Histopathology of human cochlear implants: correlation of psychophysical and anatomical measures

The cadavaric temporal bones of five subjects who underwent cochlear implantation during life (2 Nucleus and 3 Ineraid) were analyzed using two-dimensional (2D) reconstruction of serial sections to determine the number of surviving spiral ganglion cells (SGCs) in the region of each electrode of the implanted arrays. The last psychophysical threshold and maximum-comfortable sensation level measured for each electrode were compared to their respective SGC count to determine the across-electrode psychophysical variance accounted for by the SGC counts. Significant correlations between psychophysical measures and SGC counts were found in only two of the five subjects: one Nucleus implantee (e.g., r=-0.71; p<0.001 for threshold vs. count) and one Ineraid implantee (e.g., r=-0.86; p<0.05 for threshold vs. count). A three-dimensional (3D) model of the implanted cochlea was formulated using the temporal-bone anatomy of the Nucleus subject for whom the 2D analysis did not result in significant correlations between counts and psychophysical measures. Predictions of the threshold vs. electrode profile were closer to the measured profile for the 3D model than for the 2D analysis. These results lead us to hypothesize that 3D techniques will be required to asses the impact of peripheral anatomy on the benefit patients derive from cochlear implantation.