Date of Degree
PhD (Doctor of Philosophy)
Speech Pathology and Audiology
Carolyn J. Brown
The purpose of this study was to investigate how design changes in Cochlear Nucleus cochlear implants (CIs) (CI24M, CI24R, CI24RE and CI422) affected electrode impedance and ECAP measures, and to determine if these design changes affected post-lingually deafened adults and children with congenital hearing loss in a similar way.
Results of this study showed that electrode impedance was inversely related to the area of the electrode contacts in the array: lowest for the full-banded CI24M CI and highest for adults who used the CI422 device which has the smallest electrode contacts of all four devices. The noise floor of the NRT system likely plays a significant role in the finding that CI users with older devices (the CI24M, and CI24R CIs) had higher ECAP thresholds than individuals with the CI24RE electrode array. The position of the electrode array in the cochlea was also found to have a significant effect on ECAP measures. CI users with modiolar hugging (the CI24R and CI24RE CIs) electrode arrays were found to have lower ECAP thresholds than CI users whose electrode arrays were seated more laterally in the cochlear duct (e.g. the CI24M and CI422 implants). The position of the electrode contacts relative to the modiolus of the cochlea was found to be related to slope of the ECAP growth functions. The lowest slopes were found in CI24RE users. It also had a significant impact on the width of the channel interaction function. Electrode arrays seated further from the modiolus have significantly more channel interaction than electrode arrays that hug the modiolus of the cochlea.
Differences between results recorded from post-lingually deafened adults and children with congenital hearing loss were minimal. The difference only reflected on the ECAP slopes. Slopes in children with congenital hearing loss were significantly steeper than those recorded from adults. This may indicate that children with congenital hearing loss may have better neural survival than adults with acquired hearing loss.
In conclusion, the results of the current study show evidence of the effects of variations in design and function of the implanted components of the Nucleus CI. Perhaps the most significant finding from the current data set is that electrode arrays located closer to the modiolus of the cochlea have lower thresholds and exhibit less channel interaction than electrode arrays that are positioned more laterally. An argument could be made that lower stimulation levels and less channel interaction may result in better outcomes and/or longer battery life. For CI candidates who do not have significant residual acoustic hearing, the CI24RE implant might be a better choice than the more recently introduced CI422 electrode array.
Since the Nucleus cochlear implant (CI) was first introduced into clinical practice in the mid-1980’s, there have been several changes in the design of the intracochlear electrode array and in the neural response telemetry (NRT) system used to record electrically evoked compound action potentials (ECAP). ECAPs are the most direct measure of the peripheral auditory system that available today. The purpose of this study was to assess the impact that these changes (CI24M, CI24R, CI24RE and CI422) had on electrode impedance and on ECAP measures in both post-lingually deafened adults and children with congenital hearing loss.
These results demonstrate that variation in design and function of the implanted components of the Nucleus CI are not without consequences. The most significant finding from the current data set is that children CI users may have better neural survival than adult users. Also, electrode arrays located closer to the modiolus of the cochlea have lower thresholds and exhibit less channel interaction than electrode arrays that are positioned more laterally. There is no data to suggest that a lateral wall placement of the electrode array is linked to poorer performance on measures of speech perception, however, an argument could be made that changes in electrode design that result in greater channel interaction could potentially have a negative rather than positive impact on performance and, for example, use of a more modiolar hugging design might be preferable for CI candidates who do not have significant residual acoustic hearing. Specifically, the CI24RE implant might be a better choice than the more recently introduced CI422 electrode array if hearing preservation is not an issue.
publicabstract, channel interaction, cochlear implant, ECAP slope, ECAP threshold, electrode array, evoked compound action potential
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