Recurrent laryngeal nerve monitoring in thyroid and parathyroid surgery: the University of Michigan experience

OBJECTIVES/HYPOTHESIS: Intraoperative monitoring of the recurrent laryngeal nerve (RLN) is finding increasing acceptance during thyroidectomy. Recently, a laryngeal surface electrode was introduced to enable another form of noninvasive monitoring of the RLN. The present report examines the University of Michigan experience with RLN monitoring using the postcricoid surface electrode. STUDY DESIGN: All patients undergoing partial or total thyroidectomy or parathyroidectomy from January 1999 to July 2001 were considered candidates for the study. Audiologists trained in intraoperative electrophysiological techniques performed all of the monitoring. METHODS: Data collected on each patient included 1) stimulation threshold for a laryngeal compound muscle action potential on initial RLN identification, 2) stimulation threshold of the laryngeal compound muscle action potential on completion of the procedure, and 3) flexible fiberoptic evaluation of the larynx at the initial postoperative visit and at the 3-month follow-up visit. The average duration of follow-up was 9.8 months with a range of 3 to 60 months. RESULTS: The average minimum current required for stimulation on first identification of all nerves was 0.57 mA (+/-0.48 mA). After completion of the procedure a mean threshold level of 0.42 mA (+/-0.55 mA) was obtained during direct RLN stimulation. Post-dissection stimulation of the RLN on the side of tumor dissection was 0.92 mA (+/-0.65 mA) compared with a stimulation threshold of 0.76 mA (+/-0.57 mA) for the nontumor side. CONCLUSIONS: Electromyographic monitoring of the RLN using a postcricoid surface electrode provides a safe, simple, and effective method for intraoperative monitoring during thyroid or parathyroid surgery. Further, evoked electromyography confirms RLN integrity at the conclusion of surgery.