Objectives: We observed significant contractions in the cricothyroid muscle (CTM) after recurrent laryngeal nerve (RLN) stimulation in some patients. We aimed to evaluate whether these contractions resulted from the laryngeal-muscle movement due to the contraction of other intrinsic muscles or actual CTM contraction, with objective real-time intraoperative electromyography (EMG) recordings.
Methods: This study was performed prospectively in 106 consecutive patients who underwent intraoperative neural monitoring-guided primary thyroid surgery due to various thyroid diseases between February-2015 and February-2016. After completion of the thyroidectomy procedure; the RLN, vagus nerve (VN), external branch of the superior laryngeal nerve (EBSLN), plexus pharyngeus (PP), and contralateral EBSLN (CEBSLN) were stimulated and the responses from the CTM and CPM were recorded and evaluated by EMG through needle electrodes.
Results: 182 CTMs of 106 patients, with the mean age of 45, were evaluated regarding their innervation patterns. Positive EMG waveforms were achieved from 181 CTMs with EBSLN stimulation. A total of 132 (74%) positive EMG responses were recorded after the stimulation of 179 RLNs. The mean amplitude obtained with CTM EMG with RLN stimulation was 5.5% of that with EBSLN stimulation. The CTM amplitude was 39% of the vocal cord amplitude with RLN stimulation. Positive EMG responses of 96 CTMs (55%) with VN stimulation were recorded. The mean amplitude through CTM EMG with VN stimulation was 6% of that with EBSLN stimulation. Positive EMG responses were achieved from 10 (0.6%) CTMs with the stimulation of 170 PPs. The mean amplitude obtained from CTMs with PP stimulation was 4.3% of that with EBSLN stimulation. Positive EMG amplitudes of 35 (67%) CTMs were obtained with stimulation of 52 CEBSLN. Temporary vocal cord paralysis was detected in six patients (5% of patients and 3.3% of the nerves) postoperatively.
Conclusion: The RLN contributes significantly to the innervation of the CTM. Despite the findings associated with the contribution of the PP and CEBSLN to the CTM innervation, further studies are needed. We are of the opinion that these are among the significant factors that contribute to the differences in clinical findings between patients with EBSLN injuries.