Warning criterion to predict recurrent laryngeal nerve injury with percentage reduction of the amplitude of V2/R2d in neuromonitoring thyroidectomy

Objective: To evaluate the contribution of amplitude reduction compared vagal stimulation at the end of thyroid dissection (V2) to the most distal RLN stimulation during thyroidectomy in predicting postoperative vocal cords paralysis (VCP). Methods: Patients with intact preoperative RLN function who underwent monitored thyroidectomy between August 2017 and April 2018 were included. We routinely tested the exposed RLN at the lowest proximal end (R2p signal) and the most distal end near the laryngeal entry point (R2d signal), and then routinely detected the vagal nerve at the horizontal plane of the inferior pole of thyroid with 2mA stimulation current. The cut-off value was calculated with Receiver Operating Characteristic curve. Rates of specificity, sensitivity, negative predictive value, positive predictive value (PPV) for V2/R2d and R2p/R2d were compared. Results: Percentage reduction of the amplitude of V2/R2d ranged from 34.8% to 76.7%. Twentytwo (1.5%) nerves developed temporary VCP, in which one nerve with VCP showed no significant amplitude reduction at the end of the surgery. There was no permanent or bilateral VCP. Sensitivity, specificity, PPV, NPV, and accuracy for the amplitude reduction of V2/R2d> 60% were 95.5%, 99.8%, 99.9%, 98.2%, respectively, for R2p/R2d were 99.5%, 99.2%, 63.6%, 99.9%, 97.7%, respectively. Conclusion: Percentage reduction of the amplitude of V2/R2d is a reliable and practical warning criterion for RLN injury. When the amplitude reduction > 60% surgeons should consider the possibility of postoperative VCP and correct some surgical maneuvers. (C) 2021 Oto-Rhino-Laryngological Society of Japan Inc. Published by Elsevier B.V. All rights reserved.