In Response to Letter to the Editor Regarding: Is Cricothyroid Muscle Twitch Predictive of the Integrity of the EBSLN in Thyroid Surgery?

We thank Dr. Wu and colleagues for their interest in our article and appreciate the comments. In addition, we thank them for drawing attention to the clinical significance of the external branch of superior laryngeal nerve (EBSLN), sometimes referred to as the neglected nerve in thyroid surgery. In the letter, the authors commented that the new electromyography (EMG) endotracheal tube, needle, or surface electrodes using an intraoperative neuromonitoring system (IONM) could objectively and quantitatively evaluate the cricothyroidmuscle (CTM).Moreover, they pointed out that the procedure of EBSLN stimulation and CTM twitch assessment was incomplete in our study. Finally, they suggested that testing CTM twitch is not only a diagnostic tool but also a useful and effective preventive tool for EBSLN injury. Three well-designed studies mentioned in their letter were conducted on the new adjunctive devices for IONM system. In these studies, EBSLNs were intentionally explored and the exposed nerves were stimulated with the probe under direct vision. However, intentional dissection for visual identification of EBSLN is not established as a standard procedure in thyroidectomy. In addition, not every surgeon routinely performs the dissection for visual identification of EBSLN because it may be invasive and harmful to the nerve and to the postoperative voice outcome. We conducted our study in consideration of this practical situation. Visually unidentifiable EBSLNs might be superficial to the inferior constrictor muscle (ICM) under the thin overlying fascia (Friedman type I) or deep-seated in the ICM (Friedman types II and III). These anatomical characteristics might lead to an incomplete stimulation of the unidentified EBSLN. Considering these limitations, we made great efforts to avoid false positive and negative results in the CTM twitch test and described them in the Methods section. In our study, a minimal single dose of the nondepolarizing muscle relaxant (approximately 0.5 mg/kg bodyweight of rocuronium) was administered to ensure the full return of muscular activity, and the stimulating probe was initially placed in contact with the CTM to confirm the recovery from the muscle relaxant and to verify the integrity of the device circuit. In addition, the probe was positioned at least 2 cm away from the CTM to avoid a collateral electric stimulus. Although most surgeons agree with the efficacy of CTM twitch in the prediction of nerve integrity, there have been no large and prospective studies demonstrating the diagnostic values of CTM twitch for neurostimulation, especially compared to the postoperative EMG. Our data showed high-positive and low-negative predictive values (92.5% and 16.7%, respectively) Finally, we concluded that CTM twitch would not be a perfect diagnostic tool enough to replace the postoperative EMG in other research such as that on postthyroidectomy dysphonia. However, CTM twitch would be still a useful preventive tool for EBSLN injury during thyroid surgery. Also, we share our opinion with Dr. Wu that electrophysiologic approaches are optimum and promising for the prevention of EBSLN injury in thyroid surgery due to its quantitative and objective evaluations. In addition, we expect further researches on the evaluation and prevention of EBSLN injury.