Download Evidence for Benefits of Bilateral Hearing Aids

C HAPTER T WENTY-FOUR
Evidence for Benefits of Bilateral Hearing Aids
Monique Boymans
Wouter A. Dreschler
Introduction
In a large-scale retrospective study (Boymans,
Goverts, Kramer, Festen and Dreschler submitted a) we
evaluated case history data and subjective fitting results
in a population of 1000 subjects included from eight audiological centers in the Netherlands. They were all members of PACT (Platform of Audiological en Clinical testing). Each center selected 125 consecutive hearing aid
fittings and analyzed the clinical files of these subjects.
In a prospective study (Boymans, Goverts, Kramer,
Festen and Dreschler submitted b) the same audiological
centers participated, and we included 214 subjects that
were willing to start a trial period with bilateral hearing
aids. All subjects were tested unilaterally and bilaterally,
after a trial period that was long enough to decide between a unilateral or a bilateral fitting (trial durations varied individually from 4 weeks to several months). For the
purpose of evaluating the bilateral benefit we applied functional tests on speech perception in noise with spatially
separated sound sources and horizontal localization. For
the prediction of the bilateral benefit, we applied diagnostic tests with headphones on speech perception in noise
for the individual ears and binaural tests like the binaural
masking level difference (BMLD) and the inter-aural
time difference (IATD). In this paper the main results will
be discussed. For the purpose of the subjective evaluation
we composed an extensive questionnaire, called AVETA,
used for the retrospective and prospective study.
This paper is part of a large nation-wide study on the
benefits of bilateral hearing aid fittings in the Netherlands. The study is designed to assess the added value
of fitting a second hearing aid, to develop tools to evaluate this objectively, and to predict a positive effect from
diagnostic tests.
Due to the aging population in Western Europe,
an increase in the number of hearing aid users is
foreseen and as a result there is a growing pressure
on the budgets available for hearing aid fitting.
Therefore, local governments and health insurance
companies are considering different options for reducing the financial reimbursements for hearing
aids. One of the options is to cut the financial compensation for the second hearing aid. It is important
to investigate the potential benefit of bilateral hearing aids in order to facilitate decisions on the basis
of clinical evidence.
Experimental Set-Up
We started with a systematic review of the literature
on bilateral hearing aid fittings from the period 1980 until 2002. In addition, we conducted two evaluation studies in Dutch audiological centers.
Review of Literature
Method
Address correspondence to: Monique Boymans, Ph.D., Academic
Medical Center, Dep. Audiology D2, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands, M. [email protected], tel.: 0031-20-5663503.
The objective of this systematic review was to evaluate the advantages of a bilateral hearing aid fitting
over a unilateral fitting and where possible to point out
the different indication criteria. To describe these
Wouter A. Dreschler, Ph.D., Academic Medical Center, Dep. Audiology D2, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands,
[email protected], tel.: 0031-20-5663434.
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Hearing Care for Adults
First Author
Random Obj/subj Control Number
Selection
Score
Allen, 2000
+
o
+
48
+
26,5
Anderson, 1996
-
s
+
53
-
27
Balfour, 1992
+
s
+
15
-
28
Bodden, 1997
-
o
-
5
-
19
Bronkhorst, 1990
-
o
+
28
-
26
Brooks, 1981
-
s
+
204
-
20
Brooks, 1984
-
s
+
571
-
21
Byrne, 1992
+/-
o
+
87
-
27
Carter, 2001
+/-
o
-
4
-
19
Chung, 1986
+/-
s
+
150
-
28,5
Davis, 1982
+/-
o
+
572
-
29
Day, 1988
+/-
o
-
51
-
32
Dreschler, 1994
+/-
o
-
12
+
25
Erdman, 1981
-
s
-
30
-
27,5
Festen, 1986
+
o
+
24
-
30,5
Gelfand, 1987
+/-
o
+
86
+
36,5
Gelfand, 1995
-
o
-
6
-
24,5
Haggard, 1982
-
o
+
29
-
25,5
Hawkins, 1984
+
o
+
23
-
29
Helfer, 1992
+
o
+
18
-
31,5
Hurley, 1993
-
o
-
9
-
22
Hurley, 1998
-
o
+
40
+
25
Hurley, 1999
-
o
+
142
+
24,5
Jauhiainen, 2001
-
o
+
500
-
19
+/-
o
o
+
-
19
12
+
-
28
25
Markides, 1982a
-
o
+
96
-
20,5
Markides, 1982b
-
s
+
31
-
22
McKenzie, 1990
-
o
-
13
-
23,5
+/-
o/s
-
20
+
39,5
Köbler, 2002
Leeuw, 1991
Moore, 1992
Mueller, 1981
-
o
-
24
+/-
24
Nabelek, 1980
-
o
+
34
-
32,5
Nabelek, 1981
+/-
o
-
21
-
32
Naidoo, 1997
+
o
-
15
-
32,5
Novick, 2001
+
o
-
10
+
22
Punch, 1991
+/-
o/s
-
17
+/-
27,5
-
o/s
-
224
-
22
Robillard, 1996
Silman, 1984
-
o
+
67
+
27,5
Silman, 1993
+/-
o
+
66
+
33,5
Stephens, 1991
+
s
+
29
+/-
31
Vaughan-Jones, 1993
+/-
o
-
56
-
29,5
Yueh, 2001
+/-
s
+
60
+
36
Table 1. Summary of the main methodological aspects
that determine the score according to the criteria of
Chalmers. The following codes are used:
• Randomization: +; if mentioned +/–; if or test or
population selection is randomized.
