Original Paper Audiology Neurotology
Audiol Neurotol 2009;14:106–114 DOI: 10.1159/000159121
Received: December 12, 2007 Accepted after revision: June 26, 2008 Published online: October 2, 2008
Speech Performance and Sound Localization in a Complex Noisy Environment in Bilaterally Implanted Adult Patients Isabelle Mosnier a–d Olivier Sterkers a–e Jean-Pierre Bebear f Benoit Godey g Alain Robier h Olivier Deguine i Bernard Fraysse i Philippe Bordure j Michel Mondain k Didier Bouccara a, c, d Alexis Bozorg-Grayeli a, c–e Stéphanie Borel a Emmanuèle Ambert-Dahan a Evelyne Ferrary a, c, d a
AP-HP, Hôpital Beaujon, Service d’ORL, Clichy, b AP-HP, Hôpital Louis Mourier, Consultation d’ORL, Colombes, Inserm Unit-M 867, d Institut Fédératif de Recherche Claude Bernard Physiologie et Pathologie, IFR02, and e Université Denis Diderot-Paris 7, Paris, f Hôpital Pellegrin, Service ORL, Bordeaux, g Hôpital Pontchaillou, Service ORL, Rennes, h Hôpital Bretonneau, Service ORL, Tours, i Hôpital Purpan, Service ORL, Toulouse, j Hôpital Hôtel-Dieu, Service ORL, Nantes, and k Hôpital Gui de Chauliac, Service ORL, Montpellier, France c
Key Words Cochlear implant ⴢ Sound localization ⴢ Speech perception ⴢ Speech in noise ⴢ Profound deafness ⴢ Simultaneous implantation
Abstract Objective: To evaluate speech performance, in quiet and noise, and localization ability in adult patients who had undergone bilateral and simultaneous implantation. Study Design: Prospective multi-center study. Methods: Twenty-seven adult patients with profound or total hearing loss were bilaterally implanted in a single-stage procedure, and simultaneously activated (Med-El, Combi 40/40+). Subjects were assessed before implantation and at 3, 6 and 12 months after switch-on. Speech perception tests in monaural and binaural conditions were performed in quiet and in noise using disyllabic words, with speech coming from the front and a cocktail party background noise coming from 5 loudspeakers. Sound localization measurements were also performed in background noise coming from 5 loudspeakers positioned from –90° to +90° azimuth in the horizontal plane, and using a speech stimulus. Results: There was a bilateral advantage
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at 12 months in quiet (77 8 5.0% in bilateral condition, 67 8 5.3% for the better ear, p ! 0.005) and in noise (signal-tonoise ratio +15 dB: 63 8 5.9% in bilateral condition, 55 8 6.9% for the better ear, p ! 0.05). Considering unilateral speech scores recorded in quiet at 12 months, subjects were categorized as ‘good performers’ (speech comprehension score 660% for the better ear, n = 19) and ‘poor performers’ (n = 8). Subjects were also categorized as ‘asymmetrical’ (difference between their 2 unilateral speech scores 620%, n = 11) or ‘symmetrical’ (n = 16). The largest advantage (bilateral compared to the better ear) was obtained in poor performers: +19% compared to +7% in good performers (p ! 0.05). In the group of good performers, there was a bilateral advantage only in cases of symmetrical results between the 2 ears (n = 10). In the group of poor performers, the bilateral advantage was shown in both patients with symmetrical (n = 6) and asymmetrical results (n = 2). In bilateral conditions, the sound localization ability in noise was improved compared to monaural conditions in patients with symmetrical and asymmetrical performance between the 2 ears. No preoperative factor (age, duration of deafness, use of hearing aids, etiology, etc.) could predict the asymmetrical performance, nor which ear would be the best. Conclusion: This study
Dr. Isabelle Mosnier Service ORL, Hôpital Beaujon, AP-HP 100 Boulevard du Général Leclerc FR–92118 Clichy Cedex (France) Tel. +33 140 875 571, Fax +33 140 870 186, E-Mail
[email protected]
