Objectives: The distribution and extent of excitable spiral ganglion neurons (SGNs) have been investigated using electrically evoked auditory brainstem response (EABR) during either pre- or perioperative period. In this study, we investigated the EABR with extracochlear stimulation (eEABR) as a preoperative testing tool to estimate these factors.
Methods: Sixteen male Sprague-Dawley rats were used in this study. Experiments were conducted in 9 rats with normal hearing and 7 rats that were partially deafened with ouabain treatment. Each experiment involved the following steps in common: (1) The extracochlear stimulating electrode placement at three different sites along the axis of the cochlea and eEABR recordings. (2) Cochleostomies and 4-channel intracochlear array implantation. Then, EABR recordings with various electrode pair combinations. (3) After electrophysiological measurements, cochleae harvesting for histopathological evaluation. The slope characteristics of the amplitude growth function measured from eEABR and EABR, frequency-specific auditory thresholds, and the density of SGNs were compared.
Results: There were similar trends in slope changes on different sites of stimulation with both types of stimulation in normal hearing animals, a monotonically increasing slope with increasing distance between bipolar pairs. In addition, eEABR slopes showed significant correlations with EABR slopes when the expected cochlear regions of stimulation were similar in normal hearing animals. In partial-deaf animals, the auditory thresholds at several frequencies had a significant correlation to the eEABR slopes of each extracochlear electrode at the apical, middle, and basal cochlear positions. It showed that increasing stimulating regions of the cochlea revealed a differential effect on eEABR slopes according to the neural conditions.
Conclusion: Our Results indicated that eEABR slopes showed significant spatial correlations to the functionality of the auditory nerve. Therefore, eEABR tests at various cochlear positions possibly can be used for estimating the extent of excitable SGNs in cochlear implant candidates prior to implantation.
Keywords: Cochlear Implants; Electric Stimulation; Evoked Potentials, Auditory, Brain Stem; Spiral Ganglion.