Effects of Delayed Second Cochlear Implant

Effects of Delayed Second Cochlear Implant
Journal of Otology 2006
Vol. 1
No. 2
·119·
Original Article
Effects of Delayed Second Cochlear Implant
XIA Rui-ming1, WU Xi-hong2, JANG Zi-gang1, JING Yuan-yuan1,
LIN Yun-juan1, YU Li-sheng1
1. Department of Otolaryngology, People’s Hospital, Peking University, P. R. China
11, Xizhimen South St, 100044 Beijing, P.R. China
2. Speech and Hearing Research Center, Peking University, China
R.M. Xia, X.H. Wu, Z.G. Jang, Y.Y. Jing, Y.J. Lin, L.S.Yu
Abstract Objective Since Helms' successful bilateral cochlear implantation with good results in 1996, there
have been increasing number of reports on bilateral cochlear implantation. Most second device have been
implantated within one year after the first. Considering effects of long time auditory deprivation, it is not clear
whether a delayed second cochlear implant serves to add additional benefits and how it may interact with central
nervous system plasticity. Methods Three cases who received delayed second cochlear implants at People's
Hospital of Peking University from 2002 to 2005 were reviewed. The interval between the first and second
implants was longer than 2 years in all three patients. Sound perception, and unilateral/bilateral speech
discrimination in quiet and noise were evaluated. In addition, GAP detection test was conducted in one patient.
Results In one case, having both implants on provided improved performance compared to using only one
implant both in quiet and noise. Presumably due to visual interference from lip-reading or short interval between
second implant and testing, one patient showed no improvement from using the second implant either in quiet or
noise, while the last case demonstrated additional benefits from the second implant only in quiet. In all three
patients, performance in recognizing the four tones in Mandarin was superior over word recognition. Conclusions
Considerable plasticity in the cerebral auditory center is preserved, despite long acoustic deprivation in some
children who have received unilateral cochlear implant. Delayed second implants can result in significant
improvements in some of these children. Visual interference from lip-reading may be an obstacle during retraining.
The better recognition of tones in the Mandarin language may represent a different sound discrimination
mechanism in the auditory system, although it may also be related to the signal processing mechanisms of the
implant used (MED-EL COMBI 40+).
Keywords bilateral cochlear implant; auditory deprivation
Introduction
Cochlear implantation has been established as an
effective and safe method of rehabilitation for those
with profound deafness. But for economic reasons,
unilateral implantation is considered to be the
mainstream today. Numerous studies indicate that after
unilateral cochlear implantation many patients show
high levels of speech understanding, allowing them to
conduct
everyday
communication,
such
as
face-to-face conversation and using telephone. This
Corresponding author: Lisheng Yu, M.D., Department of otolaryngology, People's Hospital, Peking University, 11, Xizhimen South St, 100044 Beijing, P. R. China
E-mail: [email protected]
auditory prosthesis has greatly improved life qualities
of the cochlear implant users, and facilitated children's
mental development, making normal education
available to pediatric recipients [1-5]. However, results
also showed that speech discrimination scores were
just passable in competing noise[6]. It is well known
that noise reduction and acoustical orientation abilities
of the human auditory system depend on the time,
level, and spectral differences between sound signals
sensed by the two ears; and that subjects with normal
binaural hearing have better speech perception scores
in background noise or in reverberant environment[7, 8].
Bilateral cochlear implantation may be an effective
solution to rebuild binaural hearing. In 1996, Helms et
al. first performed a bilateral cochlear implantation to
restore patient's binaural hearing and reported benefits
·120·
Journal of Otology 2006 Vol. 1 No. 2
from bilateral stimulation[12]. Since then, the number of
bilateral implantation has increased gradually. Several
reports have demonstrated that, compared with
unilateral implantation, bilateral implantation helps
rebuild binaural hearing, significantly promotes user's
speech understanding, especially in background noise,
and restores spatial orientation abilities [13-16]. However,
most bilateral implantations have been performed
within one year of the first implant. Considerating
effects of long time auditory deprivation, it is not clear
whether a significantly delayed second cochlear
implant will provide additional benefits and whether
there is sufficient plasticity in the auditory center after
a long delay following the first implant. From 2002 to
2005, four patients received second cochlear
implantations at the People's Hospital of Peking
University, with delays longer than two years. Contact
with one of the four patients was lost during follow-up.