demonstrates a bilateral advantage (at 12 months after the implantation) in speech intelligibility and sound localization in a complex noisy environment. In quiet, this bilateral advantage is shown in cases of poor performance of both ears, and in cases of good performance with symmetrical results between the 2 ears. No preoperative factor can predict the best candidates for a simultaneous bilateral implantation. Copyright © 2008 S. Karger AG, Basel
Introduction
It is well known that single cochlear implantation restores speech recognition and communication abilities, in quiet, in adult patients with severe to profound hearing loss. However, a unilateral cochlear implant usually provides poor speech perception performance in noisy environments, and horizontal auditory localization accuracy in quiet is around chance (mean angular error around 60° in unilateral conditions, whereas the chance error rate was 65°) [Litovsky et al., 2004; Neuman et al., 2007; Nopp et al., 2004; Verschuur et al., 2005]. Consequently, the advantage of binaural benefit on speech comprehension in noise and sound localization abilities has been analyzed by several authors in the last 5 years [Gantz et al., 2002; Laszig et al., 2004; Litovsky et al., 2004, 2006; Müller et al., 2002; Ramdsen et al., 2005; Schleich et al., 2004; Schön et al., 2002; Tyler et al., 2002; Buss et al., 2008]. Most studies on bilateral implantation used separated speech and noise sources, with the speech coming from the front, and the noise coming from the side or from the front, in order to examine the 3 components that contribute to the advantages of binaural hearing: headshadow effect, binaural squelch effect and summation effect or binaural redundancy. All the studies found a significant improvement in performance with both implants due to the head-shadow effect, which is the benefit of adding a second implant on the side of the better signalto-noise ratio. Most studies also demonstrated a positive effect of bilateral implantation on the squelch effect (benefit due to the spatial separation between the signal source and the noise source) and on the summation effect (benefit resulting from redundant information provided to the 2 ears); these 2 components require central binaural processing. Nevertheless, these benefits remain smaller in terms of speech intelligibility than those gained through the head-shadow effect [Murphy and O’Donoghue, 2007]. Despite these studies showing the positive benefit of binaural hearing, simultaneous bilateral implantation Speech Performance, Sound Localization and Bilateral Implants
still remains a matter of debate, mainly because of the cost-effectiveness of the procedure and of the additional risk of a bilateral surgery that could increase the rate of complications. Little is known about the potential benefit of bilateral implantation in complex and natural listening environments, such as multiple noise sources, and about sound localization in noise. Speech comprehension with 5 uncorrelated noise sources has only been evaluated by Ricketts et al. [2006], who showed a significant advantage of bilateral implantation compared to the better ear in 16 adult patients, at 5 months postactivation, with an improvement in speech comprehension score of 9% for words. Moreover, some previous studies showed that several patients had asymmetrical performance between the 2 ears. In this population, the advantages of binaural stimulation and factors that could predict these asymmetrical results needed to be evaluated. The objective of the present study was to investigate the speech performance and the sound localization in a complex noisy environment in 27 adult patients with a bilateral profound or total hearing loss, who had been bilaterally implanted 12 months earlier in a simultaneous procedure. In addition, the study aimed to determine the predictive factors for improvements resulting from the bilateral implantation.
