Multichannel cochlear implants: Relation of histopathology to performance

Objectives: To determine the relationship of surviving neural elements to auditory function in multichannel cochlear implant temporal bones. Study Design: Case series of all 14 existing multichannel cochlear implants in our temporal bone collection. Methods. Devices included Nucleus 22 (n = 11), Nucleus 24 (n = 1), Ineraid (n = 1), and Clarion (n = 1). Morphologic evaluation of structural elements including spiral ligament, stria vascularis, hair cells, peripheral processes, and spiral ganglion cells was performed. Clinical performance data were obtained from patient charts. For eight patients, nonimplanted contralateral temporal bones were available and paired comparisons were made. Results: Despite frequent absence of hair cells and peripheral processes, all bones had at least some remaining spiral ganglion cells. Percent of normal remaining structures were unrelated to auditory performance with the implant for any of the structural elements. Ganglion cell count in segment III showed significant negative correlations to speech discrimination scores for words and sentences (Rhos = -.687 and -.661, P <= 5.03 and .04) as did segment IV and total ganglion cell count with word score (Rhos = -.632 and -.638; P <= 5.05). Spiral ganglion cell survival did not differ between implanted and nonimplanted ears, with the exception of segment 1, which had fewer cells in the implanted ear (P <= .028). Conclusions: Performance variability of cochlear implants cannot be explained on the basis of cochlear neuronal survival. Although hair cells and peripheral processes were frequently absent or greatly diminished from normal, all subjects had at least some spiral ganglion cells. And, in this series, there was an inverse relationship between survival of ganglion cells and performance.