• Objective or subjective test.
• Control group: +; if more than one group have
been researched.
• Total number of subjects invited in the study.
• Selection criteria: +; if clear criteria are given.
• Average score.
Evidence for Benefits of Bilateral Hearing Aids
advantages, literature was searched systematically by
selected keywords.
The studies that were selected for this review
had to meet to the following criteria. The review was limited to the period 1980 to 2002. In addition, all subjects
described were adults with bilateral hearing loss. The
following keywords were used: deafness, hearing loss,
hearing aid/hearing instrument, stereophonic/binaural/bilateral, auditory amplification, benefit, speech perception, localization, spatial perception, and deprivation.
The search was carried out with three medical databases (Medline, EMBase, and Science Citation Index).
This resulted in 238 papers. Among these, ten review papers were found in the existing literature (Bentzen 1980;
Byrne 1981; Libby 1981; Markides 1989; Cashman,
Corbin, Riko and Rossman 1984; Van Wyk 1993; Kimberley, Dymond and Gamer 1994; Agnew 1997; Klein
and Weber 1999; Dillon 2001). These reviews were not
used in this systematic review.
Forty-two papers were considered suitable for scoring in the context of this study. The methodological
quality of the studies and the robustness of the clinical
and experimental evidence were evaluated by two independent persons according to the criteria of Chalmers
et al. (1981), which scores papers with respect to randomization, the use of a control group, the total number
of subjects included, the use of objective selection criteria, etc. With the help of this scoring system, it is possible to weight the degree of evidence provided by the different papers. However, the weighted factors applied
leave room for discussion. Besides blinding, randomization is almost impossible in audiological research with
respect to the use of one or two hearing aids. Consequently, the studies described in this review never met
the exact criteria of a randomized control trial (RCT). If
the guidelines for writing a systematic review were enforced strictly, none of the papers would be appropriate.
The results are summarized in table 1.
To include the most important papers, we considered
the studies with a score of 25 and higher as the core of
this review. This resulted in the inclusion of 28 studies.
Results of the Review
Speech Intelligibility
Speech intelligibility is one of the most important
goals for the individuals with hearing-impairment. Most
studies concentrate on speech perception in noise and
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in reverberation, because these are the most critical listening situations. The fitting of bilateral hearing aids introduces two sources of improvement: the binaural
squelch effect and the removal of head-shadow effects.
The squelch effect is the true binaural component and
can be described as the difference (in dB) in the critical
signal-to-noise ratio (S/N ratio) between monaural and
binaural listening. However, the benefits of bilateral fittings for speech intelligibility appear to be related primarily to the compensation of head-shadow effects.