Patients and Methods Selection Criteria Subjects enrolled in this study were adult patients with a postlingual bilateral profound or total hearing loss. They were required to have: a maximum of 10% open set disyllabic word recognition in quiet environment, with, if possible, appropriate conventional hearing aids; a duration of severe to profound hearing loss of less than 20 years; a difference in duration of profound hearing loss between the 2 ears of less than 5 years; fluency in the French language; no malformations of the cochlea or middle ear. All patients received both cochlear implants (Med-El Combi 40+, Innsbruck, Austria) in a simultaneous procedure, and implants were simultaneously activated. The same speech coding strategy CIS (continuous interleaved sampling) was used in both ears, although each ear underwent independent mapping. No modification of the device programming was done during the testing. Before implantation, all patients were required to sign an informed consent form. The study was approved in September 2002 by the local ethical committee (Saint-Louis, Paris, No. 61D0/22/A). Subjects Twenty-seven patients, enrolled in 7 ear, nose and throat (ENT) departments of tertiary referral hospitals [Beaujon (Clichy), Pellegrin (Bordeaux), Pontchaillou (Rennes), Bretonneau
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Study Design All tests were performed in 3 listening conditions: monaural right and left cochlear implants (without previous practice), and binaural cochlear implants. Sound localization and speech comprehension measurements were performed in a sound-treated room using 5 loudspeakers (Monacor MKS-40, frequency response: 80–18000 Hz) positioned at 45° intervals in the frontal hemifield, ranging from –90° to +90°. Each loudspeaker was numbered. Speakers were positioned at ear level, and at a distance of 1.2 m from the center of the subject’s head. Participants were instructed to face the loudspeaker positioned at 0° azimuth without moving their head during testing. The output level for each loudspeaker was controlled and calibrated in order to obtain the same value at the level of the subject’s head. The same material (loudspeakers, compact disk with recorded Fournier lists) was used in all the ENT departments. Speech comprehension tests were performed before implantation, and 3, 6 and 12 months after activation, using the same procedure in all the hospitals. Test materials consisted of 50 lists of 10 disyllabic words (Fournier word tests) recorded in quiet and in cocktail party background noise. Speech was always presented at 70 dB SPL from the loudspeaker placed at 0° azimuth. The same noise was presented simultaneously from the 5 loudspeakers, including the central one that presented the speech target. The signal-to-noise ratio (SNR) was +5, +10 and +15 dB. Randomization of test lists presented for each patient was carried out independently at each test site. The disyllabic words results were scored as the percentage of words correctly identified. In order to find which patients had the most benefit from bilateral implantation, subjects were arbitrarily categorized into distinct groups based on unilateral speech scores recorded 12 months after activation in quiet. They were divided into ‘good performers’ (speech comprehension score 660% for the better ear, n = 19) and ‘poor performers’ (n = 8). Subjects were also categorized as ‘asymmetrical’ (difference between their 2 unilateral speech scores 620%, n = 11) or ‘symmetrical’ (n = 16), thus defining the better and the poorer ear, a distinction that is more suitable than ‘left and right’ used for the ‘symmetrical performers’. Sound localization measurements were performed 12 months after activation. The test stimuli, disyllabic words, were presented in a random sequence from each of the 5 loudspeaker locations for a total of 3 times with a 10-second interstimulus interval, at an intensity level varying from 60 to 80 dB SPL. The competing sound material was a cocktail party background noise coming from the 5 loudspeakers. In order to test only the localization, without interference from the hearing performance, the SNR was adapted for each subject and each listening condition (i.e. monaural right and monaural left cochlear implant, and binaural co-
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Quiet 100
80 Correct responses (%)
(Tours), Purpan (Toulouse), Hôtel-Dieu (Nantes), Gui de Chauillac (Montpellier)] were included in this study. The mean age of the patients was 45 years (range: 24–69, median: 46). The mean duration of deafness was 3 years (range: 1–9). The cause was unknown in 14 cases, genetic in 10 cases, traumatic in 1 case, due to otosclerosis in 1 case and to meningitis in 1 case. Before being implanted, patients had been bilateral (n = 17) or unilateral (n = 2) users of hearing aids for 18 years (range: 3–41, median: 14). Eight patients had never tried hearing aids because of a sudden total bilateral hearing loss. Partial insertion of the electrode array on 1 side was reported for 3 patients (10–11 out of 12 electrodes).
Poorer ear Better ear Bilateral
n.s. n.s.
n.s.
