and
oval window) that is forced into oscillation when tympanometric measurements
are being performed (Gruber, 2002:1). The probe unit provides the acoustic
signal (probe signal) that serves as the reference for acoustic immittance
measures. For the vast majority of instruments, the probe signal is a tone of a
specified frequency and sound pressure level. Since the sound pressure level
of the probe tone within the ear canal varies as a function of mobility, it is
possible to record these changes in mobility as a function of pressure (Wiley &
6
Fowler, 1997:8). Currently there are instruments that provide for multiple probe
frequencies (i.e. 226 Hz, 678 Hz, 1000 Hz).
The most commonly used probe tone in conventional tympanometry has been a
226 Hz probe signal (Fowler & Shanks, 2002:175; Roush, 2001:67). This probe
tone has some definitive advantages when testing the adult ear as the adult
middle-ear system is stiffness-dominated (compliance) at this frequency and the
effects of mass and friction are minor (Kei et al., 2003:21; Fowler & Shanks,
2002:187; Petrak, 2002:1; Purdy & Williams, 2000:9). However when higher
probe tone frequencies are used, the middle ear no longer acts as a masscontrolled system and mass components, particularly of the eardrum and
ossicles, become more significant (Fowler & Shanks, 2002:187; Meyer et al.,
1997:192). It has been found that tympanograms collected from infant ears
progress differently than those collected from adult ears especially when a low
frequency probe tone is used (Keefe et al., 2000:444; Purdy et al., 2000:9;
Palmu et al., 1999:207; Meyer et al., 1997:190; Sutton et al., 1996:10; Holte,
Margolis & Cavanaugh, 1991:20).
A review of the literature reveals that there is controversy and debate among
professionals at which age tympanometry becomes reliable for detecting MEE
in infants (Northern & Downs, 2002:80; Purdy & Williams, 2000:9; Meyer et al.,
1997:189). The choice of probe tone, most sensitive to the detection of MEE, is
also not fully agreed on (Northern & Downs, 2002:80; Fowler & Shanks,
2002:187; Petrak, 2002:2; Purdy et al., 2000:9; Meyer et al., 1997:189). For
some years it has been recognised that conventional low-frequency probe tone
tympanometry is inappropriate for infants below 7 months of age, because of
poor sensitivity and high false positive as well as false negative results (Petrak,
2002:1; Purdy & Williams, 2002:9; Sutton, 2000:1; Meyer et al., 1997:189).
Studies have shown that infants below four months of age may demonstrate
what appear to be normal 226 Hz tympanograms with confirmed MEE, and
abnormal tympanograms in normal ears (Purdy & Williams, 2000:9; Paradise et
al, 1976 in Meyer et al., 1997:189; Keefe et al., 1996:372). It was originally
7
believed that infant ear canal walls were so compliant that a movement of the
entire wall occurred that imitated the type A-tympanograms (Holte et al.,
1991:20). The reason for false positive and false negative results are unclear
although it has been speculated that the anatomical differences of the adult and
infant middle ear transmission system, may be contributory factors (Petrak,
2002:2; Keefe & Levi, 1996:362; Meyer et el., 1997:190; Holte et al., 1991:20).
During development of the infant ear, several anatomical changes take place,
which influence the mechanical properties of the ear canal. These changes in
acoustic response have been attributed to these physical changes in the
external and middle ear after birth by some researchers (Meyer et al, 1997:190;
Keefe & Levi, 1996:362).
1.3
RATIONALE
A number of studies have shown that there may be a better correlation between
the presence of middle ear effusion and the shape of the tympanogram when a
high-frequency probe tone is used in young infants (Kei et al., 2003:27; Keefe et
al., 2000:461; Meyer et al., 1997:194; Sutton et al., 1996:15). Thornton et al.
(1993:320) found that the percentage of successful admittance measurements
obtained using a 226 Hz probe frequency was 68%, but when a 1000 Hz probe
tone was used, this figure rose to 87%. Meyer et al., (1997:194) and Sutton et
al., (1996:10) supports the argument that a high frequency probe tone is able to
identify the presence of pathology that is unrecognized by conventional 226 Hz
tympanometry. In a study by LaRossa, Mitchell and Cardinal (1993:34) 226 Hz
tympanometry results were not repeatable or reliable in identifying MEE in
infants in the Neonatal Intensive Care Units. According to Petrak (2002:2) a
higher frequency probe tone is needed to collect tympanograms that will be
useful in identifying middle ear effusion in infants. Other research comparing
high frequency tympanometry to OAE and/or auditory brainstem response
results (Rhodes, Margolis, Hirsch & Napp, 1999:806; Hirsch, Margolis &
Rykken, 1992:181) indicates better sensitivity to conductive pathology for 678
Hz and 1000 Hz tympanograms than conventional low frequency probe tone
tympanometry.
8
Despite the advantages of high frequency tympanometry in identifying massrelated middle ear pathology, high frequency / multi-frequency probe tones are
not commonly used.
The major barrier to incorporating high-frequency
tympanometry into the routine test battery is that the measures are more
complex and not as easily understood as 226 Hz tympanograms (Fowler &
Shanks, 2002:186). There are no accepted guidelines on classifying,
interpreting high-frequency tympanograms or validated criteria for distinguishing
normal from abnormal tympanograms in infants (Petrak, 2002:1; Purdy &
Williams, 2000:9; Sutton et al., 1996:11). According to Petrak (2002:2) and
Sutton (2000:2), as further research continues to support high frequency
tympanometry, the best choice for a tympanometric probe frequency in infants
under four months of age is 1000 Hz. Currently there are still many questions
regarding the sensitivity and specificity of 1000 Hz tympanometry to the
presence of MEE in infants (Purdy & Williams, 2000:24). To date, there is no
universal normative data classification system in place for high frequency probe
tone measures, as for the 226 Hz probe tone in adults (Petrak, 2002:2).
Normative values have mainly been determined for adults and children from
three to sixteen years of age (Palmu et al., 2001:178). Thus there is a dearth of
research on tympanometry in infants, especially those with the highest risk and
incidence of otitis media, i.e. those under 2 years of age (Sininger 2003:380;
Northern & Downs, 2002:80; Palmu et al., 2001:178).
Fowler and Shanks
(2002:202) emphasize the need for additional studies on infants with normal
ears and confirmed middle ear pathology before guidelines can be established
for the reliable use of high frequency tympanometry in distinguishing normal
from pathologic ears in this population. In a recent study by Kei et al. (2003:20),
characteristics of 1000 Hz tympanograms for neonates with normal Transient
Evoked OAEs, and a single peaked admittance tympanogram, were described.
This may serve as a guide for detecting middle ear effusion in infants.
As newborn hearing screening programs are rapidly expanding, a need exists
for a simple and objective method to evaluate and interpret middle ear status in
infants.
As previously mentioned, the results of otoacoustic emissions and
auditory brainstem responses depend on the condition of the middle ear.
9
Middle ear pathology must therefore be separated from pathology of the
cochlea and brainstem for appropriate follow-up and referrals (Fowler and
Shanks, 2002:202). According to Kei et al., (2003:27) there is potential for high
frequency tympanometry to be utilized in neonatal hearing screening programs
to identify possible middle ear dysfunction. It may also serve to decrease the
number of infants in which a definite audiological diagnosis is delayed due to
chronic MEE.
1.4
PROBLEM STATEMENT
“Because transient middle-ear dysfunction may be more prevalent in neonatal
ears than cochlear or sensorineural hearing losses, there is a critical need for a
better understanding of middle-ear functioning in neonates” (Keefe et al.,
2000:443). Tympanometry is effective in identifying various forms of middle ear
dysfunction, and can be used to help interpret OAE and ABR measurements in
the same ear (Keefe et al., 2000:444). It has been suggested that because
middle ear effusion is likely to reduce or eradicate OAEs and elevate AABR
thresholds, it may be a major reason for failure of neonatal hearing screening in
infants and account for much of the false positive results (Sutton et al.,
1996:16).
Conventional tympanometric measurement in neonatal ears has been found to
be unreliable due to anatomical differences between the adult and infant ear
(Petrak, 2002:2; Purdy & Williams, 2000:9; Palmu et al., 1999:207; Meyer et al.,
1997:190; La Rossa, Mitchell & Cardinal, 1993:32). Tympanometry using a
higher frequency probe tone appears to overcome this problem.
However
validation of high frequency probe tone tympanometry for measuring middle ear
status in neonates and young infants has not yet been established (Fowler &
Shanks, 2002:175; Sutton et al., 1996:10).
10
1.5
DIVISION OF CHAPTERS
A description of the sections included in this study is provided in Table 1.1.
TABLE 1.1 Outline and description of chapters
INTRODUCTION, BACKGROUND AND RATIONALE
CHAPTER 1
Chapter one provides a background and rationale for the
present study and highlights the need for a clinical tool to
differentiate between conductive and sensori-neural hearing
loss in early infancy.
CRITICAL REVIEW OF MIDDLE EAR ASSESSMENT
IN INFANTS
CHAPTER 2
Chapter two provides an overview of the structures involved
in the assessment of the middle ear system. Knowledge
thereof, and a thorough understanding of the principles of
immittance measures, is essential to critically evaluate the
procedures currently used for the assessment of the infant
middle ear. Included is a comparison of the advantages and
disadvantages of these methods.
RESEARCH METHODOLOGY
CHAPTER 3
A detailed description of the methodological approach and
research design followed during this study will be given. A
description of the sample, materials and apparatus used,
procedures used for assembling and analysing data will be
discussed.
RESULTS AND DISCUSSION
CHAPTER 4
The results obtained from the empirical research will be
presented in accordance with the main- and sub-aims
formulated for the study. The findings of this study will be
discussed within the broader framework outlined in the first
three chapters. Present results will be compared to other
studies.
CONCLUSIONS AND RECOMMENDATIONS
CHAPTER 5
The study will be concluded by a summary of the significant
findings. This chapter will also include a critical evaluation,
the clinical implications and recommendations of this study.
11
1.6
DEFINITION OF TERMS
A definition of the most important terms relevant to the context of this study is
provided in Table 1.2.
TABLE 1.2 Terminology list
IMMITTANCE
Measures of acoustic impedance (opposition to flow of
energy) or acoustic admittance (ease with which energy
flows) within the middle ear system.
Refers to both
tympanometry and recording of stapedial reflexes.
TYMPANOMETRY
Measures of a) opposition to energy flow or b) ease of
energy flow in the ear canal as a function of changes in air
pressure. Performed by introduction of an acoustic signal
and measurement of sound pressure level of the signal in the
ear canal as pressure is varied above (+) and below (-)
atmospheric pressure.
ACOUSTIC REFLEX
Stapedial muscle reflex, elicited in response to an acoustic
signal of sufficient intensity, which causes an increase in
acoustic impedance measured at the eardrum.
PROBE TONE
A continuously delivered signal into the ear by the probe
speaker. Acoustic immittance in analysed by monitoring of
probe-tone sound pressure level in the ear canal by means of
the probe microphone. Low (226 Hz) or high (678 / 1000 Hz)
tones can be used to measure middle ear mobility. A 1000
Hz probe tone was utilized for tympanometry and acoustic
reflex measurements during this study.
PEAK ADMITTANCE
The point of maximum mobility on a tympanogram which
indicates the degree of energy flow within the middle ear
system. In the current study this was measured at the point
of maximum positive deflection on the tympanogram.
TYMPANOMETRIC
PEAK PRESSURE
The pressure value where maximum mobility occurs (peak of
the tympanogram) and which approximates pressure within
the middle ear space. In the current study this was
measured at the corresponding peak admittance point.
12
TABLE 1.2 continued…
ACOUSTIC
SUSCEPTANCE (Ba)
ACOUSTIC
CONDUCTANCE (Ga)
ACOUSTIC
ADMITTANCE (Ya)
1.7
Interaction between compliance elements and mass
elements of a system. Susceptance is positive at lower
frequencies when a system is stiffness-controlled and
negative when it is controlled by mass at higher frequencies.
Recorded simultaneously as Ga tympanograms in the current
study and analysed in terms of peak admittance and
tympanometric peak pressure. Number of extrema also
determined and shape classified as mass or stiffness
dominated.
Relates to the impact of friction elements on susceptance
and refers to the resistance to energy flow. Recorded
simultaneously as Ba tympanograms in the current study and
analysed in terms of peak admittance and tympanometric
peak pressure. Number of extrema also determined and
shape classified as mass or stiffness dominated.
Represent ease of flow of acoustic energy and is determined
by susceptance (B) and conductance (G) components of the
middle ear system. Peak admittance and tympanometric
peak pressure values obtained from Ya tympanograms used
for compilation of normative values.
SUMMARY
Chapter one has provided an orientation and rationale for the current study by
presenting relevant information on the effect of middle ear effusion on general
development and the necessity for inclusion of a test for middle ear function as
part of a screening protocol. The lack of research in the literature in the area of
high frequency immittance in infants has been highlighted and considered in the
formulation of the rationale and problem statement for the study. An outline of
the chapters to follow has been included.
13
2. CRITICAL REVIEW OF MIDDLE EAR ASSESSMENT IN INFANTS
The main aim of this chapter is to provide an overview of the
controversies surrounding the assessment of the infant middle
ear. An outline of the structures involved in the assessment of
middle ear functioning, the physics underlying middle ear
measurements, and the developmental changes that occur in the
infant middle ear will be reviewed. Based on this theoretical
foundation, current methods for assessment of middle ear
functioning in infants will be critically reviewed in support of the
search for an accurate, objective test method.
2.1
INTRODUCTION
The accurate assessment of middle-ear status in newborns and young infants is
becoming increasingly important due to the widespread implementation of
universal newborn hearing screening (UNHS) programs. Newborn hearing
screening initiatives and legislation for implementation thereof has received
extensive attention in research and in practice over the past ten years. In the
1994 Position Statement, the Joint Commission on Infant Hearing endorsed the
goal of universal detection of infants with hearing loss and encouraged
continuing research to develop and to improve methodologies for identification
and intervention of hearing loss (JCIH, 1994:3). This was extended by the Year
2000 Position Statement, which stated that all infants’ hearing should be
screened using objective, physiologic measures in order to identify those with
congenital or neonatal onset hearing loss (JCIH, 2000:6). The targeted hearing
loss for UNHS programs was defined by the JCIH as, “permanent bilateral or
unilateral, sensory or conductive hearing loss, averaging 30 – 40 dB or more in
the frequency region important for speech recognition (approximately 500
through 4000 Hz)” (JCIH, 2000:7).
Current newborn hearing screening
protocols typically include the use of Otoacoustic Emissions (OAE) and
14
Automated Auditory Brainstem Response (AABR) procedures to detect hearing
loss in infants
(Keefe et al., 2003:389; Kei et al., 2003:21; Mencher et al.
2001:5; Roush, 2001:58,63; JCIH, 2000:7; Koivunen et al., 2000:212).
However, successful recording of these measures necessitates normal or near
normal middle ear functioning and can therefore be affected by middle ear
pathologies or temporary conductive dysfunction, not otherwise detected
(Fowler & Shanks, 2002:122; Roush, 2002:19; Yeo et al., 2002:797; Koivunen
et al., 2000:212; Sutton, et al., 1996:9; Thornton et al., 1993:319). Hence, a
significant issue in newborn hearing screening programs is the problem of false
positive test results, referring to ears with normal hearing that do not have
sufficiently strong OAE and/or ABR responses at the time of the newborn
screening test.
A widely held view is that middle ear dysfunction, most often transient in nature
during the neonatal and perinatal periods, is responsible for such false-positive
outcomes (Keefe et al., 2003:389). False-positive test results can have costly
financial implications due to unnecessary follow-up and re-testing expenses.
For developing countries this may have a significant impact on sustaining
newborn screening programs.
Margolis, Bass-Ringdahl, Hanks, Holte and
Zapala (2003:384), also highlight this point, stating that there is an urgent need
for a test of middle ear function in infants to distinguish sensorineural hearing
loss from middle ear pathology in order to (a) identify screening fails caused by
transient external- or middle ear conditions, (b) to determine the need for
medical management of middle ear disease and (c) to determine the need and
timing of follow-up procedures such as auditory brainstem response testing.
As acoustic immittance measurements can objectively identify, and are
sensitive to, middle ear disorders, it is widely used in audiologic assessment
procedures (Palmu et al., 1999:207; Wiley & Fowler, 1997:116). Since acoustic
immittance measurement is not a behavioural test, is not time consuming, is
non-invasive, and is relatively easy to administer, it can be used to assess
middle ear functioning in the difficult-to-test and paediatric population (Wiley &
Fowler, 1997:2; Silman & Silverman, 1991:71). However, the clinical utility of
15
tympanometry to assess middle ear functioning has been clearly established for
all populations except infants less than 6 months of age (Fowler & Shanks,
2002:186; Meyer et al., 1997:190; Holte, Margolis & Cavanaugh, 1991:1). More
recent research has aimed to provide clarification on the issue of middle ear
assessment in infants (Kei et al., 2003:27, Margolis et al., 2003:391, Meyer et
al., 1997:193, Sutton et al., 1996:15).
However, due to the underlying
difficulties in assessment of the infant ear, there are still no clear guidelines and
generally accepted standards or norms for assessment of the infant middle ear.
These issues and underlying fundamentals of the assessment of the infant
middle ear will be reviewed in the following sections.
2.2
CURRENT ISSUES IN THE ASSESSMENT OF MIDDLE EAR
FUNCTIONING IN INFANTS
Objective measurement of middle ear function continues to be refined and
currently technology offers the opportunity for improved diagnosis of middle ear
disorders by using multi-frequency and multi-component tympanometry (Lilly,
2005:6). Can tympanometry however be used reliably in the assessment of the
infant middle ear? This has been questioned extensively in literature dating
back from the 1970’s, when Paradise, Smith and Bluestone (1976, in Meyer et
al., 1996:189) were the first to cast suspicion over tympanometric use with
infants younger than seven months.
These authors found that normal
tympanograms could co-exist with confirmed middle ear effusion when a
conventional low frequency probe tone is used.
Research has consistently demonstrated that tympanograms collected from
infant ears progress differently than those collected from adult ears and that the
acoustic response of the external and middle ear systems vary significantly over
the first years after birth (De Chicchis, Wendell Todd & Nozza, 2000:101; Holte
et al., 1991:2; Keefe & Levi, 1996:363; Meyer et al., 1997:194; Petrak, 2002:2).
As an answer to the question of the most suitable probe tone for infant middle
ear measures, contradictory results were reported in earlier research. Holte et
al., (1991:23) concluded that the probe tone frequency of choice for infants
16
below four months of age is 226 Hz because it is least affected by individual
maturational differences and tympanometric patterns are more interpretable
than at higher frequencies. In contrast, Meyer et al., (1997:194) reported that
the use of a high frequency probe tone may have greater diagnostic sensitivity
to middle ear pathology than conventional 226 Hz tympanometry.
Attempts to explain contradicting and varying results have been made, as is
seen in Keefe and Levi, (1996:367) and Keefe et al., (2000:444) who noted that
a confounding factor in neonatal tympanometry is that the introduction of
positive or negative pressure into the neonatal ear canal, increases or
decreases the ear-canal volume, respectively, and due to extremely compliant
ear canal walls in the infant ear, this in turn affects acoustic immittance
measurements. Similar results were obtained by Holte et al., (1991:21) who
indicated that prior to one month of age, many tympanograms recorded in the
negative to positive (- / +) direction of air pressure change resulted in ear canal
collapse and subsequent uninterpretable tympanograms. This was overcome
and avoided by the use of a positive to negative (+ / -) direction of air pressure
change. As a result, recording in the + / - direction was recommended as an
important procedural step in tympanometry in infants (Holte et al., 1991:21).
It is evident therefore, that the choice of probe tone for performing
tympanometry in neonates was not fully agreed on. However, a number of
studies have shown that there may be a better correlation between the
presence of middle ear effusion and the shape of the tympanogram when a
high-frequency probe tone is used in young infants (Kei et al., 2003:27; Keefe et
al., 2000; Purdy & Williams, 2000:10; Meyer et al., 1997:194; Marchant et al.,
1984:593), and in recent year it has become widely accepted that conventional
tympanometry is not an effective test for middle ear function in young infants
(Kei et al., 2003:21; Lantz, Petrak & Prigge, 2004:3; Marchant, McMillan,
Shurin, Johnson, Turczyk, Feinstein & Panek, 1984:594; Margolis et al.,
2003:384; Sutton et al., 1996:9,10).
17
Evidence of this is seen in Thornton et al. (1993:320) who found that the
percentage of successful admittance measurements obtained using a 226 Hz
probe frequency was 68%, but when a 1000 Hz probe tone was used, this
figure rose to 87%. Meyer et al., (1997:194) and Sutton et al., (1996:10) support
the argument that a high frequency probe tone is able to identify the presence
of pathology that is unrecognized by conventional 226 Hz tympanometry.
There is increasing interest in high frequency tympanometry, motivated by the
need to have an objective test of middle ear function in newborn hearing
screening programs.
More research is however needed to establish the
reliability and validity of high frequency tympanometry for assessment of the
infant middle ear system, and to establish guidelines and norms for accurate
interpretation of results (Purdy & Williams, 2000:22).
In order to understand
this controversy and the respective effects of low versus high frequency probe
tone immittance measures on infant middle ear assessment, knowledge of the
fundamental underlying physics and anatomical background of the infant ear is
required. The anatomical structures and developmental changes, relevant to
the assessment of the infant middle ear, will therefore be reviewed in the
following sections.
Knowledge of the underlying structures and physical principles involved in
acoustic measures of middle ear functioning is fundamental to understand the
assessment of the infant middle ear and the questions surrounding it. The
following section provides this information.
2.3
ANATOMICAL STRUCTURE AND PHYSIOLOGY RELEVANT TO THE
ASSESSMENT OF THE INFANT MIDDLE EAR
Anatomical differences between the infant and adult middle ear transmission
system have been regarded as the reason for the varying acoustic responses,
and this has also been regarded as the reason for high false positive and false
negative results when using conventional tympanometry in the infant population
18
(Petrak, 2002:2; Meyer et al., 1997:190; Keefe and Levi, 1996:361; Holte, et al.,
1991:1).
2.3.1 Middle ear structure and function
The middle ear is part of an interrelated system including the nose,
nasopharynx, Eustachian tube and mastoid cavities. The close communication
of the middle ear with these surrounding structures impacts on both the cause
of middle ear disease and the assessment thereof (Silman & Silverman,
1991:128). The middle ear itself consists essentially of the tympanic membrane
and the middle ear cavity, which contains the three middle ear ossicles and is
essential in the clinical diagnosis of middle ear pathologies (Martin & Clark,
2000:251).
The middle ear, or tympanic cavity, is a small air-filled, mucosa-lined cavity
within the petrous portion of the temporal bone. It is flanked laterally by the
tympanic membrane and medially by a bony wall with two openings, the
superior oval (vestibular) window and the inferior round (cochlear) window. The
superior boundary of the epitympanic recess forms the ‘roof’ of the middle ear
cavity. The middle ear space contains a mechanical system composed of three
ossicles, the malleus, incus and stapes, which collectively transmit the
mechanical energy of sound from the tympanic membrane to the fluid-filled
inner ear via the oval window (Marieb, 1995:527; Martin & Clark, 2000:251).
