Recurrent laryngeal nerve injury with incomplete loss of electromyography signal during monitored thyroidectomy-evaluation and outcome

Abstract

Purpose During monitored thyroidectomy, a partially or completely disrupted point of nerve conduction on the exposed recurrent laryngeal nerve (RLN) indicates true electrophysiologic nerve injury. Complete loss of signal (LOS; absolute threshold value < 100 mu V) at the end of operation often indicates a postoperative vocal cord (VC) palsy. However, the evaluation for the injured RLN with incomplete LOS and its functional outcome has not been well described. Methods Three hundred twenty-three patients with 522 RLNs at risk who underwent standardized monitored thyroidectomy were enrolled. The RLN was routinely stimulated at the most proximal (R-2p signal) and distal (R-2d signal) ends of exposure after thyroid resection to determine if there was an injured point on the RLN. Pre- and postoperative VC function was routinely examined. Results Twenty-nine RLNs (5.6 %) were detected with an injury point. Five nerves had complete LOS and other 24 nerves had incomplete LOS where the R-2p/R-2d reduction (% of amplitude reduction compared with proximal to distal RLN stimulation) ranged from 22 to 79 %. Postoperative temporary VC palsy was noted in those five RLNs with complete LOS (final vagal signal, V-2< 100 mu V) and four RLNs with incomplete LOS (R-2p/R-2d reduction 62-79 %; V-2 181-490 mu V). In the remaining 20 nerves with R-2p/R-2d reduction <= 53 % (V-2 373-1623 mu V), all showed normal VC mobility. Overall, false negative results were found in two RLNs (0.4 %) featuring unchanged V-2 and R-2p/R-2d but developed VC palsy. Conclusions Testing and comparing the R-2p/R-2d signal is a simple and useful procedure to evaluate RLN injury after its dissection and predict functional outcome. When the relative threshold value R-2p/R-2d reduction reaches over 60 %, surgeon should consider the possibility of postoperative VC palsy.