*** ***
6 months
12 months
*
60
40
20
0 Preop.
3 months Time
Fig. 1. Speech performance in quiet, as function of time (n = 27).
Test material was disyllabic words presented at 70 dB SPL from the loudspeaker placed in front. Results are expressed as means 8 SEM. Performance improved over time (3, 6, 12 months, p ! 0.002) for each ear alone (poorer, better) and in bilateral conditions (p ! 0.0001, two-way Anova). Individual speech perception scores were compared using paired t test. * p ! 0.05, *** p ! 0.001; preop. = before operation; n.s. = not significant.
chlear implants) in order to obtain a 50% correct speech comprehension score for disyllabic words coming from the central loudspeaker. After each stimulus presentation, subjects reported the loudspeaker number corresponding to the perceived sound location. For each loudspeaker, the number of correct responses was noted, but in case of false responses the indicated loudspeaker was not reported. Therefore, results were expressed as the mean percentage of correct responses per loudspeaker, and not as the root mean square error. Statistical Analysis Values are expressed as means 8 SEM. A two-way ANOVA [factors: time (3, 6 and 12 months) and ear (poor, better and bilateral)] was applied to analyze the speech perception scores across conditions and groups of patients. Individual speech perception scores were compared using paired t test. A 2 test was applied to analyze the sound localization data. Pre- and postoperative data among groups of patients were compared using Student’s t test. For all comparisons, p ! 0.05 was considered as significant.
Results
Speech Performance in Quiet Speech performance in quiet, in both unilateral and bilateral conditions, is shown in figure 1. Speech scores were better in bilateral conditions compared to the reMosnier et al.
100
***
100
80
***
n.s.
80
**
**
60
40
20
Correct responses (%)
Correct responses (%)
*
n.s.
*
60
40
20
0 Poorer
Better
Bilat.
Poorer
Good performers
Better
Bilat.
Poor performers
0 Poorer
Better Symmetrical
Bilat.
Poorer
Better
Bilat.
Asymmetrical
Fig. 2. Speech performance in quiet in good (n = 19) and poor (n = 8) performers, 12 months after the implantation. ‘Good performers’ were defined as patients with a speech comprehension score 660% for the better ear in quiet, 12 months after the implantation. Results are expressed as means 8 SEM. Individual speech perception scores were compared using paired t tests. * p ! 0.05, ** p ! 0.01, *** p ! 0.001.
Fig. 3. Speech performance in good performers with symmetrical
sults obtained by the better ear at 12 months (77 8 5.0% in bilateral condition, n = 27; 67 8 5.3% for the better ear, n = 27; paired t test, p ! 0.005, fig. 1). Performance improved over time (3, 6 and 12 months, p ! 0.002) for each ear alone (poorer, better) and in bilateral conditions (p ! 0.0001; two-way ANOVA).
testing at 12 months demonstrated a binaural advantage compared to the results obtained by the unilateral condition (95 8 3.1% in bilateral condition, 83 8 3.4% in unilateral condition, paired t test, p ! 0.02, fig. 3). In the 9 subjects with asymmetrical performance, there was no significant bilateral advantage over the better ear (83 8 6.0% in bilateral condition, 82 8 3.8% for the better ear). Among the group of 8 patients with poor performance, the 6 patients with symmetrical speech scores between the 2 ears showed an improvement in speech scores in the bilateral condition in quiet at 12 months (43 8 9.6% in bilateral condition, 26 8 6.2% in unilateral condition, paired t test, p ! 0.05), as did the 2 patients with asymmetrical performance (70 and 50% in bilateral condition, 40 and 30% for the better ear).