During development of the infant ear, several physical changes take place,
which influence the mechanical properties of the ear canal. Various authors
regard these as contributory factors to tympanometry outcome differentials in
adult and infant populations (Petrak, 2002:2; Holte et al., 1991:2; Silman &
Silverman, 1991:121). The factors include
•
An increase in size of the external ear, middle ear cavity and mastoid.
A typical ear-canal diameter for a one month old infant is 4,4 mm
compared with 8mm for an adult (Keefe & Levi, 1996:362).
The
stiffness component of the middle ear is largely determined by the
19
tympanic cavity volume and it is postulated that there is an overall
decrease in stiffness with an increase in cavity size and maturation
(Meyer et al., 1997:190).
•
Decrease in the overall mass due to changes in bone density and
gradual loss of mesenchyme clinging to middle ear ossicles (Meyer et
al., 1997:190). The osseous portion of the ear canal is not completely
formed until about 1 year of age.
As previously mentioned
pressurising the ear canal may result not only in movement of the
tympanic membrane, but also in distension of the ear canal walls
(Holte et al., 1991:2).
•
Change in the orientation of the tympanic membrane. The eardrum in
the infant ear canal is orientated at a flatter angle with respect to the
ear-canal axis (Keefe & Levi, 1996:362).
•
Tightening of the ossicular joints and close coupling of the stapes to
the annular ligament results in changes in resistance (Meyer et al.,
1997:190).
•
Early changes include the fusing of the tympanic ring. This process
involves mechanical changes, which influence the tympanogram
shape in neonates.
2.3.2 Tympanic membrane
The tympanic membrane forms the boundary between the external auditory
canal and middle ear cavity and is therefore the most visible structure of the
middle ear. The membrane is a thin, translucent, connective tissue membrane,
covered by skin on its external face and by a mucosa lining internally. It is
shaped like a flattened cone, with its apex protruding medially into the middle
ear. It has both a tense portion (pars tensa) as well as a smaller, more flaccid
portion known as pars flaccida (Roush, 2001:5; Marieb, 1995:527).
The
20
orientation of the structures of the tympanic membrane and middle ear changes
from infancy to adulthood. The infant tympanic membrane is almost horizontal,
the lateral process of the malleus is most prominent and the pars flaccida is
thicker and more vascular. Conversely the adult membrane is more vertical,
has a less prominent lateral process of the malleus, and the pars flaccida
appears more vascular (Roush, 2001:36). When sound waves enter the
external meatus, the tympanic membrane moves in response to incoming
sound waves and transmit the vibratory patterns to the middle ear ossicles
(Marieb, 1995:528). Pathological conditions, such as middle ear effusion, affect
the movement of the ossicles and in turn that of the tympanic membrane,
causing a decrease in the transmission capability.
2.3.3 Eustachian tube and its relation to middle ear function
The middle ear space behind the tympanic membrane in a healthy ear is
normally filled with air.
This air space connects to the nasopharynx (the
superior-most part of the throat), by the Eustachian tube running obliquely
downward to link the middle ear cavity with the nasopharynx. In an adult, the
tube is between 30mm and 40mm in length and comprises one-third bone at the
middle ear end and two-thirds cartilage. The adult Eustachian tube lies in a
relatively vertical position providing protection for the middle ear. (Marieb,
1995:528).
In contrast, an infant Eustachian tube is short, horizontal and
composed of relatively flaccid cartilage. The nearly horizontal position for the
infant’s Eustachian tube allows for retrograde reflux of bacteria from the
nasopharynx into the middle ear (Northern & Downs, 2002:65).
Because
transient middle ear dysfunction is more prevalent in neonatal ears than
cochlear or sensorineural hearing losses, there is a need for a better
understanding of the middle ear functioning in neonates.
In the following
section a brief review of the functioning of the Eustachian tube will be given.
The Eustachian tube is thought to have a number of functions of which the most
important is that of equalizing the pressure between the middle ear and the
external air pressure, and to ventilate the middle ear system (Wiley & Fowler,
21
1997:67). The mucosa of the middle ear absorbs oxygen from the contained air
thereby reducing the middle ear pressure. The Eustachian tube facilitates the
passage of air from the nasopharynx to replace the absorbed oxygen. The
Eustachian tube opens momentarily during swallowing, yawning or jaw
movements (active opening) or if the pressure gradient existing between the
aural and pharyngeal cavities overcome the closing forces of the tube (passive
opening). If, due to obstruction or dysfunction, the Eustachian tube is unable to
open, air in the middle ear will be absorbed by the blood vessels in the lining of
the middle ear (Northern & Downs, 2002:65). As no air is able to go up the
Eustachian tube to replace the decreasing air in the middle ear, a vacuum
develops. A non-functioning Eustachian tube may therefore lead to negative
middle ear pressure resulting in retraction of the tympanic membrane. This will
in turn cause fluid, which is normally secreted by the mucous membrane lining,
to be sucked into the middle ear space resulting in the formation of middle ear
effusion (Martin & Clark, 2000:255). This is known as serous otitis media and is
often associated with middle ear disorders and conductive loss of hearing
(Wiley & Fowler, 1997:68). As equalizing pressure changes in the middle ear
cavity is the primary role of the Eustachian tube, the development of negative
middle ear pressure can be viewed as a marker of Eustachian tube dysfunction
(Palmu et al., 2001:62).
The ability to equilibrate over- and under-pressure by swallowing or jaw
movement has been shown to be poorer in children than adults and markedly
poorer in infants. Poorer muscle control has been suggested as the reason for
the reduced occurrence of opening of the Eustachian tube resulting in reduced
pressure equalisation in children (Northern & Downs, 2002:65). Furthermore,
due to the anatomical differences between the adult and infant Eustachian tube,
as previously discussed, the higher incidence of middle ear disorders in the
early years of life implies that Eustachian tube immaturity can be linked to the
development of these disorders (Northern & Downs, 2002:65). A more detailed
review of pathological conditions of the middle ear, linked to Eustachian tube
dysfunction, will therefore be provided.
22
2.4
FACTORS AFFECTING MIDDLE EAR STATUS IN INFANTS
Middle ear effusion (MEE), otherwise known as otitis media with effusion
(OME), is a common condition infants experience at some time during the early
years of life (Northern & Downs, 2002:65; Roush, Drake & Sexton, 1992:63).
The presence of middle ear effusion can cause persistent conductive hearing
loss and although rare, untreated chronic otitis media can result in serious
medical conditions such as cholesteatoma, meningitis, and sensorineural
hearing loss (Martin & Clark, 2000:258).
As previously mentioned, the Eustachian tube plays an important role in the
ventilation of the middle ear, and is often associated with the development of
otitis media, when dysfunctional (Northern & Downs, 2002:71). Otitis media, or
inflammation of the middle ear, is highly prevalent in young children, especially
during the first two years of life (Northern & Downs, 2001:65; Roush, 2001:11).
Although it has been classified in many ways, Roush (2001:11) describes two
basic forms: Acute otitis media and serous otitis media. Other authors have
included terms such as secretory, suppurative and non-suppurative otitis media
(Northern & Downs, 2002:66; Martin & Clark, 2000:258; Hogan et al., 1997:1).
Acute otitis media usually presents with sudden onset as the result of viral or
bacterial agents invading the middle ear tissues, causing otalgia, fever and
general discomfort. Acute otitis media often occurs during upper respiratory
tract infections, as the mucous lining of the middle ear is continuous with that of
the pharynx. Acute upper respiratory tract illnesses are frequently treated with
prescription antimicrobials, and over use is known to occur, which is associated
with the development and spread of bacteria with antimicrobal resistance. In
the United States evidence-based guidelines to improve the judicious use of
antimicrobial agents in children have been developed by the Centres for
Disease Control and Prevention (Garbutt, Jeffe and Shackelford, 2003:143). In
a study to assess compliance with the judicious use of antimicrobials in children
with acute otitis media, Garbutt et al., (2003:144) hypothesized that as acute
otitis media accounts for 30% of all pediatric antimicrobial prescriptions, over
23
diagnosis of acute otitis media and over treatment with antimicrobials is thought
to occur (Garbut et al., 2003:143).
The second form, Serous otitis media or otitis media with effusion (OME),
involves secretion of fluid from the mucous membrane lining of the middle ear.
In contrast to acute otitis media, OME is generally characterized by non-infected
(serous) effusion drawn into the middle ear cavity as a result of poor Eustachian
tube function. Thus, OME is defined as inflammation of the middle ear with fluid
present, but without obvious signs of ear infection (Roush, 2001:12).
It is
important to note that OME is more likely to escape detection than acute otitis
media as it is usually asymptomatic, except for the temporary hearing loss
caused by the middle ear effusion (Roush 2001:12).
The nature and prevalence of middle ear disorders is also of specific relevance
to the implementation of newborn hearing screening programs, as these are
among the most common diseases affecting young children (Roush et al.,
1992:63). As previously noted, objective measures used in newborn hearing
screening (such as evoked otoacoustic emission or ABR testing) can be
adversely affected by the presence of MEE (Kei et al., 2003:384, Sininger,
2003:380, Thornton et al., 1993:319) and can result in false positive referrals
due to transient middle ear conditions (Margolis et al., 2003:384. As noted by
Trine, Hisrch and Margolis (1993:401), the middle ear is involved in the delivery
of the evoking stimulus to the cochlea and in the transmission of the OAE to the
ear canal. The recording of an otoacoustic emission therefore has a twofold
dependence upon the transmission characteristics of the middle ear.
Estimates of prevalence of MEE in infants vary, but many studies substantiate
that most children experience at least one episode of otitis media in early
childhood with peaking in prevalence between 6 and 12 months of age
(Northern & Downs, 2002:65; Gliddon & Sutton, 2001:77; Roush et al.,
1992:63). Gliddon and Sutton (2001:84) reported a rise in the prevalence of
abnormal tympanometry from 2% neonatally to 39% at eight months of age. In
view of the context of the current study it is important to note that race, birth
24
history and home environment has been identified as increased risk factors for
otitis media (Woods, 2003:687; Gliddon & Sutton, 2001:78).
There is also
evidence that higher numbers of siblings, male sex and lower maternal socioeconomic status are associated with more persistent MEE (Gliddon & Sutton,
2001:78). A study to determine prevalence of middle ear disorders in black
children in a specific South African context has shown a prevalence of 13%
among four to five year old children (Bhoola & Hugo, 1995:19); however as
prevalence is known to be higher in the first years of life, an elevated incidence
can be predicted in the infant population generally.
Identification of children with OME should therefore be one of the primary
objectives of screening procedures to identify screening failures caused by
transient external- or middle ear conditions, to establish the need for medical
management and to determine the need and timing of follow-up procedures
(Margolis et al., 2003:384). As correct diagnosis dictates treatment, the Centres
for Disease Control and Prevention (Garbut et al., 2003:143) recommend the
use of specific diagnostic criteria to prevent the over diagnosis and treatment of
otitis media.
These criteria include the presence of otorrhea of middle ear
origin, or the presence of middle ear effusion and signs or symptoms of acute
local or systematic illness. Pneumatic otoscopy is recommended to confirm the
presence of middle ear effusion (Dowell, Marcy, Phillips, Gerber & Schwartz,
1998 in Garbutt et al., 2003:143).
Various other methods have also been
employed to assess middle ear functioning and to aid in the diagnosis of otitis
media. Questions regarding the reliability, suitability and merit of performing
different methods of middle ear assessment on the infant ear have been raised
in various reports (Casselbrant, Gravel, Margolis & Marchisio, 2002:95; Holte &
Margolis, 2002:387; Kei et al., 2003:21; Palmu et al., 1999:207; Sininger,
2003:378). A critical review of methods of middle ear assessment, with specific
relevance to assessment of the infant middle ear, will be given in the following
section.
25
2.5
CRITICAL REVIEW OF MIDDLE EAR ASSESSMENT METHODS FOR
INFANTS
“There is still no better, quicker, or less expensive single audiologic procedure for assessing the
status of the middle ear, cochlea, eighth nerve, and lower auditory brainstem than performing a
complete immittance test battery.” Hall and Mueller (1997:177)
This classic quote by Hall and Mueller (1997:177) clearly points out that the
most commonly used and reliable tool in the identification and diagnosis of
middle ear pathology, is acoustic immittance measures (Holte & Margolis,
2002:387; Northern & Downs, 2002:210). Immittance measurement has
widespread clinical use and has an established role in the audiological
assessment of hearing and in the diagnosis of middle ear pathology (Palmu et
al., 1999:207).
In infant testing, however, use has been controversial as previously described.
Alternative test methods and procedures have been investigated in an attempt
to identify a test protocol with both high sensitivity (rate of correct classification
for affected individuals) and specificity (rate of correct classification for
unaffected individuals) in the diagnosis of middle ear effusion in infants.
Factors, such as age and developmental status, state of alertness and available
instrumentation also influence the selection of methods to assess middle ear
functioning (Roush, 2001:33). Prior to a review of immittance measures, an
overview of two other methods to establish middle ear status in infants,
otoscopy and Otoacoustic emission testing will be described.
Practical
implications for their application in neonatal and infant populations are included.
2.5.1 Otoscopy
Visualisation of the major landmarks of the tympanic membrane and middle ear
signs of external ear disease and canal obstruction can aid in the diagnosis of
middle ear dysfunction (Roush, 2001:36). The tympanic membrane is the most
visible structure of the middle ear, and is consequently the most important in
making clinical judgements about middle ear disease via otoscopy (Govender,
26
1998:33). Sutton et al., (2002:3), recommend that otoscopy should be carried
out on all infants prior to tympanometry to rule out occlusion of the external
auditory canal by wax, vernix or debris. However, otoscopic investigation is
very difficult to perform in newborns and infants because the tympanic
membrane is often obscured from view by vernix, the neonatal ear canal is very
narrow, and the tympanic membrane has a different orientation and appearance
(Gliddon & Sutton, 2001:78).
Difficulty in visualizing the infant tympanic
membrane is often also increased by lack of infant co-operation (Palmu et al.,
1999:208).
Assessment of otoscopic results relies purely on subjective evaluation, skill, and
experience of the examiner (Govender, 1998:39). While disorders of the ear
canal are often visible through otoscopic inspection, disorders of the middle ear
are often not apparent based on otoscopic inspection of the tympanic
membrane
and
thus,
require
more
advanced
evaluation
techniques
(Casselbrant et al., 2002:388, Wiley and Fowler, 1997:11).
Pneumatic otoscopy allows application of positive and negative pressures in the
ear canal to visually assess the movements of the tympanic membrane
(Casselbrant et al., 2002:95).
In a study to determine the usefulness of
pneumatic otoscopy in predicting middle ear effusion in children, Govender
(1998:101), compared the sensitivity and specificity of pneumatic otoscopy and
tympanometry as compared to myringotomy to confirm the presence of middle
ear effusion.
Results indicated that, when compared to myringotomy,
pneumatic otoscopy showed good sensitivity (80%) and moderate specificity
(60%) while tympanometry showed excellent sensitivity (96%) and good
specificity (80%) (Govender, 1998:103).
This finding is consistent with the
reports of others who noted that tympanometry was more accurate than
pneumatic otoscopy in detection of middle ear effusion (Casselbrant et al.,
2002:95; Govender, 1998:103; Holte & Margolis, 2002:388).
Nozza et al.,
(1994:310) also found that otoscopy alone had good sensitivity but only fair
specificity. Roberts et al., 1995 (in Gliddon & Sutton, 2001:78) found that interobserver agreement was poor for pneumatic otoscopy during the first three
27
days of life, but that by 2 weeks it had returned to acceptable levels, suggesting
that this test is not valid for use on very young infants.
Otoscopy therefore does not prove to be a sufficient single diagnostic indicator,
which is both sensitive and specific, for the identification of middle ear effusion
in infants and should be used as an adjunct to other screening procedures
(DeConde Johnson, 2001:485). Furthermore, as middle ear effusion is most
difficult to detect with otoscopy in the paediatric population (Pellet, Cox &
MacDonald, 1997:181) and is a technique that requires significant experience
for reliable interpretation of results, it is not always routinely performed on
neonates (Northern & Downs, 2002:79). Therefore it is necessary to turn to
other procedures for the assessment of middle ear functioning in the infant
population.
As OAEs are frequently used in newborn hearing screening
programs, this proves a viable option to be reviewed as a method of middle ear
assessment in addition to the assessment of hearing.
2.5.2 Otoacoustic Emissions (OAEs)
Successful recording of OAEs not only requires a normal functioning cochlea,
but also necessitates normal or near-normal middle ear function (Koivunen et
al., 2000:212). Consequently present OAEs are a useful indication of normal
middle ear function and transmission, whilst absent OAEs can be an indication
of either a middle ear or a sensory pathology. When used in conjunction with
ABR screening, the OAE becomes very useful, since the OAE is more sensitive
to middle ear effusion than the ABR screening (El-Refaie et al., 1996:7). Before
being measured in the external auditory canal, all OAEs must pass through the
middle ear and hence changes in sound conduction of the middle ear, such as
middle ear effusion and negative pressure within the middle ear, affect the
detection of otoacoustic emissions in the external auditory canal. This is due to
the fact that the presence of middle ear effusion and negative pressure within
the middle ear, changes elasticity and suppresses conduction of sounds
through the middle ear (Yeo et al., 2002:794). OAE measurement relies on the
middle ear conductive mechanism first to transmit the stimuli to the inner ear
28
and then to convey the cochlear emissions, by way of reverse transmission,
back into the ear canal where recording takes place (Yeo et al., 2002:797,
Sutton et al., 1996:10). Middle ear effusion therefore affects OAEs both by
reducing their transmission from the cochlea through the middle ear and by
attenuating the stimulus reaching the cochlea.
Results of recent studies have confirmed that middle ear effusion affects the
expression rate of Spontaneous OAEs, Transient evoked OAEs and Distortion
Product OAEs and would aid in evaluating the middle ear condition. (Yeo et al.,
2002:798). Strong statistical significance has been found between abnormal
tympanometry results and OAE failure (Thornton et al., 1993:322; Sutton et al.,
1996:10). Casselbrant et al., (2002:96) also pointed out that since some ears
with MEE have minimal hearing loss, the presence of MEE reduces the
specificity of OAEs as a screening technique for detection of significant hearing
impairment. However, OAEs may be effective as a combined screen for middle
ear disease and sensori-neural hearing loss.
Using OAEs as a combined test for middle ear functioning and cochlear integrity
appears promising, yet, passing an OAE test cannot serve as a “gold-standard”
(Kei et al., 2003:26) for normal middle ear function as OAEs have been found to
be present in ears with middle ear dysfunction in children and adults (Margolis
et al., 2003:385, Thornton et al., 1993:320). Furthermore an absent recording
of OAEs does not provide exact information regarding the degree or
configuration of the hearing loss and is unable to differentiate between a pure
sensory-neural hearing loss, and conductive hearing loss with underlying middle
ear conditions, and consequently it is of limited value in the diagnosis of middle
ear function when no OAEs are recorded (Koivunen et al., 2000:216, Northern
& Downs, 2002:234; Sininger 2003:380).
Because transient middle-ear
dysfunction may be more prevalent in neonatal ears than cochlear or sensorineural hearing losses, there is a critical need for a better understanding of
middle ear functioning in infants (Keefe et al., 2000:443).
29
Although AABR and OAE testing are widely used in newborn hearing screening
programs, they do not provide diagnostic information in their screening mode.
ABR and OAE screening does not effectively distinguish between mild
sensorineural hearing loss and conductive hearing loss.
More complicated
diagnostic ABR procedures that compare air-and-bone conduction can be used
effectively to differentiate conductive hearing loss from sensorineural hearing
impairment. However, the analysis time and level of expertise that this type of
instrumentation demands of the user makes it too difficult, costly, and therefore
inappropriate as a screening tool (Shahnaz, 2002:1).
Furthermore, as
previously mentioned passing an OAE test in the strictest sense, cannot
guarantee normal middle ear function as OAEs have been found to be present
in some ears with middle ear effusion (Kei et al.,2003:26; Thornton et al.,
1993:323).
Therefore an objective method for middle ear assessment is
essential for use in conjunction with OAE and AABR screening procedures for
correct diagnosis and identification of otitis media.
As a test of middle ear function, immittance measurements have proven to be
the best single diagnostic indicator for the identification of middle ear pathology
Hall and Mueller (1997:177). Immittance measures have widespread clinical
use and are established to accurately identify middle ear pathology in adults
and older children (Palmu et al., 1999:207). A review of immittance
measurements, as a technique for middle ear assessment, will therefore be
provided in the following section.
2.5.3 Immittance measurement
Immittance is a generic term that encompasses tympanometry and acoustic
reflex
measurements.
Current
commercially
available
immittance
instrumentation measures acoustic admittance and / or its components,
acoustic
susceptance
and
acoustic
conductance
(Lilly,
2005:24).
Tympanometry, as currently defined, refers to measures of acoustic admittance
that are taken at various pressure points, and is an objective procedure used to
evaluate the mechanical characteristics of the tympanic membrane and middle
30
ear as pressure changes are created in the ear canal (Roush, 2001:67). All
determinations of middle ear function are indirectly made by measurements
made in the plane of the tympanic membrane (Martin & Clark, 2000:152). These
values are graphed to form a tympanogram (Fowler & Shanks, 2002:175).
Tympanometry and recording of acoustic reflexes are objective tests of
tympanic membrane mobility, middle ear pressure and of brainstem auditory
activation (Sininger 2003:37) and the primary rationale for the clinical use of
immittance measures is that they are sensitive to middle ear disorders.
Additionally an acoustic immittance procedure requires no behavioural
response from the patient and is a feasible method of middle ear assessment in
the infant population (Wiley & Fowler, 1997:2).
Immittance measures have
potential for improving diagnostic accuracy of otitis media in clinical practice,
and have long been accepted as a reliable and valid method of assessing
middle ear function in children above the age of approximately seven months of
age, however, suspicion has been cast over the usefulness thereof for infants
below this age in earlier reports (Gliddon & Sutton, 2001:78; Palmu et al.,
2001:178; Purdy & Williams, 2000:9; Meyer et al., 1997:189, 190).
The following sections will expand on the current outlook, as found in recent
literature, on the use of immittance measurements. The implications for the
infant population will be reviewed with renewed interest in high frequency
tympanometry. An in-depth discussion of the fundamentals of immittance is
however necessary to evaluate its application in infants critically and this will
follow in section 2.6.1. A discussion of the underlying physical and acoustic
principles of acoustic immittance will also be included.
Knowledge of the
underlying structures and physical principles involved in acoustic measures of
middle ear functioning is important to understand the assessment of the infant
middle ear and the controversies surrounding it. These aspects are therefore
reviewed in the following section.
31
2.6
CRITICAL
REVIEW
OF
IMMITTANCE
MEASUREMENTS:
PRINCIPLES AND APPLICATION IN INFANTS
Developmental changes in the infant middle ear influence reliable and valid use
of immittance measures in this population. In order to understand the effects of
these developmental changes and other influencing factors, a discussion of the
underlying physical and acoustic principles of acoustic immittance is provided in
the subsequent sections.
