Estimation of surviving spiral ganglion cells in the deaf rat using the electrically evoked auditory brainstem response.

A procedure was developed to record the electrically evoked auditory brainstem response (EABR) in the rat with sufficiently little stimulus artifact to permit systematic measurements of the first positive wave (P1), the compound action potential (CAP) of the auditory nerve. Our principal aim was to verify the theoretical prediction that maximum P1 amplitude is directly proportional to the number of excitable auditory nerve fibers. This was carried out in animals with graded lesions of the spiral ganglion induced by perfusion of the cochlea with different concentrations of neomycin. Two series of observations confirmed the theoretical prediction. Several measures of P1, including maximum amplitude, and slopes of the P1 and P1-N1 growth functions, were highly correlated with the number of surviving spiral ganglion cells. Correlation coefficients (r) ranged from 0.75 to 0.92. Amplitudes of the later waves exhibited much lower correlations with spiral ganglion cell counts. These findings suggest that measurement of the CAP in deaf humans, possibly as wave I of the EABR, should provide quantitative information about the status of the nerve, which could be useful in screening candidates for cochlear implants, prescribing the optimum device for individual patients, and determining how benefits derived from such devices relate to the condition of the auditory nerve.