Correlation of final evoked potential amplitudes on intraoperative electromyography of the recurrent laryngeal nerve with immediate postoperative vocal fold function after thyroid and parathyroid surgery

IMPORTANCE: Thyroid and parathyroid surgery are among the most common operations in the United States. Recurrent laryngeal nerve (RLN) injury is an infrequent but potentially detrimental complication. OBJECTIVE: To correlate the final evoked potential amplitudes on intraoperative electromyography (EMG) after stimulation of the RLN with immediate postoperative vocal fold function after thyroid and parathyroid surgery. DESIGN, SETTING, AND PARTICIPANTS: Retrospective observational study at a tertiary academic medical center.We included 674 patients (with 1000 nerves at risk) undergoing thyroid or parathyroid surgery from July 1, 2008, through June 30, 2012. INTERVENTIONS: Thyroid and parathyroid surgery. MAIN OUTCOMES AND MEASURES: The association of final evoked potential amplitudes on EMG after thyroid and parathyroid surgery with vocal fold function as determined by postoperative fiberoptic laryngoscopy. RESULTS: Three patients experienced permanent vocal fold paresis (VFP) secondary to intraoperative RLN transection. Of the remaining 997 RLNs at risk, 22 (2.2%) in 20 patients exhibited temporary VFP on fiberoptic laryngoscopy after extubation. Eighteen patients experienced unilateral temporary VFP, and 2 experienced bilateral VFP without the need for tracheostomy or reintubation. Of the 22 RLNs, postdissection EMG amplitudes were less than 200 μV (true-positive findings) in 21 and at least 200 μV (false-negative finding) in 1. Of the 975 RLNs (97.5%) with normal function, postdissection EMG amplitudes were at least 200 μV (true-negative findings) in 967 and less than 200 μV (false-positive findings) in 8. In regard to immediate postoperative VFP, sensitivity, specificity, positive and negative predictive values, and accuracy of postdissection EMG amplitudes of less than 200 μV were 95.5%, 99.2%, 72.4%, 99.9%, and 99.1%, respectively. CONCLUSIONS AND RELEVANCE: Intraoperative nerve monitoring of the RLN with EMG provides real-time information regarding neurophysiologic function of the RLN and can predict immediate postoperative VFP reliably when a cutoff of 200 μV is used. The high negative predictive value means that the surgeon can presume with confidence that the RLN has not been injured in the presence of a potential of at least 200 μV. This information would be useful in patients for whom bilateral thyroid surgery is being considered.