Cochlear nerve injuries caused by cerebellopontine angle manipulations. An electrophysiological and morphological study in dogs.

Changes in the response from the cochlear nerve in dogs resulting from cerebellopontine angle (CPA) manipulations were correlated with histological changes in the nerve. The aim of this study was to determine the mechanisms underlying hearing deficits incurred as a result of manipulations in the CPA. Compound action potentials (CAP) were recorded from the cochlear nerve in response to click stimulation before, during, and after cerebellar and eighth nerve retractions were performed under anesthesia. The retractions were carried out to elicit different degrees of change in the latency and waveform of the CAP. About 30 minutes after completion of the manipulations, the dogs were perfused with a fixative and their cochlear nerves and brain stems were prepared for histological studies. The results showed that retraction of the eighth nerve caused a disintegration of the myelin sheath, and there were multiple and extensive foci of petechial hemorrhage and thromboses of the vasa nervorum of the cochlear nerve. In two dogs in which retraction was carried to a point at which the N2 peak of the CAP was abruptly obliterated, there was a separation of the central and peripheral myelin junction (Obersteiner-Redlich (OR) zone) and bleeding from the vasa nervorum at the OR zone. In the dogs in which the changes in the CAP had almost recovered before fixative perfusion, there were petechial hemorrhages within the cochlear nerve trunk, thus showing that improvement of electrophysiological responses may not always correlate with the absence of morphological changes.