When listening with two hearing aids, the difference (in dB) of the critical S/N ratio between near-ear
and far-ear listening is about 6–7 dB smaller than for listening with one aid (Markides, 1982a). Köbler and
Rosenhall (2002) used a fixed S/N ratio of +4dB, and
they found a statistically significant advantage of 5% in
speech intelligibility when the subjects were fitted bilaterally. Festen and Plomp (1986) investigated the
speech-reception threshold (SRT) in noise with one
and with two hearing aids in a group of 24 hearing-aid
users. The critical S/N ratio measured (the S/N ratio at
50% speech perception) proved to be hardly better with
two hearing aids than with one hearing aid for subjects
with moderate hearing losses when speech and noise
came from the frontal direction. However, a significant
benefit for bilaterally fitted hearing aids was present in
subjects with a pure tone average PTA (.5,1,2 kHz)
larger than 60 dB, and if the speech and noise sources
were spatially separated. Day, Browning and Gatehouse (1988) also concluded that subjects with severe
hearing losses experienced more benefit from two
hearing aids than from one. Bronkhorst and Plomp
(1989) showed that the binaural advantage due to head
shadow effects decreased when the hearing loss at
high frequencies was more severe. So, the binaural advantage depended on the audiometric configuration
of the hearing loss. In 1990, the same authors
(Bronkhorst and Plomp, 1990) found that the binaural
advantage due to a spatial separation of speech and
noise was milder for small hearing losses than for more
severe hearing losses. In contrast to this study, Moore,
Johnson, Clark and Pluvinage (1992) showed a binaural
advantage for almost all hearing losses when speech
and noise were separated. However, in Moore’s test design one ear was blocked for the unilateral condition.
This suggests that contribution of the unaided ear was
mainly responsible for the fact that the benefit from bilateral fitting depends on the degree of hearing loss.
Moore et al. did not find differences in binaural advantage for linear and compression hearing aids.
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Hearing Care for Adults
Hawkins and Yacullo (1984) determined the S/N ratio necessary for a constant performance level of word
recognition for normal hearing and for hearing-impaired
listeners with bilaterally symmetrical mild-to-moderate
sloping sensorineural hearing losses in different reverberation times, for unilateral and bilateral fittings, using
omni-directional or directional microphones. The results
for bilateral conditions (averaged across two microphone conditions in the three reverberant environments) were 2–3 dB more favorable than the results for
unilateral conditions. This bilateral advantage was independent of microphone type and reverberation time.
These two advantages were additive (at least at the two
shorter reverberation times) because no interaction between the two was found. The results indicated that the
optimum performance in noise was achieved when hearing-impaired subjects were bilateral hearing aids with directional microphones in rooms with short reverberation times. Nabelek and Mason (1981) measured the effects of unilateral and bilateral fittings for 15 subjects
with bilateral sensorineural hearing losses in noise and
in reverberation. Word recognition scores were significantly higher in bilateral listening modes. The advantage
of bilateral listening did not depend strongly on reverberation time or the use of hearing aids. The scores improved by 7% for a reverberation time of 0.1 s and 3.4%
for a reverberation time of 0.5 s.
Leeuw and Dreschler (1991) found better critical
S/N ratios for speech intelligibility in noise (SRT-test)
tested by normal-hearing listeners using two BTE hearing aids compared to one BTE hearing aid (mean difference 2.5 dB). There was a significant advantage of bilateral over unilateral amplification, which proved to be dependent on the type of microphone (omni-directional or
directional) and the azimuth of the noise source, except
for 0°. Contrary to the results of Hawkins and Yacullo
(1984), the bilateral advantage in speech intelligibility is
found to be highest with directional microphones.
Dreschler and Boymans (1994) measured SRTs in noise
with a spatial separation between the speech and noise
in 12 hearing-impaired subjects. The results showed
better SRTs for the subjects using bilateral hearing aids.
Bilaterally fitted subjects make better use of the spatial
separation between speech and noise sources, resulting
in 5 dB better SRT thresholds. In addition, the authors
applied a dichotic discrimination task, where 3-syllable
words and 4-syllable numbers were presented simultaneously from +45° and –45° azimuths. Results show a
clear bilateral improvement in speech discrimination for
the speech material that was presented from the (unilat-
erally) unaided side. For the words and for the numbers,
this effect was statistically significant. Helfer (1992) investigated that neither age nor peripheral hearing loss
was related strongly to the amount of benefit obtained
from binaural presentation.
Not all studies support the findings of improved
speech intelligibility. Allen, Schwab, Cranford and Carpenter (2000) found significant evidence of binaural interference for two out of 48 elderly subjects (p<0.05). Although the small number can easily be explained by normal variability in differences between speech scores,
this finding may indicate that for some individuals,
speech intelligibility scores with two ears can be poorer
than with the better ear alone. Bodden (1997) argued
that the binaural function of the ears should be restored
by hearing aids. When hearing loss deteriorates the binaural function, signal processing should be used as compensation. The experiments of Markides (1982a) confirm that the effects of the headshadow compensation
are more important than the effects of binaural squelch.
In a study with only four subjects, Carter, Noe and Wilson (2001) found better word-recognition scores for a
unilateral fitting than for a bilateral fitting in a one, two
and three pair dichotic digit task. The scores were
higher for the condition with a hearing aid in the right
ear, than for the condition with a hearing aid in the left
ear.