2.6.1
Fundamentals
and
principles
of
acoustic
immittance
measurements
Acoustic immittance is a collective term that is used to describe the transfer of
acoustic energy, whether measured in terms of acoustic admittance (flow of
energy, Ya) or acoustic impedance (opposition to the flow of acoustic energy,
Za) or both terms (Ferekidis, 2003:59; Roush, 2001:66; Wiley & Fowler,
1997:1,8; Silman & Silverman,1991:71,77). The character of the energy system
under measurement is signified by the associated term, acoustic, and is
indicated by the subscript, a, appended to each abbreviation (Wiley &
Stoppenbach, 2002:161; Wiley & Fowler, 1997:8). Acoustic admittance (Ya)
refers to the ease with which sound energy flows through an acoustic system,
while acoustic impedance (Za) refers to the opposition to the flow of sound.
Acoustic admittance and impedance are direct reciprocals and therefore if an
acoustic transmission system, such as the human middle ear, has a high
acoustic admittance, it has low acoustic impedance. Conversely if the middle
ear has a low acoustic admittance, it has high acoustic impedance (Wiley &
Stoppenbach, 2002:161; Wiley & Fowler, 1997:1). Both terms have been used
to describe acoustic measurements of middle ear function, although current
commercially available acoustic immittance instruments typically provide
measures in terms of acoustic admittance (Wiley & Stoppenbach, 2002:161;
Shahnaz, 2002:1; Roush, 2001:66; Wiley & Fowler, 1997:7; Jerger & Northern,
1980:1). An example of how impedance and admittance tympanograms are
graphically recorded and illustrated by current commercially available
32
tympanometers is given in Figure 2.1.
Note that because impedance and
admittance are reciprocal quantities, the phase relations are reversed, and
therefore a mirror image is obtained (Fowler & Shanks 2002:163).
Figure 2.1
Graphic illustration of impedance (Z) vs. admittance (Y)
tympanograms (Adapted from Wiley & Fowler, 1997:3)
In order to effectively understand and use acoustic immittance measures,
knowledge of the fundamental physical and acoustic principles, coupled with
knowledge of the basic anatomy and physiology of the middle ear structures
involved in these measures, is essential.
Regardless of the specific instrument used to assess acoustic admittance or
impedance, the basics of measurements are the same and require at least
three primary subsystems as illustrated in Figure 2.2: a) a sound pressure
source, b) a means of varying and monitoring air pressure changes in the ear
33
canal and c) an analysis system for monitoring the SPL of the probe signal
(Wiley & Fowler, 1997:8). If acoustic reflex measures are performed, a fourth
subsystem (d) providing a test signal source for eliciting of the acoustic reflex
would be included in the measurement system.
a) PROBE UNIT
(Sound pressure source /
Signal Driver)
226 Hz / 678 Hz / 1000 Hz
b) AIR PUMP
and
MANOMETER
Probe
Tip
c) ANALYSIS SYSTEM
(Recording of acoustic
immittance measures)
d) SECOND SOUND
SOURCE
(Eliciting of acoustic reflex)
Figure 2.2
Diagram of electroacoustic immittance instrumentation
(Adapted from Wiley & Fowler 1997:9 and Wiley & Stoppenbach, 2002:168)
The measurements of acoustic immittance is performed by introducing an
acoustic signal (probe tone) to the ear and measuring the sound pressure level
(SPL) of the signal in the ear canal as air pressure changes are varied and
monitored (Wiley & Fowler, 1997:8). The SPL of the probe tone, measured at
the probe tip, serves as an indirect index of acoustic admittance or impedance
as it is directly proportional to the acoustic impedance offered by the ear at the
point of measure (Wiley & Fowler, 1997:8). Higher SPL measurements of the
probe tone indicate higher levels of acoustic impedance or conversely lower
levels of acoustic admittance, offered by the ear under measurement.
34
Although the ideal point of measure is the tympanic membrane, it is not feasible
to place a probe tip next to the eardrum using commercially available
instruments. Hence, the tip of the probe unit is the point at which the acoustic
immittance analysis system receives input and implies plane of measurement
(Wiley & Fowler, 1997:11).
Measures are therefore termed compensated
acoustic measure, referring to the extraction of ear canal effects (Wiley &
Fowler, 1997:12).
Acoustic immittance measures in the human ear can be explained on the bases
of mechanical, electrical, and mathematical principles. In the human ear, a
specified peak compensated acoustic impedance value would represent the
total opposition to sound flow offered by the ear under test at the lateral surface
of the tympanic membrane. This measure represents the acoustic impedance
at the input to the middle ear (Wiley & Fowler, 1997:13).
The middle ear system is composed of different mechanical structures. These
react to sound and pressure waves in a variety of ways, transferring energy and
causing changes in state of the mechanical structures of the middle ear system.
These do not always occur instantaneously when a force is applied.
An
acoustic impedance measure is determined by the complex relation of the
applied force (sound pressure) to the velocity (or sound flow) (Wiley & Fowler,
1997:13). The way in which different structures oppose sound flow is complex
across middle ear components. The overall acoustic impedance or admittance
measured at the probe tip is determined by the multifaceted contributions of the
volume of the external ear canal, the tympanic membrane, the interconnected
cavities of the middle ear, the ossicular chain, and the coupling of the stapes
footplate to the oval window of the cochlea. For example, the introduction of
substantial (positive or negative) ear canal pressure effectively stiffens the
tympanic membrane and middle ear transmission system which affects
immittance measures proportionately (Wiley & Fowler, 1997:14).
It is clear
therefore that a number of mechanical variables within the middle ear affect
immittance measurements.
A brief discussion of the mechanical variables
within the middle ear is presented below.
35
2.6.2
The middle ear as a mechanical system
In the ear, the tympanic membrane is mechanically coupled with the middle ear
ossicles to the oval window – the interface between the middle and inner ear. It
is this entire system (membrane, middle ear and oval window) that is forced into
oscillation when sound waves enter the ear canal (Gruber, 2002:1). Under
normal circumstances, when a sound wave enters the ear canal, it progresses
inward until it reaches the tympanic membrane where some of its energy sets
the membrane into vibration and some is reflected. The tympanic membrane is
stimulated by sound energy, which in turn, sets the ossicular chain into motion.
Movement of the malleus, which is attached to the tympanic membrane, sets
the incus into motion, which in turn triggers the stapes to move. The stapes is
set in the opening of the inner ear, the oval window. Sound is then conducted
via the stapes through the oval window into the cochlea. The mobility of the
ossicular chain therefore dictates to a great extent, the quantity of sound
perceived by an individual. If the middle ear system is stiffened or disturbed for
any reason, the amount of energy transmitted to the oval window will be altered
(Martin & Clark, 2000:253).
The tympanic membrane and ossicular chain also act as an impedancematching device to bring sound from the air into the cochlea. Since the cochlea
is fluid-filled, it presents a reflective barrier to airborne sound. The impedancematching capability of the middle ear is primarily related to the ratio of the area
of the tympanic membrane to that of the stapes of the footplate (Martin & Clark,
2000:254).
There are three factors that determine how much energy is
accepted or reflected and in what frequency ranges. These factors are the
mass of the system, the stiffness of the system, and its resistance (Jerger and
Northern, 1980:3).
2.6.3
Mass, stiffness and resistance components of the middle ear
The acoustic impedance measured at the tympanic membrane is controlled by
the mass of the middle ear ossicles, the stiffness of the ossicular ligaments and
36
muscles, the stiffness of the tympanic membrane and round window membrane,
the stiffness of the air contained in the tympanum, the mass and friction that
result from air movement within the tympanum, and, finally, the impedance,
primarily resistance, offered by the coupling of the stapes footplate to the
cochlea at the oval window (Lilly, 1993 and Zwislocki, 1976 in Wiley & Fowler,
1997:14).
In terms of the relative contributions of the mass, stiffness and resistance
components to the overall opposition to energy flow, the components may be
categorized as in-phase (those that occur simultaneously with the applied force)
and out-of-phase (those that precede or lag behind the applied force). The inphase component of impedance is known as resistance (R) and the out-ofphase component is known as reactance (X). The complex relations between
resistive and reactive components determine the total acoustic impedance
(Fowler and Shanks, 2002:162).
In the human ear energy transfer is initiated when sound waves are presented
to the ear canal and sound pressure is applied to the tympanic membrane
(Fowler and Shanks, 2002:162).
The effects of each of the mechanical
components are determined by the frequency of the sound entering into the ear
canal. Sound waves, entering the ear canal and impinging upon the tympanic
membrane, have both inward and outward motions that exert forces on the
middle ear structures. Consequently we have to consider the incoming force
not only in the positive direction, but also in the negative direction, and all points
in time between maximum positive and maximum negative deflection.
The
acceptance or reflection of energy in the middle ear is, largely, frequency
dependent (Jerger & Northern, 1980:3).
Mathematically, the impedance
equation can be expressed as follows, indicating the effect of frequency (Jerger
and Northern 1980:3):
___________________
Z = √R² + 6.28 fM – k²
6.28f
Resistance
└──┬──┘
└──┬──┘
Mass
Stiffness
Z= impedance
R = Resistance
f = frequency M = Mass k = stiffness
37
An admittance Cartesian plot is presented in Figure 2.3 to illustrate the effect of
frequency of measurement tone on the resultant acoustic admittance.
Stiffness controlled
90°
Bj
678 Hz
Admittance
Vector
System
Susceptance (B)
226 Hz
0° (G)
1000 Hz
- Bj
- 90°
Figure 2.3
Mass controlled
Influence of frequency on admittance vector system (adapted
from Wiley & Stoppenbach, 2002:166, 189)
If the stiffness of a given system increases, the effect of the increased stiffness
will be to weaken transmission in the low frequencies. If we increase the mass
of the system, the effect will be to weaken transmission in the high frequencies.
Thus measures made using low frequency probe tone provide information on
the stiffness (reciprocal = compliance) characteristics of the middle ear, while
measures made with high frequency probe tones provide information regarding
the mass characteristics of the middle ear, particularly of the eardrum and
ossicles (Fowler & Shanks, 2002:187). This proves to be especially useful in
testing of the infant middle ear, which is a mass-dominated system (Holte et
al., 1991:20). Meyer et al., (1997:193) found the use of a 226 Hz probe tone in
the neonate and infant to be less sensitive to measuring the acoustic
characteristics of the middle ear as it is more sensitive to measuring the
stiffness-dominated system of the adult-ear. Further findings indicated that the
use of a high frequency probe tone was more suitable for use in infants as it is
38
more sensitive to changes in mass-dominated systems (Meyer et al.,
1997:194). Despite the advantages of high frequency tympanometry in
identifying mass-related middle ear pathology, high frequency and multi
frequency tympanometry probe tones are not commonly used.
The major
barrier to incorporating high frequency tympanometry into the routine test
battery is that the measures are more complex and not as easily understood as
226 Hz tympanometry (Fowler & Shanks, 2002:187).
Developmental changes that occur in the infant ear canal (see section 2.3.1)
are postulated to contribute and have an effect on the stiffness, mass and
resistance components of the infant middle ear (Petrak, 2002:1; Meyer et al.,
1997:190; Holte et al., 1991:20).
As previously described stiffness of the
middle ear controls the low frequency responses and is largely determined by
the tympanic volume and to a lesser extent the fibrous nature of the tympanic
membrane (Meyer et al., 1997:190). It is postulated that there is an overall
decrease in the stiffness with an increase in cavity size and maturation. The
orientation of the neonatal tympanic membrane also changes post-natally due
to inward displacement of the tympanic ring to the more vertical position of the
adult tympanic membrane. The ossicles predominately determine the mass of
the middle ear and controls high frequency responses. Developmentally there
is a net overall decrease in the mass components in the middle ear due to the
decreased density of the stapes through internal bone erosion, and the gradual
loss of mesenchyme clinging to the ossicles.
The resistive component of
admittance comprises mainly the frictional forces opposing motion. There is a
change in resistance during maturation due to tightening of the ossicular joints,
closer coupling of the stapes to the annular ligament and a resultant change in
the reactive force exerted by the cochlear fluids (Meyer et al., 1997:190). The
developmental changes result in a mass-governed middle ear transmission
system gradually changing to the more adult-like stiffness-dominated system.
The preceding overview of the mechanical properties of the middle ear, which
included the effects of mass and stiffness elements in the adult and infant
39
middle ear, serves as background to a review of the clinical application of
tympanometry as a method of middle ear function in infants.
2.6.4 Clinical application of tympanometry
The primary rationale for performing tympanometry on a patient is to assess the
functioning of the middle ear system and to determine the existence and
potential causes of middle ear disorders (Petrak, 2002:1; Palmu et al.,
1999:207; Gaihede & Ovesen, 1997:215; Wiley & Fowler, 1997:1,39; LaRossa
et al., 1993:32). Consequently tympanometry has an established role in the
audiological assessment of hearing and in otological studies as an objective tool
for diagnosing middle ear pathologies (Palmu et al., 1999:207; Wiley & Fowler,
1997:2).
There are several tympanometric methods ranging from vector
(single-frequency, single component) tympanometry to multi-component and
multi-frequency tympanometry.
A brief description of these methods will
follow.
2.6.4.1
Vector tympanometry
Vector (single frequency, single component) tympanometry involves the
measurement of one component, generally the acoustic admittance vector.
Conventionally a single, low frequency probe tone (typically 226 Hz) has been
used. By using only one probe-tone frequency, the view of the middle ear is
restricted, almost as if you were to try to estimate hearing sensitivity by testing
thresholds at only one frequency. Disorders that exert an influence on middle
ear mechanics only at the high frequencies, such as those adding mass to the
system, will not be evident using only the low-frequency probe tone (Wiley &
Fowler, 1997:40).
Measurement of vector tympanograms involves the application of pressure,
which is varied from a positive pressure to a negative pressure. The resulting
changes in acoustic admittance are measured and plotted as a tympanogram.
At the extremes of pressure, the tympanic membrane and middle ear system
40
stiffen and the ear canal effectively becomes a hard-walled cavity.
This
necessitates specific consideration when testing the infant ear, as ear canal wall
distensibility has been held responsible for unusual tympanometric shapes
(Holte et al., 1991:19). The infant ear canal and tympanic membrane are more
vulnerable to changes in middle ear pressure and therefore a high negative
pressure could cause a collapse of the canal whilst a high positive pressure
could result in distension of the ear canal walls (Holte et al., 1991:21). Wiley
and Fowler (1997:59) supported this notion stating that as the infant ear canal is
cartilaginous, it cannot be modeled as a hard-walled cavity as it is in adults.
Little acoustic energy passes through the tympanic membrane and the majority
of acoustic energy is reflected into the ear canal. At the pressure extremes,
therefore, the acoustic admittance of the middle ear is minimal. As the ear
canal pressure approaches atmospheric pressure (0 daPa) less energy is
reflected and more pass into the middle ear. Near atmospheric pressure, the
normal tympanic membrane passes most of the acoustic energy into the middle
ear; therefore, the acoustic admittance is at its highest value.
The normal vector tympanogram, therefore, has a single, positive peak near
atmospheric pressure. Because the normal middle ear system is dominated by
compliant acoustic susceptance, manufacturers commonly call the single
component compliance, although in reality the single component is the vector
component, acoustic admittance.
(Wiley & Fowler, 1997:40,41).
The
susceptance (B) and conductance (G) components fully determine the
admittance (Y) of the middle ear system. The conductance component is in
phase with the delivered probe tone, whereas the susceptance is an out-ofphase component. Consequently the susceptance and conductance elements
can be separated by analysis of the phase of the reflected probe tone (Lantz,
Petrak & Prigge, 2004:38). This is used by middle ear analysers to plot B/G
tympanograms and simultaneous recording of B and G components is known
as multi-component
tympanometry.
An
overview of
multi-component
tympanometry is provided in the following section.
41
2.6.4.2
Multi-Component tympanometry
Multi-component tympanometry refers to measurement of the two components,
susceptance (Ba) and conductance (Ga), which provide an adequate view of the
magnitude and direction of the admittance.
Evaluation of these two
components is more important for the interpretation of tympanograms measured
with high frequency probe tones than for tympanograms measured with a 226
Hz probe tone because of the variety of tympanometric shapes that occur in
normal as well as abnormal ears at higher probe frequencies (Wiley & Fowler,
1997:56). The various shapes indicate the relative contribution of mass and
stiffness to the admittance tympanogram and are used to separate normal from
pathological tympanograms and to determine the cause of the abnormalities
reflected in the tympanogram. Knowing whether a tympanogram has increased
mass or stiffness allows the clinician to identify the probable cause of the middle
ear disorder that has resulted in the changes in the tympanogram (Wiley &
Fowler, 1997:56).
Holte et al., (1991:23), suggesting that the complex
components of admittance must be measured to calculate the meaningful
values of static admittance magnitude, also noted the importance of performing
multi-component tympanometry.
Pathologic conditions alter the shape of tympanograms and therefore the goal
of the diagnostic use of tympanometry is to separate the changes that are
caused by pathologic conditions from the changes that are associated with
normal variability (Wiley & Fowler, 1997:44). Infant testing, however, has been
a controversial issue because of ambiguous results. Mass effects are better
evaluated with the component tympanometry and at high probe tone
frequencies.
2.6.4.3
Tympanometric peak pressure (TPP)
A rough estimate of the resting pressure within the middle ear is indicated from
the pressure location of the tympanograms peak (Wiley & Fowler, 1997:2).
Acoustic admittance is at its highest value when the pressures on both sides of
42
the tympanic membrane are equal. The diagnostic value of measuring TPP is
that it can detect the presence of negative pressure in the middle ear. If the
Eustachian tube becomes blocked by disease, negative pressure develops in
the middle ear before the development of effusion as the liquids from the
tissues surrounding the middle ear are drawn into the middle ear.
The
presence of negative middle ear pressure can therefore indirectly indicate a
problem with Eustachian tube function, often associated with the initial stages of
otitis media (Wiley & Fowler, 1997:2).
Tympanometric peak pressure is
considered to have little clinical value for screening purposes, due to amount of
normal pressure variation, which may lead to over-referral rates in screening
programs (Wiley & Fowler, 1997:49). This is in contrast to studies by Thornton
et al., (1993:320) who suggested that in assessment of infant middle ear
function by high frequency tympanometry, positive middle ear pressure was the
best indicator of middle ear dysfunction as correlated to OAE pass / fails results.
Further limitations are the fact that an unique tympanometric pattern does not
exist for every possible middle ear disorder, and thus clinical expertise and
classification systems for different tympanometric shapes are necessary for
correct diagnosis.
2.6.5 Classification systems in tympanometry
Various classification systems have been developed for conventional low
frequency tympanometry. However, due to the complex notching patterns that
can occur when the infant middle ear is assessed at different probe frequencies,
no universal classification system exists at present. According to the Vanhuyse
classification system (Vanhuyse et al. 1975 in Fowler & Shanks, 2002:191),
tympanometric shapes can be classified according to the number of extrema in
the susceptance and conductance tympanograms and assumptions can be
made about the contributions of mass and stiffness elements. As notching is a
more prevalent in neonatal and infant ears compared to adult ears at similar
frequencies (Holte et al., 1991:12), this classification system appears useful for
application in the infant population. Figure 2.4 provides a graphic illustration of
the Vanhuyse classification system and how mass and stiffness dominancy can
43
be derived from tympanograms shape. As the infant middle ear is a massdominated system with a lower resonant frequency, tympanograms will
progress differently compared to tympanograms collected from an adult middle
ear, which is stiffness controlled (Lantz et al., 2004:3, Purdy & Williams,
2000:12). Therefore the Vanhuyse model is a useful classification scheme at
higher probe frequencies close to middle ear resonance where complex multipeaked tympanograms are normal.
226 Hz
1000 Hz
1130 Hz
Figure 2.4
Vanhuyse classification model (Adapted from Fowler & Shanks,
2002:189,191)
Figure 2.4 illustrates that as frequency increases, the shapes of component
tympanograms progress from single peaked to notched tympanograms.
Vanhuyse et al., (in Fowler & Shanks, 2002:191) developed a model to explain
the variety of shapes for susceptance and conductance tympanograms.
Between 90° and 45° both susceptance and conductance tympanograms are
single peaked and this pattern is designated 1B1G. As the acoustic admittance
vector rotates between 45° and 0°, the susceptance (Ba) tympanogram notches,
but conductance (Ga) remains single peaked and this pattern is designated
3B1G. When the admittance vector rotates between 0° and -45°, both the
44
susceptance and conductance tympanograms notch; this forms the 3B3G
pattern (Fowler & Shanks, 2002:191).
As the tympanometry probe tone frequency is increased and approaches the
point of middle ear resonance (where mass and stiffness are equal) the shape
of the tympanogram becomes more complex and notching occurs (Purdy &
Williams, 2000:10). The resonant frequency is the probe tone frequency where
the mass and stiffness components are in balance, and susceptance becomes
zero due to the counteractive forces of its components (Ferekidis, 2003:60). A
normal value for resonant frequency for adult ear is around 900 – 1000 Hz
(Ferekidis, 2003:60; Purdy & Williams, 2000:11). Below this value the middle
ear system is stiffness-controlled and above the normal value mass-controlled
according to the more prominent component of susceptance (Petrak, 2002:1).
Another method of middle ear assessment that therefore merits investigation is
multi-frequency tympanometry. This is an advanced, sweep frequency method
of acoustic impedance measurement and provides values for the resonant
frequency of the middle ear.
2.6.6 Multi-frequency tympanometry
Multi-frequency tympanometry allows for middle ear function to be tested over a
wide frequency range (226 – 2000 Hz) to determine the resonant characteristics
of the middle ear. By means of multi-frequency tympanometry it is possible to
directly assess the resonant frequency of the middle ear system where mass
and stiffness elements are in balance (Ferekidis, 2003:60).
Meyer et al.,
(1997:194) hypothesized that the initial use of multiple frequency tympanometry
to determine the middle ear resonance frequency may enable the audiologist to
make appropriate probe frequency selection depending on the characteristics of
each individual middle ear. Changes in resonant frequency are used to assess
the pathology of the middle ear system, especially those of the ossicular chain
as any ossicular chain discontinuity lowers the resonant point and any stiffness
(for example otosclerosis) makes it higher (Ferekidis, 2003:60, Holte &
Margolis, 2002:389).
Multi-frequency tympanometry has been shown to
45
improve test sensitivity in some cases of otitis media (Holte & Margolis,
2003:389), however due to increased testing time and required duration of
infant co-operation and the need for enhanced operation and interpretation
skills, it does appear favourable as a screening tool for middle ear function in
infants.
However, acoustic reflexes measured with a high frequency probe
tone, appear a more valuable adjunct to high frequency tympanometry in infants
as part of a the assessment of middle ear functioning.
2.6.7
Acoustic reflex testing
Gelfand (2002:205) defines the acoustic reflex as the reflexive contraction of the
middle ear muscles as a result of sound stimulation. The stapedial muscle is
attached to the stapes.
When contracted, this muscle pulls the stapes
posteriorly, thus impeding its movement into the oval window (Martin & Clark,
2000:215). The most important parameter of the acoustic reflex is the acoustic
reflex threshold (ART). This is referred to as the lowest stimulus intensity at
which a contraction of the stapedius muscle can reliably be recorded. Acoustic
reflex measurement also has a high sensitivity for the detection of middle ear
pathology (Neumann, Uppenkamp & Kollmeier, 1996:359).
The acoustic reflex is detected as a change of middle ear compliance. Two
stimuli are presented to the subject: typically a low frequency tone of 226 Hz is
used to measure the compliance of the middle ear, and an additional high level
tone of limited duration is used to elicit the acoustic reflex (Neumann et al.,
1996:360).
