Primary Neural Degeneration after Cochlear Implantation: Histological and Electrophysiological Evidence from a Mouse Model.

We demonstrate that hearing-preservation cochlear implantation induces primary neural degeneration at the cochlear apex, a region anatomically remote from the electrode array and essential for low-frequency hearing and speech recognition. Using a longitudinal mouse model, we show that afferent synapse and cochlear nerve fiber loss progresses across the cochlea and markedly exceeds inner hair cell loss at the apex. This degeneration occurs despite preserved low-frequency thresholds and is best predicted by suprathreshold ABR wave I measures, revealing neural dysfunction not detected by standard audiometry. These findings challenge the prevailing assumption that apical cochlear structures remain intact after basal electrode insertion.