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| Journal of the New Zealand Medical Association,
27-August-2010, Vol 123 No 1321
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Improved speech
discrimination after cochlear implantation in the Southern Cochlear Implant
Adult Programme
Glossary of abbreviations
SCIP: Southern Cochlear Implant
Programme
SCIPA: Southern Cochlear Implant
Programme- Adult
HINT: Hearing in Noise Test (described in
introduction)
HINT A: auditory cues alone
HINT AV: audiovisual cues
S/O: Switch- on of cochlear
implant
BKB sentence test: Bench, Kowal and
Bamford sentence lists—open-set sentence lists (i.e. response options not
prescribed in advance) similar to HINT
CID sentence test: Central Institute
for the Deaf everyday sentence lists- open-set sentence tests similar to
HINT
K-CID sentence test: Korean version of
the CID test
NVA speech recognition: Nederlands
Vereninging Audiologie—Dutch open-set speech recognition tests consisting
of monosyllabic (consonant-vowel-consonant) word lists
NU-6 words: Northwestern University
Auditory Test No. 6: open-set lists of monosyllabic words
A cochlear implant (CI) is an electronic device which
bypasses the auditory function of the outer, middle and inner ear and stimulates
the cells of the spiral ganglion (and therefore the cochlear nerve) directly.
Cochlear implants are indicated in severe and profound sensorineural hearing
loss (SNHL) when hearing aids provide insufficient information for understanding
speech.
Cochlear implantation was first performed in New Zealand in
1986.1 An initially small national programme
was based in Auckland. Cochlear implants were first performed in Christchurch in
1998. The Southern Cochlear Implant Programme (SCIP) was established in 2003.
Located in Christchurch it serves the South Island and lower North Island. It is
divided into separate Adult (SCIPA) and paediatric programmes. The SCIPA
comprises two surgeons, two audiologists and a rehabilitationist. Potential
candidates are referred to the programme, principally by audiologists and
otolaryngologists, and undergo assessment by all three components of the CI
team. Currently 80% of referrals are found to be suitable candidates.
The key audiologic test performed is aided speech
discrimination, details of which are provided below. This testing aims to
determine how well the candidates can utilise their aided hearing to understand
speech. The same testing processes are used post-implant to assess the
effectiveness of the CI.
Previous research has indicated that there are many factors
contributing to the outcome of CI including age, duration of deafness/age at
onset of deafness, anatomic issues as determined by radiology, other handicaps,
speech and language abilities, functional hearing, family and social support,
expectations/motivation, educational setting, communication mode, availability
of support services, and the intensity of post-implant rehabilitation
therapy.15,4
Many studies have shown a significant negative correlation
with duration of deafness, 4,7,9,11,12 and a
positive correlation with age at onset of deafness, hearing aid use, and
progressive hearing loss.7 In particular,
hearing loss prior to development of speech and language inhibits its
development and also reduces the effectiveness of cochlear implantation later
on.
There are differing findings in the literature regarding the
time after CI that maximal benefit is obtained. Some studies indicate that this
is reached as early as 3 months,10 others
suggest it is as late as 2 years.4 This is an
important factor to be able to counsel patients about before and after their CI,
so they will have some idea of their likely long-term outcome as they carry out
their rehabilitation.
In 2005 our Programme introduced the Nucleus®
Freedom™ (Cochlear Ltd, Lane Cove, New South Wales, Australia) device
which superseded the older technology from the same company of the Nucleus 22
and the later Nucleus 24 cochlear implants. Anecdotally we thought our
patients were experiencing more rapid improvements in speech discrimination and
slightly higher levels of peak performance, but wished to accurately investigate
this.
The objective of this study was to assess the outcome with
respect to improvement in speech discrimination test results after CI in
patients enrolled at SCIPA, and to determine how long after a CI there is a
plateau in performance. We also aimed to assess the effect of duration of
deafness and percentage of life spent deaf on results, and the effects of a new
CI device.
MethodsA retrospective review of all files of patients
receiving follow-up in Christchurch for CI between 1999 and Dec 2008 was
undertaken. Exclusion criteria were patients under age 18 (these would be under
the Paediatric Programme), and patients with no or inadequate pre-implant data.
