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Original article
Bilateral or unilateral cochlear implantation for deaf children: an observational study
  1. R E S Lovett1,
  2. P T Kitterick1,
  3. C E Hewitt2,
  4. A Q Summerfield1,3
  1. 1Department of Psychology, University of York, York, UK
  2. 2Department of Health Sciences, University of York, York, UK
  3. 3Hull–York Medical School, University of York, York, UK
  1. Correspondence to Professor A Q Summerfield, Department of Psychology, University of York, Heslington, York YO10 5DD, UK; aqs1{at}york.ac.uk

Abstract

Objective Cochlear implantation in one ear (unilateral implantation) has been the standard treatment for severe-profound childhood deafness. We assessed whether cochlear implantation in both ears (bilateral implantation) is associated with better listening skills, higher health-related quality of life (health utility) and higher general quality of life (QOL) than unilateral implantation.

Design Cross-sectional observational study.

Setting University of York.

Participants Fifty severely-profoundly deaf and 56 normally-hearing children recruited via a charity, the UK National Health Service and schools.

Interventions Thirty of the deaf children had received bilateral cochlear implants; 20 had unilateral cochlear implants.

Main outcome measures Performance measures of children’s listening skills; parental-proxy valuations of the deaf children’s health utility obtained with the Health Utilities Index Mark 3 and of their QOL obtained with a visual analogue scale.

Results On average, bilaterally-implanted children performed significantly better than unilaterally implanted children on tests of sound localisation and speech perception in noise. After conservative imputation of missing data and while controlling for confounds, bilateral implantation was associated with increases of 18.5% in accuracy of sound localisation (95% CI 5.9 to 31.1) and of 3.7 dB in speech perception in noise (95% CI 0.9 to 6.5). Bilaterally-implanted children did not perform as well as normally-hearing children, on average. Bilaterally- and unilaterally-implanted children did not differ significantly in parental ratings of health utility (difference in medians 0.05, p>0.05) or QOL (difference in medians 0.01, p>0.05).

Conclusions Compared with unilateral cochlear implantation, bilateral implantation is associated with better listening skills in severely-profoundly deaf children.

https://doi.org/10.1136/adc.2009.160325

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    Introduction

    The National Institute for Health and Clinical Excellence (NICE) has recommended that severelyprofoundly deaf children should have the option of receiving two cochlear implants, one in each ear (bilateral implantation).1 Provisional guidance recommended implantation in one ear only (unilateral implantation),2 reflecting uncertainty about the effectiveness and cost-effectiveness of bilateral implantation. We present new evidence of effectiveness which contributed to the ?nal guidance from NICE.

    Cochlear implants contain an array of electrodes, placed surgically in the inner ear, which stimulate the auditory nerve.3 4 Hitherto, it has been policy in the UK to provide unilateral implantation. This intervention is low-volume and highcost: it was provided to 243 children in the year ending March 2007 in England and Wales1 at an estimated lifetime cost of £60 000.5

    What is already known about this topic

    • Unilateral cochlear implantation usually allows severely-profoundly deaf children to acquire spoken language.

    • In the UK, guidelines have recently changed to recommend bilateral implantation for children rather than unilateral implantation.

    • Comparisons of unilaterally- and bilaterallyimplanted children have not shown consistent differences in listening skills and quality of life.

    What this study adds

    • Bilateral implantation in children is associated with improved sound localisation and speech perception in noise.

    • These advantages are maintained when confounding differences between the groups are controlled statistically.

    • Parents report an association between bilateral implantation and improved listening skills, but not improved health utility or quality of life.

