Objective: To determine whether monitoring facial nerve motor evoked potentials (FNMEPs) elicited by transcranial electrical stimulation during skull base tumor surgery is useful for predicting facial nerve outcome.
Methods: We analyzed FNMEP findings in 31 patients with skull base tumor. Surgery was performed twice in 2 of the 31 patients because of tumor regrowth. FNMEP monitoring was conducted 33 times in the present study. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supramaximal stimuli (140-550V). FNMEPs were recorded from the orbicularis oculi and oris muscles. Correlation between the final-to-baseline FNMEP ratio and postoperative facial nerve function (House & Brackmann grade) was examined.
Results: Valid FNMEPs were obtained in 26 of the 33 (78.8%) recordings from the orbicularis oculi muscle and in 31 of the 33 (93.9%) recordings from the orbicularis oris muscle. Facial nerve function correlated significantly with the FNMEP ratios in the orbicularis oculi (r = -0.52 N=26, p < 0.01) and orbicularis oris (r = -0.60, N=31, p < 0.001) muscles. An FNMEP ratio of -50% consistently predicted immediate postoperative facial palsy, although the degree of palsy differed among the patients.
Conclusions: Intraoperative FNMEP monitoring is useful for predicting facial nerve function after skull base surgery.