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Interpreting Intraoperative Facial Nerve Signals: A Surgeon-focused Comparison of TcFMEP and Direct Electrical
Shahid Iqbal1, Jonathan Dilgen2, Krish Suresh1
1Department of Otolaryngology-Head and Neck Surgery, University of California San Diego, La Jolla, CA.
Objective:
To compare the intraoperative predictive utility of transcranial facial motor evoked potentials (TcFMEP) and direct electrical stimulation-evoked compound muscle action potentials (CMAP) in identifying early postoperative facial nerve dysfunction following acoustic neuroma surgery.
Patients:
Six patients undergoing acoustic neuroma resection at a high-volume tertiary referral center who demonstrated postoperative facial nerve dysfunction (House-Brackmann grade IV-VI) were retrospectively selected. All patients had normal preoperative facial nerve function.
Intervention:
Intraoperative facial nerve monitoring using TcFMEP, free-running electromyography, and direct electrical stimulation (DES) with a Prass probe during acoustic neuroma resection via translabyrinthine, retrosigmoid, or middle fossa approaches.
Main Outcome Measure:
Early postoperative facial nerve function assessed using the House-Brackmann grading scale, and its association with postresection TcFMEP amplitude changes and DES-evoked CMAP thresholds.
Results:
Tumor size ranged from 10 to 45 mm (mean 27.3±12.6 mm). Using an institutional CMAP threshold of ≥300 µV at 0.05 mA, DES suggested preserved facial nerve integrity in 4 of 6 patients who subsequently developed poor postoperative facial nerve function. In contrast, 5 of 6 patients demonstrated a reduction in TcFMEP amplitude following tumor resection, with several exceeding previously reported prognostic thresholds associated with adverse facial nerve outcomes. Long-term follow-up demonstrated partial recovery in all patients, with final House-Brackmann grades ranging from I to III.
Conclusion:
In this selected cohort of patients with early postoperative facial nerve dysfunction, TcFMEP amplitude changes demonstrated closer concordance with postoperative facial nerve outcomes than Prass probe-evoked CMAP responses. These findings suggest that TcFMEP may provide more reliable intraoperative prognostic information than DES-evoked CMAPs for early facial nerve dysfunction following acoustic neuroma surgery.
