Estimation of surviving auditory nerve by electrically evoked auditory brainstem response (EABR) - Comparison between extra-cochlear stimulation and intra-cochlear stimulation.

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Abstract
This study investigated the possibility of using electrically evoked auditory brainstem response (EABR) for predicting surviving spiral ganglion cell populations. EABR recordings were made from six kanamycin induced deaf cats by the round window monopolar stimulation (RW) and the scala tympani bipolar stimulation (ST). On completing the electrical stimulation, each animal was sacrificed and prepared for histological examination. The spiral ganglion cell populations were estimated by classifying them into four groups (0-25%, 25-50%, 50-75%, 75-100%) as a percentage of survival cells under magnification. EABR thresholds and input-output function of amplitude were compared with microscopic findings of surviving spiral ganglion cell populations. The results were as follows: 1) The rate of amplitude growth in response to increased stimulus intensity of electrically evoked potentials from ST was gradual, but that of RW was steep. 2) The vestibular potentials were elicited by RW, and at a lower stimulus intensity the RW waveform began to become distorted by the myogenic potentials. 3) EABR threshold was a poor predictor of surviving spiral ganglion cell populations. 4) There was a correlation between surviving spiral ganglion cell populations and the slope of the input-output functions of ST EABR. However, there was no correlation between that of RW EABR and the slope.

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