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

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 μV) 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, V₂ < 100 μV) and four RLNs with incomplete LOS (R_2p/R_2d reduction 62–79 %; V₂ 181–490 μV). In the remaining 20 nerves with R_2p/R_2d reduction ≤53 % (V₂ 373–1623 μV), all showed normal VC mobility. Overall, false negative results were found in two RLNs (0.4 %) featuring unchanged V₂ 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.