    Unilateral implantation enables the majority of deaf children to acquire spoken language.6 There are three motivations for also implanting the second ear: (1) to ensure that the more responsive auditory nerve is stimulated, (2) to provide a backup in the event of device failure and (3) to create the potential for binaural hearing. Binaural hearing allows listeners to localise sound sources (by comparing the intensity and timing of sounds arriving at the two ears),7 and to improve the perception of speech in noise (by attending to whichever ear gives the better signal-to-noise ratio).8 These skills may help children to avoid hazards outdoors and to understand speech at home and at school. Evidence that children acquire these skills is needed to justify the additional surgery and extra cost of a second implant (£27 000).5

    Most attempts to demonstrate the effectiveness of bilateral implantation in children compared the listening skills of bilaterally-implanted patients when using both implants with their skills when only one implant was switched on.9 That design confounds the unilateral condition with unfamiliarity. Longitudinal designs, in which unilaterally-implanted children are assessed before and after receiving a second implant, confound the bilateral condition with maturation and experience of performance tests.10 A preferable alternative is to compare the listening skills of bilaterally- and unilaterally-implanted children.

    No random ised trials have been repor ted. Some obser vat ional comparisons have shown that bilaterally-implanted children display better sound-localisation skills than unilaterallyimplanted children.11 12 Other comparisons have not found this difference, despite using similar methods.13 Advantages for speech perception in noise have been assessed by measuring spatial release from masking (SRM; ?gure 1). Normally-hearing children display SRM with noise on either side of the head.14 15 Bilaterally- and unilaterally-implanted children have shown SRM with noise contralateral to their ?rst or only implant.13 15 16 Implanted children have not shown SRM with noise ipsilateral to their ?rst or only implant.13 15 16 If bilateral implantation is more effective than unilateral implantation in enabling speech perception in noise, bilaterally-implanted children should show SRM with noise on either side of the head.

    Measures of quality of life can contribute to assessments of the cost-effectiveness of paediatric interventions.17 Comparisons of the quality of life of unilaterally- and bilaterally-implanted children either did not show an association between bilateral implantation and higher quality of life11 or were based on retrospective estimates which combined data from adults and children.18

    Thus, three questions arise. Compared with unilaterallyimplanted children, do bilaterally-implanted children: (1) localise sounds more accurately, (2) display SRM with noise on either side of the head and (3) enjoy additional quality of life? We addressed these questions in an observational cross-sectional comparison with several differences from previous studies: larger numbers of children completed performance tests; parents estimated the children’s listening skills, health utility and quality of life; and reference data were collected from normally-hearing children. The study included an embedded comparison of outcomes from bilateral implantation in a single surgery with bilateral implantation in sequential surgeries. Variables which predict success with a unilateral implant were measured;19 confounding differences between the unilateral and bilateral groups were controlled statistically.

    Methods

    Participants

    Eligible participants were children aged 18 months to 16 years without disabilities that precluded performance testing. They had a parental declaration of either (1) normal hearing or (2) severe-profound deafness and had been using unilateral or bilateral cochlear implants for over 6 months. The deaf children used cochlear implants made by Advanced Bionics Corporation (Sylmar, California), Cochlear Ltd (Lane Cove, Australia), or Med-El GmbH (Innsbruck, Austria). The study was designed to detect differences of 1 SD between unilaterally- and bilaterally-implanted children with 90% power at p<0.05. The deaf participants were sequential volunteers recruited via a charity and the UK National Health Service. 70 eligible families with deaf children contacted the authors. 18 families declined to take part (12 with a bilaterallyimplanted child). Two children were excluded following testing because they fell asleep or did not sit still (both bilateral). Thirty bilaterally-implanted and 20 unilaterally-implanted children completed the study. They were compared with 56 normally-hearing children recruited via schools.

    Table 1 contains biographical data for the children who completed the study. Nine of the unilaterally-implanted children used a contralateral acoustic hearing aid. 12 of the bilaterallyimplanted children received their implants with under a month between surgeries (simultaneous bilaterals); 18 had more than a month between surgeries (sequential bilaterals). The average chronological age of the normally-hearing children was similar to the average length of time for which the bilaterally-and unilaterally-implanted children had used at least one implant. Thus, all three groups had a similar ‘hearing age.’

    View this table:
    Table 1

    Biographical data

    Listening tests

    Children attempted four listening tests while sitting in the centre of a circle of loudspeakers. Videos of the tests can be viewed at http://tinyurl.com/yorkspatial.