Localization
Improved localization is an advantage often mentioned in literature. It means that subjects with two hearing aids are better capable of determining from what direction a sound arrives.
Punch, Jenison, Allan and Durrant (1991) presented
objective data of this advantage. Although their research
is focused on bilateral fitting strategies, they found that
localization with bilateral hearing aids was significantly
superior to localization with unilateral hearing aids. Besides this objective advantage, Stephens et al. (1991)
found that an improvement of localization is one of the
reasons people choose for two hearing aids. Dreschler
and Boymans (1994) tested localization ability with one
and two hearing aids in the same subjects. They found
that the localization ability was significantly better with
two aids than with one. The results of Byrne, Nobel and
LePage (1992) show that the bilateral advantage also applies for subjects with moderate to severe hearing losses.
In the experiments of Köbler and Rosenhall (2002) the
subjects had to repeat sentences and indicate the side
Evidence for Benefits of Bilateral Hearing Aids
where the sentence came from. The results for localization were almost the same for the condition without hearing aids and with two hearing aids. A worse result was
found for the condition with only one hearing aid.
In contrast with other studies, Vaughan-Jones,
Padgham, Christmas, Irwin and Doig (1993) found that
the localization ability with two hearing aids was worse
than with one hearing aid in some subjects. Their conclusion is that subjects initially should be aided unilaterally and, if necessary, two aids can be considered.
Nabelek, Letowski and Mason (1980) investigated the
effect of asymmetry in sound pressure levels produced
by signals coming from two loudspeakers. By changing
the sound pressure level (when the sound level at one
side was increased by a certain amount of dB’s (ΔL), the
sound level at the other side was decreased by the same
amount) the position of the sound image in a lateralization experiment varied. In normal-hearing subjects, for
sound images on the midline, ΔL was zero. In unfitted
hearing-impaired subjects with bilateral hearing losses,
ΔL was within the normal range. However, in aided balanced (equal gains) and/or unbalanced conditions
(10 dB disparity in gains) ΔL for midline images was
outside the normal range for some bilaterally fitted
subjects. Based on these results, the authors concluded
that bilateral hearing aids could give a bias in the symmetry of the presentation levels between both ears.
Subjective Outcome Measures
The paper of Balfour and Hawkins (1992) focused
on the issue of improved sound quality. There was a
preference for a bilateral fitting in a group of 15 hearingaid users, mainly for overall impression, fullness, and
spaciousness. Also for listening to music there was an
overall preference for bilateral listening. Erdman and
Sedge (1981) analyzed the subjective preferences of 30
new-fitted hearing-impaired listeners (8 asymmetrical
and 22 symmetrical hearing losses). The subjects wore
unilaterally as well as bilaterally fitted hearing aids, for
controlled periods of time. Bilateral fittings were preferred by 90% of the hearing-impaired listeners, mainly
due to improved speech clarity, a stereo effect, balanced
hearing, better overall hearing, relaxed listening, and
better speech clarity in noise. The most frequently cited
disadvantages were problems to balance volume controls and ambient noise. In a study of Andersson,
Palmkvist, Melin and Arlinger (1996) no clear subjective
differences were found between a group with unilateral
and a group with bilateral fittings (47 unilaterally and 29
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bilaterally fitted subjects). Stephens et al. (1991) investigated the acceptance of two hearing aids. By randomly
assigning one or two hearing aids to 29 subjects and by
reversing this procedure in a crossover design, they determined the reasons why people chose two hearing
aids. These reasons were primarily acoustical. Clarity of
sound, better localization, and improved loudness were
the most frequently mentioned reasons to choose two
hearing aids. The reasons for people to choose one hearing aid were less obvious. Among them are user convenience and some psychological reasons.
Chung and Stephens (1986) describe a subjective
study of bilateral benefit with 200 subjects. Results of
the questionnaire point out that women appear to reject
bilateral fittings more often than men and that subjects
with asymmetrical hearing losses use their two hearing
aids twice as much as subjects with symmetric hearing
losses. The use is also higher for subjects with moderate to severe hearing loss. In a study of Naidoo and
Hawkins (1997), subjects listened to connected discourse in quiet and in noise and made judgments in a
paired-comparison paradigm. In another experiment
they rated different conditions on a scale from 0 to 10.
An improved sound quality and speech intelligibility due
to the second hearing aid was shown in conditions with
high noise levels, for subjects with symmetrical sensorineural hearing losses.