Some patients had CI performed by the Southern Cochlear Implant Programme
– Adult, then moved to the Northern Cochlear Implant Programme (NCIP) for
follow-up. Most of these were included, as follow-up data was sent from the
NCIP. Some patients had received their CI in NCIP and then moved to the SCIPA
for follow-up. These were included if we had a copy of their pre-implant data
sent from NCIP.
Files were obtained and the following were recorded:
age, sex, type of implant: Nucleus® Freedom™ versus older devices
– Nucleus 22 and Nucleus 24, age of onset of deafness, age at implant,
percentage of life spent deaf (calculated from above), cause of deafness where
known, and speech discrimination test results. We originally intended to measure
both age at onset of hearing loss and age when hearing aids were first fitted,
along with which sides were aided.
Due to lack of consistent information we were only able
to estimate age of onset of deafness with any degree of accuracy. Speech
discrimination was measured using the hearing in noise test (HINT), which is a
standardized test of speech recognition using sentences delivered in a sound
field of 55 dB HL sound pressure level. In determining cochlear implant
candidacy, HINT is performed without background noise, despite its name. HINT
consists of 25 equivalent 10-sentence lists, two lists are used at each
assessment.
The tests are played on a DVD with a speaker/amplifier
and are tested both with audiovisual (AV) cues (watching the presenter read the
lists whilst listening) and with only auditory (A) cues (visual display turned
off). The patient is scored for the percentage of words correctly identified on
the lists. This is carried out pre-implant with their hearing aid on, at
switch-on (s/o) of CI, then at 1 month, 3 months, 6 months, 9 months, 12 months,
18 months and 3 years post-implant where available.
The following were assessed: improvement in HINT A and
AV over time post-CI, overall improvement at 9, 12 and 18 months,
characteristics of patients lost to follow-up or with missing data, relationship
between final outcome and duration of deafness, percentage of life spent deaf
and type of implant used.
Statistical methods: All analyses were carried out
using SAS 9.1. To investigate the time course of recovery up to 9 months after
an implant, measures A and AV were analysed separately using repeated
measures analysis of variance (PROC ANOVA) with time as the repeated measure
(pre-implant, s/o, 1 month, 3 months, 6 months and 9 months), and a between subjects factor based on division of the patients into approximately three equal-sized groups based on their pro-operation scores. The Huynh-Feldt adjustment for non-sphericity was used
for repeated measures sources of variance. Chi-square tests and t-tests were
used to compare those who had complete data up to 9 months and those who did not
on pre-implant variables. Prediction of long-term outcome used data from 18
months if available and otherwise the 12 month values were used. Including 12
month values in this way increased the number of A outcome observations from 40
to 46 and the number of AV observations from 37 to 44.
Prediction of outcome was carried out using one way
analysis of variance (PROC GLM). Because of ceiling effects, particularly for
AV, Welch’s test was used if Levene’s test for the homogeneity of
variance was significant with α=0.10, although this precaution mostly
made little difference.
ResultsA total of 171 patients were on the cochlear implant
rehabilitation files at SCIPA. Of this total, 57 were excluded for the following
reasons: 11 had been implanted too recently for analysis (2 months or less), 12
had transferred from the Paediatric Programme so did not apply to our study, 17
had been implanted prior to 1997 so speech discrimination tests other than HINT
had been used for their pre-implant and post-implant assessments, nine had no
follow-up notes available, four were non-users (discussed later), and four did
not have pre-implant speech discrimination tests available.
A total of 114 patients were then left for analysis.
However, among this group there was much missing data. Only 55 had complete data
for A and 53 for AV. Comparison of those with complete data and the remaining
patients showed no significant differences in any of the background
characteristics or of pre-implant functioning.
We used complete data only for the analyses as imputation of
data was deemed inappropriate due to the ceiling effect of the results. This was
to ensure internal validity, i.e. comparing people across time for the same
people. Pre-implant characteristics are shown in Tables 1–3, and the
pre-implant scores on HINT are shown in Tables 4 and 5.
Table 1. Characteristics of those with a
cochlear implant between 1999 and 2008
Table 2. Mean HINT AV scores (% correct) at
pre-implant, switch-on (s/o), and 1–9 months post-implant: 53
patients
Table 3. Mean HINT A scores (% correct) among
those with complete data up to 9 months by grouping based on pre-implant scores
(55 patients)
Table 4. Mean HINT AV scores (% correct) among
those with complete data up to 9 months by grouping based on pre-implant scores
(53 patients)
There was significant heterogeneity in aetiology. The
recorded causes included idiopathic, genetic, maternal rubella, meningitis,
otosclerosis, endolymphatic hydrops and large vestibular aqueduct
syndrome.
