Changes in directly recorded cochlear nerve compound action potentials during acoustic tumor surgery.

Damage to labyrinthine, neural, and vascular structures may possibly account for failure to achieve hearing preservation after removal of small acoustic neuromas. The pathogenesis of auditory impairment is, however, sometimes unclear. Intraoperative monitoring of auditory evoked potentials enables us to study possible correlations between surgical maneuvers and the function of auditory structures and pathways. To this end, simultaneous recording of brainstern auditory evoked potentials and direct cochlear nerve action potentials (CNAPs) was utilized in 14 patients with small (12 to 18 mm) unilateral acoustic neuromas. The latency of the first negative component (N(1)) 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 morphologic 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, the most critical steps in hearing preservation. The mechanisms of damage are analyzed and discussed.