If the level of the additional sound is high enough to elicit the
acoustic reflex, the acoustic compliance decreases and the level of the
measurement tone increases. The increase of the measurement tone level can
be detected with a microphone placed in the ear canal. The acoustic reflex can
be a very useful part of the audiologic evaluation in infants as a present
acoustic reflex is added support for normal middle ear function in infants
(Sininger, 2003:380).
46
Studies with newborns have demonstrated that the acoustic reflex is typically
not reliably detectable when a low frequency (226Hz) probe tone is used
(McMillan, Marchant & Shurin, 1985:145; Purdy et al., 2000:21). Reflexes have
however been recorded with reasonable reliability in infants when a high
frequency probe tone and an ipsilateral stimulus in the mid-frequency range is
used to activate the reflex (Rhodes, Margolis, Hirsch & Napp, 1999:802).
Marchant et al., (1984:593) have also demonstrated that when a 660 Hz probe
tone is used for acoustic reflex measurements, absent or elevated ipsi-lateral
reflexes were closely associated with middle ear effusion.
Sutton et al., (1996:12) reported that acoustic reflexes were absent in over half
of the ears tested in their study-population of 84 special care neonates.
Although they found OAE failure to be highly associated with acoustic reflex
absence, they found that reflexes were also absent in most of the ears with
normal tympanograms and in most of the ears passing OAEs.
Thus they
concluded that acoustic reflex absence alone is not a very specific indicator of
middle ear effusion (Sutton et al., 1996:12).
Hirsch, Margolis and Rykken
(1992:186) found that the high correspondence between elevated reflex
thresholds and delayed ABR wave V latencies suggests that the combined
information from acoustic reflex thresholds and ABR-testing may be valuable in
the early detection of abnormal middle ear dysfunction.
It is apparent therefore that the ability to measure acoustic reflexes in infants
depends to a large extent on the probe tone frequency that is utilized. Similarly,
as previously mentioned, the outcome of tympanometric measures in the infant
population depends largely on the choice of probe tone frequency.
A
discussion of the conventional (226 Hz) probe tone tympanometry in
comparison to high frequency (1000 Hz) probe tone tympanometry and the
implications for infant testing follows in the next section.
47
2.6.8 Conventional tympanometry versus high frequency tympanometry
The diagnosis of middle ear effusion in neonates and infants presents specific
problems and the age at which tympanometry becomes reliable for the
detection of middle ear effusion is not fully agreed on (Meyer et al., 1997:190).
In infants under 7 months of age, the interpretation of tympanometry is
controversial, and for some years it has been recognised that conventional lowfrequency probe tone tympanometry is inappropriate because of poor sensitivity
to middle ear disease in young infants (Purdy & Williams, 2000:9; Fowler &
Shanks, 2002:187).
Paradise, Smith and Bluestone (1976, in Meyer et al.,
1997:189), were the first to cast suspicion over the use of tympanometry with
infants younger than 7 months of age.
They demonstrated that normal
tympanograms could coexist with confirmed middle ear effusion in that age
group.
Results reported by Holte et al., (1991:388) also reported that low
frequency; single-component tympanometry can produce spurious patterns in
neonates.
Questionable results and reasons for the high false negative rate when using
low frequency 226 Hz tympanometry in the infant population are unclear,
although it has been speculated that the anatomical differences and significant
variations in the acoustic response properties, between the infant and adult
middle ear transmission system may be a contributory factor (Meyer et al.,
1997:190; Keefe et al., 1996:361; Holte et al., 1991:1).
Anatomical development of the external ear canal has been suggested as a
contributor to tympanometric shape in neonates (Holte et al., 1991:1; Meyer et
al., 1997:190; Purdy & Williams, 2000:9). The osseous portion of the ear canal
is not completely formed until about 1 year of age. Thus, in the first few months
of life, pressurizing the ear canal during tympanometry may result not only in
movement of the tympanic membrane, but also in the distention of the ear canal
walls.
48
Thornton et al., (1993:320) reported that the percentage of successful
recordings using a 220 Hz probe tone was 68%, but when a 1000Hz probe tone
was used, the figure rose to 87%, supporting the notion that high frequency
tympanometry is more effective in infants. Sutton et al., (1996:16) found OAE
screening results strongly related to tympanometry when a high frequency
probe tone is used. Similar results were found by Meyer et al., (1997:193),
suggesting that the use of a high frequency probe tone is more sensitive to the
identification of middle ear effusion in the infant population. The high frequency
probe tone was found to be able to identify the presence of pathology that was
unrecognised by conventional 226 Hz tympanometry (Meyer et al., 1997:194).
As a result it was concluded that high frequency tympanometry is a useful
indicator of middle ear status, and that middle ear effusion strongly affects
OAEs in neonatal years. Negative middle ear pressure also has a significant
effect on OAE results.
Therefore, as middle ear effusion has a confirmed
impact on the successful recording of OAEs, an objective measure, such as
high frequency tympanometry, is necessary to differentiate between true
cochlear and middle ear pathology. The high degree of association between
high frequency tympanometry and presence or absence of OAEs as reported by
Sutton et al., (1996:15), supports the validity of high-frequency tympanometry
as an indicator of middle-ear status in neonates.
Currently there are no universally accepted guidelines or normative data system
on classifying or interpreting high-frequency tympanograms in neonates and
young infants (Petrak, 2002:2; Sutton et al., 1995:10). Recent studies by Kei et
al. (2003:25) and Margolis et al., (2003:385) have provided preliminary
normative values for 1000 Hz tympanometry performed on infants. Though this
provides
eminent
guidelines
for
clinical
practise
in
high
frequency
tympanometry, more results and normative values are imperative for the
development of a standardised classification or interpretation system.
Normative values from 1000 Hz tympanograms reported by Margolis et al.,
(2003:385, 386) and Kei et al., (2003:25) are presented in tables 2.1 and 2.2.
49
TABLE 2.1 Normative tympanometric values from 1kHz tympanograms by
Margolis et al., (2003:385)
Birth GA
Test CA
Test CA
TPP
Comp Y
Comp Y
(wks)
(wks)
(wks)
(daPa)
Y +200
Y -200
Y Peak
(+200)
(-400)
Mean
32.8
3.9
36.7
-9
1.4
0.6
2.2
0.8
1.5
SD
4.2
3.8
2.7
48
0.3
0.2
0.7
0.5
0.7
Max
41.0
20.1
44.7
145
2.9
1.4
5.4
3.4
4.7
Min
23.0
0.1
31.3
-188
0.8
0.4
1.0
0.1
0.3
th
5 Percentile
26.0
0.4
32.6
-93
0.9
0.4
1.3
0.2
0.6
th
32.4
2.1
36.6
-5
1.3
0.6
2.1
0.8
1.5
th
40.0
10.9
41.0
53
1.9
1.0
3.4
1.6
2.7
50 Percentile
95 Percentile
Margolis et al., (2003:386) reported descriptive statistics for characteristics and
tympanometric measures of 1000 Hz probe tone tympanometry measurements.
In contrast to previous reports of complex high frequency tympanometry
patterns, Margolis et al., (2003:388) reported that tympanograms recorded in
their study were almost always single peaked and free of irregular patterns.
Margolis et al., (2003:388) found static admittance values to be substantially
higher for infants who passed OAE screening and suggested that due to the
strong relationship between OAE pass-fail status on static admittance, many
screening failures may result from middle ear rather than inner-ear factors.
Therefore, as previously discussed, passing OAE screening suggests normal
middle ear function and given the invasive constraint of using surgical
procedures to confirm middle ear function, OAEs can be used as a control
variable in studies investigating normative immittance measures. This method
was applied in both studies by Margolis et al., (2003:387) and Kei et al.,
(2003:22).
50
TABLE 2.2 Normative Data for 1000 Hz tympanometry by Kei et al.,
(2003:25)
th
th
N
(ears)
Mean
SD
5
Percentile
95
Percentile
(Left ear)
106
1.04
0.51
0.39
1.95
(Right ear)
106
1.16
0.58
0.39
2.28
(Left ear)
106
3.20
1.11
1.54
5.09
(Right ear)
106
3.06
1.07
1.40
5.01
(Left ear)
57
0.51
0.13
0.33
0.71
(Right ear)
62
0.48
0.13
0.27
0.75
Variable
Peak Compensated Static Admittance Ypc (mmho)
Admittance at +200 daPa, Y200 (mmho)
Gradient
Tympanometric width, TW (daPa)
(Left ear)
57
97.7
30.1
46.1
144.2
(Right ear)
62
107.6
28.0
56.6
154.0
212
18.3
41.6
-58.0
86.6
212
2.13
0.77
0.13
3.54
Ear Canal Pressure, Pec (daPa)
(Ears combined)
Downward Admittance, Yd (mmho)
(Ears combined)
Kei et al., (2003:27) reported that single-peaked 1000 Hz tympanograms were
recorded in 92.2% of neonatal ears. Normative data reported by Margolis et al.,
(2003:386) and Kei et al., (2003:25) (Tables 2.1 and 2.2) provide criterion for
identifying middle ear dysfunction in newborns and infants and may be helpful
for distinguishing between screening fails caused by sensorineural hearing loss
and those caused by transient external or middle ear conditions. However, as
these studies were limited in sample size (170 neonates participated in the
study by Kei et al., (2003:22) and normative values from 65 NICU and 30 fullterm babies respectively, were reported by Margolis et al., 2003:385), results
cannot be applied universally and large scales studies are necessary for the
compilation of representative normative data.
Despite the advantages of high frequency tympanometry in identifying massrelated middle ear pathology, high frequency / multi-frequency probe tones are
not commonly used.
The major barrier to incorporating high-frequency
tympanometry into the routine test battery is that the measures are more
complex and not as easily understood as low frequency 226 Hz tympanometry
(Fowler & Shanks, 2002:187).
51
2.7
IMPLICATIONS FOR INFANT TESTING
The preceding sections commenced with a discussion of otoscopy, OAEs and
immittance measures as tests of middle ear function. As otoscopy is often very
difficult to perform in newborns and infants, and has several other
disadvantages, it does not appear advantageous as a specific diagnostic
indicator for the identification of OME in infants (Holte & Margolis, 2002:388).
OAE testing is sensitive to MEE and is often abolished by it (Yeo et al.,
2002:797) and therefore may serve a secondary role for the identification of
potential middle ear pathology. However, as no discrimination between middle
ear pathology and sensory hearing impairment can be made with absent OAE
results in isolation it is clearly an imperfect isolated test of middle ear function.
Immittance measures therefore prove to be the best single diagnostic technique
for the assessment of middle ear function in infants. Supported by current
literature it is recommended that high frequency (1000 Hz) immittance
measurements should be included in a battery of tests to identify any
abnormality in an infant’s hearing system (Lantz, Petrak & Prigge, 2004:4).
Research has indicated that there may be a better correlation between the
presence of middle ear effusion and the shape of the tympanogram when a high
frequency probe tone is used (Marchant et al., 1984:593).
Acoustic reflex
measurements complement tympanometry in the identification of middle ear
effusion and acoustic reflexes should be present in healthy infant ears when
using a 1000 Hz probe tone and ipsilateral stimulus (Sininger 2003:380).
A summary of the advantages and disadvantages of the methods typically used
for middle ear assessment in infants, as discussed in the preceding sections, is
provided in Table 2.3.
52
TABLE 2.3 Comparison of methods for middle ear assessment in infants
METHOD
Useful in identifying
complete occlusion of
ear canals and foreign
bodies
Inexpensive
REFERENCES
DISADVANTAGES
ADVANTAGES
×
Skill and experience
valuable adjunct
×
Subjective – variability among
otoscopists
×
Difficult
to
perform,
particularly in young neonates
×
No golden standard – actual
verification
only
by
myringotomy
×
×
Low sensitivity
No objective record – must be
manually documented
×
Does not distinguish cochlear
from middle ear pathology
×
OAEs
Dependent on middle
ear state – useful in
detecting middle ear
effusion
Sensitive to middle ear
pathology
Objective
Quick administration
Permanent record is
available to print out for
record and comparison
Have been found to be
present in ears with middle
ear pathology
×
Expensive cost involved
×
No accepted guidelines on
classifying or interpreting high
frequency tympanograms in
young infants
Low specificity
∼ Sutton et
1996:11
al.,
∼ Roush et
1992:63, 64
al.,
IMMITTANCE
Fairly quickly and easily
performed
Objective
Easy to obtain
High sensitivity
Permanent record is
available to print out for
record and comparison
×
Criteria developed for one
group / population may not be
suitable for use with another
∼ Govender,
1998:68
×
Cost involved
OTOSCOPY
• Tympanometry
• Acoustic reflexes
×
is
∼ Sutton et al.,
1996:9
∼ Roush et al.,
1992: 66
∼ Pellet
et
al.,
1997: 181,187
∼ Govender,
1998:68
∼ Thornton et al.,
1993:319
∼ Koivunen et al.,
2000:212
∼ Nozza et al., 311
∼ Palmu et
1999:210;
2000:264;
2002:141
Advantages and disadvantages of different methods used in the assessment of
the infant middle ear are summarized in Table 2.3. Lack of objectivity and
intricacy to perform otoscopy makes this the least resilient method of middle ear
assessment. Though OAEs are objective and sensitive to middle ear pathology
it cannot distinguish cochlear from middle ear pathology and therefore are an
imperfect test of middle ear function. It is therefore clear from Table 2.3 and the
above discussion that immittance measurements currently appears to the best
53
al.,
single diagnostic indicator for the assessment of middle ear function in
neonates and infants.
High frequency immittance measurement indicates
significant promise and requires further investigation in the infant population.
2.8
SUMMARY
This chapter provided a critical discussion on the current issues in the
assessment of the infant middle ear in terms of the physical properties and the
use of different instrumentation and test protocols. Procedures were compared
with regards to their effectiveness and utility in the identification of middle ear
effusion in the infant population. There is evidence in the literature supporting
the use of high frequency tympanometry, though various limitations currently
confine clinical use.
High frequency tympanometry currently indicates
significant promise in the infant population and there is potential for it to be
utilized in neonatal hearing screening programs to screen for middle ear
dysfunction (Kei et al., 2003:27).
Preliminary normative values may be helpful to differentiate between screening
failures caused by sensorineural hearing loss and those caused by transient
external and middle ear conditions (Margolis et al., 2003:389). However in light
of Newborn Hearing Screening Programs and the number of false positive
failures that can occur due to middle ear effusion, more detailed and large scale
investigations of high frequency immittance, as a test of middle ear function in
young infants are necessary.
The need for normative value classification systems for 1000 Hz probe tone
immittance, serve as the primary rationale and motivation for conduction of the
present study.
The theoretical aspects covered in this chapter, serve as
background to the development of the research methodology, which will be
discussed in the subsequent chapter.
The literature supports the use of high frequency probe tone immittance
measures in infants in identifying middle ear effusion. However, to be a reliable
54
tool for clinical use it is necessary to have knowledge of the characteristics and
normative values for 1000 Hz immittance measures. That raises the question:
“What characterizes normal high frequency (1000 Hz) probe tone
tympanometry and acoustic reflexes in infants and what are the variables
that need to be taken into account when interpreting these results?”
55
3
RESEARCH METHODOLOGY
Chapter 3 describes the main and sub-aims formulated
for this study. The methodological approach is discussed
and
a
description
of
research
subjects
provided.
Procedures for data-collection and analysis are included.
3.1
INTRODUCTION
In its simplest form research can be defined as the collection of methods used
to systematically produce knowledge (Neuman, 1997:6).
The research
methodology is the strategic framework for action that serves as a bridge
between the research questions and the execution of the research. It is the
precisely designed and planned nature of observation that distinguishes
research from other forms of everyday observation (Durrheim, 1999:29). When
the ultimate goal of the research is to generalise findings to a specific context in
order to assist decision-making, this “knowledge” is developed to convert
existing knowledge into products, processes and technologies (Durrheim,
1999:41).
It is generally recognised that conventional low frequency tympanometry is not
a reliable measure of middle ear functioning in infants and higher frequency
probe tones are advocated when assessing the infant middle ear (Sutton et al.,
2002:2; Purdy & Williams, 2000:9). However, there is currently no universal
criterion or classification system for identification of middle ear disorders in
infants and neonates when high frequency immittance is employed (Fowler &
Shanks, 2002:186). To aid the classification and use of high frequency
immittance in clinical practice, the need for further research have recently been
highlighted by authors (Kei et al., 2003:27; Margolis et al., 2003:384; Purdy &
Williams, 2000:23).
56
Founded on the dearth in current literature, this study aimed to describe the
characteristics of high frequency (1000 Hz) acoustic immittance results in
normal neonates and infants to establish normative data for this population.
The normative data, derived from the sample of infants with normal middle ear
functioning, may serve as a guide for identifying middle ear dysfunction in
infants and neonates.
This chapter outlines the research question and design and describes the
methodological approach to obtaining, recording and analysing of data.
3.2
HYPOTHESIS
As successful recording of OAEs are known to be sensitive to middle ear
pathology, it is hypothesized that infants who pass DPOAE hearing screening
have normal middle ear functioning, and will show a peaked 1000 Hz
tympanogram. The hypothesis was tested by comparing results obtained from
OAE testing to results of 1000 Hz tympanometry and acoustic reflex
measurement.
Since this hypothesis was confirmed, results from 1000 Hz
immittance measures could be used to derive normative data.
3.3
AIMS OF RESEARCH
The aims of the current research study are as follows:
3.3.1 Main aim
The main aim of the study is to determine and describe the characteristics and
normative values of high frequency (1000 Hz) acoustic immittance measures in
a sample of infants between the ages of 0 – 12 months.
57
3.3.2 Sub-aims
The following sub-aims were formulated to achieve the main aim of the study:
1. To describe the shape and characteristics of high frequency
admittance (Ya) tympanograms within subgroups A and B and to
determine
associations
between
OAE results
and
measured
tympanometric variables.
2. To describe examples and characteristics of susceptance (Ba) and
conductance (Ga) tympanograms within subgroups A and B.
3. To describe results of 1000 Hz probe tone acoustic reflexes,
measured with an ipsilateral 1000 Hz stimulus, for all test ears and to
compare results with OAE and tympanometry results.
4. To describe high frequency immittance norms for the subgroup of
infants demonstrating both OAE pass results in addition to peaked
1000 Hz admittance tympanograms.
3.4
RESEARCH APPROACH
The nature of the present research study and the type of data that was obtained
primarily necessitated a quantitative research approach (Terre Blanche &
Durrheim 1999:42).
Quantitative research is objective and aims to classify
features, count them, and construct statistical models in an attempt to explain
what is observed and therefore a qualitative approach involves analysis of
numerical data (Durrheim 1999:96).
The results obtained from immittance
measurements performed during this study provided a range of variables that
were statistically analysed to describe and interpret the data (Leedy & Ormrod,
2001:252).
In contrast, qualitative research is more subjective and the aim of qualitative
analysis is to obtain a complete, detailed description of data not represented by
58
numerical values (Leedy & Ormrod, 2001:147). For the purpose of this study a
secondary qualitative approach was used to lesser degree for the purpose of
subjective description and analysis of tympanogram shapes.
3.4.1 Research design
The research design addresses the planning of scientific inquiry and the
formulation of a strategy for obtaining information to answer the research
questions (Goddard & Melville, 2005:32; Leedy & Ormrod, 2001:91).
Terre
Blanche & Durrheim (1999:29) describes the research design as a bridge
between the research questions and the execution or implementation of the
research.
The research design that was implemented in the present study comprised of
analytical, descriptive, and exploratory research (Leedy & Ormrod, 2001:148,
Neuman 1997:19).
Terre Blanche and Durrheim (1999:29) describes the
advantages of employing multiple research methods as a way of addressing
different, but complimentary questions within a study. It can also be used to
enhance interpretability of statistical analysis of a primarily quantitative study, by
a qualitative narrative account. A graphical illustration of the primary research
design and methods utilized in the present study are presented in Figure 3.1
(compiled from Durrheim, 1999:39, 40).
59
Research design
Analytical
Descriptive
Exploratory
QUANTITATIVE
QUALITATIVE
QUANTITATIVE
Figure 3.1
Research methods representing the research design of the
current study
As illustrated in Figure 3.1 the research design utilised in this study consisted of
analytical, descriptive and exploratory elements. The rationales for following
these designs are as follows:
1) The research design is analytical due to the quantitative nature of the data,
which were obtained from results of physical audiologic measurements.
Inferential statistics was used to analyze the data to obtain a representation
of which immittance values can be classified as normal for infants.
2) The research design is descriptive in nature due to the fact that certain
aspects of the data (e.g. the shape of the tympanogram-forms) will be
described in terms of similarities and differences. A descriptive approach
will also be followed in the description of values and ranges of normative
values (Leedy & Ormrod, 2001:259).
60
3) The research design can also be classified as exploratory. Exploratory
studies are used to make preliminary investigations into relatively unknown
areas of research and when new knowledge is sought (Neuman 1997:19;
Terre Blanche & Durrheim, 1999:39).
Literature on high frequency
immittance measurements is limited and universal normative values for 1000
Hz probe tone measurements have been specified.
As the findings derived from the current research have the potential for practical
application, and aims to contribute towards the resolving of practical issues and
difficulties in decision making when utilizing and interpreting high frequency
tympanometry results in infants, the current study can be defined as applied
research (Durrheim, 1999:41).
3.5
ETHICAL CONSIDERATIONS
Whenever the focus of investigation is human beings, ethical implications of
what is proposed must be carefully considered (Leedy & Ormrod, 2001:107).
Ethical clearance for conducting the current study was obtained from the
Research Proposal and Ethics Committee, Faculty of Humanities, University of
Pretoria (Appendix B) and the Ethical Committee of the District Health
Department of North West Province.
Specific ethical issues that were
considered during the execution of the current study are as follows:
3.5.1 Potential harm to research subjects
The collection procedures utilised for the current study were non-invasive and
mothers or caregivers were informed verbally and in written format about the full
bearing of the investigation.
3.5.2 Informed consent
According to Leedy and Ormrod (2001:107) research subjects must be informed
about the nature of the study to be conducted and must be given a choice of
61
whether they want to participate in the study or not. Furthermore they must be
aware that they have the right to withdraw from the study at any time. A verbal
explanation of the nature of the research project and the involvement required
from the participants were provided to all possible research subjects. As two of
the fieldworkers were fluent in more than three of the national languages, this
information was conveyed in a language native to the subjects and ensured
comprehension of the investigation. Parents and/or caregivers brought their
infants for the assessment on a voluntary basis.
3.5.3 Research fieldworkers
The researcher and three other fieldworkers who had previous experience in
the research context and with the research material performed data collection.
All of the fieldworkers were qualified audiologists, apart from one fieldworker,
who was a final year audiology student.
3.6
RESEARCH SUBJECTS
Wisker (2001:23) describes a sample as a selected group upon which research
is conducted to indicate the larger whole. For this study, infants between the
ages of 0 and 12 months were selected. This age group was selected firstly as
the current study formed part of a PhD study investigating the feasibility of infant
hearing screening at maternal and child health clinics in a developing South
African community for infants 0 – 12 months of age (Swanepoel, 2004) and
secondly as this provided the opportunity to describe age specific normative
data over an extensive age range as there is discrepancy in the literature at
which age conventional 226 Hz tympanometry can be successfully used to
asses the infant middle ear .
The subjects used for the description of normal immittance values were
required to have normal middle ear functioning. Subjects were recruited from
the patients visiting the specified health clinics during the course of the data
collection period.
