Intraoperative detection of a false-negative facial motor evoked potential by direct facial nerve stimulation during vestibular schwannoma surgery: A case report

Background: False-negative transcranial facial motor evoked potential (FMEP) findings have previously been recognized retrospectively through discrepancies between preserved intraoperative FMEPs and postoperative facial palsy. To the best of our knowledge, no previous report has demonstrated a false-negative FMEP finding intraoperatively using direct facial nerve stimulation. We report the only false-negative case encountered among more than 100 cases monitored with FMEPs at our institution. Case Description: A 58-year-old man with a right Koos Grade II vestibular schwannoma underwent near-total resection with transcranial FMEP monitoring, free-running electromyography, and intermittent direct facial nerve stimulation. Reproducible FMEPs were obtained only with double-train stimulation and remained stable throughout tumor removal, with amplitudes never decreasing below 50% of baseline. Direct stimulation initially elicited stable responses at 0.4 mA at all tested sites. During the final stage of resection, stimulation at the internal auditory canal fundus produced a robust compound muscle action potential at 0.4 mA, whereas cisternal stimulation produced no response at 0.4 mA and only a minimal response at 0.7 mA. This discrepancy demonstrated severe segmental facial nerve conduction impairment despite preserved FMEPs. Postoperatively, House–Brackmann Grade V facial palsy developed and improved to Grade II by 6 months. Conclusion: To the best of our knowledge, this is the first reported case in which direct facial nerve stimulation identified a false-negative FMEP finding intraoperatively rather than retrospectively from postoperative facial palsy. Serial direct stimulation at multiple facial nerve segments should be integrated with FMEP monitoring to detect occult conduction impairment and guide safe tumor resection.