The rest three cases are reviewed in this paper,
regarding their sound and speech recognition tests
results. The impact of bilateral cochlear implantation
on speech discrimination and the role of central
plasticity in delayed bilateral implantation are
disscussed.
the right ear at ages 6 and 7, while the girl had her first
implant in the left ear at 8.
All recipients lived in Beijing and were the single
child in their families. Parents of these children were
well educated, with stable and above average incomes.
There was no family issue-related stress. All subjects
attended regular schools, with good grades, and
showed willingness to communicate with classmates.
Surveys among their classmates suggested no
significant difficulties in communication with the
patients related to their hearing conditions. Subjects 2
and 3 acquired lip-reading and speech skills through
more than two years of regular lip-reading and speech
training before their first implantation.
Tests
Test 1
Monosyllable recognition was tested in quiet and
noise under three conditions: with both implants on;
left implant on only; and right implant on only.
Monosyllable words were presented at 55 dB SPL.
When noise was added, the signal/noise (S/N) ratio
was kept at 5 dB. Azimuths of test material and noise
were 0 degree from a distance of 1.5 meters. Tests
were conducted in an anechoic chamber. No changes
were made to implant speech processor parameters.
Instructions and practices were given before testing
until subjects completely understood and were familiar
with the tests. Obtained from Beijing Tongren
Hospital, speech materials used in the tests were
composed of single-syllable words recorded in male
voice with 3 second intervals between words to give
the subject adequate response time. Word recognition
scores were based upon correct repeats by the subject.
The subject’s ability to correctly recognize the tone of
the word was simultaneously assessed. No real-time
feedback regarding response correctness was provided
Material and methods
Subjects
Three children (2 boys and 1 girl) with profound
congenital hearing impairment who were implanted
with MED-EL COMBI 40 + bilaterally were included
in this study. The mean preoperative auditory
thresholds were greater than 100 dB HL. The patients'
age ranged between 11 and 12 years. The interval
between the first and second implants was between 2
and 5 years. The 2 boys received their first implants in
Table .1 Results of word and tone recognition
Recognition score in monosyllable
Quiet
Recognition score in Chinese four-tone
Noisy (s/n: 5dB)
Quiet
Noisy (s/n: 5dB)
1
2
3
1
2
3
1
2
3
1
2
3
Subject 1
74%
58%
90%
32%
42%
54%
96%
88%
100%
86%
94%
88%
Subject 2
24%
8%
16%
10%
4%
8%
74%
60%
78%
66%
46%
62%
Subject 3
4%
8%
16%
2%
4%
6%
78%
76%
80%
40%
72%
68%
1: the first implant on 2: the second implant on 3: both on
Subject 1 demonstrated improved performance with bilateral stimulation over unilateral stimulation whether in quiet or in
noise. There was no significant difference between the two conditions in Subject 2. In Subject 3, bilateral stimulation was superior
over unilateral stimulation in quiet, but the difference was not significant in noise. All patients showed better tone recognition capability than word recognition with no significant difference between in quiet and in noise.
Journal of Otology 2006
Table .2
Test sequence
Vol. 1
No. 2
·121·
GAP threshold test and word recognition scores in Subject 1
Left ear
Right ear
Both
monosyllable
GAP threshold
monosyllable
GAP threshold
monosyllable
1
0%
NA
40%
5ms
NA
2
18%
7ms
40%
4ms
54%
3
60%
7ms
72%
5ms
76%
1. at the time of second device switch-on 2. one month after switch-on 3.five months after switch-on
to the subjects during the tests.