The mean post-implant HINT scores at nine months were 74%
for A only (78 patients, range 11–100%), and 96% for AV (74 patients,
range 62–100%). The mean scores at 18 months were 81% for A only (52
patients, range 14–100) and 98% for AV (50 patients, range 70–100).
Improvement over time following CI was assessed by comparing HINT scores
pre-implant with those at follow-up appointments, for both A and AV (Tables 6
and 7). As mentioned earlier, this was only possible for 55 patients for A and
53 patients for AV due to lack of complete data on the remaining patients.
Table 5. Age at operation vs longterm HINT A
and AV scores: 58 patients
Table 6. Age of onset vs longterm HINT A and AV
scores: 56 patients
Table 7. Percentage of life deaf vs longterm
HINT A and AV scores: 56 patients
We also assessed improvement by initial scores (for both A
and AV) divided into three groups: Table 8 (A) and Table 9 (AV).
Table 8. Implant model (Nucleus Freedom vs
Nucleus 22 and 24) vs longterm HINT A and AV scores: 58 patients
Table 9. Pre-implant HINT A score vs longterm A
and AV scores (% correct): 58 patients
Predicting longer term outcome: only 52 patients had 18
month follow-up for A and 50 for AV. However by adding in 12 month data for
those missing 18 month data, the numbers were increased to 58 and 57
respectively. The effect of age at operation, age of onset of deafness, and
percentage of life spent deaf were assessed in terms of A and AV scores at
long-term follow-up.
Age at operation was unrelated to outcome but the earlier
the onset of deafness (and higher percentage of life deaf), the poorer the
outcome, as would be expected (see Tables 5–7). There was no outcome
difference between the sexes but patients with Nucleus Freedom implants did
better at long-term follow-up than those with older implants (see Table 8). In
terms of pre-implant functioning, medium pre-implant levels of A (scores of
1–15%) were associated with lower post-implant A scores, as was also found
when looking at recovery up to 9 months.
Pre-implant AV scores had no significant effect on long-term
outcome (see Tables 9–10). Multivariate analysis was not possible due to
the small numbers of patients. The ceiling effect for AV was such that there was
little ability to predict this outcome as it was mostly very good with means all
over 90%.
Table 10. Pre-implant HINT AV score vs longterm
A and AV scores (% correct): 57 patients
We looked at the highest and lowest performers within the
group of 114 patients. We defined the very high performers as a post-implant
HINT A score of at least 95% (at most recent assessment), and low performers as
postoperative HINT A score of less than 60%. There were 43 high performers and
20 low performers out of the 114 patients.
Overall for the low performers the percentage of life spent
deaf was more than 50% except for one patient, and for the high performers about
half (22) of the patients had spent more than 50% of their life deaf while 21 of
the patients had spent less than 50% of their life deaf.
For the high performers, 15 out of the 43 had pre-implant
HINT A scores of more than 30. Nine of these were at least 25 years old at onset
of deafness and had mainly progressive aetiology (e.g. otosclerosis), the other
three being younger at onset but also progressive causes, and three with
congenital deafness. A further 15 with lower pre-implant scores were at least 25
years old at onset and mainly progressive causes and had spent less than a third
of their life deaf. One further patient had a congenital hearing loss that was
made worse by otosclerosis over time.
There were 13 patients who would not have been expected to
be high performers, with congenital deafness, implanted between ages 20-43 and
pre-implant HINT A scores ranging from 0 to 30%.
Of the 20 lowest performers, 14 had congenital deafness, 11
with pre-implant HINT A scores of less than 15%, and 19 of whom were older than
35 at implant. There were two older patients: one aged 72 with progressive
hearing loss and duration of deafness of 36 years, pre-implant HINT A score of
9%, and one aged 79 with endolymphatic hydrops as likely aetiology, duration of
deafness of 49 years, and pre-implant HINT A score of 14%.
The remaining four low performers were aged between 49 and
55 and had differing background characteristics: a 55 year old with unknown
aetiology, duration of deafness of 47 years and pre-implant HINT A score of 18,
a 53 year old with chronic middle ear disease, duration of deafness of 48 years
and pre-implant HINT A 18, a 52 year old with otosclerosis, duration of deafness
of 32 years and pre-implant HINT A score of 55, and a 49 year old with chronic
middle ear disease, duration of deafness of 4 years and pre-implant HINT A score
of 0.
