Assessment of Intraoperative Nerve Monitoring Parameters Associated With Facial Nerve Outcome in Parotidectomy for Benign Disease

Importance Prior studies suggest that the use of facial nerve monitoring decreases the rate of immediate postoperative facial nerve weakness in parotid surgery, but published data are lacking on normative values for these parameters or cutoff values to prognosticate facial nerve outcomes. Objective To identify intraoperative facial nerve monitoring parameters associated with postoperative weakness and to evaluate cutoff values for these parameters under which normal nerve function is more likely. Design, Setting, and Participants This retrospective case series of 222 adult patients undergoing parotid surgery for benign disease performed with intraoperative nerve monitoring was conducted at an academic medical institution from September 13, 2004, to October 30, 2014. The data analysis was conducted from May 2018 to January 2019. Main Outcomes and Measures The main outcome measure was facial nerve weakness. Receiver operating characteristic curves were generated to define optimal cut point to maximize the sensitivity and specificity of the stimulation threshold, mechanical events, and spasm events associated with facial nerve weakness. Results Of 222 participants, 121 were women and 101 were men, with a mean (SD) age of 51 (16) years. The rate of temporary facial nerve paresis of any nerve branch was 45%, and the rate of permanent paralysis was 1.3%. The mean predissection threshold was 0.22 milliamperes (mA) (range, 0.1-0.6 mA) and the mean postdissection threshold was 0.24 mA (range, 0.08-1.0 mA). The average number of mechanical events was 9 (range, 0-66), and mean number of spontaneous spasm events was 1 (range, 0-12). Both the postdissection threshold (area under the curve [AUC], 0.69; 95% CI, 0.62-0.77) and the number of mechanical events (AUC, 0.58; 95% CI, 0.50-0.66) were associated with early postoperative facial nerve outcome. The number of spasm events was not associated with facial nerve outcome. The optimal cutoff value for the threshold was 0.25 mA, and the optimal cutoff for number of mechanical events was 8. If a threshold of greater than 0.25 mA was paired with more than 8 mechanical events, there was a 77% chance of postoperative nerve weakness. Conversely, if a threshold was 0.25 mA or less and there were 8 mechanical events or less, there was 69% chance of normal postoperative nerve function. No parameters were associated with permanent facial nerve injury. Conclusions and Relevance Postdissection threshold and the number of mechanical events are associated with immediate postoperative facial nerve function. Accurate prediction of facial nerve function may provide anticipatory guidance to patients and may provide surgeons with intraoperative feedback allowing adjustment in operative techniques and perioperative management.