62
3.6.1 Selection criteria
Selection of subjects in the study fulfilled the following criterion:
Health clinic
All infants who attended the Refentse or the Eersterust health clinic in the
Hammanskraal region during the specified data collection period were possible
subjects for inclusion in the current study. The researcher and fieldworkers
visited these clinics and data were obtained by performing an audiometric test
protocol, developed for the purpose of this study, on infants at the clinic.
Age
For the purpose of this study all subjects had to be between the ages of 0 and
12 months.
Co-operation
Subjects had to preferably be in natural sleep or demonstrate a low state of
alertness at the time of testing. In infants that were awake, general co-operation
was required to the extent that the probe or electrode placement was tolerated
for the full duration of the test. Lack of infant co-operation complicated testing,
as movement and crying resulted in the recording of artifacts.
Due to the
importance of obtaining reliable data, only infants, who were co-operative and
not resistant to testing, were included in the study sample.
Mothers or caregivers
Subjects had to be accompanied by their mothers or caregivers to provide
biographical information and a case file had to be available for each subject to
ensure that the medical history and other important information on the subject,
which the mother or caregiver was unable to supply, was available. The health
clinic files also served as a means to record when an infant had been included
in the study, to ensure that subjects were not included in the study more than
once.
63
3.6.2 Inclusion and exclusion criteria for compilation of norms
For the compilation of normative values only ears displaying a clearly
discernable peaked admittance tympanogram, in combination with an OAE
pass result were included. If a subject failed OAE screening, displayed a flat
tympanogram response curve or if it was not possible to complete both OAE
and tympanometry procedures for the same ear, the data were excluded from
the statistical analysis and were only used for a descriptive analysis.
As a result of the data exclusions, tympanometry and OAE results were
statistically analysed for rendering of normative values from 809 (79%) of
infants (424 males and 385 females).
Data of infants included in the final
analysis passed OAE testing in one or both ears, maintained an acceptable
probe seal for tympanometric measurement in the corresponding ear or ears,
and displayed a peaked tympanogram response curve in the same ear.
3.6.3 Subject selection procedures
Non-probability sampling (Leedy & Ormrod, 2001:218) was used in selecting
research subjects. This method of subject-selection is also often referred to as
random sampling Wisker (2001:23) and is motivated as a means of sampling as
it ensures that all possible candidates have an equal chance of being selected.
Subjects were selected according to the selection criteria set out for the
purpose of this research project.
Subjects were recruited from the patient toll visiting the 6 week-immunization
clinics at the specified locations selected for sampling. These locations were
selected because of the large number of infants receiving services on a daily
basis at these centres. Parents or caregivers of infants were informed about
the research study and the opportunity to have their infant’s hearing and middle
ear status assessed, and infants were brought for assessment on a voluntary
basis.
All infants who were brought for assessment and who fulfilled the
selection criteria were included in the research study. As previously mentioned
64
lack of infant co-operation complicated testing and had the potential to impair
reliable recording of results, therefore infants who were in natural sleep or in a
low state of alertness enjoyed preference in the selection of subjects over
infants who were awake and crying.
3.6.4 Description of subjects
Due to the geographical representation of the research subjects, the majority of
infants were of black ethnic origin, except for two infants. 510 infants, ranging
in age from 0 – 52 weeks participated in the study. A distribution of the ages of
the infants is presented in Figure 3.2.
140
132
NUMBER OF SUBJECTS
120
100
85
80
70
62
60
40
28
22
20
18
27
19
14
15
9
9
0
0-3
4-7
8 - 11 12 - 15 16 - 19 20 - 23 24 - 27 28 - 31 32 - 35 36 - 39 40 - 43 44 - 47 48 - 52
AGE IN WEEKS
Figure 3.2
Age distribution of infants (n = 510)
A greater number of infants were found to attend at the age-groups of zero to
three weeks, four to seven weeks, twelve to fifteen weeks, twenty four to twenty
seven weeks, and thirty six to thirty nine weeks. This can be attributed to the
fact that infants are generally brought to the maternal health care clinics for
check-up and immunization procedures at 6 week time intervals. 26 % (132) of
65
infants were between the ages of zero to three weeks. The mean chronological
age of infants that participated in the study was 12.8 weeks, with a standard
deviation of 12.58 weeks.
The gender distribution of the subjects was
approximately equal with 248 (49%) female infants and 262 (51%) male infants
included in the study.
Statistical analysis indicated that no statistically
significant gender-effect (p > 0.5) was evident in results obtained from 1000 Hz
tympanometry.
3.7
MATERIAL AND APPARATUS
The material and apparatus used for the collection of data in this current study
included apparatus for the recording of quantitative audiologic measures (dual
OAE and AABR screening device and a GSI® Tympstar middle ear analyzer),
in addition to a recording sheet (Appendix A) that was compiled on which all
these variables were documented.
The material and apparatus are listed and discussed below:
Data recording sheet
A recording sheet (see Appendix A) was compiled to record all variable acoustic
immittance and OAE measurement values, and biographical information
obtained from a short structured interview with the mother or caregiver who
accompanied the infants.
Middle-Ear Analyzer
A GSI® TympStar Version 2 Middle-Ear Analyzer was used to perform acoustic
immittance measurements.
The GSI® TympStar was calibrated in January
2003 before research commenced and re-calibrated after 300 infants were
tested.
As the middle ear analyzer was transported to the specified health
clinics for recording of data, calibration checks were performed according to
specifications provided by the manufacturer in the hard-walled test cavities prior
to commencement of testing each day. Data from immittance measurements,
performed with the GSI® TympStar, were recorded on the recording sheet
66
(Appendix A, section C) and a printout of all tympanograms and reflexes were
made.
Test parameters for tympanometry and acoustic reflex measurements made
with the GSI® TympStar were as follows:
Tympanometry
∼ For recording of tympanograms the probe tone frequency of the
tympanometer was set to 1000 Hz with a positive to negative
pressure sweep of 200daPa per second as recommended for infants
(Holte et al., 1991:23) and to reduce recording time.
∼ For each infant a Ya (admittance) tympanogram, and a simultaneous
Ba (susceptance) and Ga (conductance) tympanogram, was recorded
in the left and right ear.
∼ The point of maximum deflection was marked on each tympanogram
to obtain the uncompensated peak admittance value with the
corresponding pressure value at this point.
Reflexes
∼ Acoustic reflexes were recorded with a 1000 Hz ipsilateral stimulus
using a 1000 Hz probe tone. Present reflex thresholds were defined
as the lowest intensity eliciting a reflex response with a deviation
greater than 0.02. To be accepted as a true, the deviation had to be
repeatable and had to indicate reflex growth with increase in intensity
and a decrease in amplitude at lower intensities.
OAE and AABR screener
A handheld dual OAE and AABR GSI AUDIOscreener™ was used for hearing
screening measurements. This device does not require the use of a computer
and uses a single probe tone to conduct OAE and ABR measurements. The
system uses ‘real-ear’ calibration to allow calibration within the test ear.
Distortion Product (DP) OAE and click evoked ABR measurements were made
67
with this device. OAE measurements were used as the first screening step and
AABR as the second screening step for those subjects who failed the first OAE
screen. As OAEs are also known to be sensitive to middle ear functioning, the
OAE pass / fail results was used as a cross-validation to identify infants with
normal middle ear functioning.
The test parameters that were used for OAE testing with the GSI
AUDIOscreener™ were as follows:
The default screening protocol, setting ‘DPOAE 2’, was selected on the GSI
AUDIOscreener™ for screening neonates and infants in this study.
Five
frequencies were assessed for each ear and a pass criterion was based on
passing at least four of the five frequencies evaluated. The stimulus parameters
are in agreement with the guidelines by the American Speech-Language and
Hearing Association (ASHA, 1997) and are presented in Table 3.1.
TABLE 3.1 OAE stimulus parameters (DPOAE 2)
2000 Hz
3000 Hz
4000 Hz
5000 Hz
6000 Hz
L1/L2 ratio
65/55
65/55
65/55
65/55
65/55
F1 (Hz)
1750
2550
3250
4250
4950
F2 (Hz)
2100
3100
3950
4950
6000
Fdp (Hz)
1400
2000
2550
3550
3900
F1/F2
1.2
1.2
1.2
1.2
1.2
The recordings of OAEs were based on a F2 centre method and the
frequencies were measured in a downward order starting with the highest and
ending with the lowest using a linear averaging method of analysis.
3.8
DATA COLLECTION PROCEDURES
Research subjects were recruited from the specified health clinics. The health
clinics were visited by the researcher and a three-stage procedure was
performed to obtain data. The three stages are illustrated in Figure 3.3:
68
1. Acquiring of biographical
information
2. Otoacoustic Emission Testing
3. High frequency immittance
measurements
Figure 3.3
Three-stage data collection procedure
As two infants were occasionally tested simultaneously with the OAE and
immittance equipment, OAE and immittance testing was performed in
randomized order with immittance measures conducted first in certain cases
and OAE testing first in other cases.
Stage 1: Biographical information
Mothers or caregivers of infants were informed that a voluntary hearing
screening service was available to test their infant’s hearing sensitivity and
middle ear functioning and that the information would form part of a research
project. If the mother/caregiver did not fully understand English or Afrikaans,
this information and further explanation of procedures was provided by one of
the two fieldworkers able to speak other African languages.
The following procedure was followed to obtain biographical information:
Mothers who wished to have their infant’s hearing screened brought their
infants to the test rooms for a hearing screening.
Caregivers were requested to bring their infants preferably if he / she was
asleep or quiet and settled.
69
The testing procedures were explained to mothers / caregivers and testing
only commenced if consent was obtained.
In a short structured interview with the mother or caregiver, biographical
information
and
a
medical
case
history
was
obtained
mother/caregiver, supplemented by the infant’s medical file.
from
the
Data were
recorded on the data sheet (Appendix A, sections A & B).
Stage 2: Otoacoustic emission (OAE) testing
The subsequent procedures were followed during OAE testing:
The infant was placed in a comfortable position.
After investigation of the infant’s ear an appropriate sized disposable probe
tip was selected and inserted into the infant’s ear.
The OAE screening module on the GSI AUDIOSCREENER was selected
and testing commenced for the test ear. This procedure was repeated for
the other ear.
If an infant did not pass the initial DPOAE-hearing screening, an Automated
Auditory Brainstem Response Test was performed as a second phase
hearing screening (not included in analysis and description of results for this
study).
Results were recorded on the data recording sheet (Appendix A) and also in
the infant’s medical file.
Stage 3: High frequency immittance measures
Immittance measurements were performed according to the following
procedures:
An appropriate sized probe tip was selected and inserted into the infant’s ear
to ensure a good seal was obtained
70
The probe tone frequency of the tympanometer was set to 1000 Hz with a
positive to negative pressure sweep of 200 daPa per second to reduce
recording time.
For each infant a Ya (admittance) tympanogram, and a simultaneous Ba
(susceptance) and Ga (conductance) tympanogram was recorded in the left
and right ear.
The cursor option on the GSI TympStar was used to mark the peak
(measured at point of maximum positive deflection) of every tympanogram
to provide the points of maximum peak admittance (mmho) and
corresponding peak pressure (daPa) in each tympanogram.
In the case of double peaked tympanograms measures were taken form the
highest peak.
If no peak was present, this was recorded as a “flat” tympanogram response
curve.
Recording
of
acoustic
reflexes
followed
successful
recording
of
tympanograms. Reflexes were recorded with a high frequency probe tone of
1000 Hz and at an ipsilateral test frequency of 1000 Hz.
Reflex testing commenced at 70 dB HL and reflex threshold seeking was
conducted by a 10 dB increase and 5 dB decrease in intensity level.
Maximum testing level was 110 dB.
Tympanometry and reflex results were printed for each subject.
Cross-infection precautions were maintained and all equipment was frequently
cleaned with a sterilising solution. Sterile gloves were worn by all examiners
during testing procedures.
The different procedures that were successfully performed within the total case
sample, in addition to procedures that were not successfully completed are
presented in Figure 3.4.
71
1020
962
936
936
935
809
TOTAL NO. OF EARS
850
763
OAE's (94%)
765
TYMPS (92%)
680
595
TYMPS NOT PERFORMED
(2,5%)
510
TYMPS AND OAE's (92%)
425
340
PEAKED TYMPS & OAE
PASS (79%)
255
ACOUSTIC REFLEX (75%)
170
85
26
0
PROCEDURES PERFORMED
Figure 3.4
Procedures performed (n = 1020 ears)
As illustrated in Figure 3.4, OAEs were performed in 962 out of the total number
of 1020 ears (94%). OAE and tympanometry testing were performed in
randomised order, but due to state of alertness or lack of co-operation, it was
not possible to obtain tympanometric measures from all infants. OAE testing
and tympanometry was performed on 936 (92%) ears. Within this group an
OAE pass result, in combination with a peaked tympanogram, was recorded
from 809 (79%) of ears.
Data derived from the last mentioned group was
analyzed for the compilation of normative tympanometric values.
Acoustic
reflex measurement was performed in 763 (75%) ears.
3.9
DATA PREPARATION AND ORGANISATION PROCEDURES
Data obtained from OAE and immittance measures, in addition to biographical
information, was recorded onto a data recording sheet (Appendix A). This raw
quantitative data was coded to organise data into a suitable format which could
be transformed into a data set in machine-readable format (Durrheim, 1999:98).
This data capturing allowed for computerised analysis of data. The coded data
72
was entered into a statistical computer program (SAS statistical package) for
statistical analysis.
3.9.1 Division of case sample into two subgroups based on middle ear
function
The main aim of the study was addressed by an initial process of differentiating
the sample of infants into two subgroups, based on judgment of immittanceand OAE measurement results. As discussed in Chapter 2, the recording of
evoked otoacoustic emissions is dependent on the status of the middle ear and
can be adversely affected by the presence of middle-ear effusion and negative
middle ear pressure (Roush, 2001:50). As a screening tool, the technology
consequently also appears to be useful for the detection of middle ear effusion
in the infant population (Koivunen et al., 2000:212; Thornton et al., 1993:319).
For the purpose of this study, results yielded from a DPOAE screening were
acclaimed to differentiate the sample of infants into a group with normal middle
ear functioning (OAE pass result) and a group with postulated middle ear
effusion (OAE refer result). It was beyond the purpose of this study to clinically
confirm the presence of middle ear effusion by invasive methods such as
myringotomy, however as successful recording of OAEs are known to be
sensitive to middle ear pathology, it was hypothesized that infants who pass
DPOAE screening had normal middle ear functioning and infants who failed
DPOAE screening had possible middle ear pathology (not excluding the
possibility of sensory loss).
The validity of this method is supported by results by Sutton et al. (1996:10), in
which the relationship between OAEs and tympanometry was investigated.
Results indicated that OAEs are sensitive to middle ear effusion and are usually
abolished by it, and it was concluded that abnormal tympanometry is strongly
associated with OAE failure (Sutton et al., 1996:13,14). A similar classification
method was used by Margolis et al., (2003:387).
73
OAEs were successfully recorded from 964 ears, signifying that it was not
possible for OAE measurement in 56 (5,5%) of infant ears. A combination of
OAE measurement and successful recording of 1000 Hz admittance
tympanograms was achieved in 936 ears of 510 healthy infants between the
age of 0 and 52 weeks (Mean CA = 12.8 weeks). Lack of infant co-operation
and state of alertness was the foremost reason that deterred the recording of
OAEs in addition to tympanometry measurements in all infant ears. Only cases,
in which a combination of an OAE and tympanometry result could be obtained
in the same ear or in both ears, were included in the analysis of results.
A graphic representation of the procedure followed for the division of the groups
is given in Figure 3.5
OAE MEASUREMENT RESULTS FOR TOTAL CASE
SAMPLE *
(n = 936 ears)
OAE PASS
GROUP A (n = 869)
Normal middle ear
functioning
Figure 3.5
OAE FAIL
GROUP B (n = 67)
? Sensory neural hearing
loss
? Middle ear effusion
OAE results acclaimed for division of groups
(* OAEs and tympanograms recorded)
For the purpose of discussion of results in the subsequent chapters, the group
with normal middle ear functioning is referred to as Group A, and the group with
postulated middle ear effusion is referred to as Group B.
74
In summary, as the main aim of the current study was to determine and
describe characteristics and normative values for 1000 Hz probe tone
immittance measures, it was essential for infants used in the compilation of
normative values to have normal middle ear functioning.
A screening OAE
procedure was used in conjunction with immittance measurements to identify
infants with normal middle ear functioning.
As the recording of an OAE is
dependent on middle ear state, it also appears to be useful for detecting MEE in
infants. The first phase of the study therefore set out to differentiate the sample
into a group of infants with normal and abnormal middle ear functioning, as
correlated with an OAE pass or fail result. The total case sample of 936 ears
was divided into two groups, depending on outcome of OAE testing. Group A
(869 ears) was classified as cases with normal middle ear functioning
corresponding to an OAE pass result. Group B (67 ears) was classified as
infants with postulated middle ear effusion, corresponding to an OAE fail result.
Discussion of subsequent results will be based on results obtained within these
subgroups.
An initial comparison between results of OAE testing and
tympanogram shape will be pursued to determine associations between OAE
results and tympanogram shape.
3.9.2 Procedures for classification of tympanograms
Comparable to the procedure used for the division of the case sample
according to OAE results, a similar process was used to differentiate the sample
into a group with normal and a group with abnormal tympanograms. Recorded
tympanograms were classified in terms of their shape and configuration.
Results were subsequently compared to results of OAE testing to determine the
relationship and agreement between results of the two procedures.
A visual inspection of the configuration of admittance (Ya) tympanograms was
applied as assessment for pass-fail differentiation of tympanograms. As there
are still no generally accepted guidelines on the classification or interpretation of
high frequency tympanograms in neonates (Kei et al., 2003:27; Sutton et al.,
1996:11) classification of admittance (Ya) tympanograms was based on whether
75
a discernable peak was present or not.
The presence of a single peak or
notching (double peak) was accepted as normal, while tympanograms with flat,
even traces were considered to be suggestive of effusion (Purdy & Williams,
2000:16; Sutton et al., 1996:11).
The characteristics of normal 1000 Hz probe tone Y-admittance tympanograms
were determined and described in terms of peak admittance and corresponding
tympanometric peak pressure values. Only ears displaying both a pass OAE
result in addition to a peaked 1000 Hz probe tone admittance tympanogram in
the same ear, were included in the compilation of normative values.
A flowchart to illustrate the research process is provided in Figure 3.6. This is
supplemented by a graphic illustration of the research process and use of data
categorized according to the main and sub-aims in Figure 3.7.
76
77
Figure 3.6
Threshold
value
analysis
Statistical
Double
peaked
Flowchart of research process
Single
peaked
with normal middle ear functioning
probe tone immittance measures for infants
Compilation of normative values for 1000 Hz
acoustic admittance
acoustic admittance
analysis and description
Undiscernible peak for
tympanograms
Classification of
OAE FAIL
Tympanogram shape
B
Discernible peak for
NO
analysis and description
present?
YES
Tympanogram shape
1000 Hz Acoustic reflex
OAE PASS
A
RESULTS FROM OAE AND IMMITTANCE TEST
PROCEDURES
MAIN AIM:
To describe normative values for high frequency (1000Hz) acoustic
immittance measures in a sample of infants, between 0 – 12 months of age.
TOTAL SAMPLE
OF EARS n = 936
Group A: (n = 869)
Group B: (n = 67)
0AE Pass = Postulated
normal middle ear functioning
OAE Fail = Postulated middle
ear pathology
Comparison of
test results
Single
peaked
1000Hz Acoustic reflex
Absent
Present?
Threshold?
Incidence?
Main
aim…
Ya
Ba
Ga
daPa
daPa
Figure 3.7
Ga
Ba
Double
peaked
Descriptive analysis
mmho
mmho
Ya
mmho
Descriptive analysis
Sub
Aims
2
Single
peaked
Flat
mmho
Double
peaked
Sub
Aims
3
Analysis of
tympanogram
shape
Analysis of
tympanogram
shape
Sub
Aim 1
daPa
daPa
1000Hz Acoustic reflex
Present?
Threshold?
Incidence?
Absent
Description
Illustration of research process according to main and sub-aims
78
3.10
DATA ANALYSIS
According to Leedy and Ormrod (2001:252) data analysis is a process of
attempts to answer the question “What do the data mean?”
Statistical
procedures are used to analyse quantitative data allows for extraction of
meaning and inferences about the characteristics of the data (Leedy & Ormrod,
2001: 258; Durrheim, 1999:96).
This process involves various ways of
organising data. The quantitative data obtained in the study were analysed by
statistical methods to provide a means through which numerical data could be
made more meaningful.
Descriptive and inferential statistics (Leedy & Ormrod, 2001:259) were used to
describe the data and allowed for the drawing of conclusions about the sample
population. Statistical methods that were utilised to test for differences between
the mean values of variables, (i.e. gender, left and right ears, age effects and
differences in measures between subgroup A and group B) included t-test
analysis (Durrheim, 1999:119), ANOVA and Mann-Whitney analysis (Leedy &
Ormrod, 2001:278). Descriptive statistics in the form of measures of variation,
dispersion and standard deviations (Leedy & Ormrod, 2001:268) were used to
determine
and
describe
normative
values
for
1000
Hz
probe
tone
tympanograms and acoustic reflexes.
3.11
SUMMARY
This chapter provided a description of the methodological approach
implemented for the collection and analysis of the data to realise the main and
sub-aims of this study. The research design and ethical considerations were
described, followed by a description of the context where the research was
conducted. Details of the selection criteria and research subjects included in
the study were provided. A delineation of the material and apparatus, including
specific test parameters was given and procedures for data-collection and
analysis were included.
79
4. RESULTS AND DISCUSSION
Chapter 4 presents the results obtained from analysis of
data acquired during the study.
Results are discussed
according to the main aim and sub-aims formulated at the
onset of the study. Significant findings are discussed and
recommendations made based on the current research.
4.1
INTRODUCTION
The main aim of the study was to determine and describe the characteristics and
normative values of high frequency (1000 Hz) acoustic immittance measurement
results in a sample of infants (N = 510) attending maternal health care clinics.
Methodological and data collection procedures, as described in Chapter 3, served
as the basic structure for accrual and analyses of information to answer the
research questions posed at the onset of this study. The large sample of infant
ears on which OAE, high frequency tympanometry and acoustic reflexes were
performed, allowed for the compilation of comprehensive normative data for 1000
Hz probe tone acoustic immittance measures.
Results from the test procedures were initially analysed for left and right ears
separately but as no statistically significant difference was observed, ears were
viewed as independent variables.
P-values of Paired T-test analysis (Leedy &
Ormrod, 2001:278; Durrheim, 1999:119) to test for differences between left and
right ears are shown in Table 4.1.
80
TABLE 4.1 P-values for differences between left and right ears
P = 0.7451
Statistically significant
difference?
No
P = 0.1301
No
P = 0.5980
No
P = 0.6885
No
P = 0.6071
No
•
Peak admittance (mmho) of Ga
tympanograms
Peak pressure of Ga tympanograms
P = 0.0743
No
•
Acoustic reflex threshold value
P = 0.7917
No
Variable analysed for statistically significant
difference between Left and Right ears
• Peak admittance (mmho) of Ya
tympanograms
• Peak pressure (daPa) of Ya
tympanograms
• Peak admittance (mmho) of Ba
tympanograms
• Peak pressure (daPa) of Ba
tympanograms
•
P-value
As is evident from Table 4.1, paired t-tests revealed no statistically significant
differences in the immittance data for the left and right ears individually as all pvalues were > 0.05.