One of the few studies not showing a bilateral
benefit is that of Robillard and Gillain (1996). The
conclusion of their satisfaction sur vey is that bilateral
fittings are not superior to unilateral fittings for different listening conditions. Therefore, the authors recommend a better utilisation of bilateral aids with professional follow-up as well as an increased use of inthe-ear hearing aids.
Other Factors
One aspect frequently described in the selected papers is the occurrence of a deprivation effect. When the
hearing organ is stimulated insufficiently, speech discrimination ability can deteriorate gradually (Silman,
Gelfand and Silverman 1984; Gelfand, Silman and Ross
1987; Silman, Silverman, Emmer and Gelfand 1993;
Gelfand 1995; Hurley 1999). People that have been fitted
unilaterally and who have bilateral hearing losses develop a deprivation effect in the unaided ear. The magnitude of the unaided ear effect does not appear to be
age related (Hurley 1998).
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Hearing Care for Adults
Interim Discussion
This review underlines the fact that there are important methodological limitations in the field of Audiology that affect the methodological quality of the
papers needed for a systematic review. Probably the
most important problem is the lack of (double)
blinding, which is simply impossible for studies on
the bilateral benefit of hearing aids over unilaterally
fitted hearing aids. Randomization is also a difficult
issue because a clinician has to take into account
that ever y subject has his/her own audiological
characteristics. Although there are some discrepancies between studies, there is material evidence that
bilateral hearing aids provide clear benefits for most
bilaterally hearing-impaired subjects. Most studies
in this review regard hearing-impaired listeners with
symmetric hearing losses. No clear criteria to prescribe one or two hearing aid(s), could be derived
from the literature.
After the systematic review we had three questions
to be answered.
• Does a bilateral hearing aid fitting work? The focus
is on the efficacy based on objective outcome data.
• Does it help? The focus is on effectiveness, based on
subjective outcome data.
• Can we predict the effect of a bilateral hearing aid fitting?
To get more information about the fitting practices,
of the last few years, and the subjective experiences of
the subjects we did a retrospective study with 1000 subjects. To measure the effect of a second hearing aid and
to investigate if we could predict the positive effect we
also conducted a prospective study.
Table 2. Average critical S/N ratio for the condition with the unilateral
hearing aid at the speech side( 2nd column: ipsi-lateral side) and the hearing aid at the noise side (3rd column, contra-lateral, and for the bilateral
condition (n = 214). Lower values indicate better results.
Critical S/N ratio in dB
Ipsi lateral side
Contra lateral side
Unilateral
-4,6 dB
-1,6 dB
Bilateral
-5,0 dB
-4,9 dB
Speech Reception Threshold Test with Spatially
Separated Sources
Two loudspeakers were positioned in front of the
subject. The left loudspeaker was placed at –45 degrees
and the right loudspeaker at +45 degrees. The noise
was an interfering (time-reversed) signal of the other
gender. In conditions with speech from the left-hand
side, the interfering signal came from the right-hand
side and vice versa. The sentences (VU98, see Versfeld,
Daalder, Festen and Houtgast 2000) were presented
randomly from the left and the right hand side. The
noise was presented at 65 dB(A).
In table 2 the average critical signal-to-noise ratios
are presented for the ipsi- and contra-lateral condition,
lower values representing better results. In this table
the results for a female and a male voice have been
averaged.
The ipsi-lateral condition (second column) is the
condition with the unilaterally fitted hearing aid at the
speech side; this is the most favorable condition. In the
bilateral condition the second hearing aid is added to
the noise side. Nevertheless, an improvement of 0.4 dB
is shown. This is a purely binaural effect (binaural
squelch effect).
Does Bilateral Fitting Work?
Functional Tests
To measure the effect of a second hearing aid, we
used a Speech Reception Threshold Test (SRT-test)
with spatially separated sound sources and a localization
test. Both measurements were conducted with a unilateral and a bilateral fitting for all subjects. The ear of the
unilaterally fitted hearing aid was according to the preference of the individual subject.
Table 3. The percentage of errors (within 45 degrees, between 45–90
degrees, more than 90 degrees and the total errors) of the localization
test, for the unilateral condition (2nd column) and the bilateral condition (3rd column).
Percentage of errors
Unilateral (avg R/L)
Bilateral
< 45 degrees
38,3
28,3
45 - 90 degrees
7,2
5,1
> 90 degrees
2,5
1,3
Total
48,0
34,7
Evidence for Benefits of Bilateral Hearing Aids
The contra-lateral condition (third column) is the
most difficult condition, with the unilaterally fitted hearing aid at the noise side. In the bilateral condition the
second hearing aid is added to the speech side. An improvement of 3.3 dB is shown, due to the combined effect of elimination of the head shadow and the effect of
binaural squelch.
