Across-site patterns of electrically evoked compound action potential amplitude-growth functions in multichannel cochlear implant recipients and the effects of the interphase gap

Electrically evoked compound action potential (ECAP) measures of peak amplitude, and amplitude-growth function (AGF) slope have been shown to reflect characteristics of cochlear health (primarily spiral ganglion density) in anesthetized cochlear-implanted guinea pigs. Likewise, the effect of increasing the interphase gap (IPG) in each of these measures also reflects SGN density in the implanted guinea pig. Based on these findings, we hypothesize that suprathreshold ECAP measures, and also how they change as the IPG is increased, have the potential to be clinically applicable in human subjects. However, further work is first needed in order to determine the characteristics of these measures in humans who use cochlear implants. The current study examined across-site patterns of suprathreshold ECAP measures in 10 bilaterally-implanted, adult cochlear implant users. Results showed that both peak amplitude and slope of the AGF varied significantly from electrode to electrode in ear-specific patterns across the subjects' electrode arrays. As expected, increasing the IPG on average increased the peak amplitude and slope. Across ears, there was a significant, negative correlation between the slope of the ECAP AGF and the duration of hearing loss. Across-site patterns of ECAP peak amplitude and AGF slopes were also compared with common ground impedance values and significant correlations were observed in some cases, depending on the subject and condition. The results of this study, coupled with previous studies in animals, suggest that it is feasible to measure the change in suprathreshold ECAP measures as the IPG increases on most electrodes. Further work is needed to investigate the relationship between these measures and cochlear implant outcomes, and determine how these measures might be used when programming a cochlear-implant processor.