All these tests were implemented in November
2005, three years and two months after Subject 1's
second implantation and four months after the second
implantation for Subjects 2 and 3.
Test 2
The subject was asked to read a short essay in a
natural manner. Four normal-hearing Mandarin
speaking adults scored the understandability of the
reading on a scale from 0 to 4, with 0 representing
normal reading and 4 indicating severely impaired
understandability. The three patients were tested
separately.
Test 3
This test was conducted in Subject 1 at the time of
first fitting of the second device, and at one month and
five months later, and measured the subject's ability to
detect a GAP between continuous noise and to
correctly recognize monosyllable words.
Testing was conducted in an EMI. A loudspeaker
was set 1.5 meters from the midpoint of subject's head
at the level of the tested ear. Gaussian white noise was
presented for a duration of 2.5 seconds at 60dB SPL. A
GAP was inserted into the noise. The duration of GAP
was gradually reduced from 21 to 0 ms. Step length
was 16ms, minimum at 1 ms. The GAP threshold (the
shortest GAP that the subject could reliably detect) was
approached in a one-up/three-down mode. The subject
was allowed to practice before the test. Evaluations
were performed to the left implant in Subject 2 one
month after binaural stimuli and right implant in
subject 3 five months after binaural stimuli
respectively.
Monosyllable words recognition test was
conducted in the same fashion as in Test 1.
Results
Test 1
Table 1 shows results of monosyllable word
recognitions and Mandarin tone tests from the three
subjects.
Subject 1 demonstrated improved performance
with bilateral stimulation over unilateral stimulation
whether in quiet or in noise. There was no significant
difference between the two conditions in Subject 2. In
Subject 3, bilateral stimulation was superior over
unilateral stimulation in quiet, but the difference was
not significant in noise. All patients showed better tone
recognition capability than word recognition with no
significant difference between in quiet and in noise.
Test 2
All subjects received a score of 1, representing
minimally impaired articulation capability that did not
cause significant deterioration in understandability.
Test 3
The results of GAP threshold test and speech test
for Subject 1 are shown in Table 2.
Discussion
The three patients in this study received the same
model of devices in both sides. They received their
first implantation at similar ages. Post-operative speech
abilities, family environments and school performance
were similar. However, their experiences during the
early language development and the delay between the
first and second implantations were different. An
investigation of their post-implantation auditory/speech
functions may therefore help define the effects of
bilateral implantation and elucidate plasticity of central
auditory system.
Monosyllable words were used in this study to
focus on testing sound signal processing and to
minimize interference from associative and meaning
effects [18]. In normal hearing, advantages of binaural
hearing have been attributed to three effects: the head
shadow effect, the squelch effect and the summation
·122·
effect. Users of bilateral cochlear implants may also
benefit from these effects in rebuilding binaural
hearing. Consequently, bilateral implantation can
provide significant advantages over unilateral
implantation, including improved speech recognition
in various listening situations and sound localization[8,
13-16]
.
Subject 1 showed zero word recognition and was
unable to detect GAP when the second device was first
switched on and used unilaterally. This may be due to
lack of adequate sound signal processing in the
auditory system on the side of the second device as a
result of long auditory deprivation. It takes time for
auditory cortices to regain speech recognition function
[17]
. The good word recognition scores on the opposite
side indicates that the auditory and speech systems on
the two sides may develop relatively independent of
each other. Binaural hearing depends not only on input
from both ears, but more importantly on the integration
process at the cortical level, which develops after birth,
possesses high degree of plasticity and can be lost
under certain circumstances.
Subject 1 consistently demonstrated improvement
in GAP detection and word recognition performance
along with time either using the first implant, the
second implant or both. This is an indication that even
in the presence of intact auditory-speech pathways
with well preserved functions, gaining binaural hearing
in bilateral CI users will still require a process of
learning and training. While bilateral cochlear
implantation provides an opportunity for improved
speech recognition, training following implantation is
of crucial importance.
