Effects of cerebellar retractions on the cochlear nerve: an experimental study on rhesus monkeys

Cerebellar retractions were performed in the cerebellopontine (CP) angle of rhesus monkeys to study the relationship between changes in brainstem auditory evoked potentials (BAEP) and/or compound action potentials (CAP) from the cochlear nerve, and morphological changes in the cochlear nerve in order to investigate the nature of cochlear nerve injuries caused by this surgical procedure. The changes in the BAEP and/or CAP from the cochlear nerve that were seen during cerebellar retraction were similar to those seen in human patients during operation. After completion of each experiment the temporal bones were prepared for histological examination. Hemorrhages often developed at the fundus of the internal auditory canal as a direct result of cerebellar retraction. These hemorrhages were located at the area cribrosa, where the branches of the internal auditory artery penetrate the cribriform plate. As the hemorrhages appeared to be due to avulsion rupture of these branches, they were considered to be the probable cause of the sudden loss of all the components of the auditory responses. The cochlear nerve fibers, especially those at the basal turn of the cochlea, avulsed simultaneously at the area cribrosa in most cases.