Intraoperative detection of a false-negative facial motor evoked potential by direct facial nerve stimulation during vestibular schwannoma surgery: A case report.
- 1Department of Neurosurgery, Kansai Medical University, Hirakata, Osaka, Japan.
- 2Department of Orthopedics, Kansai Medical University, Hirakata, Osaka, Japan.
Abstract
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.
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BibTeX
@article{nonaka2026intraoperative,
title = {{Intraoperative detection of a false-negative facial motor evoked potential by direct facial nerve stimulation during vestibular schwannoma surgery: A case report.}},
author = {Masahiro Nonaka and Takeshi Itakura and Katsuya Ueno and Mayuko Miyata and Haruna Isozaki and Yi Li and Junichi Takeda and Takehiro Suyama and Ryosuke Matsuda},
journal = {Surgical neurology international},
year = {2026},
month = oct,
doi = {10.25259/sni_941_2026},
url = {https://europepmc.org/article/MED/42974267},
note = {CC BY-NC-SA},
}
RIS
TY - JOUR
TI - Intraoperative detection of a false-negative facial motor evoked potential by direct facial nerve stimulation during vestibular schwannoma surgery: A case report.
AU - Masahiro Nonaka
AU - Takeshi Itakura
AU - Katsuya Ueno
AU - Mayuko Miyata
AU - Haruna Isozaki
AU - Yi Li
AU - Junichi Takeda
AU - Takehiro Suyama
AU - Ryosuke Matsuda
PY - 2026
DA - 2026/10/16
JO - Surgical neurology international
DO - 10.25259/sni_941_2026
UR - https://europepmc.org/article/MED/42974267
AB - 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.
SN - 2229-5097
SN - 2152-7806
ER -