SPECIAL REPORT:THE CLINICAL EFFECTIVENESS OF COCHLEAR IMPLANTS
Bilateral Implantation: One Ear Good, Two Ears Better? Susan Thomas, Medical Correspondent
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ochlear implantation is the currently accepted gold-standard for hearing improvement2. Until recently, binaural stimulation with one implant and one standard acoustic hearing aid was considered best practice38 based on cost benefit analyses, with data clearly indicating that advantages could be obtained from this method39. However, this has led to the natural proposition; if a hearing aid in the non implanted ear presents benefits, then surely a second CI could provide even greater benefit. A number of studies from the UK, Europe and the USA have looked into the suitability, cost-effectiveness and potential advantages of bilateral implantation in both child and adult populations.
Increased Vocalization and Listening in Children The major studies in the literature are crosssectional/repeated measure studies from the USA40,41,42. All concluded that bilaterally implanted children were significantly better than children with one implant plus a hearing aid in all three test areas of sensitivity to sound50,51,52 speech perception50 and speech production52. However, although there is no convincing data to contradict these results, the small sample sizes (n=2050), poor design, and a lack of consideration of confounding factors meant that the evidence from them could not be broadly generalized. Nevertheless, based on this initial success, it seemed that there was probably an additional benefit for children from having two CIs37. Two studies published in 2010 43,44 have indeed confirmed the reliability of this initial data and placed it in practical context. Profoundly deaf bilaterally implanted children are, for example, more likely to use vocalization to communicate53, and to listen more effectively53,54, compared with unilaterally implanted children. Importantly, these results are independent of age at implantation and length of preceding deafness53.
Improved Sensitivity to Sound, Speech Recognition, and Directionality in Adults Too Results on the sensitivity to sound and speech perception has tended to demonstrate a binaural
The advantages of bilateral CIs correspond with the primary benefits of bilateral hearing, that is, improved speech perception in quiet and in noise.� advantage 43,44,45,46. Summerfield et al (n = 24)55, measured self-reported spatial hearing, hearing quality and speech-specific hearing with a significant benefit for spatial hearing in particular, at 3 and 9 months post bilateral CI. Additionally, it was reported by Verschuur and colleagues (n = 20)56 that bilaterally aided participants make significantly fewer errors in sound direction detection, regardless of where speakers are positioned.
The Head Shadow Effect and Low Frequency Preservation – New Considerations Litovsky and colleagues (n = 37)57 measured speech perception in simultaneously implanted adults, finding significant binaural gains in all outcomes. In particular, bilaterally implanted participants were able to improve the head shadow effect when in noise (which occurs when speech and noise come from different directions producing a difference in hearing www.primarycarereports.co.uk | 11