Localization
For the localization test 5 loudspeakers were used
(–90º, –45º, 0º, +45º, +90º). Mixed sounds were randomly
presented from different loudspeakers. All sounds were
presented at an average level of 65 dB(A) with adequate
roving. There was one target sound: the telephone bell.
The subject had to indicate where the target sound
came from. The order of presentations was randomized,
in total six measurements for each loudspeaker box.
In table 3 the results of the unilateral condition
(average for the right and the left side) and the bilateral condition are shown. Most errors were made within
45 degrees. There was a clear reduction of errors when
a second hearing aid was added, in total 13%.
Does Bilateral Fitting Help?
Questionnaires
In the retrospective study, all 1000 subjects were
asked to complete an extensive questionnaire, two
years after their hearing aid approval. This questionnaire is called the AVETA (“Amsterdamse Vragenlijst
Eén of Tweezijdige Aanpassingen”). The AVETA is
partly based on existing questionnaires: a selection of
293
questions from the Amsterdam Inventory of Auditory
Disability and Handicap (AIADH, Kramer, Kapteyn,
Festen and Tobi 1995), from the Abbreviated Profile of
Hearing Aid Benefit (APHAB, Cox and Alexander
1995), and the International Outcome Inventory for
Hearing Aids (IOI-HA, Cox et al. 2000, Kramer,
Goverts, Dreschler, Boymans and Festen 2002). Most
of the questions were asked for the unaided condition,
the unilateral condition and the bilateral condition. The
214 subjects in the prospective study were asked to
complete the same questionnaire. The difference between both groups is that in a retrospective study the
subjects had already chosen for a unilateral or bilateral
fitting and this is not the case for the hearing-impaired
subjects of the prospective study.
Results of the Retrospective Study
Eventually 505 questionnaires were useful for further
analyzes. This response-group is representative for the total group (n = 1000) with respects to age and hearing loss.
The response group consisted of 210 unilaterally fitted subjects and 295 bilaterally fitted subjects. The results of both groups are presented in table 4. The first column shows the different outcome parameters. The second and third columns show the results of the unilateral
group for the unaided and aided condition, respectively.
The last three columns show the results of the bilateral
group, for the unaided condition, the unilateral condition, and the bilateral condition. All values are scored on
a four-point scale, and higher values correspond with
better results. The unilaterally fitted group scored significantly better for all factors with one hearing aid than
without a hearing aid, except for the comfort of loud
Table 4. The subjective outcome measures of the unilaterally fitted group (n = 210) and the bilaterally fitted group (n = 295), for unaided, unilateral, and bilateral conditions (only for the bilateral group). Higher values indicate a more positive result on a four-point scale.
Unilateral group (n = 210)
Bilateral group (n = 295)
unaided
unilateral
bilateral
unaided
unilateral
bilateral
Detection
2,3 ± 0,8
3,0 ± 0,8
n.a.
2,0 ± 0,7
2,7 ± 0,7
3,1 ± 0,7
Discrimination
2,7 ± 1,1
3,2 ± 0,9
n.a.
2,3 ± 1,1
2,8 ± 1,0
3,2 ± 1,0
Speech in quiet
2,2 ± 0,8
2,6 ± 0,8
n.a.
1,8 ± 0,7
2,3 ± 0,8
2,7 ± 0,9
Speech in noise
1,8 ± 0,8
2,4 ± 0,8
n.a.
1,6 ± 0,6
2,3 ± 0,7
2,6 ± 0,9
Localization
2,1 ± 1,0
2,3 ± 1,0
n.a.
1,9 ± 0,9
2,4 ± 0,9
2,7 ± 0,9
Comfort of loud sounds
3,3 ± 0,5
2,2 ± 0,6
n.a.
3,4 ± 0,5
2,2 ± 0,7
2,1 ± 0,8
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Hearing Care for Adults
Table 5. The subjective outcome measures of the subjects who preferred a unilateral fitting (n = 14) and of the subjects who preferred a bilateral
fitting (n = 200) for the unaided condition, the unilateral condition and the bilaterally condition. Higher values indicate a more positive result on
a four-point scale.