Only four out of 20 of the lowest performers’ scores
had not improved post-implant compared with pre-implant measurements. The
remaining lowest performers had still improved significantly, despite having a
low post-implant score compared with the average for the group. The mean
post-implant score for HINT A for the 20 lowest performers was 33 (range 14-56),
compared with a mean pre-implant score of 15 (range 0-55).
Four patients are now not using their cochlear implants. One
patient was found to be a malingerer, the patient is able to converse on the
telephone without a hearing aid or cochlear implant. The second patient has
superficial siderosis of the CNS, presumably with a large neural component
towards his hearing loss meaning the device is ineffective. The third patient
had a right petrous apex chondrosarcoma treated with surgery and radiotherapy
and a left acoustic neuroma treated with radiotherapy. He could not perceive any
sound from his right CI despite pre-implant testing suggesting a functioning
auditory nerve on the right. The final patient had many psychosocial issues and
significant negative thought processes which were thought to be the main cause
of failure.
DiscussionOur medium-to-long term results after CI in adults compare
favourably with others in the literature. This can be difficult as most studies
have used different speech perception tests, so direct comparisons can not be
made, however, the difference between pre-implant and post-implant scores can
still be seen.
Our patient group was defined as adults receiving a CI, and
includes those with prelingual and postlingual deafness, which can make it more
difficult to draw comparisons. However our result of a mean of 74% correct HINT
(A) score at 9 months and 81% at 18 months compared with a mean pre-implant
score of 15% compares well even with studies of adults with postlingual deafness
only (who would be expected to have a better result).
A Manchester study of 34 patients with postlingual deafness
aged at least 65 years having a CI between 1989 and 2002 (with a mean duration
of deafness of 11 years) had similar results to our study with a BKB sentence
test mean score of 73% at 9 months (compared with a mean of 0 pre-implant),
despite a shorter duration of deafness.2 A
Minnesota group of 33 patients over the age of 18 (average 52 years) with
postlingual deafness who had CI before 2002 achieved a mean CID sentence test
score of 54% at one year compared with 14%
pre-implant.3
A Korean group of 13 postlingually deaf patients (average
duration of deafness 9 years) with CI between 1988-1998 had a mean K-CID
sentence test score of 52% at 12 months (pre-implant scores were not
recorded).4 A study of 37 postlingually deaf
adults with CI between 1989-1997 in the Netherlands obtained an average score in
NVA open-set speech recognition of 36% at very long-term follow-up (6 years or
more after CI).5
A Tennessee study of 27 patients over age 50 with
postlingual deafness, with CI before 2004 had mean HINT score of 5.4%
pre-implant, and 66% at mean of 4 years
post-implant.6 A study of 33 postlingually deaf
patients in the Netherlands implanted before 1994 obtained mean scores on speech
perception testing (test not specified) of 28% at 12 months compared with 0%
pre-implant.8
A Swedish study of patients aged over 16 (mean 50 years) and
mean duration of deafness of 15 years with CI before 1994 had a mean open set
spondee word score of 20% at one year (0%
pre-implant).9 A study of 89 postlingually deaf
CI patients in Antwerp with average age 58 years showed a mean post-implant NVA
phoneme score of 47%-68% (differing means depending on differential age groups)
at 12 months compared with 5-7%
pre-implant.13
Studies have shown a significant increase in health-related
quality of life following CI, which correlates with results in speech perception
tests.12 A quality-of-life study conducted by
our programme has shown significant differences between implanted patients and
patients on the waiting list for CI and also in the quality-of-life of their
significant others.14
Patients with lower scores on speech perception can still
obtain the benefits of being able to hear environmental sounds (e.g. a dripping
tap, the doorbell, improvement in driving awareness etc), all four of the
patients whose speech discrimination did not improve did receive some benefit
from improved hearing of environmental sounds with their implant.
As previously mentioned there are many factors which affect
the success or otherwise of cochlear implantation. In this study we have clearly
shown that patients with a longer duration of deafness and a higher proportion
of their lives with deafness, had significantly poorer results with cochlear
implantation. This is consistent with previous findings in the literature
4,7,9,11,12.
