[Postoperative facial and vestibular nerve palsy: experimental study of its pathophysiological mechanisms]

The 7th and 8th cranial nerves were shifted in the cerebellopontine (CP) angle of dogs by cerebellar retractions that were similar to those performed in humans with monitoring of auditory evoked brainstem responses (ABR). Postoperatively, the vestibular, facial nerves, and brainstem were histologically examined. Caudal-to-rostral shifts of the nerves could induce vestibular and/or facial nerve damages. The most vulnerable portion of the vestibular nerve was located between the vestibular ganglions and the area vestibularis-the most lateral end of the internal auditory canal. This indicated that due to traction force derived from surgical interventions, the nerves and vessels were avulsed at the fundus of the internal auditory canal. The vestibular nerve may be potentially injured more easily and frequently than the cochlear and facial nerves in retromastoid craniectomies with lateral decubitus position in humans. Direct injuries of the facial nerves in the CP angles were not observed in this study. It was elucidated that the facial nerve was usually injured in the facial canal proximal to the geniculate ganglion due to traction force derived from manipulations in the CP angle. It is likely that as facial nerve edema progresses postoperatively, the facial nerve is gradually compressed within the narrow labyrinthine portion of the facial canal. This may be the cause of delayed postoperative facial nerve palsy. The importance to recognize how not only cochlear but also vestibular and facial nerve are injured by the usual manipulations in the CP angle is stressed.