[Pathophysiology of cochlear nerve injury incurred through surgical manipulation in the cerebellopontine angle....

Cochlear nerve injuries caused by surgical manipulation in the cerebellopontine (CP) angle were electrophysiologically and morphologically investigated in dogs. Operative procedures similar to those performed in the CP angle in humans were performed. Lateral-to-medial retraction of the cerebellar hemispheres applied traction force to the cochlear nerve. Brainstem auditory evoked potentials and compound action potentials from the intracranial portions of the cochlear nerves were recorded during the procedures. As a result of the traction force produced by manipulations in the CP angle, the Schwann-glial junctions of the cochlear nerve were separated in some dogs. The exit portions of the cochlear nerve fibers and the branches of the internal auditory artery from the tractus spiralis foraminosus at the fundus of the internal auditory canal. This finding may explain occasional occurrence of postoperative high frequency hearing loss among patients who undergo surgical manipulation in the CP angle. In some cases, massive hemorrhage and exudation of plasma were observed in the deep portion of the modiolus, where they compressed the cochlear nerve trunk. This is apparently one of the causes of intraoperative failure of cochlear function. In this study, no correlations between electrophysiological and morphological findings were established.