Recurrent Laryngeal Nerve with Loss of Signal During Monitored Thyroidectomy: Percentage Reduction in Sum of the Amplitude of Left and Right Channel

Purpose The prognostication for the injured recurrent laryngeal nerve (RLN) with incomplete loss of signal (LOS) and its function outcome have not been well unified. A warning criterion was proposed to predict RLN injury during monitored thyroidectomy. Methods A retrospective review of prospectively collected data from consecutive 357 patients with 560 nerves at risk was conducted. Vocal cords mobility with laryngoscope was performed preoperatively, on the second day, and once a month postoperatively until complete recovery. Different cutoff values of the percentage reduction in sum of the amplitude of left and right channel at the end of the surgery, for postoperative vocal cord paralysis (VCP) prediction were compared. Results Percentage reduction in sum of the amplitude of left and right channel at the end of operation ranged from 30.2 to 63.6% in 27 nerves with incomplete LOS (absolute amplitude value of final R 2 > 100 μV with reduction > 50% of R 1 ). Seven (1.25%) nerves experienced transient postoperative VCP, in which one nerve with postoperative VCP showed no amplitude reduction. The positive predictive value of VCP for the sum amplitude reduction exceeding 30, 40, 50, and 60% was 22.2, 40, 85.7, and 100%, respectively. Accuracy was 96.1, 98.2, 99.6, 99.4%, respectively. Conclusion Percentage reduction in sum of the amplitude of left and right channel is a meaningful method to improve the accuracy of VCP prediction. When the sum amplitude reduction ≥ 50%, surgeons should consider the possibility of postoperative VCP and correct some surgical maneuvers.