Good and Poor Performers If we consider the good performers (speech scores 660% in quiet at 12 months, n = 19) and the poor performers (n = 8), speech scores were still better in bilateral conditions compared to the results obtained by the better ear in both groups (fig. 2). Furthermore, the magnitude of improvement was higher (p ! 0.05, Student’s t test) in poor performers (48 8 7.7% in bilateral condition, 29 8 4.8% for the better ear, improvement of 19 8 2.9%, paired t test, p ! 0.01) than in good performers (89 8 3.5% in bilateral condition, 82 8 2.4% for the better ear, improvement of 7 8 1.0%, paired t test: p ! 0.05). Symmetrical and Asymmetrical Results Among the group of 19 patients with good performance, 10 patients had symmetrical speech scores between the 2 ears. In these 10 patients, results of speech Speech Performance, Sound Localization and Bilateral Implants
(n = 10) and asymmetrical (n = 9) scores, 12 months after the implantation. The group of asymmetrical performers had a difference between their 2 unilateral speech scores of 20% or more in quiet at 12 months. Results are expressed as means 8 SEM. Individual speech perception scores were compared using paired t test. * p ! 0.05, *** p ! 0.001; n.s. = not significant.
Speech Performance in Noise Similarly to the quiet condition, performance at SNR of +15 dB improved over time (3, 6 and 12 months, p ! 0.0001) for each ear alone (poorer, better) and in bilateral condition (p ! 0.01, two-way ANOVA; fig. 4). For the 3 periods after implantation (3, 6 and 12 months), a difference of about +5–10% was observed beAudiol Neurotol 2009;14:106–114
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Table 1. Speech performance in noise at various SNRs
3 months
SNR (dB) +15 +10 +5
6 months
12 months
better ear
bilateral
better ear
bilateral
better ear
bilateral
4286.8 (22) 3488.0 (15) 1685.5 (18)
4986.0 (22) 4587.4 (15) 2286.6 (18)
5787.0 (21) 4889.4 (15) 2486.9 (17)
6285.5 (21) 5788.0 (15) 3488.1 (17)b
5586.9 (25) 4888.8 (17) 3388.0 (18)
6385.9 (25)a 5388.4 (17) 4288.6 (18)c
Results are expressed in percentages as means 8 SEM. The numbers of patients are indicated in parentheses. Comparison of performance between bilateral implants and the better ear: a 8 8 3.4% (p < 0.05, paired t test); b 10 8 4.3% (p < 0.05, paired t test); c 9 8 3.8% (p < 0.05, paired t test).
Noise 100
80 Correct responses (%)
tween the performance in unilateral and bilateral conditions (table 1). Nevertheless, statistical significance was only achieved at 6 months for an SNR of +5 dB, and at 12 months for SNRs of +5 and +15 dB. The statistical analysis in the groups of good and poor performers, and in the groups of asymmetrical and symmetrical patients, showed no significant bilateral advantage over the unilateral condition, probably because of the small number of patients in each group (data not shown).
Poorer ear Better ear Bilateral
*** *
60
n.s.
** *
n.s.
40
20
Sound Localization in Noise Sound localization in noise was analyzed in patients (good and poor performers) with symmetrical (n = 16) and asymmetrical performance (n = 11) between the 2 ears, in unilateral and bilateral conditions (fig. 5). In patients with symmetrical performance, the mean identification of the speech stimuli in noise was improved with both implants compared to either implant alone for each loudspeaker (2 analysis, p ! 0.05). In patients with asymmetrical performance, statistically better results with bilateral implants were observed in extreme loudspeaker locations (–90° and +90°, 2 analysis, p ! 0.002), and in intermediate loudspeaker locations situated on the opposite side of the activated implanted ear (–45° and +45°), no matter which ear was the better (p ! 0.001) or the poorer one (p ! 0.0003). When the noise was in front of the subject, bilateral implantation brought statistically better performance compared to the poorer ear (p ! 0.01), but not compared to the better one. Analysis of the individual data showed that differences in sound localization abilities among patients were not strictly correlated to speech performance. Indeed, in the group of the 19 patients with good performance, localization ability was not improved in bilateral condition in 6 110
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0 Preop.
