CHANGES IN DIRECTLY RECORDED COCHLEAR NERVE COMPOUND ACTION-POTENTIALS DURING ACOUSTIC TUMOR SURGERY

Damage to labyrinthine, neural, and vascular structures may possibly a ccount for failure to achieve hearing preservation after removal of sm all acoustic neuromas. The pathogenesis of auditory impairment is, how ever, 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 thi s end, simultaneous recording of brainstem 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 l atency of the first negative component (N-1) of the CNAPs proved extre mely sensitive in detecting intraoperative auditory damage and was a g ood predictive index of postoperative auditory outcome. Evaluation of temporal and morphologic CNAP patterns allowed identification of coagu lation close to the cochlear nerve, drilling of the internal auditory canal, and removal of the intrameatal portion of the tumor, the most c ritical steps in hearing preservation. The mechanisms of damage are an alyzed and discussed.