Unilateral group (n = 14)
Bilateral group (n = 200)
unaided
unilateral
bilateral
unaided
unilateral
bilateral
Detection
2,7 ± 0,8
3,4 ± 0,7
3,4 ± 0,6
2,0 ± 0,7
2,8 ± 0,8
3,6 ± 0,5
Discrimination
3,1 ± 0,7
3,4 ± 0,7
3,4 ± 0,7
2,3 ± 0,8
2,8 ± 0,7
3,4 ± 0,6
Speech in quiet
3,1 ± 0,8
3,6 ± 0,4
3,5 ± 0,5
2,4 ± 0,8
3,0 ± 0,7
3,6 ± 0,5
Speech in noise
2,3 ± 0,7
2,8 ± 0,6
2,6 ± 0,9
1,6 ± 0,6
2,3 ± 0,7
3,1 ± 0,7
Localization
2,5 ± 0,8
3,2 ± 0,4
3,2 ± 0,8
1,9 ± 0,8
2,4 ± 0,8
3,3 ± 0,7
Comfort of loud sounds
3,2 ± 0,9
2,3 ± 0,7
2,3 ± 0,8
3,5 ± 0,7
2,8 ± 0,8
2,3 ± 0,8
sounds. The bilaterally fitted group showed the same
trend. Better results for the aided condition than for the
unaided condition except for the comfort of loud sounds.
In the bilateral group the scores for the bilateral condition were – on average – higher than the scores for the
unilateral condition, but the interpretation of this result
is complicated by the fact that not every subject answered all questions for the unilateral condition.
In order to compare the unilaterally fitted group and
the bilaterally fitted group, subgroups were composed
matched for gender, age, degree of hearing loss and audiometric asymmetry (Boymans et al. submitted a). The
bilateral sub-group (n = 250) scored significantly higher
than the unilateral subgroup (n = 104) for detection of
sounds (p<0.05), speech in quiet (p<0.05) and localization (p<0.05). For comfort of loud sounds, the unilaterally fitted group scored significantly better than the bilaterally fitted group (p<0.01).
Results of the Prospective Study
The results of the prospective study are similarly
presented in table 5. During the trial period the subjects
could experience different situations unilaterally and bilaterally. Eventually only 14 subjects preferred a unilateral fitting and 200 subjects preferred a bilateral fitting.
On average the group who preferred a unilateral fitting
scored better in the unaided condition than the group
who preferred a bilateral fitting (except for the comfort
of loud sounds). As expected, no differences were found
between the unilateral and the bilateral condition for
subjects who preferred a unilateral fitting (third and
fourth column). There is a trend that subjects who prefer a bilateral fitting score better in speech in noise than
the group who preferred a unilateral fitting (2.6 for the
unilateral group and 3.1 for the bilateral group).
No significant differences were found between the
subjective outcomes in the bilateral condition for the retrospective group and the prospective group.
Can We Predict Bilateral Benefit?
From Case-History Data
In the 1000 files of the retrospective study, we investigated the effect of case-history data (age, standard audiometric results and fitting results) on the preference
for unilateral or bilateral fittings. In the total group, 590
subjects were fitted bilaterally (59%). For mild hearing
losses in the better ear (up to 40 dB) relatively more
unilateral fittings were found. For losses above 40 dB
the percentage of bilateral fittings varied between
48–73%. As expected a bilateral fitting was more frequently seen in subjects with symmetrical hearing losses
(Davis and Haggard 1982). However, in the bilaterally
fitted group, audiometric asymmetry was found up to
50 dB. When we consider the effects of case-history
data on the overall subjective benefit (AVETA scores)
of the second hearing aid in the bilaterally fitted group,
we did not find significant effects for occupation-related
factors (full-time/part-time; work in noise, communication demands), audiometric parameters (degree of
hearing loss and maximum speech discrimination
scores), or fitting parameters (new users/experienced
users, type of hearing aid, and level of technology incorporated in the hearing aid).
Evidence for Benefits of Bilateral Hearing Aids
295
From Diagnostic Tests
Final Discussion and Conclusions
In the prospective study, diagnostic tests were
implemented to enhance the prediction of the added
value of a second hearing aid with a priori tests with
headphones.
The survey of the literature showed many advantages of bilateral fittings, relative to unilateral fittings,
but this survey also showed that it is difficult to obtain
evidence about the bilateral benefits because of methodological limitations. Some of these limitations also apply on the experimental work presented in this study.
For the correct interpretation one has to keep in mind
that blinding was not possible, and that the selection of
subjects in these studies partly determines the findings.
