Sarnthein et al.: FNMEPs analyzed by the threshold-level method 1 Facial nerve motor-evoked potentials during skull base surgery to monitor facial nerve function by the “threshold-level” method

Object During surgeries that put the facial nerve at risk for injury, its function can be continuously monitored by transcranial facial nerve motor evoked potentials (FNMEPs) in facial nerve target muscles. Despite their advantages, FNMEPs are not yet widely used. While most authors use a 50% reduction in FNMEP response amplitudes as a warning criterion, in this paper the authors’ approach was to keep the response amplitude constant by increasing the stimulation intensity and to establish a warning criterion based on the ”threshold-level” method. Methods The authors included 34 consecutive procedures involving 33 adult patients (median age 47 years) in whom FNMEPs were monitored. A threshold increase greater than 20 mA for eliciting FNMEPs in the most reliable facial nerve target muscle was considered a prediction of reduced postoperative facial nerve function, and subsequently a warning was issued to the surgeon. Preoperative and early postoperative function was documented using the House-Brackmann grading system. Results Monitoring of FNMEPs was feasible in all 34 surgeries in at least one facial nerve target muscle. The mentalis muscle yielded the best results. The HouseBrackmann grade deteriorated in 17 (50%) of 34 cases. The warning criterion was reached in 18 (53%) of 34 cases, which predicted an 83% risk of House-Brackmann grade deterioration. Sensitivity amounted to 88% (CI 64%-99%) and specificity to 82% (CI 57%-96%). Deterioration of FNMEPs and a worse House-Brackmann grade showed a high degree of association (p < 0.001). The impact of FNMEP monitoring on surgical strategy is exemplified in an illustrative case. Conclusions In surgeries that put the facial nerve at risk, the intraoperative increase in FNMEP stimulation threshold was closely correlated to postoperative facial nerve dysfunction. Monitoring of FNMEPs is a valid indicator of facial nerve function in skull base surgery. It should be used as an adjunct to direct electrical facial nerve stimulation and continuous electromyographic monitoring of facial nerve target muscles. DOI: https://doi.org/10.3171/2012.12. FOCUS12386. Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-76317 Journal Article Accepted Version Originally published at: Sarnthein, J; Hejrati, N; Neidert, M C; Huber, A M; Krayenbühl, N (2013). Facial nerve motor evoked potentials during skull base surgery to monitor facial nerve function using the threshold-level method. Neurosurgical Focus, 34(3):E7. DOI: https://doi.org/10.3171/2012.12. FOCUS12386. Sarnthein et al.: FNMEPs analyzed by the threshold-level method 1 Facial nerve motor-evoked potentials during skull base surgery to monitor facial nerve function by the “threshold-level” method Johannes Sarnthein, PhD; Nader Hejrati,cand med; Marian C. Neidert, MD; Alexander Huber, MD; Niklaus Krayenbühl, MD 1 Klinik für Neurochirurgie, UniversitätsSpital Zürich, Switzerland 2 Klinik für Ohren-, Nasen-, Halsund Gesichtschirurgie, UniversitätsSpital Zürich 3 Center for Integrative Human Physiology, Universität Zürich, Switzerland Corresponding author: PD Dr. Johannes Sarnthein Klinik für Neurochirurgie, UniversitätsSpital Zürich, 8091 Zürich, Switzerland johannes.sarnthein@usz.ch Tel +41 44 255 5672 Fax +41 44 255 4399 Running title: FNMEPs analyzed by the threshold-level method