[Cochlear nerve injuries caused by manipulations in cerebellopontine angle: Part II. An electrophysiological and morphological study in rhesus monkeys]

Cochlear nerve injuries caused by the manipulations in the cerebellopontine angle were electrophysiologically and morphologically investigated in rhesus monkeys. The brainstem auditory evoked potentials and/or the compound action potentials recorded from the intracranial portions of the cochlear nerve so often deteriorated due to cerebellar retractions. In these cases, the morphological changes were often verified at the most distal portions of the cochlear nerves at the fundus of the internal auditory canal (area cribrosa): the cochlear nerve fibers were pulled out from the bony foramina at the area cribrosa and the branches of the internal auditory artery were also avulsed and ruptured. Sudden loss of the auditory evoked potentials was occasionally experienced during cerebellar retractions in the monkeys as occurred in human CP angle surgery. In these cases, massive hemorrhages were found at the area cribrosa. This indicates that sudden loss of hearing function during CP angle manipulations is caused by vascular accident of the internal auditory artery. It was emphasized that the operative procedures should be done with the operative direction toward the cochlea and that the operative procedures away from the cochlea should be avoided, because they may effect traction force on the cochlear nerve.