3 months
6 months
12 months
Time
Fig. 4. Speech performance in noise (signal-to-noise ratio +15 dB),
as function of time (n = 27). Test material was disyllabic words presented at 70 dB SPL from the loudspeaker placed in front, with cocktail party background noise presented simultaneously from the 5 loudspeakers. Results are expressed as means 8 SEM. Performance at SNR +15 dB improved over time (3, 6, 12 months, p ! 0.0001) for each ear alone (poorer, better) and in bilateral conditions (p ! 0.01, two-way ANOVA). Individual speech perception scores were compared using paired Student’s t test. * p ! 0.05, ** p ! 0.01, *** p ! 0.001; preop. = before operation; n.s. = not significant.
of them. Out of these 6, 3 had symmetrical performance. Moreover, in the group of the 8 patients with poor performance, speech localization was improved with both implants in 2 of them. Prognostic Factors Because of the intersubject variability in performance and the different results obtained between the 2 implantMosnier et al.
100
Left ear Right ear Bilateral
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50 n.s. n.s.
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LS3 0°
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Fig. 5. Sound localization in noise in unilateral and bilateral conditions, 12 months after the implantation. a Patients with symmetrical performance between the 2 ears (n = 16). The mean identification of the speech stimuli in noise was improved with both implants compared to either implant alone for each loudspeaker (2 analysis, p ! 0.05). b Patients with asymmetrical performance between the 2 ears (n = 11). Considering asymmetrical performance, the loudspeakers (LS) were not defined as usual as left or right, but as ‘poorer’ and ‘better’, the ‘poorer’ loudspeaker being that located in the axe of the poorer ear (defined as –90° in this
graph). Better results with bilateral implants were statistically observed in extreme loudspeaker locations (–90° and +90°, 2 analysis, p ! 0.002), and in intermediate loudspeaker locations (–45° and +45°) of the loudspeaker located on the opposite side of the activated implanted ear, no matter which ear was the better (p ! 0.001) or the poorer one (p ! 0.0003). When the noise was in front of the subject, bilateral implantation brought statistically better performance compared to the poorer ear (p ! 0.01), but not compared to the better one. The dotted line represents the chance performance (correct responses: 1/5). n.s. = Not significant.
ed ears, pre- and postoperative data and postoperative fittings of each ear were compared between good and poor performers, and between patients with symmetrical and asymmetrical results. Among these different groups, there was no difference in mean age, etiology, mean duration of hearing loss, hearing aid experience in each ear and duration of the surgery. In cases of asymmetrical results, there was no difference between the better ear and the poorer ear in the postoperative fittings (rate of stimulation and comfort level), nor between right or left being the better ear (right ear in 5 patients and left ear in 6 patients). In 25 patients, all the 12 electrodes were activated in both ears. Two patients had only 10 activated electrodes on 1 side; 1 had good performance on this side, the other showed symmetrically poor performance.
