Cochlear Device Implant in the Worse-Hearing Ear Poses No Disadvantage

Since their inception some 20 years ago, multi-channel cochlear implants (CI) have provide improved speech perception over conventional hearing aids in patients with handicapping hearing impairment. These devices have demonstrated significant improvements in quality of life, yielding a life-altering intervention with favorable cost-utility. However, there is variability in cochlear implant outcomes, somewhat influenced by age at deafness, duration of deafness, severity of hearing loss and psychological factors, but large gaps remain in the understanding of what distinguishes patients with different levels of benefit.

The choice of ear has been implicated as one of the clinical decisions that can affect speech perception outcome in an implant recipient. Although there is strong evidence to suggest that individuals with residual hearing experience superior speech perception performance with a cochlear implant compared with patients that have bilateral profound hearing loss, the role of the hearing status of the implanted ear has not been heretofore addressed with the study designs or controls used.

Now, a team of researchers from the Johns Hopkins University has found that there was no functional disadvantage to implanting the worse hearing ear, implying that trophic influences of residual hearing in the central nervous system may be more important to implant outcome than trophic effects in the spiral ganglion, which is directly stimulated by the device and conveys coded electrical signals generated by the prosthesis to the brain. These findings imply that the better ear could be preserved for the continued use of a hearing aid with the implant, or the use of future technologies.

The choice of ear has become a greater dilemma as more patients with residual hearing and larger inter-aural differences are implanted. Cochlear implantation was initially restricted to those adults with a profound (pure tone average 90 dB or greater) sensorineural hearing loss and no benefit from hearing aids. The demonstration in patients with residual hearing that speech perception with a multi-channel cochlear implant also exceeds the benefit from hearing aids led the US Food and Drug Administration approval to include individuals with severe sensorineural hearing loss (pure tone average between 71 and 90 dB HL) in one or both ears, including an open-set sentence recognition score of 30 percent or less.

Previous research suggest that despite higher spiral ganglion cell (SGC) counts in the better hearing ear, there is no functional advantage to implanting this ear rather than the worse hearing ear. However, the effect of ear choice on cochlear implant outcome for subjects with residual hearing, however, has not been examined within the same patient population in which other confounding factors have been controlled.

A team of researchers set out to determine whether the advantage associated with residual hearing is adversely affected by implanting the ear with profound hearing loss in patients with asymmetric losses. In a retrospective analysis of patients implanted at Johns Hopkins since 1992, they tested the hypothesis that among patients with similar levels of residual hearing in the non-implanted ear, speech perception outcome is the same whether or not the implanted ear has profound or severe levels of hearing loss. Howard W. Francis MD, Jennifer D. Yeagle, MEd., CCC-A, Toni Brightwell, MS CCC-A, and Holly Venick, MS CCC-A, from The Listening Center, Department Otolaryngology-Head and Neck Surgery, Johns Hopkins University, Baltimore, MD, are authors of "Central Effects of Residual Hearing: Implications for Choice of Ear for Cochlear Implantation." They will present their findings at the annual meeting of the Triological Society http://www.triological.com/default.htm being held May 4-6, 2003, at the Gaylord Opryland Hotel, Nashville, TN.

Methodology: Post-lingually deafened adults implanted at Johns Hopkins between 1991 and 2002 were classified according to pure-tone averages (PTA) as bilateral severe (n = 20), asymmetric severe-profound (n = 23) and bilateral profound (n = 43). There was no significant difference in the age at onset and duration of deafness, between the 3 patient groups. The bilateral severe and severe-profound groups had comparable levels of severe hearing loss in their non-implanted ears whereas severe-profound and bilateral profound subjects had comparable levels of profound hearing loss in their implanted ears. Speech perception performance was evaluated using CNC words, HINT sentences in quiet and CID sentences via recorded presentation at 70 dB SPL.

Results: Of adults with post-lingual hearing impairment, 27 fulfilled criteria for severe-profound asymmetric hearing loss and had speech perception data available within the first year of implantation. Twenty-three of these subjects were implanted in the profound ear and three in the severe ear. Of 23 adults with bilateral severe hearing impairment, data were available for 20 subjects within the first year of implantation. Subjects in the severe-profound and bilateral severe groups were matched to patients with bilateral profound sensorineural hearing loss by age at onset of hearing loss, duration of hearing loss and age at implantation.

The severe-profound and bilateral severe groups were functionally similar except for the level of hearing present in the implanted ear. As expected, the implanted ears of the severe-profound group (worse ear implanted) had a similar mean PTA to that of the bilateral profound group, whereas their contralateral ears had a mean PTA that was almost identical to that of the bilateral severe group. Speech discrimination scores prior to cochlear implantation were not significantly different between the severe-profound and bilateral severe groups. Both groups of patients had significantly higher pre-operative scores compared to the matched bilateral profound group.

Conclusions: The presence of residual hearing in the implanted ear did not impart superior hearing outcome compared to subjects with profound hearing loss in the implanted ear and similar levels of residual hearing in the contralateral ear. There was no significant difference in speech perception scores between subjects in the severe-profound and bilateral severe groups. These results suggest that within the first year after cochlear implantation, the hearing status of the implanted ear in patients with at least one severely hearing impaired ear has no significant impact on speech discrimination outcomes.

The comparison of these matched reciprocal severe-profound groups suggests that among patients with severe hearing loss in one ear and profound hearing loss in the other, speech perception results are the same regardless of which ear is implanted. Patients with unilateral and bilateral severe hearing impairment have superior speech perception results compared to patients with bilateral profound hearing loss, regardless of the hearing status of the implanted ear. These data further support the pre-eminence of central versus peripheral effects of auditory deprivation as a mediator of cochlear implant outcome.The results of this study suggest that residual hearing in one or both ears has trophic effects on central auditory areas that receive bilateral inputs from the auditory periphery. The preserved capacity of the central auditory pathway to process coded speech information may therefore supercede any negative effects of auditory nerve degeneration on the input signal.