Intraoperative facial nerve electromyography parameters to optimize postoperative facial nerve outcome in patients with large unilateral vestibular schwannoma

Background: Decision-making for large sporadic vestibular schwannomas (VS) resection guided by the intraoperative change in supramaximal facial nerve (FN) amplitude and latency response to optimize post-operative FN outcome. Methods: Prospectively study of 43 patients, from January to December 2018, of large sporadic VS with preoperative normal FN function at our center. Tumors were removed through retrosigmoid (81%) or translabyrinthine (19%) approaches with FN monitoring. Intraoperative pre- and post-VS resection supramaximal (2 mA) amplitude and latency responses at the proximal FN root were recorded. Results: Total, near-/subtotal VS resections (TR, NTR, STR) were achieved in 51%, 38%, and 11% of tumors, respectively, guided by no more than 40% decrease in supramaximal amplitude. Pre- and post-resection supramaximal amplitude and latency responses were lower and longer, respectively, in NTR+STR than in TR. At day 8, FN function was grade I–II in 77% of patients and grade III–V in 23%, and after 6 months, it was in grade I-II in 95% and grade III in 5%, and there was no significant difference between TR and NTR+STR. Facial palsy occurred in older patients and in the case of severe FN adhesion. At day 8, pre- and post-resection supramaximal amplitude but not latency responses were different between FN grade III-V and grade I-II. Serviceable hearing was preserved in 28% of large VS. Conclusions: Intraoperative FN monitoring guided VS resection in large VS so that 49% retained some residual tumor. Accordingly, 95% good postoperative FN function and significant hearing preservation were achieved after 6 months.