Interpreting Intraoperative Facial Nerve Signals: A Surgeon-focused Comparison of TcFMEP and Direct Electrical Stimulation During Acoustic Neuroma Surgery

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.