quiet and in noise, in a relatively large number of adult patients. The most robust effect was seen in quiet at 12 months postactivation with an improvement of 10 8 0.3% for disyllabic words. The magnitude of improvement was slightly lower in noise (8 8 1.0% at SNR +15 dB). Most previous studies have reported this bilateral advantage, although large differences in methodologies were observed across studies [Gantz et al., 2002; Laszig et al., 2004; Litovsky et al., 2004, 2006; Müller et al., 2002; Ramdsen et al., 2005; Schleich et al., 2004; Schön et al., 2002; Tyler et al., 2002; Buss et al., 2008]. These differences concerned the population (number of included patients ranging from 1 to 37 and the duration of profound hearing loss), methods of implantation (sequential implantation with various intervals between the 2 surgeries or simultaneous implantation), duration of experience with both implants before evaluation (1–17 months), speech and noise test materials, speech coding strategies and mainly procedures of evaluation. Indeed, some studies used a fixed SNR (benefit expressed as an increase in the percent-correct score for this SNR) that can create a ceiling effect. Other studies used an adaptive SNR, with
Discussion
Speech Performance in Quiet and in Noise This study showed a significant benefit of bilateral implantation (implanted simultaneously on both sides) compared to the better ear for speech understanding, in Speech Performance, Sound Localization and Bilateral Implants
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the benefit expressed as the gain of the speech reception threshold (SNR at a speech perception score of 50%) that reduces the ceiling performance effect. Speech understanding in noise was usually investigated using 2 separate speech and noise sources to evaluate the head-shadow effect, squelch effect and binaural redundancy. We used a more realistic method, with 5 loudspeakers in a horizontal plane to simulate a difficult noisy environment and test materials at different fixed SNR to minimize the ceiling effect. The magnitude of improvement of speech understanding compared to unilateral performance in such difficult conditions, even at poor SNR (+5 dB), is a strong and convincing argument for the advantage of bilateral implantation. Our results were in good agreement with the study of Ricketts et al. [2006], who reported an average benefit of 9%, 4–7 months after implantation, using 5 loudspeakers surrounding the patient’s head and words at a fixed SNR of +10 dB. Moreover, it was reported that in 6 patients evaluated at different fixed SNR, the largest average bilateral advantage was generally found at the poorer SNR. In our study, the magnitude of bilateral advantage at 12 months, tested in a larger population, was similar at +5 dB (9 8 3.8%, n = 18) and at +15 dB (8 8 3.4%, n = 25). Nevertheless, the bilateral advantage is observed as soon as 6 months after the implantation at the poorer SNR, probably because only the best performers are able to identify the words at low SNR. Sound Localization in Noise Our results indicate that speech localization in noise was better under the bilateral than unilateral condition in patients with symmetrical and asymmetrical performance between the 2 ears. This study used a speech signal accompanied by noise coming from 5 loudspeakers positioned in a frontal hemifield. Results were simply expressed as the mean percentage of correct responses per loudspeaker. These results corroborate those of previously published studies, which reported a marked improvement in sound localization in quiet with bilateral, compared to unilateral, implantation [Grantham et al., 2007; Neuman et al., 2007; Nopp et al., 2004; Schoen et al. 2005; Verschuur et al., 2005]. In those studies, subjects were only tested in quiet, using various numbers of loudspeakers (5–17) placed in a frontal plane at ear level, extending usually from –90° to +90°, and various types of stimulus (broadband noise, noise burst, tone burst, speech samples). Results indicated a lower mean localization error with bilateral listening (24–29°) than with unilateral listening (1 45°), which is around chance. Verschuur et al. 112
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[2005] noticed that these performances were still poorer than those of hearing aid users (10°) or normal hearing listeners (2–3°) for the same test methodology. A previous study also reported that subjects localized the speech signal better than other stimuli [Grantham et al., 2007]. Interestingly, in our study, the analysis of individual data indicated that there were large individual differences, with a lack of improvement in localization abilities in noise in 12 of the 27 patients. For these patients, we did not find any correlation with the speech performance. In the group of 6 good performers with poor localization abilities, no preoperative factors can explain these results: similar duration of hearing loss between the 2 ears (1–9 years), and hearing aid experience before implantation in 3 patients with a similar duration between the 2 ears (9, 22 and 33 years). Our test compared speech localization in bilateral and unilateral conditions in the presence of 5 noise sources, which is a more realistic, but also more difficult method than testing in quiet. This could explain why improvements in localization were not found in some cases in this study. Nevertheless, Grantham et al. [2007] reported considerable intersubject variability, ranging from less than 10 to 80° in tests performed in quiet using 17 loudspeakers, and did not find any relationship between the subject error patterns and factors such as age or duration of hearing loss. Guidelines for Bilateral Implantation The cost-effectiveness of bilateral implantation has been studied in a prospective randomized trial in the UK comparing spatial hearing, quality of hearing and quality of life in 2 groups of 12 unilaterally implanted patients receiving the second implant 1 month (bilateral-immediate group) or 12 months (bilateral-delayed group) later [Summerfield et al., 2006]. The authors concluded that the gain in quality of life with the second implant is too small to achieve an acceptable cost-effectiveness ratio. Considering this issue and the functional results, another goal of this study has been to predict the best candidates for a bilateral implantation, in order to establish guidelines. Therefore, we categorized subjects as ‘good’ or ‘poor’ performers, with symmetrical or asymmetrical results, depending on their unilateral speech performance. Seventy percent of patients were good performers and 41% had asymmetrical results between the 2 ears. The largest bilateral advantage (+19% in quiet) was obtained in poor performers, in both patients with symmetrical and asymmetrical results. In case of good performance on 1 side, there was a bilateral advantage only in patients with similar performance on the other side. Mosnier et al.
