Intraoperative facial motor evoked potential monitoring with transcranial electrical stimulation during skull base surgery

Objective: To address the limitations of standard electromyography (EMG) facial nerve monitoring techniques by exploring the novel application of multi-pulse transcranial electrical stimulation (mpTES) to myogenic facial motor evoked potential (MEP) monitoring. Methods: In 76 patients undergoing skull base surgery, mpTES was delivered through electrodes 1 cm anterior to C1 and C2 (M1–M2), C3 and C4 (M3–M4) or C3 or C4 and Cz (M3/M4–Mz), with the anode contralateral to the operative side. Facial MEPs were monitored from the orbicularis oris muscle on the operative side. Distal facial nerve excitation was excluded by the absence of single pulse responses and by onset latency consistent with a central origin. Results: M3/M4–Mz mpTES (n=50) reliably produced facial MEPs while M1–M2 (n=18) or M3–M4 (n=8) stimulation produced 6 technical failures. Facial MEPs could be successfully monitored in 21 of 22 patients whose proximal facial nerves were inaccessible to direct stimulation. Using 50, 35 and 0% of baseline amplitude criteria, significant facial deficits were predicted with a sensitivity/specificity of 1.00/0.88, 0.91/0.97 and 0.64/1.00, respectively. Conclusions: Facial MEPs can provide an ongoing surgeon-independent assessment of facial nerve function and predict facial nerve outcome with sufficiently useful accuracy. Significance: This method substantially improves facial nerve monitoring during skull base surgery.