COMPARISON OF RESPONSE AMPLITUDE VERSUS STIMULATION THRESHOLD IN PREDICTING EARLY POSTOPERATIVE FACIAL-NERVE FUNCTION AFTER ACOUSTIC NEUROMA RESECTION

Objective: This study aimed to better predict the early postoperative facial nerve (FN) function after acoustic neuroma (AN) resection. Stud y Design: This study was a prospective series. Setting: The surgery wa s conducted in a tertiary referral center. Patients: A total of 44 pat ients undergoing AN resection with cranial nerve monitoring were obser ved for at least 1 year after surgery. Main Outcome Measures: The pred ictive value of amplitude of the FN stimulus response on the early pos toperative FN function was measured. Results: Cranial nerve monitoring in AN surgery was used to obtain the stimulation threshold and facial electromyograph response amplitudes to FN stimulation proximal and di stal to the tumor at 0.2 V above threshold. Thirty-eight of forty-four patients studied had a low postresection threshold (less than or equa l to 0.1 V). Of these (10), 26% sustained a postoperative FN dysfuncti on of House-Brackmann (HB)grades 3-6. In an effort to improve the pred ictive value from cranial nerve monitoring, the response amplitude to suprathreshold stimulation was compared with the threshold and FN func tion. Eighty-nine percent of patients with an amplitude of greater tha n or equal to 200 mu V had a grade 1-2 early postoperative FN function , whereas only 41% of patients with < 200 mu V had a grade 1-2 early p ostoperative FN function (p = 0.00035). Eighty-eight percent of patien ts with both a low threshold and high amplitude had a grade 1-2 early postoperative FN function, whereas the remaining 12% of patients had a grade 3-6 FN function (p = 0.0032). The false-positive rate of thresh old alone in predicting a grade 1-2 FN function was 26% compared to 12 % for low threshold and high amplitude combined. Conclusions: The use of FN threshold and amplitude together is superior to threshold alone as a predictor of early postoperative FN function.