Effects of long-term use of a cochlear implant on the electrically evoked compound action potential

Since the early 1990s, it has been possible to measure electrically evoked compound action potentials (ECAPs) from Nucleus cochlear implant users. Recording the ECAP does not require active participation by the subject, and the recordings are not adversely affected by attention or sleep, making this response an ideal tool for monitoring long-term changes. Previous research from our laboratory (Hughes et al, 2001) has shown that ECAP thresholds and slope of the ECAP growth functions are relatively stable over time. However, this conclusion was based on results obtained from a fairly limited number of study participants, each of whom used the Nucleus CI24M cochlear implant and were followed for less than two years.

To evaluate the effect of long-term use of a cochlear implant on ECAP thresholds and slope of the ECAP input/output function for both pediatric and adult cochlear implant recipients.

A longitudinal study that describes how ECAP thresholds and growth functions change over a period of 96 mo following initial activation. Changes over time in ECAP threshold and slope of the ECAP growth function were analyzed, and effects of the subject's age, type of CI (cochlear implant), and stimulating electrode are included in the analysis.

134 Nucleus CI users participated in this study. All were profoundly deaf. This subject pool included 84 individuals (40 adults and 44 children) who used the Nucleus CI24M cochlear implant and 50 individuals (21 adults and 29 children) who used the Nucleus CI24R cochlear implant.

Electrodes 5, 10, 15, and 20 were stimulated, and ECAP growth functions were measured for each subject at regular intervals following the initial activation of the device.

Small increases in mean ECAP thresholds were observed for both pediatric and adult CI users between an "early" visit that occurred within 3-6 mo following hookup and a "late" visit that occurred 4.8-6 yr later. For adults, the average increase in ECAP threshold was 3.94 CL (clinical programming units for Nucleus CIs). For children, the average increase was 4.16 CL. These differences, while small, were statistically significant. Slope of the ECAP growth functions measured over the same time interval did not change significantly. On average, pediatric CI users had ECAP thresholds that were 4-5 CL units higher than the adult CI recipients. The most striking outcome from this study, however, was the finding that when compared with postlingually deafened adults, pediatric CI users had ECAP growth functions that were substantially steeper. The differences between the results obtained from children and those obtained from adults were statistically significant and largely independent of device type or stimulating electrode.

Results from this study show ECAP thresholds and growth functions to change very little over a 5-6 yr observation interval suggesting that long-term use of a CI is not likely to have a significant negative impact on the response of the peripheral auditory system. Pediatric CI users were shown to have, on average, higher ECAP thresholds and steeper ECAP growth functions than postlingually deafened adult CI users. This finding suggests potential differences between the two patient populations either in terms of the current fields within the cochlea or the effective distance between the stimulating electrode and the stimulable neural tissue.