Abstract
BACKGROUND: The application of intraoperative neurophysiological monitoring (IONM) has been accepted to avoid injury of a recurrent laryngeal nerve (RLN). Loss of the neuromonitoring signal indicates nerve injury and is subdivided into segmental type and global type nerve paralysis. This study aimed to determine the course of vocal cord function recovery after definitive loss of signal (LOS) types. METHODS: This retrospective study included 1,442 patients (with 2,752 nerves at risk) who had thyroidectomies between January 2018 and December 2021. Preoperative and postoperative vocal cord functions were evaluated by laryngoscopic examination. RESULTS: LOS occurred in 168 of 1,442 (11.7%) patients and 171 of 2,748 (6.2%) nerves at risk during surgery. Of LOS nerves of benign tumors, 74.2% showed global type. In cancer cases, segmental paralysis was more common, accounting for 51.3% of LOS nerves. Of nerves with segmental LOS in cancer patients, 55.3% needed partial layer resection for RLN invasion. Intraoperative recovery was seen in 9 of 62 nerves (14.5%) with segmental LOS and 32 of 109 (29.4%) nerves with global type LOS. The vocal cord palsy rate on postoperative days (PODs) 2-3 was lower after global type nerve paralysis (63.6%) than after segmental loss (84.9%). At 6 months postoperatively, the rate of vocal cord paralysis in benign tumors was not significantly different between segmental type and global type (P=0.586). However, cancer patients with segmental LOS significantly more often had vocal cord dysfunction than those with global LOS at 6 months postoperatively (rate of nerve palsy: segmental 40.0% vs. global 3.4%) (P<0.001). CONCLUSIONS: The intraoperative recovery rate and early nerve recovery rate are significantly higher for patients with global LOS than for patients with segmental LOS. Cancer patients with segmental LOS significantly more often had vocal cord dysfunction than those with global LOS at 6 months postoperatively.