Facial nerve motor-evoked potential monitoring during skull base surgery predicts facial nerve outcome

Objective: To determine whether monitoring facial nerve motor-evoked potentials (FNMEPs) elicited by transcranial electrical stimulation during skull base tumour surgery is useful for predicting facial nerve outcome. Methods: This study analysed FNMEP findings in 26 patients with skull base tumours. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supramaximal stimuli (180-550 V). FNMEPs were recorded from the orbicularis oculi and oris muscles. The correlation between the final-to-baseline FNMEP ratio and postoperative facial nerve function was examined. Results: Postoperative facial nerve function correlated significantly with the FNMEP ratios in the orbicularis oculi (p=0.004) and orbicularis oris (p<0.001) muscles. An FNMEP ratio of <50% consistently predicted immediate postoperative facial palsy, although the degree of palsy differed among patients. All patients had satisfactory facial nerve function (House and Brackmann grades I and II) postoperatively if the FNMEP ratio remained at >50%. Conclusions: Intraoperative FNMEP monitoring can be useful for predicting facial nerve function after skull base surgery. This new method is a valuable adjunct to conventional facial nerve monitoring.