Delta fMEP predicts facial nerve dysfunction after surgical resection of vestibular schwannoma: A quantitative application of transcranial facial nerve evoked potential

Abstract

Abstract Without direct facial nerve (FN) stimulation, transcranial facial motor evoked potential (fMEP) may considerably reflect the functional integrity and postoperative outcome of FN in cerebellopontine surgery. This study investigated the “threshold method” of fMEP to prognosticate the FN function after large vestibular schwannoma (VS) surgery. The medical records from 58 patients with large VS (maximal diameter > 2.5 cm) who underwent microsurgical resection were retrospectively reviewed. The threshold was the stimulation intensity to elicit the baseline amplitude of fMEP, and its change from the baseline to the end of surgery was defined as the delta fMEP. House-Brackman grade (HBG) of FN was documented at 1 week, 1 month, 6 months, and 1 year after surgical resection, classifying the patients presenting HBG 3 or more as FN dysfunction group. Receiver operating characteristics and binary logistic regression for the risk of postoperative FN dysfunction were performed. During surgery, the fMEP stimulation threshold increased in 27 (46%) patients. The delta fMEP value for FN dysfunction had a higher predictive value at long-term follow-up (cut-off = 15V, area under curve [AUC] 0.82 and 0.81 at postoperative 6 months and 1 year, respectively) than at the short-term follow-up (cut-off = 27.5V, AUC 0.78 and 0.74 at postoperative 1 week and 1 month, respectively). In regression analysis, the delta fMEP higher than its optimal cut-off was significantly associated with FN dysfunction in all follow-up period. These results suggested that the delta fMEP may serve as a valuable indicator of FN dysfunction after surgical resection of large VS, especially for long-term outcomes.