Evaluation of eighth nerve integrity by the electrically evoked middle latency response.

A reliable objective test for estimating the number and distribution of surviving eighth nerve fibers needs to be identified for selection of candidates for cochlear implantation. Kanamycin and ethacrynic acid administration in guinea pigs resulted in graded amounts of eighth nerve degeneration over time. The electrically-induced middle latency response (EMLR) was acutely recorded in these animals at specific post-drug times, followed by the immediate killing of the animals, histologic preparation, and spiral ganglion cell density determination. Significant progressive spiral ganglion cell loss was noted by 4 weeks that increased over time. While EMLR threshold remained stable over time, the slope of the EMLR input/output function decreased with increasing post-drug intervals in a manner directly correlated with reduction in spiral ganglion cell density.