No additional benefit of a bilateral implantation was observed in cases of asymmetrical results with a poor contralateral ear. These results were consistent with the study of Litovosky et al. [2006], who demonstrated no benefit of bilateral implantation compared to the better ear in 10 ‘asymmetrical’ patients, but an improvement in performance compared to unilateral conditions in 24 ‘symmetrical’ patients (adaptive SNR method, with speech and noise coming from the front). Our inclusion criteria led us to obtain a homogeneous population of 27 patients characterized by a short and similar duration of profound hearing loss between the 2 ears. Analysis of other individual preoperative factors showed similar etiologies and similar duration of hearing aid experience between the 2 ears in 89% of patients (n = 24), even in cases of asymmetrical performance. Preoperative data were also similar between good and poor performers. Based on these preoperative factors, the predictability of postoperative performance and of the better ear, in case of asymmetrical results, is therefore impossible to report, which is in agreement with some other studies [Gantz et al., 2002; Litowsky et al., 2006; Ramdsen et al., 2005]. The reasons for the asymmetrical performance obtained in some patients remain to be investigated. The number of activated electrodes, the stimulation rate and the comfort level were similar in both sides. Variations in the neural survival rate or small differences in the electrode position in the cochlea, with lack of adjustment of speech coding strategy, are possible explanations.
tients with a postmeningitic or a posttraumatic profound hearing loss, due to the risk of cochlear ossification. Our study, which included a relatively high number of patients tested in a difficult noisy environment, demonstrated a significant benefit of bilateral implantation for speech intelligibility in all patients with poor performance and in patients with good performance and symmetrical results, which corresponds to 18 out of the 27 patients (67%). The ability to localize sounds in noise is also restored in most patients. As in our study, none of the published papers found preoperative factors that could predict the postoperative performance, nor which patients are the best candidates for bilateral implantation. In our opinion, these issues are convincing arguments to recommend a simultaneous bilateral implantation in subjects of the working population, who need to perform well in their professional life, and in blind patients. In addition, this study demonstrates an important benefit of bilateral implantation in patients with poor performance on both ears. This leads us to recommend a second implant in patients who obtained poor results with a unilateral implant. In summary, this study demonstrates the benefit in speech intelligibility and sound localization provided by bilateral implantation in a complex noisy environment. Further examinations about electrode placement and its relationship to postoperative performance are needed in order to optimize harmonious mapping strategies in bilateral implanted patients. Thus, bilateral hearing rehabilitation should be a concern in future cochlear implant systems.
Conclusion Acknowledgments
Considering these results and the high cost of bilateral implantation should we recommend it in a simultaneous, or in a sequential procedure in patients with profound bilateral hearing loss? It is now established that a bilateral simultaneous implantation is required in pa-
References
Speech Performance, Sound Localization and Bilateral Implants
The authors would like to thank the medical and audiological staff of all participating departments who carried out the tests and collected the data. The authors would also like to thank MED-EL, whose support made this work possible and Samia Labassi for her helpful comments.
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