In the prospective study, objective benefits were the
elimination of the head shadow and the binaural effect
for speech intelligibility in spatially separated background noise, and an improvement in localization. The
binaural advantage for speech intelligibility was on average 3.3 dB (0.4 for the binaural squelch and 2.9 for the
head-shadow effect), which is less than found by
Markides (1982a). This may be caused by differences in
the experimental set-up and the selection of subjects.
The advantage of horizontal localization was a decrease
of 13% in the errors made. So, this study underlines that
bilateral fitting works and that it can be assessed by objective test results.
The next important issue is whether bilateral benefit
is relevant for the hearing aid user. The subjective benefits were clear. In the retrospective study the bilaterally fitted subgroup (n = 250), matched for gender, age, degree
of hearing loss, and audiometric asymmetry, scored significantly higher than the unilateral sub-group (n = 104),
for detection of sounds, speech in quiet, and localization.
For comfort of loud sounds the unilaterally fitted group
scored significantly better than the bilaterally fitted
group. This could perhaps be explained by binaural summation (Haggard and Hall 1982). The findings of the
prospective study confirm the results of the retrospective
study. A complication of the interpretation is that the unilateral group is relatively small. On the other hand, the
small number of subjects opting for a unilateral fitting underlines the preference of subjects, once they have experienced the use of two hearing aids. So, this study indicates that bilateral fitting helps and is judged to be worthwhile by a large percentage of hearing aid users.
The final question to be answered is whether we can
predict which subjects will benefit most from a bilateral
fitting. The audiogram turned out to be a relatively poor
predictor. In the retrospective study, some subjects with
bilateral fittings had asymmetrical hearing losses up to
50 dB, while other subjects with unilateral hearing aid
fittings had symmetrical hearing losses. Separate from
• A Speech Reception Threshold (SRT) test in fluctuating noise for each ear separately was conducted to
measure speech intelligibility in background noise.
The S/N ratio for the better ear for the total group
(n = 214) was about 9dB higher than for an age
matched group of normal hearing subjects (Versfeld
and Dreschler 2002).
• A test of the Binaural Masking Level Difference
(BMLD) was conducted to measure the capacity of
the auditory system to process phase differences between signals. This is important for the cocktail
party effect. For the BMLD test the scores for the
hearing impaired subjects were poorer than for normal hearing subjects but there was also considerable
overlap between both groups.
• A test of the Inter Aural Time Difference (IATD) was
conducted to measure the capacity of the auditory
system to distinguish arrival times of sounds between ears, important for the localization of sounds.
There was a clear difference between the hearingimpaired group and the normal hearing group for
the IATD outcomes. For some subjects this test
turned out to be too difficult.
There were only a few significant correlations between the diagnostic tests and the functional tests and
questionnaires. Poorer S/N in the better ear was significantly related to higher hearing losses (p<0.001), to
poorer maximum discrimination scores (p<0.001), and to
the bilateral benefit for speech perception with spatially
separated sound sources (p<0.01). BMLD test results
showed no significant correlations with other outcome
measures. IATD test results were only significantly correlated to the maximum discrimination scores (p<0.01).
A stepwise multiple linear regression analysis was
conducted to predict outcome measures from audiometric and diagnostic data. The results show that the diagnostic tests used in this study were not able to improve
the (relatively poor) prediction of the bilateral benefit
based on the degree of hearing loss in the better ear.
296
Hearing Care for Adults
audiological factors, there were a number of subjective
preferences for unilateral or bilateral fittings. For example, unilateral fittings were preferred in order to keep
one ear available for unaided use of the telephone or because of the sound quality of the own voice. Bilateral fittings were preferred because of an improved overall
sound quality, a better balance between both ears, and
better localization.
It turned out that the prediction of the benefit of the
second hearing aid was not improved by using BMLD
and IATD results. The SRT-test for each individual ear
is the most predictive measurement for the benefit of bilateral hearing aids, in combination with the degree of
hearing loss in the better ear.
Not only benefits from a bilateral hearing aid should
be considered. Hearing aids have shown to be useful to
avoid a deprivation effect (Silman et al. 1984, 1993;
Gelfand et al. 1987; Gelfand 1995; Hurley 1999). Therefore, based on the proven benefits in this and other studies, to avoid deprivation in the unaided ear, bilateral
amplification should be the first choice in cases of bilateral hearing loss.
Acknowledgements
We like to thank all employees of the audiological
centers who contributed to this study and especially
Joost Festen, Theo Goverts, and Sophia Kramer of the
Free University Medical Center (VUMC) in Amsterdam. This work was financially supported by the Health
Care Insurance Board (CvZ) and the Dutch Ministry of
Health (VWS).
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