Vulnerability of hearing function during acoustic neuroma surgery.

Intraoperative monitoring of auditory evoked potentials reveals possible correlations between surgical maneuvers and the function of auditory structures and pathways in cerebellopontine angle surgery. Direct cochlear nerve action potentials (CNAPS) furnish realtime information on the function of the cochlea and cochlear nerve and were utilized in 14 patients undergoing removal of small (12-18 mm) unilateral acoustic tumors. The latency of the first negative component (N1) of the CNAPs proved extremely sensitive in detecting intraoperative auditory damage and was a good predictive index of postoperative auditory outcome. Evaluation of temporal and morphological CNAP patterns allowed identification of coagulation close to the cochlear nerve, drilling of the internal auditory canal and removal of the intrameatal portion of the tumor as the most critical steps in hearing preservation.