Consequently the data of immittance measures were
combined and results for left and right ears will be discussed as independent
variables.
Onwards the n-value will refer to the number of ears upon which the
specific test procedures were performed as opposed to the number of infants (N).
Results will be presented according to the sub-aims formulated at the onset of the
study and an interpretation and discussion follows each presentation of the results.
81
4.2
DESCRIPTION OF ADMITTANCE (Ya) TYMPANOGRAM SHAPES AND
CHARACTERISTICS WITHIN SUBGROUPS A AND B
As discussed in Chapter 3 (Figure 3.5) results rendered from OAE testing were
utilized to divide the data in subgroups A and B. 869 ears (93%) that passed the
initial OAE-screening, were considered to have normal middle ear functioning and
were assigned to Group A. 67 ears (7%) failing OAE screening were assigned to
Group B.
Though sensory hearing loss could not be excluded at this time,
research has shown that a high incidence of false-positive test results can occur in
hearing screening programs due to transient middle ear effusion (Kei et al.,
2003:21, Roush, 2001:26). Furthermore, as the incidence of confirmed sensoryneural hearing loss is relatively low, (one to two out of every 1000 in the USA), one
in 67 subjects will have a negligible effect on the result. Thus 93% of the total
sample of ears were therefore assumed to have normal middle ear functioning
(OAE pass result), in addition to 7% of ears with possible middle ear effusion.
1000 Hz probe tone admittance tympanograms were analysed in terms of shape
and characteristics, and compared to OAE results. Conclusions were drawn from
associations between OAE pass or fail and corresponding tympanometric results to
indicate normal or abnormal middle ear functioning.
Pass-fail results obtained for the recorded 1000 Hz probe tone admittance
tympanograms are presented in Figure 4.1.
82
88%
12%
PASS TYMP (Discernible peak, n = 823)
FAIL TYMP (Undiscernible peak, n = 112)
Figure 4.1
Results of admittance (Ya) tympanometry (n = 936)
Figure 4.1 shows the relationship between ears in which normal, peaked
tympanograms (pass) were recorded and ears displaying tympanograms
suggestive of middle ear effusion (flat, with no discernable peak). 88% (n = 823) of
the ears of the total case sample displayed normal, peaked tympanograms, while
flat tympanograms, were recorded in 12% (n = 112) of the test ears.
Similar results for classification of tympanograms by shape were described by
Sutton et al. (1996:11), although the authors further sub-classified tympanograms
in terms of measurements from maximum peak compliance, raw data revealed
91% (153/168) peaked tympanograms and 9% (15/168) flat tympanograms (Sutton
et al.,1996:11). In describing characteristics of 1000 Hz tympanograms, Kei et al.,
recorded single peaked tympanograms in 92% (225/244) of their test sample.
Pass-fail results for 1000 Hz tympanometry described by Margolis et al. (2003:388)
varied for infants of different age groups. Test subjects of at least two weeks
chronological age, revealed higher pass rates for tympanometry compared to
subjects tested in the neonatal period.
Margolis et al., (2003:389) found that
although pass results for newborns may be lower than for older infants, the pass
83
rates are sufficiently high that 1000 Hz tympanometry may be useful for identifying
potential middle ear problems.
4.2.1 Associations between results of OAE-testing and tympanogram shape
As there is no “gold standard” for the identification of middle ear effusion in infants,
tympanometry as identification method of pathology could be validated by
comparing results of tympanometry with other tests of auditory function. The effect
of middle ear effusion on recording of OAEs has been described in earlier chapters
and hence a correlation between test results obtained for tympanometric
measurements and OAE-test results was performed. A significant concurrence
between OAE and tympanometry results (pass associated with pass and
conversely) would suggest the effect middle ear effusion can have on OAEs in
addition to the sensitivity of 1000 Hz tympanometry to identify middle ear effusion.
Similar analysis was described by Sutton et al., (1996:10) in a study investigating
the relationship between OAEs and tympanometry.
A comparison between
reported and current results will be given.
Associations between results obtained from OAE-testing and tympanogram shape
are presented in Figure 4.2 and Figure 4.3.
84
PEAKED TYMP
900
FLAT TYMP
60
800
Number of ears
700
600
500
809
400
300
200
53
100
14
0
OAE PASS (n = 869)
Figure 4.2
OAE FAIL (n = 67)
Relationships between OAE and Tympanometry results (n
= 936)
As can be derived from Figure 4.2, OAE pass results were recorded in 869 ears
and OAE fail results were recorded in 67 ears. Of the ears that passed OAE
testing (n = 869), 93% (809) of ears also produced a peaked tympanogram; whilst
OAE pass results were obtained in 7% (60) of ears that displayed flat
tympanograms. Of the ears in which an OAE fail result (n = 67) was recorded,
79% (53) displayed flat tympanograms, while 21% (14) of ears with an OAE fail
result displayed peaked tympanograms.
Mann-Whitney (Leedy & Ormrod,
2001:278) and t-test analysis (Durrheim, 1999:119) verified statistically significant
differences between the group with OAE pass and peaked tympanogram results,
and the group with OAE fail combined with flat tympanogram results (p < 0.05).
Figure 4.3 more prominently illustrates the associations between OAE pass and
peaked tympanogram results, and between OAE fail and flat tympanogram results.
85
OAE FAIL (n = 67)
OAE PASS (n = 869)
60
7%
53
79%
809
93%
Figure 4.3
14
21%
PEAKED TYMPANOGRAM
PEAKED TYMPANOGRAM
FLAT TYMPANOGRAM
FLAT TYMPANOGRAM
Independent relationships between OAE results and
peak or flat tympanograms (n = 936)
It is clear within the separate subgroups that an OAE pass result was highly
associated with a peaked tympanogram result, and conversely an OAE fail result
was highly associated with a flat tympanogram response curve. This indicates that
high frequency (1000 Hz) tympanometry shows good sensitivity and specificity to
the assessment and identification of normal and abnormal middle ear functioning.
These results are in agreement to results by Sutton et al., (1996:12), who
concluded that a 1000 Hz probe tone showed greater sensitivity for identification of
middle ear pathology, as opposed to a 678 Hz probe tone. Sutton et al., (1996:12)
found that when comparing 678 Hz probe tone tympanometry to OAE results,
about half (16/33) of the OAE fails had abnormal tympanograms. In contrast,
however, this study indicated a significantly higher association of 79% (53/67)
between OAE-fail, and abnormal tympanogram results. This supports the finding
that 1000 Hz tympanometry shows greater to the identification of middle ear
pathology.
Similar results were also published by Kei et al., (2003:26), who
reported 92.3% of ears passing OAE testing displayed single peaked 1000 Hz
tympanograms.
A disparity was noted between current results and that of
86
Thornton et al., (1993:320) who did not find tympanogram shape to have a
significant effect on the outcome of OAE measurement.
Though significant
associations between OAE pass results and peaked tympanograms, and between
OAE fail results and flat tympanograms were obtained, there was evidence from
results of the current research to confirm the notion that passing OAEs cannot
serve as a gold standard for normal middle ear function (Kei et al., 2003:26) as 7%
(60) of ears with OAE pass results, displayed flat 1000 Hz tympanogram response
curves.
In agreement to Sutton et al., (1996:15), results obtained during this study are
consistent with the conclusion that OAEs are sensitive to middle ear effusion and
are usually abolished by it. Significant associations were found between OAE pass
and peaked tympanograms results, and also between OAE fail and flat
tympanograms results. It can therefore be concluded that the outcome of OAE
testing is highly associated with tympanogram shape when a 1000 Hz probe tone
is employed.
To assess the effect of peak pressure and admittance values in admittance
tympanograms on OAE outcome, a discussion of these values, as related to OAE
results will follow in the next section.
4.2.2 Maximum admittance and tympanometric peak pressure values for Ya
tympanograms within subgroups A and B
Maximum (peak) uncompensated acoustic
admittance and corresponding
tympanometric peak pressure were analysed for admittance (Ya) tympanograms
within subgroups A and B. Statistically significant differences (p < 0.05) (Durrheim
1999:119) were observed between results obtained from measurements for Group
A, classified to have normal middle ear functioning and Group B, classified to have
middle ear effusion.
Mann-Whitney analysis (Leedy & Ormrod, 2001:278) of
87
results from Group A and Group B revealed highly significant differences for peak
admittance (p = 0.000) and for tympanometric peak pressure (p = 0.000).
OAE results were considered the criterion for the initial classification of ears into
Group A (normal middle ear functioning) or Group B (abnormal middle ear
functioning). As discussed in section 4.2.1, 93% of ears with an OAE pass result,
also displayed a peaked tympanograms, whilst 7% of ears with an OAE pass result
displayed no discernable tympanogram peak. Of the total number of ears in which
peaked tympanograms were recorded (n = 823 ears), 809 presented with a
corresponding OAE pass result, whilst only 14 ears corresponded to an OAE fail
result.
Values obtained and analysed from admittance (Ya) tympanograms are presented
in the following section.
In ears with discernable tympanograms peaks values
were measured at the point of maximum displacement on the peak, whilst for the
7% of tympanograms where no discernable peaks were present, the highest point
on the tympanogram was marked to obtain a maximum admittance value (mmho)
with a corresponding pressure value (daPa).
In the case of double peaked
tympanograms, measures were taken from the highest peak.
The distribution of admittance and tympanometric peak pressure (TPP) values for
the recorded tympanograms in Groups A and B are presented in Figures 4.4 and
4.5.
88
60%
50%
OAE PASS - GROUP A (n=870)
40%
OAE FAIL - GROUP B (n=64)
30%
20%
10%
0%
0.1 - 0.9 1.0 - 1.5 1.6 - 1.9 2.0 - 2.5 2.6 - 2.9 3.0 - 3.5 3.6 - 3.9 4.0 - 4.5 4.6 - 4.9 5.0 - 5.5 5.6 - 5.9
>6
Ya Admittance (mmho)
Figure 4.4
Distribution of maximum Ya-admittance values for ears
with OAE pass and refer results (n = 934)
The mean value for uncompensated acoustic admittance for peaked Ya
tympanograms was recorded at 2.85 mmho, with a standard deviation of 1.13
mmho.
Maximum peak admittance corresponding to an OAE pass result was
recorded at 9.64 mmho and minimum peak admittance relating to OAE pass result
was 0.86 mmho. For the OAE pass group, the majority (77%) of the admittance
values were 2 mmho and greater, compared to the majority (81%) of admittance
values for the OAE refer group being less than 2 mmho. Although an overlap of
results is visible, it is evident from Figure 4.8 that there is a clear trend toward
lower Ya-admittance values for the OAE refer group.
Though the range for
89
acoustic admittance relating to an OAE pass result was quite big, it is evident from
Figure 4.8, that 75% of ears (n = 606) displayed admittance values between 1.6
and 3.5 mmho.
45%
40%
OAE PASS GROUP (n = 854)
OAE FAIL GROUP (n = 63)
35%
30%
25%
20%
15%
10%
5%
0%
<-200
-195 - -150
-145 - -100
-95 - -50
-45 - 0
5 - 50
55 - 100
105 - 150
155 - 200
Peak Pressure (daPa)
Figure 4.5
Distribution of tympanic peak pressure values for ears
with OAE pass and refer results (n = 917)
Figure 4.5 shows the distribution of middle ear pressure results for ears with OAE
pass and refer results. The mean pressure value for ears passing OAE testing
was 0.13 daPa with a standard deviation of 60.93 daPa. The highest extreme
pressure value related to an OAE pass resulted was 185 daPa, and the lowest
extreme was measured at -275 daPa. For the OAE refer group the majority (62%)
of tympanic peak pressure values were greater than 105 daPa, compared to the
majority (62%) of tympanic peak pressure values for the OAE pass group being
between -45 and 50 daPa. It is evident from Figure 4.5 that compared to the OAE
90
pass group, there is clear trend indicating more positive pressure values for the
OAE refer group. Additionally 8% of the OAE refer group indicated peak pressure
values less than -200 daPa, compared to a 1% incidence in the OAE pass group.
This indicates that positive peak pressure values exceeding 105 daPa, as well as
negative pressure values of ≤ -200 daPa are more prone to OAE refer results,
compared to pressure values for the OAE pass group.
Similar results were
reported by Thornton et al., (1993:321) who found that high positive middle ear
pressures (>150 daPa) correspond with OAE failures.
OAE pass results were highly associated with tympanic peak pressures values
greater than -100 daPa and smaller than 100 daPa, with 87% of ears in the OAE
pass group falling within this region. These results are in agreement with Trine,
Hirsch & Margolis (1993:406) who reported that negative middle ear pressure ≤ 100 daPa reduced the overall amplitude and reproducibility of TEOAEs. They
concluded that middle ear pressure affects the amplitude, reproducibility, and
spectral characteristics of TEOAEs (Trine et al., 1993:406).
Thornton et al.,
(1993:321) found vast overlapping in middle ear pressures for groups that failed
and passed OAE testing and concluded that though the effect of middle ear
pressure was statistically significant, it seems unlikely that it could account for
more that a small percentage of cases that fail OAE testing. Results from the
current study and that of previous reports are however consistent with the finding
that there is an apparent relationship between middle ear pressure, measured
using a 1000 Hz probe tone, and OAE pass or fail results in neonatal and infant
ears (Thornton et al., 1993:322).
Among the group of ears with peaked tympanograms, a number of double peaked
tympanograms were recorded.
A description of results from double peaked
tympanograms is presented in the next section.
91
4.2.3 Double peaked tympanograms
The number of double peaked tympanograms recorded in this study revealed that
5% of ears (n = 41) in which a peaked tympanograms (n = 823) were recorded,
displayed double peaked admittance response curves. This is notably higher than
the number of double peaked tympanograms recorded by Kei et al., (2003:24) who
reported recording of double peaked 1000 Hz tympanograms for only 3 out of 224
ears (1%). The discrepancy in results between the present study and that of Kei et
al. (2003:24) may be attributed to differences in sample size (122 neonates in
study by Kei et al. in comparison to 512 neonates in present study) and possibly
even more so to age differences between samples (mean age of 3.26 days in
study by Kei et al. compared to a mean age of 15.74 weeks in the present study).
Holte et al., (1991:13) found that as probe frequency increased more
tympanometric types and shapes were recorded for all age groups, but that by the
age of 4 months, the majority of infants had single peaked tympanograms. The
oldest subject in whom a double peaked tympanogram was recorded in the present
study was aged 44 weeks.
The
relationship
between
male
to
female
infants
with
double
peaked
tympanograms is illustrated in Figure 4.6.
64%
36%
Male
Figure 4.6
Female
Gender relation of ears displaying double peaked
tympanograms (n = 41)
92
As is evident in Figure 4.6, double peaked tympanograms were recorded in 64%
(n=27) male infants compared to 36 (n=15) female infants. This gender difference
is an interesting finding which may validate further research as the number of male
to female ears with double peaked tympanograms, displayed a ratio of almost two
to one. As double peaked tympanograms are generally less common, no large
scale study has been performed to assess gender relations in double peaked
tympanograms.
According to Thornton et al., (1993:320), when using a high frequency probe tone
with an infant’s small ear canals, double peaked tympanograms are quite normal.
Consequently tympanograms which displayed a double peak were judged as
normal and included in the group of ‘peaked’ tympanograms, classified as normal.
Double peaked tympanograms were also included in the case sample used for
compiling normative values for 1000Hz tympanometry by Margolis et al.,
(2003:387).
Further validation for judging double peaked tympanograms as normal, was
performed by exploration of OAE results in the group displaying double peaked
tympanograms. Results are illustrated in Figure 4.7.
12%
88%
OAE PASS
Figure 4.7
OAE FAIL
OAE test results of ears that displayed double
peaked tympanograms (n = 42)
93
Congruent to reports by Thornton et al, (1993:320) double peaked tympanograms
can be judged as normal, and current results revealed that, compared to OAE
testing, 88% (n = 37 out of 42) of ears with double peaked tympanograms showed
OAE pass results. 12% (n = 5 out of 42) of ears displaying double peaked
tympanograms failed OAE testing. Further comparisons between OAE test results
and the presence of double peaked tympanograms, revealed that within the total
case sample of OAE pass results (n=869), 37 double peaked tympanograms were
recorded, while 5 out of the 67 ears with an OAE fail result displayed a double
peaked tympanogram.
Due to the greater contribution of elements of mass in the newborn middle ear
system, as opposed to elements of stiffness in the adult middle ear, tympanometric
patterns observed in newborn infants do often not conform to the classic patterns
found in older infants, children, and adults.
Low frequency probe tone
tympanograms are more likely to show notching or complex patterns in infants
(Purdy et al., 2000:10), while more informative tympanometric recordings in
neonates are derived using higher probe tone frequencies (Shahnaz, 2003:3).
Shahnaz (2002 in Shahnaz 2003:3) found that at 1000Hz admittance
tympanograms had a single peak for 74% of infant ears. The author concluded that
in the infant ear, admittance tympanograms become simpler in shape as probe
frequency increases, the reverse of what is found in adult ears, where admittance
tympanograms become more complex as probe frequency increases. Holte et al.,
(1991:13), found that when using the Vanhuyse model of tympanometric shapes to
classify tympanograms according to the number of extrema in the susceptance and
conductance tympanograms, more tympanometric types were recorded for all age
groups, and the types were more complex, as probe frequency increased.
Examples of double peaked tympanograms recorded during this study are
presented in Figure 4.8.
94
c)
b)
a)
Y
Y
d)
G
Y
Y
B
G
G
B
G
Figure 4.8
G
Y
B
B
B
Examples of double peaked tympanograms
a) Double peaked tympanogram recorded from the left ear of an infant aged 36 weeks. A single peaked tympanogram was
recorded from the right ear. OAE pass result was recorded from left (and right) ear and acoustic reflex was present
bilaterally. b) Double peaked tympanogram recorded from the right ear of an infant aged 40 weeks. A single peaked
tympanogram was recorded from the left ear. OAE pass result was recorded from right (and left) ear and acoustic reflex was
present bilaterally. c) Double peaked tympanogram recorded from the right ear of an infant aged 9 days. A single peaked
tympanogram was recorded from the left ear. OAE pass result was recorded from right (and left) ear. Acoustic reflex not
recorded. d) Double peaked tympanograms recorded from right and left ears of the same infant aged 36 weeks. OAE refer
results were recorded bilaterally and acoustic reflexes were absent. A pass result was obtained on AABR screening for the
right and left ear.
Figure 4.8 depicts examples of double peaked tympanograms obtained during this
study. Though it was beyond the scope of this study to investigate the significance
and occurrence thereof, it was of interest to note that in all these examples (and
the majority of all double peaked tympanograms obtained) notching occurred in the
Ga (conductance) tympanograms at approximately the same pressure point as in
the Ya (admittance) tympanograms. When classified according to the Vanhuyse
classification system the double peaked tympanograms showed a frequent 5B3G
type tympanogram. Figure 4.8 a), b), and c) were all associated with an OAE pass
result, while d) was associated with an OAE fail result. OAE failure in Figure 4.8 d)
can be related to the considerable negative pressure at peak admittance,
measured as –210 daPa at point of maximum displacement in the admittance
95
tympanogram. This association is in agreement with Sutton et al., (1996:13) who
concluded
that
negative
pressure,
as
measured
with
high
frequency
tympanometry, had a marked effect on OAEs.
4.3
SUSCEPTANCE (Ba) AND CONDUCTANCE (Ga) TYMPANOGRAM
ANALYSIS AND DESCRIPTION
As discussed in Chapter 2, component tympanometry refers to measurement of
the susceptance (Ba) and conductance (Ga) components of a tympanogram, which
provides an adequate view of the magnitude and direction of the admittance.
Because of the variety of tympanometric shapes that can occur in normal as well
as abnormal ears at higher probe frequencies, evaluation of these two components
is important for the interpretation of tympanograms measured with high frequency
probe tones than for tympanograms measured with a 226 Hz probe tone (Wiley &
Fowler, 1997:56).
The immttance measurement instrument utilized in this study allowed for
simultaneous
measurement
of
susceptance
(Ba)
and
conductance
(Ga)
components. Ba and Ga tympanograms were successfully recorded from 889 ears
(96%). Though it was beyond the scope of this study to statistically analyse and
classify the shape of susceptance and conductance tympanograms, a number of
examples of recorded tympanograms are presented in Figure 4.9. Specifically
note the difference in notching patterns in the B/G tympanograms that are evident
across all age groups depicted. Notching was mainly evident in the susceptance
(B a) tympanograms and this pattern was in agreement with Holte et al., (1991:9)
who found that reactance shifts towards mass with high probe frequencies and
towards asymmetrical resistance, with the interactions resulting in notched
susceptance (Ba) tympanograms. Similar results were seen for all tympanograms
recorded throughout this study with a general trend for single peaked conductance
(Ga) tympanograms and notching occurring in the susceptance (Ba) tympanograms
only. As also illustrated in Figure 4.9, a second frequent shape across the age
96
range of subjects was a single peaked conductance tympanogram, in conjunction
with a single peaked susceptance tympanogram. With reference to the Vanhuyse
et al. (1975 in Fowler & Shanks, 2002:191) classification system (see Chapter 2,
Figure 2.5), these shapes can be classified according to the number of extrema in
the susceptance and conductance tympanograms, and hence the most frequent
shapes observed can be classified as 3B1G and 1B1G. It is evident from the
presented examples that a great range of variation of tympanogram shape occurs
across all age groups with the most frequent shape being the 3B1G and 1B1G
Vanhuyse type.
Initially resistance is greater than reactance and the Ba
tympanogram is expected to show a double peak gradually changing to a single
peak as the resistance-reactance relationship changes (Vanhuyse et al., 1974 in
Meyer et al., 1997:192). When the height of the susceptance (Ba) tympanogram is
greater than the height of the conductance (Ga) tympanogram and both are single
peaked (1B1G), the acoustic admittance vector is between 90° and 45° (see Figure
2.4) and relates to a stiffness-controlled middle ear (Fowler and Shanks,
2002:191). This shape can be seen in Figure 4.9 - ii) d; iii) d; iv) c; v) c; vi) b; vii) d;
viii) a and b; ix) b.
More complex configuration and notching patterns were
observed in susceptance and conductance tympanograms, while 95% of ears
displayed single peaked admittance (Ya) tympanograms.
Highly significant
associations between OAE pass results and peaked tympanograms and between
OAE fail results and flat admittance tympanograms, suggest that admittance
tympanograms have good sensitivity and specificity for middle ear status.