    The Left–Right Discrimination Test was used with all children. A spoken phrase was presented from the left or right of the child, with the side chosen randomly on each trial. The experimenter, who was blind to the location of the stimulus, recorded the direction of any eye- or head-movement made by the child within 10 s of the start of the phrase. A computer scored movements to the same side as the stimulus as correct responses. There were two versions: using loudspeakers at ±60° relative to straight ahead and at ±30°. Each version involved 20 trials. The percentage of correct responses was calculated.

    Figure 1
    Figure 1

    Spatial release from masking (SRM). SRM is measured by comparing the ability to perceive speech in three conditions. (A) In one, speech and noise come from the front (A). In the others, speech comes from the front, and noise comes from the side (B, C). In the latter conditions, the head shields one ear from the noise, improving the speech-to-noise ratio at that ear and hence improving performance. The difference in performance between conditions A and B is the amount of SRM with noise contralateral to the ?rst or only implant (illustrated in the left ear). The difference between A and C is the amount of SRM with noise ipsilateral to the ?rst or only implant.

    The Localisation Test was used only with deaf children aged 4 years and above. Three video screens were placed beneath three loudspeakers at locations separated by 60°. The screens showed pictures of different toy blocks; the blocks themselves were placed on a table in front of the child. One loudspeaker, chosen randomly on each trial, presented a speech stimulus; the task for the child was to localise the source of sound and pick up the block shown on the screen at that location. There were 30 trials. The percentage of correct responses was calculated.

    The Movement Tracking Test was used with all children to assess whether they could track sounds presented from a sequence of loudspeakers. Each child was exposed to four movement trajectories. An independent observer attempted to deduce the trajectory by observing a video-recording of the child’s head and eye movements. Performance was scored as the percentage of correct deductions.

    SRM was measured in all children aged 3 years and above using a toy-discrimination test.20 Fourteen toys were placed in front of the child. A prerecorded voice, presented against a background of pink noise, instructed children to point to toys in a random sequence. The level of the speech was ?xed. The level of the noise was varied. The speech-reception threshold (SRT)21 was measured: the minimum speech-to-noise ratio in decibels (dB) at which the child could point to the correct toy with an accuracy of 71%. The speech came from straight ahead. Children completed the test three times in an order counterbalanced over participants: with the noise from 90° to the left, 90° to the right and straight ahead. The child’s ‘first’ implant was defined as the only implant (for unilaterals), the first implant (for sequential bilaterals) or randomly assigned to be the left or right implant (for simultaneous bilaterals). SRM was calculated by subtracting the SRT with noise at the side from the SRT with noise at the front, giving two measures of SRM: with noise ispilateral, and contralateral, to the ?rst implant. A positive score indicates that the child could tolerate more noise in the condition with noise at the side.

    Parental questionnaires

    One parent of each deaf child completed the Speech, Spatial and Qualities of Hearing Scale for Parents (SSQ).22 The SSQ yields three scores, each ranging from 0 to 10, indicating abilities in speech-hearing, spatial-hearing and other qualities of hearing. Higher scores indicate greater ability.

    The same parent valued their child’s health-related quality of life (health utility) with the Health Utilities Index Mark 3 (HUI)23and their child’s general quality of life (QOL) with a visual analogue scale (VAS).24 The HUI has been validated for children aged 5 years and above. It integrates assessments of function on eight dimensions, including hearing and speaking, to yield a value on a scale where 0 corresponds to dead and 1 to perfect health. The VAS ranged from 0 to 100 with end-points labelled ‘best’ and ‘worst’ imaginable QOL. VAS responses were divided by 100 to permit comparison with previous data.25

    Statistical methods

    Outcome measures did not distribute normally, with the exception of the measures of SRM. Some data were missing because children failed to complete tests. Statistics were computed using SPSS 17.0 for Windows (SPSS, Chicago); p values are two sided.

    Primary analyses (deaf and hearing children)

    Unilaterally- and bilaterally-implanted children, and bilaterally-implanted and normally-hearing children, were compared using Mann–Whitney tests. Children were excluded from analyses of outcome measures for which their data were missing.

