Intraoperative transcranial facial motor evoked potential monitoring in surgery of cerebellopontine angle tumors predicts early and late postoperative facial nerve function

Objective We propose a novel method that predicts facial nerve function (FNF) calculated from the drop and recovery of facial motor evoked potential (FMEP) amplitude ratio during the surgery of cerebellopontine angle tumors. Methods We enrolled 73 patients with cerebellopontine angle tumor, and used a biphasic, constant current, and suprathreshold stimulation (BCS) protocol to record FMEP of the orbicularis oris. We measured the intraoperative minimum-to-baseline amplitude ratio (MBR), the final-to-baseline amplitude ratio (FBR), and the recovery value (RV). RV was measured by subtracting MBR from FBR. Using those values, we evaluated FNF both at early postoperative (EP) and late postoperative (LP) periods. Results We successfully obtained 62 FMEP readings. Facial palsies occurred in 22 patients during the EP period, and 14 patients recovered during the LP period. Both MBR and FBR showed a significant correlation with FNF in the EP period. RV showed a good predictive power of FNF recovery during the LP period for the first time. Conclusions RV is a new and useful predictor of FNF recovery. MBR can be an intraoperative predictor of FNF in the EP period. Significance FNF outcome in the early and late postoperative periods can be predicted by FMEP.