97
Figure 4.9 Random examples of tympanograms
recorded during the study at age
groups of 1, 2, 6, 10, 14, 20, 36, 40
and 44 weeks
i) Age: 1 week
Y
Y
G
G
B
B
a) 3B1G
G
B
Y
Y
G
B
B
Y
B
Y
G
b) 1B1G
G
c) 3B1G
ii) Age: 2 weeks
G
G
B
B
B
Y
B
G
Y
G
G
B
Y
Y
B
B
G
a) 3B1G
b) Left 1B1G, Right 3B1G
Y
Y
Y
G
c) Left 3B1G, Right 1B1G
d) 1B1G
B
G
Y
98
iii) Age: 6 weeks
G
G
G
B
B
B
G
G
Y
G
B
Y
B
B
B
Y
Y
Y
Y
Y
Y
G
a) 3B1G
b) 3B1G
c) 1B1G
d) 1B1G
iv) Age: 10 weeks
Y
Y
G
Y
Y
G
B
B
G
G
Y
B
B
G
Y
B
99
a) 3B1G
b) 1B1G
c) 1B1G
B
G
B
G
v) Age: 14 weeks
Y
Y
G
G
Y
B
Y
B
Y
G
G
B
B
a) 3B1G
Y
B
B
G
G
b) 3B1G
c) 1B1G
vi) Age: 20 weeks
Y
Y
G
B
G
B
B
G
Y
Y
100
a) 3B1G
b) 1B1G
B
G
vii) Age: 36 weeks
B
G
B
G
G
B
B
Y
Y
Y
G
Y
B
G
G
Y
Y
Y
Y
d) 1B1G
c) Left 3B1G, Right 3B1G → 1B1G
viii) Age: 40 weeks
B
B
G
G
b) 3B1G
a) 3B1G
B
B
B
ix) Age: 44 weeks
B
B
G
G
Y
G
G
Y
B
B
G
G
Y
Y
Y
a) 3B1G
Y
Y
b) 1B1G
a) 3B1G
Y
B
G
b) Left 3B1G, Right 1B1G
B
G
101
4.4
ACOUSTIC REFLEXES USING A HIGH FREQUENCY PROBE TONE
The acoustic reflex can be a very useful part of the audiologic evaluation of infants.
A present acoustic reflex is added support for normal middle-ear function, and as
the reflex arc involves the seventh and eighth nerve and the low brainstem, a
normal or present reflex can be useful in ruling out abnormalities such as Auditory
Neuropathy (Sininger et al., 2003:380).
Acoustic reflexes using a 1000 Hz probe tone were evaluated at 1000 Hz in 840
ears of infants who tolerated the entire test battery. Results of reflex measurement
from the present study are illustrated in Figure 4.10.
Present reflex
ALL EARS (n=914)
14%
Absent reflex
86%
15%
85%
Figure 4.10
14%
14%
13%
87%
LEFT EARS (n=459)
RIGHT EARS (n=455)
MALE EARS (n=469)
FEMALE EARS (n=445)
86%
86%
1000 Hz acoustic reflex results using a 1000 Hz probe tone
(n = 914)
Figure 4.10 shows the results of acoustic reflexes recorded in the total case
sample and results for male versus female, and left versus right ears. As is clearly
evident from Figure 4.10, no significant differences were observed between left
and right, and male and female ears (p > 0.05). Present acoustic reflexes were
102
recorded in 86% (n=785) of the ears tested. In 14% (n=129) of the ears no reflexes
were recorded. In view of the fact that present acoustic reflexes were recorded in
a significantly high proportion (86%) of the test subjects, the above results are
consistent with Weatherby and Bennett (1980:106), and support the validity of high
frequency probe tones for measurement of acoustic reflexes in infants. Wetherby
and Bennett (1980:107) demonstrated that neonatal acoustic reflexes were not
measurable using 226 Hz probe tones, but when employing high frequency probe
tones, acoustic reflexes were obtained at levels comparable to adults. This
suggests that the use of a high frequency probe tone for measurement of acoustic
reflexes is more sensitive in infants and shows promise for inclusion in audiologic
assessment protocols in infants. To arbitrate this application, a discussion of the
degree of association between acoustic reflexes and OAE’s, and between acoustic
reflexes and tympanometry will follow.
4.4.1 Comparison of acoustic reflex measurement results with OAE and
tympanometry results
A comparison of reflex measurement- to OAE results is provided in Figure 4.11 to
indicate the relationship between OAE and acoustic reflex testing results.
100%
Number of cases (%)
90%
92%
89%
80%
70%
60%
50%
40%
30%
20%
10%
8%
11%
0%
OAE PASS (n=829)
REFLEX PRESENT
OAE FAIL (n=65)
REFLEX ABSENT
Figure 4.11 Distribution of acoustic reflex compared to OAE results
103
As presented in Figure 4.11 acoustic reflex arcs were recorded in 92% (n=763) of
ears who passed OAE testing, while 8% (n=66) of ears obtaining and OAE pass
result displayed absent acoustic reflexes.
In the group of infants failing OAE
testing, 89% (n=58) also had absent acoustic reflexes, though reflexes were
present in 11% (n=7) of this group.
As OAE and acoustic reflex measurement
require normal middle ear transmission (Casselbrandt et al., 2002:96; Gelfand,
2002:208), acoustic reflex results obtained in the current study were compared with
results of 1000 Hz tympanometry. Results are presented in Figure 4.12.
100%
94%
90%
Number of cases (%)
80%
73%
70%
60%
50%
40%
30%
27%
20%
10%
6%
0%
Peaked Tym panogram (n = 776; 88%)
Reflex Present
Flat Tympanogram (n=106; 12%)
Reflex Absent
Figure 4.12 Relationship of tympanometry to acoustic reflex results
(n = 882)
Results of acoustic reflex measurement with a 1000 Hz probe tone indicate a high
percentage of present acoustic reflexes (86%).
McMillan et al., (1985:146)
reported similar findings and recorded present acoustic reflexes in 95% of infant
ears when a 660 Hz probe tone was used. Contrary to current findings, Sutton et
al., (1996:12) reported acoustic reflexes, measured with a 678 Hz probe tone, to be
104
absent in most of the ears with normal tympanograms and in most of the ears
passing OAE testing. This difference in findings can be attributed to the risk status
and young age of the neonates, as well as to the use of a 678 Hz probe tone as
opposed to a 1000 Hz probe tone. Current findings are also in contrast to reports
on poor reliability of recording present reflexes using a low frequency probe tones
in infants (Gelfand, 2002:212). This can be attributed to the fact that: 1) a 1000 Hz
probe tone was used, 2) an ipsilateral stimulus was used, and 3) a mid-frequency
(1000 Hz) stimulus was used to activate the reflex.
Significant associations between OAE pass results, peaked tympanograms and
present acoustic reflexes were evident. Normative values for present 1000 Hz
probe tone acoustic reflex measures, corresponding to an OAE pass result and a
peaked tympanogram, will be discussed in the final section of this chapter.
Preliminary normative data for 1000 Hz probe tone tympanometry compared to
OAE results have recently been reported, although previous studies were limited in
sample size and encompassed a limited age range (Kei et al., 2003:23 – 25;
Margolis et al., 2003:385 – 388).
The current study therefore presents data that
can be used to establish normative data for 1000 Hz tympanometry and acoustic
reflexes within a large sample of infants ranging in age from one day to one year.
This normative data will be discussed in the following section.
4.5
HIGH FREQUENCY (1000 Hz) IMMITTANCE NORMS
Various authors have highlighted the need to perform further studies to establish
guidelines for interpretation of high frequency immittance results, and for
distinguishing normal from pathological ears in neonates and infants (Fowler &
Shanks, 2002:202; Purdy & Williams, 2000:9; De Chicchis, 2000:98). This chapter
concludes with a description of normative values derived from the cohort of
neonates and infants included in the final analysis of the current study. The large
sample of infant ears on which OAE and high frequency tympanometry and
105
acoustic reflexes were performed allowed for compilation of comprehensive
normative data for 1000 Hz probe tone immittance measures.
The criteria for
inclusion will be discussed, followed by a description of 1000 Hz tympanometry
norms.
As normative studies of tympanometry in infants are often complicated by a lack of
an independent method for assuring only normal ears are included in the study
(Margolis et al., 2003:384), two criteria were set out for classification of normal
middle ear functioning. The first criterion was based on results from OAE-testing.
As previously discussed, an initial division was made where cases, in which an
OAE pass result was obtained, were considered to have normal middle ear
functioning (Group A). The hypothesis as described in Chapter 3 was tested by
comparing results obtained from OAE testing to results of 1000 Hz tympanometry
and acoustic reflex measurement. Current results from 1000 Hz tympanometry
indicated that the majority of (93%) of ears with an OAE pass result displayed a
discernable tympanogram peak and therefore confirmed that the hypothesis was
true.
However, as reported by Sutton et al., (1996:11) and Thornton et al.,
(1993:322), emissions can occasionally be recorded from ears with middle ear
disease and hence OAEs alone cannot be considered as the only decisive factor
for normal middle ear functioning. In 7% of ears with an OAE pass result no
discernable peak could be identified in the current study. Based on reports in
literature that 1000 Hz probe tone tympanograms with a discernable peak can be
accepted as representative of normal middle ear functioning, this was specified as
a second criterion alongside an OAE pass result (Kei et al., 2003:22; Purdy &
Williams, 2000:18, Sutton et al., 1996:13). Accordingly only ears displaying both a
pass OAE result in addition to a peaked 1000 Hz probe tone admittance
tympanogram in the same ear, were included in the compilation of normative
values. Ears classified as ‘abnormal’ indicated a fail result in either OAE-testing or
tympanometry, or both measures.
106
According to the two criteria, normative data were compiled for 809 ears.
Normative values for 1000 Hz probe tone tympanometry are presented for
maximum peak uncompensated admittance and tympanometric peak pressure
(TPP) values, measured from admittance (Ya), susceptance (Ba) and conductance
(Ga) tympanograms.
Norms for 1000 Hz probe tone acoustic reflexes are also
presented.
Uncompensated admittance values for Ya, Ba and Ga tympanograms were
measured at point of maximum positive displacement, and corresponding middle
ear pressure was measured at this point. When double peaks occurred, the peak
admittance, as described here, was obtained from the higher peak.
Table 4.2 presents norms for 1000 Hz probe tone admittance (Ya), susceptance
(B a) and conductance (Ga) tympanograms recorded for all ears included in the final
analysis of this study.
TABLE 4.2 1000Hz Admittance (Ya), Susceptance (Ba) and Conductance (Ga)
tympanometry norms (Total sample)
Ya (n=809 ears)
Ba (n=791 ears)
Ga (n=791 ears)
Variables
Peak
admittance
TPP
(daPa)
Peak
admittance
TPP
(daPa)
Peak
admittance
TPP
(daPa)
Mean
2.9
0.13
1.8
43.8
2.6
-8.7
Std Deviation
1.1
60.93
0.8
69.1
1.2
62.1
Max
9.6
185
6.1
195
9.9
180
Min
0.9
-275
0.4
-235
0.2
-285
5th Percentile
1.5
-110
0.9
-80
1.2
-120
2.60
5
1.7
45
2.3
-5
4.9
90
3.1
155
4.9
80
50th Percentile
Median
95th Percentile
107
Though a number of studies have reported preliminary normative data for
admittance 1000 Hz tympanometry, there are very limited data on results obtained
from susceptance and conductance tympanograms.
high
frequency
tympanometry
in
infants
have
Most studies investigating
measured
admittance
or
susceptance tympanograms (Kei et al., 2003:20; Margolis et al., 2003:383; Purdy &
Williams, 2000:14).
A distribution curve of the normative values (as shown in
Table 4.2) for admittance tympanograms is provided in Figure 4.13.
250
5th percentile
(1.53 mmho)
Median
(2.60 mmho)
95th percentile
(4.94 mmho)
200
Number of ears
Admittance
150
100
50
0
0.75 1.25 1.75 2.25 2.75 3.25 3.75 4.25 4.75 5.25 5.75 6.25 6.75 7.25 7.75 8.25 8.75 9.25 9.75
Admittance value
Figure 4.13 Distribution of admittance results for Ya tympanograms
(n= 809)
Figure 4.13 shows the distribution of uncompensated peak admittance values
recorded for the total case sample (n = 809). The mean admittance value is 2.85
mmho.
As indicated in Figure 4.13 the admittance values that account for
normality between the 5th and 95th percentile, ranges from 1.53mmho to
4.95mmho, with a median of 2.60 mmho.
These results correspond highly to
normative values from 1000 Hz admittance tympanograms for full-term babies
108
reported by Margolis et al., (2003:386). A comparison between results from the
present study and results reported by Margolis et al., (2003:386) is presented in
Table 4.3.
TABLE 4.3 Comparison between results obtained from the current study
and results by Margolis et al., (2003:386)
This study
Results by Margolis et al., 2003:386
Mean
2.8
2.7
SD
1.2
1.3
5th Percentile
1.5
1.2
50th Percentile
2.6
2.5
95th Percentile
4.9
4.8
It is clear that results obtained in the current study showed similar results to that of
Margolis et al., (2003:389). Margolis et al., (2003:389) also reported a single cutoff
value of 0.6 mmho for static admittance (peak-to-negative tail difference), though
this was not compared in the current study.
As peak admittance values, as presented for the total sample in Table 4.2,
indicated statistically significant differences between male and female ears, gender
specific norms are presented separately in Table 4.4 and Table 4.5. A comparison
between Ya admittance normative data male and female ears is provided in Table
4.6.
109
TABLE 4.4 1000Hz tympanometry norms (Female ears)
FEMALE
Ya (n=385 ears)
Ga (n=376 ears)
Ba (n=376 ears)
Variables
Peak
admittance
TPP
(daPa)
Peak
admittance
TPP
(daPa)
Peak
admittance
TPP
(daPa)
Mean
2.6
-4.9
1.7
35.9
2.3
-17.4
Std Deviation
0.8
60.5
0.7
69.4
0.9
61.2
Max
8.7
185
5.6
195
9.0
165
Min
0.9
-205
0.5
-235
0.2
-280
5th Percentile
1.4
-115
0.75
-85
1.1
-135
2.4
5
1.6
35
2.1
-15
4.2
90
2.7
155
4.2
75
th
50 Percentile
Median
th
95 Percentile
TABLE 4.5 1000 Hz tympanometry norms (Male ears)
MALE
Ya (n=424 ears)
Ba (n=415 ears)
Ga (n=415 ears)
Variables
Peak
admittance
TPP
(daPa)
Peak
admittance
TPP
(daPa)
Peak
admittance
TPP
(daPa)
Mean
3.1
4.7
1.9
50.8
2.9
-1.0
Std Deviation
1.3
60.9
0.8
68.1
1.3
61.9
Max
9.6
160
6.1
190
9.9
180
Min
1.1
-275
0.4
-210
0.9
-285
5th Percentile
1.7
-100
0.9
-70
1.3
-110
50 Percentile
Median
2.9
10
1.8
55
2.5
5
95 th Percentile
5.4
95
3.3
150
5.4
85
th
110
TABLE 4.6 Comparison between peak admittance values in Ya
tympanograms in male and female ears
Variable
Ya Peak admittance (mmho)
MALE
FEMALE
Mean
3.1
2.6
Std
deviation
1.3
0.8
Max
9.6
8.7
Min
1.1
0.9
5
Percentile
1.7
1.4
50 th
Percentile
2.9
2.4
5.4
4.2
th
Median
95 th
Percentile
Peak admittance values presented in Tables 4.4, 4.5 and 4.6 indicated a
statistically significant difference (p = 0.00) between male and female ears for
maximum uncompensated acoustic admittance, with admittance being higher for
male infants than female infants. No significant difference was observed for peak
pressure values (p = 0.16) in Ya tympanograms between genders.
In contrast to Kei et al., (2003:20) who reported a significant ear effect with right
ears showing significantly higher mean peak compensated static admittance and
who found no significant gender effects or interaction with results, this present
study revealed no statistical significant differences between right and left ears for
peak uncompensated acoustic admittance (p = 0.75) or peak pressure values (p =
0.13) in Ya tympanograms, whilst a significant difference between male and female
ears for peak uncompensated acoustic admittance were found. Similar results
111
have been reported by Palmu et al., (2001:182) who investigated infant ears at 7
and 24 months of age with 226 Hz probe tone tympanometry.
They found a
statistically significant difference between acoustic admittance values for male and
female ears, with the admittance values for males being significantly higher than
for female ears. Differences in middle ear and tympanic membrane sizes were
impeached for these results (Palmu et al., 2001:183).
In the adult population
differences between male and female ears have been widely reported for 226 Hz
probe tone tympanometry (Fowler & Shanks, 2002:178).
This is of important
consideration when assessing infant ears, as peak admittance values for 1000 Hz
probe tone tympanometry are significantly lower in female in comparison to male
ears.
Several investigations have provided some preliminary data with regards to various
acoustic admittance parameters in the assessment of middle ear function in
infants. There is evidence from those studies and others that these variables are
age dependant (De Chicchis et al., 2000:97, Holte et al., 1991:20).
When
assessing the infant middle ear age-related changes in acoustic admittance values
need careful consideration. These changes have shown to increase with age and
therefore necessitate age specific normative data to avoid false-positive results
due to inappropriate normative values (Meyer et al, 1997:192; Holte et al.,
1991:12). A study by De Chicchis et al., (2000:97) investigating developmental
changes in acoustic admittance measurements of children aged 6 months to 5
years, indicated statistically significant effects showing increases in acoustic
admittance and ear canal volume as ages increased (De Chiccis et al., 2000:99).
In another study Palmu et al., (1999:210, 211) investigated the diagnostic value of
tympanometry using a 226 Hz probe tone in infants and found that middle ear
compliance increases with age and is quite low in infants. They concluded that the
compliance limit of >0.2 ml for normal ears is too high for the infant population, and
that normal infant ears commonly have compliance values of less the 0.2 ml when
a 226 Hz probe tone is used (Palmu et al., 1999:211).
112
Normative data were therefore calculated for neonates and infants across four age
groups: 1) 0 weeks of age, 2) 0 – 4 weeks of age, 3) 5 – 28 weeks of age and 4) 29
– 52 weeks of age. Means and standard deviations for the acoustic admittance
variables across these four age groups are shown in Table 4.7.
Statistically
significant differences were obtained for peak admittance values between all age
groups. In comparison to the older age groups, infants in the 0 – 4 week age
group presented with much lower mean and standard deviations for peak
admittance. The range of values form the 5th to the 95th percentile for the 0 – 4
weeks age group was 2.2 mmho, compared to 2.7 and 3.4 mmho for infants 5 – 28
weeks and 29 – 52 weeks of age, respectively. Ya admittance values increased in
a rather orderly manner from the youngest group, those zero weeks of age, to the
oldest group, those infants who range in age from 29 to 52 weeks. Additionally an
increasing range of variability accompanies increasing age, illustrated by the higher
standard deviation values as infants become older.
A graphic representation of
the increase in mean admittance values and standard deviations as a function of
age is provided in Figure 4.14A.
Figure 4.14 illustrates the mean values for peak admittance (Figure 4.14A) and
tympanometric peak pressure (Figure 4.14B) across the four age groups. The
error bars in Figure 4.14 represent the standard deviations observed within these
groups.
Mean tympanometric peak pressure data are graphically represented in Figure
4.14B. No significant differences or age-related trends were observed for this
measurement variable and the mean pressure values were very similar across all
age groups approximating 0 daPa. Higher standard deviations for tympanometric
peak pressure were however noted in the older age groups (5 – 28 weeks and 29 –
52 weeks), indicating increased variability in peak pressure with increase in age.
These results are similar to pressure values for 1000 Hz probe tone tympanometry
reported by Margolis et al., (2003:386) with a 90% range for tympanometric peak
pressure of -133 daPa (5th percentile) to 113 daPa (95th percentile). The lower
113
standard deviation and 90% range for peak pressure observed in the 0 – 4 weeks
age group (-75 daPa to 80 daPa / 5th – 95th percentile), compared to -120daPa to
90 daPa (5th – 95th percentile) and -130 daPa to 105 daPa (5th to 95th percentile) in
the 5 – 28, and 29 – 52 week age groups respectively, indicate more stringent
criteria for normality in the younger age groups.
A summary of the normative tympanometric data for the different age groups is
presented in Figure 4.15.
114
115
2.2
0.9
7.7
1
1.2
Mean
Std Deviation
Max
Min
5th Percentile
2.0
3.4
50 Percentile
Median
95th Percentile
th
Peak
admittance
Variables
70
-10
-70
-130
185
48
-2
TPP
(daPa)
0 WEEKS
(n = 73 ears)
3.7
2.2
1.4
1
7.7
0.8
2.4
Peak
admittance
80
-5
-75
-185
185
49
-1
TPP
(daPa)
0 – 4 WEEKS
(n = 250 ears)
4.97
2.7
1.7
0.86
8.5
1.0
2.9
90
10
-120
-275
160
65
0.1
TPP
(daPa)
5 – 28 WEEKS
(n = 457 ears)
Peak
admittance
AGE
TABLE 4.7 Norms for 1000 Hz admittance tympanometry across four age groups
7
3.4
2.1
1
9.6
1.5
3.8
Peak
admittance
105
15
-130
-205
170
72
3
TPP
(daPa)
29 – 52 WEEKS
(n = 98 ears)
A
Peak Y a acoustic admittance (mmho)
6
5
4
3
2
1
0
0 weeks
0 - 4 weeks
5 - 28 weeks
29 - 52 weeks
Age
B
Tympanometric peak pressure (daPa)
100
80
60
40
20
0
-20
-40
-60
-80
0 weeks
0 - 4 weeks
5 - 28 weeks
29 - 52 weeks
Age
Figure 4.14 Mean values for Ya tympanometric variables across age
groups: (A) peak admittance in mmho, (B) tympanometric
peak pressure in daPa. (Error bars represent plus and
minus one standard deviation)
116
0 – 4 weeks (n = 250 ears)
9
ADMITTANCE(mmho)
8
-75 daPa
th
5 %ile
-5 daPa
median
80 daPa
th
95 %ile
7
6
5th %ile
5
median
4
95th %ile
3
2
1
0
5 – 28 weeks (n = 457 ears)
9
ADMITTANCE (mmho)
8
-120 daPa
th
5 %ile
10 daPa
median
90 daPa
th
95 %ile
7
6
5th %ile
5
median
4
95th %ile
3
2
1
0
29 – 52 weeks (n = 98 ears)
9
ADMITTANCE (mmho)
8
-130 daPa
5th %ile
15 daPa
Median
105 daPa
95th %ile
7
6
5
5th %ile
median
4
95th %ile
3
2
1
0
Figure 4.15 Peak admittance and pressure norms
117
Normative values for acoustic reflexes were investigated in the current study. As
previously indicated in Figure 4.12, acoustic reflexes were recorded in 94% of ears
displaying peak 1000 Hz tympanograms. Normative values for 1000 Hz probe
tone acoustic reflexes were however compiled for 727 ears that displayed both a
peaked tympanogram, in addition to an OAE pass result.
These norms are
presented in Table 4.8.
TABLE 4.8 1000 Hz probe tone acoustic reflex norms (n = 727)
MALE & FEMALE
(n=727 ears)
MALE
FEMALE
( n=379 ears)
(n=438 ears)
VARIABLES
Reflex threshold
(dB)
Reflex Threshold
(dB)
Reflex
Threshold (dB)
Mean
93
92
94
Std Deviation
9
9
9
Max
110
110
110
Min
60
65
60
5th Percentile
80
75
80
50th Percentile
(median)
95
95
95
95th Percentile
105
110
105
As is evident from Table 4.8, no significant difference was observed between reflex
thresholds obtained for male and female ears.
All ears considered together
indicated a mean reflex threshold of 93dB, with a standard deviation of 9dB. The
90% range varied from 80dB to 105dB. Though the usefulness and diagnostic
significance of these preliminary normative values may be limited until further
research is conducted which include ears with pathology, the presence or absence
of acoustic reflexes still remain a useful part in the audiological evaluation in
118
infants. This can aid in the differential diagnosis of infants who failed newborn
hearing screening and who display abnormal results on other tests of hearing
function. However, the normative values for acoustic reflexes indicated a 95th
percentile of 105dB (Table 4.8), which is very close to the maximum output of the
equipment (110dB).