    Secondary analyses (deaf children)

    The aim was to assess whether conclusions about the effectiveness of bilateral compared with unilateral implantation were biased by missing data and confounding differences between the groups. Multiple imputation methods were not used because reliable predictors of ‘missingness’ could not be identified. Missing data were imputed as the median of the other group (either unilateral or bilateral). Multiple linear regression analyses were then carried out to control the influence of confounds. Two measures which met the assumptions of linear regression26 were analysed: (1) a composite localisation score, calculated as the mean of the imputed scores for Left–Right Discrimination at ±60° and ±30°, Localisation and Movement Tracking; (2) SRM with noise ipsilateral to the ?rst implant. The Localisation test was used only with children aged 4 years and above; accordingly, the analysis of the composite score included children above this age. Likewise, the analysis of SRM included children aged 3 years and above. The number of variables in the model was limited to four by the sample size;26 the choice of confounds was informed by previous research19 and by differences observed between the groups (table 1).

    Ethics

    Approval was obtained from the North West Research Ethics Committee of the National Research Ethics Service. Parents gave informed written consent.

    Results

    Primary analyses

    Listening tests

    The bilateral group scored significantly better than the unilateral group on tests of left–right discrimination, localisation, movement tracking and SRM with noise ipsilateral to the ?rst implant (table 2 and supplementary data available online). As predicted, both groups showed a similar amount of SRM with noise contralateral to the ?rst implant. There was overlap in individual scores between groups. The normally-hearing group performed signi?cantly better than the bilateral group on all tests except left–right discrimination with loudspeakers at ±60° and SRM with noise contralateral to the ?rst implant.

    View this table:
    Table 2

    Results of listening tests

    Parental questionnaires

    Ratings on the speech-hearing and spatial-hearing sections of the SSQ were significantly higher for the bilateral group than the unilateral group (table 3). There were no significant differences between the groups in health utility or QOL.

    View this table:
    Table 3

    Results from questionnaire

    There were no significant differences between the simultaneous and sequential bilaterally-implanted children on either the listening tests or the parental questionnaires (see supplementary data available online).

    Secondary analyses

    38 children were old enough to provide a composite localisation score: data were imputed for four children on one of the tests used to form the composite and for one child on three of the tests. The mean composite score was 74.0% (95% CI 65.3% to 82.7%) for the bilateral group and 53.1% (44.7% to 61.5%) for the unilateral group. 46 children were old enough to provide a measure of SRM with noise ipsilateral to the ?rst implant: data were imputed for ?ve children. The mean was +2.75 dB (95% CI +1.37 dB to +4.12 dB) for the bilateral group and ?0.58 dB (?2.93 dB to +1.76 dB) for the unilateral group.

    When the influence of covariates was held constant, bilateral implantation was associated with a signi?cant increase of 18.5% in composite localisation score and a significant increase of 3.7 dB in SRM with noise ipsilateral to the ?rst implant (tables 4, 5).

    View this table:
    Table 4

    Results of multiple linear regression with composite localisation score as the measure of outcome

    View this table:
    Table 5

    Results of multiple linear regression with SRM with noise ipsilateral to the fi rst implant as the measure of outcome

    Discussion

    The bilaterally-implanted children displayed four important listening skills. On average, they distinguished sounds on the left from sounds on the right, they discriminated among three possible sound-source locations, they tracked moving sounds, and they displayed improved speech perception when a masking noise was moved from the front to either side of their head. On average, the unilaterally-implanted children performed more poorly, at levels that were often close to chance. Previous comparisons of unilaterally- and bilaterally-implanted children have not shown consistent differences in localisation skills;11,–,13 nor have they demonstrated that bilaterally-implanted children show SRM on both sides of the head.13 15 16 By recruiting a larger sample, we found such differences.

    The median family income of the implanted children exceeded the national average (£30 000).27 Outcomes from implantation are positively associated with higher socio-economic status.19 Therefore, both groups may have shown atypical outcomes. Nonetheless, the bilaterally-implanted children performed worse than normally-hearing children with a similar average hearing age, showing that bilateral implantation had not restored normal listening skills.