The vast majority of these people still had very significant
improvement in speech discrimination. Although long periods of deafness without
hearing aids also contribute to poor results, our retrospective review was not
able to adequately capture this data.
The age of the patient at implantation did not affect their
outcome. Although just over 30% of our patients were aged over 60, the vast
majority were under 80 years of age at the time of implantation. The literature
confirms somewhat poorer results of cochlear implantation over the age of 80. We
would expect that, in time, a higher proportion of our patients may be in the
older age group.
Our impression of improved performance with the new Freedom
device was confirmed with a mean auditory alone (A) score of 89% versus 73% with
the older devices. This result could be somewhat confounded by the increased
rehabilitation support available to our patients after 2004.
It is always disappointing when expensive technology is not
used. We are undertaking an audit of our assessment processes to try to avoid
inappropriate implants in those patients who are psychologically not able to
benefit from the technology.
Our patient with superficial siderosis and another patient
from the Northern Cochlear Implant Programme of New Zealand have been
reported in the literature as the first two failures of CI in this
condition.16 We have subsequently successfully
implanted a patient with this debilitating
condition.17
An interesting finding was that in terms of pre-implant
functioning, the “middle group” of patients with pre-implant A
values of 1-15% had a lower mean final score at 9 months than those with
pre-implant scores of 0% (and those with higher pre-implant scores, as
expected). This has also been noted in the
literature.9 We have no explanation for this
finding.
In terms of the time at which near-maximal benefit is
obtained, we appeared to have a near plateau at 6 months, with mean HINT A
scores of 75% compared with 79% at 9 months and 81% at 18 months. For AV scores,
this was difficult to assess due to the ceiling effect (very high scores near
100%) which restricts the opportunity to observe changes. This is probably why
AV scores peak faster.
This “plateau” at 6 months is in general
consistent with findings from previous research. The Manchester study previously
mentioned, 2 found that most benefit occurred
during the first 9 months. The Netherlands study of 33 patients had a
plateau at 6 months,8 and the larger
Netherlands study, 13found a peak at 6-12
months. An Iowa study of adults with postlingual deafness reached a plateau in
speech discrimination 6-9 months after CI.
Differing results were from the Korean
study,4 which did not obtain a plateau until 2
years. A Californian study of 46 adults implanted before 1994 with a mean age of
53 years and duration of deafness between 2-69% of lives achieved at least 80%
of their 12-month performance levels on open-set CID sentences at the first
visit at 3 months, but for a more difficult test (NU-6 words), most improvement
occurred in the first 6 months.10
There was much missing data which meant we used a smaller
sample than initially intended. This is not entirely unexpected as many of the
patients do not live locally and have to travel long distanced for follow-up. We
noted that patients who had implants in 2004 were far more likely to have
incomplete follow-up. This is likely to be because there was a changeover of
rehabilitationist that year.
To assess for exclusion bias, we analysed background
characteristics of the patients who were not used due to missed data and found
there was no difference. Also, it can often be the case that the patients who do
not attend for follow-up are actually performing better and feel that they do
not need any further rehabilitation.
In conclusion, our study has enabled us to inform patients
presenting for CI at SCIPA that our results compare very favourably with
world-wide standards, that maximal benefit in terms of speech recognition
appears to occur at around 6 months post-implant. Patients with a shorter
duration of deafness or percentage of life spent deaf do better on average, but
that patients with very poor pre-implant functioning may in fact perform better
at long-term follow-up then those with slightly better pre-implant
functioning.
Competing interests: None known.
Author information: Justine Bradley,
Otolaryngology Trainee, Philip Bird Consultant Otolaryngologist/Otologist,
Department of Otolaryngology, Head and Neck Surgery, Christchurch Hospital,
Christchurch; Penny Monteath, Rehabilitationist, Southern Cochlear Implant
Programme–Adult, J Elisabeth Wells, Biostatistician, Department of Public
Health and General Practice, University of Otago, Christchurch
Acknowledgements: The Southern Cochlear
Implant Programme–Adult. Plus special thanks to Beth Kempen,
Audiologist.
Correspondence: Philip Bird, Department of
Otorhinolaryngology, Head and Neck surgery, Christchurch Hospital, Christchurch.
Fax: +64 (0)3364 0273; email: phil.bird@chchorl.co.nz
References:
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