Therefore the diagnostic significance is limited and the
absence of acoustic reflexes alone cannot confirm the presence of abnormality in
any age (McMillan et al., 1985:148)
Ipsilateral acoustic reflex measurement, as utilized in this study, offers advantages
over contralateral testing in infants, as results cannot be confounded by
contralateral ear factors, and testing is easier as an earphone need not be
positioned in the contralateral ear. Further studies are however needed to define
the role of ipsilateral acoustic reflexes, with different stimuli in the infant population.
4.6
Summary
This chapter presented the results obtained from the clinical research investigation
as described in Chapter 3. Systematic analysis of data was employed to derive
meaning from raw data.
Results and findings were presented and critically
discussed according to the main and sub-aims formulated for this study within the
context of previous research.
1000 Hz acoustic immittance results were described for infants failing and passing
OAE testing. A high incidence of peaked tympanograms and present acoustic
reflexes, suggestive of normal middle ear function, could be obtained in infants
using a 1000 Hz probe tone. Double peaked tympanograms were recorded in
4.5% of the total sample, and 64% of these were recorded from male ears. For
ears passing OAE testing and displaying a peak 1000 Hz tympanogram normative
values were compiled. Age appropriate and gender specific normative data were
also described and merit consideration when assessing the infant middle ear.
119
Peaked tympanograms and present acoustic reflexes were highly associated with
OAE pass results and conversely flat tympanograms and absent reflexes were
more associated with OAE fail results.
High frequency (1000 Hz) immittance
measures therefore prove to be a valid measure of middle ear function in infants.
Current normative values and that of previous studies on 1000 Hz tympanometry
provide criteria for identifying middle ear dysfunction in infants and can assist in
distinguishing between screening fails caused by sensorineural hearing loss and
those caused by transient middle ear conditions.
120
5. CONCLUSIONS AND RECOMMENDATIONS
Chapter 5 presents conclusions drawn from the current
research results and recommendations for clinical practice and
for future research.
A critical evaluation of the study is
included,
5.1
INTRODUCTION
Objective measurement of middle-ear function continues to be refined and in
recent years high frequency tympanometry has enjoyed added awareness and
is increasingly being implemented in neonatal and infant hearing assessment
protocols. In accordance with a dearth in current research, this study explored
and validated the use of high frequency acoustic immittance measures in a
large group of infants and described age- and gender related normative values.
To date, no large scale studies have been reported on high frequency
tympanometry and acoustic reflex testing in the neonatal and infant population
and limitations in subject size and the shortfall of age appropriate normative
data in previous studies have been identified.
Although the use of high
frequency tympanometry has generally been proven and accepted as superior
over conventional 226 Hz tympanometry in infants below 7 months of age (Lilly,
2005:24; Kei et al., 2003:27; Margolis et al., 2003:391; Purdy & Williams,
2000:10; Meyer et al., 1997:194:); the need exists for classification systems and
norms for interpreting these results to ensure effective diagnosis of middle ear
pathology in infants (Sutton et al., 1996:15).
Conclusions from the research findings in the current study will be presented in
the following section to ascertain the significance of the obtained results and to
determine implications for clinical practise.
121
5.2
CONCLUSIONS
This study has investigated and highlighted the use of high frequency probe
tone (1000 Hz) tympanometry as a method of middle ear assessment in infants.
Conclusions are discussed in Table 5.1 against the framework of the main and
sub-aims formulated for the study.
TABLE 5.1 Conclusions according to sub-aims
•
Sub aim 1 – Admittance (Ya) tympanogram shape and characteristics
within subgroups
∼
The total case sample of 936 ears was divided into two groups, depending on
outcome of OAE testing. 869 ears (93%) that passed OAE-screening were
considered to have normal middle ear functioning and were assigned to Group
A. 67 ears (7%) failed OAE screening, were postulated to have possible middle
ear pathology and were assigned to Group B.
∼
88% (n = 823 ears) of ears in the total case sample displayed peaked
admittance tympanograms and in conjunction with an OAE pass, this was
considered confirmation of normal middle ear functioning. Flat tympanograms,
considered indicative of middle ear pathology, were recorded in 12% (n = 112)
of the test ears.
∼
Significantly high associations were observed between tympanogram shape
and OAE outcome: 93% of ears with OAE-pass results (n = 869) displayed
peaked 1000 Hz admittance tympanograms. In 7% of ears (60/869) in which
flat tympanograms were recorded, OAE’s were however still found to be
present.
79% of ears failing OAE testing displayed flat (abnormal)
tympanograms. Results of this study showed greater agreement between OAE
failure and abnormal 1000 Hz tympanomgrams compared to results by Sutton et
al., (1996:12) who reported only about half of OAE fails (16/33) to correspond
with abnormal 1000 Hz tympanograms.
∼
This study has confirmed that abnormal 1000 Hz tympanometry is strongly
associated with OAE failure, and that normal, peaked 1000 Hz tympanometry is
strongly associated with OAE pass results.
122
∼
88% of Ya tympanograms displayed discernable peaks. Double peaked
tympanograms were recorded in 4.5% of the total sample and comprised 5% of
ears (41) with peaked tympanograms (n = 823). A considerable gender effect
was observed within the group displaying double peaked tympanograms, and
64% were male. The incidence of double peaked tympanograms were much
higher in the present study compared with results by Kei et al., (2003:24) who
reported recording of double peaked tympanograms for only 3 out of 224 ears
(1%).
Differences in sample size and age of subjects could account for
discrepancy in results.
∼
Statistically
significant
differences
(p
=
0.00)
were
observed
in
Ya
tympanograms between the group passing OAE testing (mean admittance 2.5
mmho) and the group failing OAE testing (mean admittance 1.78 mmho)
∼
For ears displaying a peaked Ya tympanogram in conjunction with an OAE pass
result, the
mean
value for uncompensated acoustic admittance
(Ya)
tympanograms was recorded at 2.85 mmho, with a standard deviation of 1.13
mmho. Maximum admittance values corresponding to an OAE pass result were
recorded at 9.64 mmho and minimum admittance relating to OAE pass result
was 0.86 mmho.
∼
The mean pressure value in admittance tympanograms for ears passing OAE
testing and displaying a peaked Ya tympanogram, was 0.13 daPa with a
standard deviation of 60.93 daPa. The highest extreme pressure value related
to an OAE pass resulted was 185 daPa, and the lowest extreme was measured
at -275 daPa.
87% of ears passing OAEs displayed peak pressure values
bigger than -100 daPa and smaller and equal to 100daPa.
•
Sub aim 2 – Characteristics of susceptance (Ba) and conductance (Ga)
tympanograms
∼
Notching was mainly evident in the susceptance (Ba) tympanograms and this
pattern was in agreement with Holte et al., (1991:9) who found that reactance
shifts towards mass with high probe frequencies and towards asymmetrical
resistance, with the interactions resulting in notched susceptance (Ba)
tympanograms.
∼
A great range of variation of B/G tympanogram shapes occurred across all age
groups with the most frequent shape being the 3B1G and 1B1G Vanhuyse type.
123
∼ Complex configuration and notching patterns were observed in susceptance
and conductance tympanograms, making interpretation of B/G tympanograms
more difficult (Holte et al., 1991:22).
∼ Mass and stiffness related elements can be assessed by 1000 Hz B/G
tympanograms, but prior to more definite results and large scale investigations
on the interpretation of B/G tympanograms, the use of Y-admittance
tympanograms appear superior as a screening tool for middle ear functioning in
infants to ease interpretation and classification. Highly significant associations
between OAE pass results and single peaked admittance tympanograms (93%)
and between OAE fail results and flat admittance tympanograms (79%)
obtained in the current study suggest that admittance tympanograms have good
sensitivity and specificity for assessment of middle ear status in infants. Kei et
al., (2003:27) reported single-peaked tympanograms, indicative of normal
middle ear functioning, in 92.2 % of neonate ears.
•
Sub aim 3 – 1000 Hz probe tone acoustic reflexes
∼
Ipsilateral acoustic reflexes at 1000 Hz were evaluated with a high frequency
(1000 Hz) probe tone.
Reflex thresholds were found to be present in 86%
(n=760) of the ears tested.
∼
A mean threshold of 93 dB with a standard deviation of 9 dB and a 90% range
of 80 – 105 dB was obtained.
∼
The higher percentage of present acoustic reflexes obtained in the current study
compared to previous studies in infants was attributed to the fact that a 1000 Hz
probe tone and an ipsilateral mid-frequency (1000 Hz) stimulus was used to
activate the reflexes (Wetherby & Bennet 1980:107).
∼
Good agreement was observed between acoustic reflex presence or absence
and normal and abnormal OAE and tympanometry results. This validated the
use of OAE pass results and peaked admittance tympanograms as a control
variable for normal middle ear functioning with identification of the subgroup of
subjects used for the description of normative tympanometric values.
∼
Acoustic reflexes appear a good adjunct to 1000 Hz tympanometry for the
confirmation of middle ear functioning in infants below seven months of age.
124
•
Sub aim 4 – High frequency immittance norms
∼
Age and gender specific normative values for 1000 Hz acoustic admittance,
susceptance and conductance tympanograms were compiled for ears displaying
peaked tympanograms in conjunction with OAE pass results.
∼
An increase in mean admittance and standard deviation values were observed
across age groups. Mean admittance for infants aged 0 – 4 weeks was 2.4
mmho, with a standard deviation of 0.8, compared to 2.9 mmho with standard
deviation 1.0 and 3.8 mmho with standard of deviation of 1.5 for infants aged
5 – 28 and 29 – 52 weeks respectively.
∼
In agreement to Palmu et al., (2001:181) age specific normative values are
needed for the interpretation of tympanometry obtained from infants and young
children. Results obtained from this study may serve as a preliminary normative
data for the interpretation of age specific tympanograms.
5.3
IMPLICATIONS OF FINDINGS
This study has provided normative values and results for 1000 Hz
tympanometry and this may serve as a guide to further research and as
preliminary norms for the identification of normal and abnormal high frequency
tympanograms.
However, as this study did not include results of abnormal
tympanometry, more research on data of abnormal ears is needed to fully
understand the effect of abnormal middle ear functioning on tympanograms
recorded with a high frequency probe tone.
The prospect of employing high frequency tympanometry for timely identification
and treatment of middle ear effusion in infants may serve as an important
adjunct to tests currently used for audiological diagnosis in the infant population,
to differentiate between middle ear pathology and sensori-neural hearing loss.
This may also serve as a means to reduce high false positive test outcomes
during neonatal hearing screening program relating to transient middle ear
effusion. Thus high frequency tympanometry demonstrates promise for
inclusion in neonatal audiological assessment procedures and hearing
screening programs.
125
Previous studies investigating developmental changes and characteristics of
high frequency tympanometry in infants have utilized susceptance and
conductance tympanograms (Holte et al., 1991:3, Meyer et al., 1997:191) while
others have utilized admittance tympanograms (Kei et al., 2003:23, Margolis et
al., 2003:385, Meyer et al, 1997:191, Sutton et al, 1996:11, Thornton et al.,
1993:320) in analysis and description of results. Purdy and Williams (2000:19)
reported that the majority of studies investigating high frequency tympanometry
measured admittance (Ya) or susceptance (Ba) tympanograms and that
conductance (Ga) tympanograms had a limited diagnostic role in infants. Their
recommended test protocol consequently suggested the use of susceptance or
admittance tympanograms (Purdy et al., 2000:22) for measurement of high
frequency tympanograms in infants.
Conductance (Ga) tympanograms recorded in the present study showed notable
similarities with admittance (Ya) tympanograms when a simple visual analysis
was applied (see Figure 4.12 for examples). Analysis of B/G tympanograms in
terms of number of extrema can determine contribution of mass and stiffness
related systems in the middle ear. The most frequent shapes observed in the
current study were classified as 3B1G and 1B1G. The diagnostic value of the
assessment of the mass- and stiffness related elements in the infant middle ear
still remains to be determined (Purdy & Williams, 2000:22).
Owing to high associations observed between results of admittance
tympanograms and OAE outcome, with a significant association of 93%
between OAE pass results and peaked admittance tympanograms and of 79%
between OAE fail results and flat admittance tympanograms, total admittance
(Ya) appears a valid method of categorisation of 1000 Hz tympanograms in
infants. Due to greater variation in notching patterns that occur in susceptance
and conductance tympanograms and complicates interpretation, an additional
benefit of the use of admittance tympanograms is that it facilitates and simplifies
interpretation as less variability in notching patterns occur.
126
5.4
CRITICAL EVALUATION OF RESEARCH PROJECT
A critical evaluation in the form of strengths and limitations of the current study
is provided in Table 5.2.
TABLE 5.2 Strengths and limitations of the current study
STRENGTHS
The study encompassed a large sample of subjects on which high frequency
immittance measurements were performed, which allows for increased sensitivity of
normative values that were compiled.
1000 Hz probe tone tympanometry and acoustic reflexes were analyzed and age and
gender specific norms were reported.
The use of an OAE pass result as a control variable for normal middle ear functioning
proved successful and useful for identification of normal middle functioning and
correlated well to peaked tympanogram results and present acoustic reflexes.
LIMITATIONS
Poor infant co-operation prevented the whole test procedure to be performed on all
infants.
Breast or bottle feeding to pacify distressed infants in addition to visual
distraction proved successful in enhancing infant co-operation, though 100% success
rate could not be obtained.
Due to failure of subjects to return for follow-up OAE screening it had not been
possible to confirm or reject sensori-neural hearing loss in cases with a combination
of a peaked tympanogram and absent OAEs. A further limitation was due to the fact
that ABR screening results were not considered in the analysis of the results for
subjects that underwent AABR testing,
Uncompensated acoustic admittance and tympanometric peak pressure were the
only variables analysed in this study. For more detailed analysis and for comparison
between studies tympanometric width, compensated static admittance and
tympanometric gradient could be included as measurement variables in subsequent
investigations.
127
5.5 CLINICAL
GUIDELINES
FOR
INTERPRETATION
OF
HIGH
FREQUENCY IMMITTANCE MEASURES
∼
High frequency tympanometry in combination with acoustic reflexes proves
a useful and sensitive measure of middle ear functioning in infants.
Peaked tympanograms and present acoustic reflexes strongly indicate
normal middle ear functioning.
∼
High frequency admittance (Ya) tympanograms are easier to interpret and
proves a suitable measure if the assessment of mass and stiffness-related
elements are not the primary objective.
∼
In agreement with previous reports, double peaked tympanograms indicate
good agreement with OAE pass results, and can therefore be interpreted
as normal.
∼
OAE pass results were highly associated with tympanic peak pressures
values greater than -100 daPa and smaller than 100 daPa, with 87% of
ears in the OAE pass group falling within this region. Positive middle ear
pressure >150 daPa may be an important indicator of the presence of
middle ear effusion in infants.
∼
Age related normative data described in this study offer guidelines for the
interpretation of high frequency tympanometry and reflexes in infants.
5.6
o
RECOMMENDATIONS FOR FUTURE RESEARCH
Further research is needed on larger subgroups with abnormal middle ear
functioning for comparisons to be made between results of normal and
abnormal measures of high frequency tympanometry.
o
More research on the validation of admittance tympanograms in relation to
OAE outcome and other tests of middle ear assessment is necessary to
further validate the use high frequency (1000 Hz) Ya tympanograms as a
single indicator of middle ear functioning in infants.
128
o
Classification
systems
based
on
single
component
admittance
tympanograms should be explored and developed to ease classification
and interpretation of 1000 Hz tympanograms. This could provide a more
scientific base for interpretation of high frequency tympanometry opposed
to the current practise employing a simple pass criteria based on the
presence of any peaked tympanometric pattern.
5.7
FINAL COMMENTS
The prospect of employing high frequency tympanometry to assess middle ear
functioning in infants, addresses one of the major challenges of early
identification and differentiation between middle ear pathology and sensorineural hearing loss in infants. High frequency admittance tympanograms prove
useful as part of a paediatric audiologic test battery for infants less than twelve
months of age, though it is important to remember that concerns about hearing
sensitivity after referral from newborn hearing screen cannot be dismissed on
the basis of a flat tympanogram (Holte & Margolis, 2002:390).
1000 Hz
admittance tympanometry proves a valid and useful method for identification
middle ear pathology in infants. Normative values, as described in this study,
offer guidelines for further research into universal normative values and a
classification system for high frequency acoustic immittance measures in
infants.
5.8
SUMMARY
Chapter 5 provided conclusions and recommendations based on the results
obtained
in
this
study.
Significant
findings
were
highlighted
and
recommendations for clinical practise provided. High frequency tympanometry
shows great promise for timely diagnosis of middle ear dysfunction in infants
and for differentiation between true and false positive results from hearing
screening.
129
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APPENDIX A: Data recording sheet
DATA SHEET
ID NO.
Respondent no
V1
1-4
SECTION A ~ IDENTIFYING INFORMATION
a) Gender
Male 1 Female
2
V2
5
b) Child's Age
Weeks
V3
6-7
c) Mother's Age
Years
V4
8-9
d) Home Language
Tswana
1
Sepedi
2
Shangaan
3
Zulu
4
English
5
Afrikaans
6
Other
7
e) Race
Black
Coloured
Indian
White
1
2
3
4
f) Primary Caregiver
Mother
1
Father
2
Both
3
Grandparents
4
Extended family
5
Foster parents
6
V5
10
V6
11
V7
12
141
g) Educational Qualifications
i. Biological Mother
< St. 6
St. 6-8
St. 8-10
Diploma/Degree
Postgraduate
1
2
3
4
5
V8
13
V9
14
ii. Biological Father
< St. 6
St. 6-8
St. 8-10
Diploma/Degree
Postgraduate
1
2
3
4
5
h) Average Household Income (p/m)
<R500
1
R501 – R1000
2
R1001 – R2000
3
R2001 – R 5000
4
R5000+
5
i) No. of children (Biological mother)
Born
Still living
j) Marital status
Married
Never married
Divorced
Widow
k) Housing
Own house/flat
Informal housing
Renting
With others
of Biological parents
1
2
3
4
1
2
3
4
V10
15
16-17
18-19
V11
V12
V13
20
V14
21
V15
22
SECTION B ~ RISK INDICATORS
a) Family History of childhood Hearing loss
Yes 1 No 2 Info unavailable 3
142
b) Hyperbillirubinemia
Levels requiring blood transfusion/exchange
Yes 1 No 2 Info unavailable 3
V16
23
If levels are known, are they in excess of the following amounts,
Birth weight (grams)
Bili level
≤ 1000
10.0
1001 – 1250
10.0
1251 – 1500
13.0
1501 – 2000
15.0
2001 – 2500
17.0
2500 +
18.0
Yes 1 No 2 Info unavailable
c) Congenital infections
V17
24
V18
25
V19
V20
V21
V22
V23
V24
V25
V26
26
27
28
29
30
31
32
33
d) Craniofacial defects (Head and neck)
Yes
1 No
2
V27
34
e) Birth weight < 1500g
Yes 1 No 2 Info unavailable
3
V28
35
f) Bacterial meningitis
Yes 1 No 2 Info unavailable
3
V29
36
g) Asphyxia
Apgar 0-4 at 1min and/or 0-6 at 5min
Yes 1 No 2 Info unavailable 3
V30
37
If ‘Yes’ specify at:
1 min
5 min
V31
V32
Yes
1
No
3
2
If 'Yes', specify:
Toxoplasmosis
Cytomegalovirus
Syphillis
Herpes
Rubella
Measles
HIV
Malaria
YES
1
1
1
1
1
1
1
1
NO
2
2
2
2
2
2
2
2
38-39
40-41
h) Ototoxic medications
Used for more than 5 days (e.g. gentamycin, tobramycin, kanamycin,
streptomycin, aminoglycosides and loop diuretics combined with amino’s)
Yes
1 No
2 Info unavailable
3
V33
42
143
i) Persistent pulmonary hypertension / persistant fetal
circulation. Prolonged mechanical ventilation ≥ 5 days
V34
Yes 1 No 2 Info unavailable 3
43
j) Syndrome present
Yes
1 No
2
44
V35
If ‘yes’, specify syndrome:
45 - 46
V36
k) Admitted to the NICU
Yes
1 No
2
If ‘Yes’, for how long?
No of days
V37
47
V38
48-50
SECTION C ~ IMMITTANCE
a) 1000 Hz Tympanogram
i. Y – Admittance
i.i Performed
RIGHT
Yes No
LEFT
Yes No
i.ii Discernable peak
Yes No
Yes
i.iii Admittance (mmho)
,
No
,
i.iv Pressure (daPa)
i.v Double peak
Yes No
Yes
No
RIGHT
Yes No
LEFT
Yes No
i.vi Time taken (min)
V39R
V39L
V40R
V40L
V41R
V41L
V42R
V42L
V43R
V43L
V44R
V44L
51
52
53
54
,
55-58
,
59-62
63-66
67-70
71
72
73-74
75-76
ii. B – Susceptance
ii.i Performed
ii.ii Admittance (mmho)
ii.iii Pressure (daPa)
,
,
V45R
V45L
V46R
V46L
V47R
V47L
77
78
,
79-82
,
83-86
87-90
91-94
144
iii. G– Conductance
RIGHT
Yes No
iii.i Performed
iii.ii Admittance (mmho)
LEFT
Yes No
,
V48R
V48L
V49R
V49L
V50R
V50L
,
iii.iii Pressure (daPa)
95
96
,
,
97-100
101-104
105-108
109-112
b) 1000 Hz Probe Tone Reflex
i. Performed
RIGHT
Yes No
LEFT
Yes
No
ii. Threshold present
Yes
Yes
No
No
iii. Threshold value (dB)
V51R
V51L
V52R
V52L
V53R
V53L
113
114
115
116
117-119
120-122
SECTION D ~ HEARING SCREENING
a) First Screen
i. OAE
ii. AABR
RIGHT
Pass Refer
LEFT
Pass Refer
Pass
Pass
Refer
Refer
V54R
V54L
V55R
V55L
123
124
125
126
iii. Time taken:
iii.i OAE
min
V56
127-128
min
V57
129-130
iii.ii AABR
145
b) Follow-up Screen
i. Returned?
Yes
1 No
ii. OAE
iii. AABR
2
V58
RIGHT
Pass Refer
LEFT
Pass Refer
Pass
Pass
Refer
131
V59R
V59L
V60R
V60L
Refer
132
133
134
135
SECTION E ~ DIAGNOSTIC ASSESSMENT
a) Returned?
Yes
1 No
2
V61
136
b) Hearing loss?
None
1 Bilateral 2 Unilateral 3
V62
137
V63
138
c) Type of hearing loss?
S/N
1
Conductive
2
Mixed
3
AN
4
d) Ear
Left
1 Right
e) Degree of hearing
i.
Right ear
Mild (15-30 dB)
Moderate (31-50dB)
Severe (51-70dB)
Profound (71dB+)
ii.
Left ear
Mild (15-30 dB)
Moderate (31-50dB)
Severe (51-70dB)
Profound (71dB+)
2 Both
3
V64
139
1
2
3
4
V65
140
1
2
3
4
V66
141
loss?
146
COMMENTS
147
APPENDIX B: Ethical Clearance
148
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