Effect of Cochlear Implantation on Residual Spiral Ganglion Cell Count as Determined by Comparison with the Contralateral Nonimplanted Inner Ear in Humans

Abstract

It is generally assumed that at least a minimal number of spiral ganglion cells is essential for successful speech perception with a cochlear implant. Although the insertion of a multichannel cochlear implant frequently results in loss of residual hearing in the implanted ear, this outcome does not imply that significant damage to residual populations of spiral ganglion cells has occurred. The purpose of the current study was to compare spiral ganglion cell counts in implanted and nonimplanted cochleas in 11 patients for whom both temporal bones were available and in whom a multichannel cochlear implant had been placed unilaterally. The temporal bones were processed for light microscopy by standard techniques. The cochleas were reconstructed by 2-dimensional methods. Spiral ganglion cell counts of the implanted and nonimplanted sides were compared by a paired t-test (2-tailed). The mean spiral ganglion cell counts for implanted and nonimplanted ears were not statistically different in the most basal three segments of the cochlea. However, the mean spiral ganglion cell count in segment 4 (apical segment) and the mean total spiral ganglion cell count were lower in the implanted cochleas than in the nonimplanted cochleas (p < .01). The results of this study suggest a modest decrease in the total spiral ganglion cell count in the implanted ears as compared to the nonimplanted ears, principally in the apical segment. Possible interpretations of this finding are discussed.

Neurotrophins and electrical stimulation for protection and repair of spiral ganglion neurons following sensorineural hearing loss

“…Using a variety of animal models, a number of studies have demonstrated rescue effects associated with ES (Hartshorn et al, 1991;Kanzaki et al, 2002;Leake et al, 1999;Leake et al, 1991;Leake et al, 1992;Leake et al, 1995;Lousteau, 1987;Miller et al, 1995;Mitchell et al, 1997). In contrast, other studies report no trophic effects associated with ES per se (Araki et al, 1998;Coco et al, 2006;Li et al, 1999;Shepherd et al, 1994;Shepherd et al, 2005;Widijaja et al, 2006); these findings are consistent with a recent temporal bone study of 11 cochlear implant patients that showed no evidence of enhanced SGN survival in the implanted ear compared to the contralateral deafened control ear (Khan et al, 2005).…”

Combining Cell-Based Therapies and Neural Prostheses to Promote Neural Survival

“…SM = scala media; SV = scala vestibuli comparisons between these experiments [42]. However, a lack of ES-induced survival effects on SGNs reported here is supported by evidence from human temporal bones following cochlear implant use [43][44][45][46][47], although clinical studies lack the control offered by animal experiments.…”

“…However, to date it has not been possible to show a positive correlation between cochlear implant performance and SGN density [45,46,57,58]. The conductive nature of the cochlear fluids leads to relatively low spatial precision of SGN activation by cochlear ES meaning that higher SGN densities may not offer improvements in resolution with contemporary devices.…”

Frequency Map for the Human Cochlear Spiral Ganglion: Implications for Cochlear Implants

“…However, radial nerve fibers begin to degenerate soon after the OC is damaged (Otte et al 1978;Hinojosa et al 1983; Leake and Hradek 1988;Nadol et al 1989;Nadol 1990;McFadden et al 2004). In contrast, SG cell somata degenerate relatively slowly in humans, and significant populations often survive even after decades of deafness, and/or after many years of CI use, even when radial nerve fibers are largely absent (Fayad et al 1991;Nadol 1997;Khan et al 2005). For these reasons, some contemporary Bperimodiolar^electrode arrays (e.g., the Contouri electrode from Cochlear Limited, Sydney, Australia, and the HiFocusi with positioner and new Helixi electrodes from Advanced Bionics, Sylmar, CA, USA) have been designed to place stimulating electrodes in close proximity to the modiolus to directly excite the SG cell somata within Rosenthal_s canal.…”