Estimating eighth nerve survival by electrical stimulation.

Determining nerve survival is important in selecting patients for cochlear implants, and in predicting outcomes from such implants. In search of a possible method we deliberately destroyed nerve fibers (ganglion cells) in 17 cat cochleas to produce a range of degenerations. Months later, we electrically stimulated these ears (and seven controls) and recorded electrical ABR input-output functions. Cats with no surviving ganglion cells showed no ABR activity. Cats with 5%-10% surviving cells had ABRs which typically had normal thresholds but decreased input-output functions. The suprathreshold slopes of these input-output functions reliably predicted ganglion cell survival for all degrees of degeneration. Thus, perceptual (or electrical) threshold is a poor indicator of nerve survival. Loudness growth (or growth in the electrically-induced auditory brainstem response) is a good index of surviving ganglion cells.