There was no significant difference in articulation
capabilities among the three subjects. However,
Subject 1's word recognition scores were noticeably
better under all test conditions than the other two
subjects. We speculate that the difference may be
related to the longer delay for the second implant,
which may lead to increased auditory deprivation, and
the shorter interval between second implantation and
testing in the latter two patients. In addition, both
Subjects 2 and 3 had received systematic lip-reading
training for longer than 2 years, which may result in
suppression of auditory recognition by dependence on
visual cues.
These results suggest that a person's learning
experience at a young age plays an important role in
his auditory/speech function development. Speech
development depends on integritation of information
from auditory, visual and somesthetic systems. While
Journal of Otology 2006 Vol. 1 No. 2
auditory cues play a dominant role in language
development, inputs from other systems are also
capable of facilitating speech functions and can be
competitively repellent to the auditory system. The
development of speech areas in the brain depends on
natural language acquisition at young ages [9]. It is
possible that speech development in Subjects 2 and 3
may have been substantially influenced by visual input,
which results in invasive deprivation to auditory
cortices, leading to poor monosyllable word
recognition scores even after long time use of bilateral
CIs.
GAP threshold, the shortest.GAP that can be
reliably detected by a subject, represents the sensitivity
of the auditory system in sensing a sudden sound
alteration in time. For adults with normal hearing or
mild hearing loss, GAP threshold correlates with
speech recognition [10, 11]. This is reflected in results of
test 3 in Subject 1. The subject's GAP threshold
reached 7 ms(within normal limits) at one month
following second device switch-on, indicating regain
of time discrimination on the side of the second
implant, and continued to improve afterward along
with benefits from binaural effects.
While landmarks of their brain development were
apparently
comparable,
the
three
children
demonstrated
significantly
different
hearing
deprivation effects. Longer delays between the two
implantations and shorter interval between the second
device switch-on and testing may count for some of the
differences. Two types of auditory development
deprivation may have been involved in these patients:
deprivation from lack of auditory input(as in all three
subjects) and from substitution by visual cues (as in
Subjects 2 and 3). Different from Subject 1, who
developed no substitute language function, Subjects 2
and 3's auditory language development was further
suppressed by their dependence on visual cues from
lip-reading acquired through training at young ages.
This may also have affected the plasticity of their
central auditory systems. The data from these three
patients seem to suggest that pure acoustic deprivation
has limited effects on the development of the auditory/
language system and the patient may retain useful
plasticity in the central auditory system for hearing
rehabilitation. In contrast, auditory deprivation from a
combination of lack of sound input and substitute
communication skills may have increased impact on
plasticity of the central auditory system. Obviously,
more data are needed to better understand the degree of
the plasticity loss under such circumstances and if it is
Journal of Otology 2006
Vol. 1
No. 2
recoverable.
The data also serve to emphasize the importance of
differentiating auditory language dysfunction from
sound deprivation and from visual input interference
when
engaging
post-implantation
patients.
Individualized retraining strategies are necessary and
interference by lip-reading and sign language should
be minimized, while auditory cues should be
emphasized.
The Mandarin tones are based upon frequency
modulation of the vowel elements, resembling music
to a certain extent. The three patients in this study
showed minimal difficulties in recognizing the four
tones in Mandarin, both in quiet and noise. This may
be due to the fact that tone recognition is an essentially
central, process, separate from speech recognition,
with minimal dependence on cochlear processing or on
binaural effects. It is also possible that the signal
processing mechanism of the implants used in these
patients may affect tone recognition as well.
Conclusions
Bilateral cochlear implantation can provide
additional benefits in speech recognition in both quiet
and noise for patients even when the second implant is
delayed for longer than 2 years. However, the benefits
can be affected by auditory deprivation and tend to
diminish in patients with invasive deprivation such as
by visual input suppression. Whether this disadvantage
can be overcome needs further investigation.
Patients implanted with MED-EL COMBI 40 +
demonstrated excellent Mandarin tone recognition,
which may represent an independent mechanism.
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