    In any study, missing data can bias results. In any observational study, differences in outcome may be due not to differences in treatment but to confounding differences between groups. In the present study, significant differences in performance between the bilateral and unilateral groups were sustained following imputation of missing data and statistical control over three confounding variables. The method of imputation was deliberately conservative and may therefore underestimate the size of effects.

    Parental judgements indicated that, compared with the unilaterally implanted group, the bilaterally-implanted group had better skills in spatial hearing, and hearing for speech. This result mirrors the superior performance of the bilateral group on tests of localisation and speech perception. However, questionnaire responses by the same parents did not reveal any advantages associated with bilateral implantation in either health utility or QOL. Significant differences in questionnaire reports of everyday listening but not of quality of life were also found in an observational comparison of five unilaterally and ?ve bilaterally-implanted children11 and in a randomised trial comparing 12 unilaterally and 12 bilaterally-implanted adults.28 This pattern of results may arise for any of four reasons. First, bilateral implantation may not be associated with improved health utility or QOL. Second, advantages may take longer to emerge than the 19 months postsecond implantation that was the average in our sample, or the 9-month28 and 22-month11 follow-ups of the previous studies. Third, in the current study and previously,25 parents of unilaterallyimplanted children gave high ratings of health utility and QOL, leaving little headroom for any advantage associated with bilateral implantation to be shown. Fourth, the gain in health utility associated with bilateral implantation may be as small as +0.03.5 28 Neither the present study nor previous studies11 28were designed to detect a difference of this size.

    Conclusion

    The present study demonstrates, more rigorously than previous studies, that bilateral implantation of severely-profoundly deaf children is associated with better localisation of sound and perception of speech in noise, in the laboratory and in everyday life. To inform estimates of cost-effectiveness, larger samples than the 50 implanted children who participated here are needed to quantify the advantages, if any, from bilateral implantation to health-related and general quality of life.

    Acknowledgments

    We are grateful to the participating children and their parents. The deaf children were cared for at: Birmingham Children’s Cochlear Implant Programme (Mr D Proops and Ms K Hanvey), Emmeline Centre for Hearing Implants, Cambridge (Mr P Axon and Ms M Adlington), Great Ormond Street Cochlear Implant Programme (Mr M Bailey and Dr K Rajput), Manchester Cochlear Implant Programme (Professor R Ramsden and Ms L Henderson), Nottingham Cochlear Implant Programme (Professor G O’Donoghue and Ms T Twomey), Royal National Throat Nose and Ear Cochlear Implant Programme (Mr J Lavy and Ms W Aleksy), Scottish Cochlear Implant Programme (Miss M Shanks and Miss A Allen), South of England Cochlear Implant Centre (Mr P Ashcroft, Mr M Pringle, Ms J Brinton and Ms J Eyles), St Georges Hospital Cochlear Implant Program (Mr D Selvadurai and Ms E McKendrick), St Thomas’ Hospital Paediatric Cochlear Implant Programme (Mr A Fitzgerald O’Connor and Ms S Driver), Yorkshire Cochlear Implant Service (Mr C Raine and Ms J Martin).

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    Footnotes

    • Funding RESL is supported by Deafness Research UK. During this study, PTK was supported by the Royal National Institute for Deaf People. Travel and accommodation costs for participating families were reimbursed by a grant from Advanced Bionics UK Ltd.

    • Competing interests Advanced Bionics UK Ltd, a manufacturer of cochlear implants, organises an annual conference. The company has reimbursed travel and accommodation costs incurred by RESL, PTK and AQS in attending that conference. AQS has received a research grant, which funds a fellowship for PTK, from Advanced Bionics UK Ltd.

    • Ethics approval Ethics approval was provided by the North West Research Ethics Committee of NRES and by local research ethics committees in the following locations: South Birmingham, Nottingham (Committee 1), Central Manchester, Ayrshire & Arran.

    • Provenance and peer review Not commissioned; externally peer reviewed.

    • Patient consent Obtained from the parents.