Hearing preservation after acoustic neuroma surgery using intraoperative direct eighth cranial nerve monitoring

Since the advent of brainstem auditory evoked response audiometry and computerized tomography, small acoustic neuromas are found more frequently. The patients often have serviceable hearing and wish to preserve it during complete tumor removal. Since 1978, we operated on thirteen patients with acoustic neuromas using the retrosigmoid suboccipital approach. Our goal in these cases was to preserve hearing. We began using intraoperative direct eighth nerve monitoring in 1983. This allowed us to rapidly assess cochlear nerve function during excision of small acoustic neuromas. Intraoperative monitoring was used in 5 of 13 cases, and in three patients, hearing was preserved. In the 8 cases where intraoperative monitoring was not used, hearing was preserved in only two patients. Our overall success rate of simultaneous total tumor removal hearing preservation was 38%. Tumor size varied from intracanalicular to one with a 3.0 cm protrusion medial to the porus acousticus. Hearing was preserved in 54% of cases where tumor size was less than 1.5 cm. We find continuous monitoring of direct eighth nerve evoked action potentials to be extremely valuable and a rapid indication of reversible cochlear nerve trauma.