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          VOLUME 50 / ISSUE 2

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2 - The effect of the intraoperative neuromonitoring to the external branch of the superior laryngeal nerve identification and contribution of the nerve to the motor function of the thyroaritenoid muscle

Nurcihan Aygün, Evren Besler, Fevzi Celayir, Emre Bozdağ, Bülent Çitgez, Gürkan Yetkin, Mehmet Mihmanlı, Mehmet Uludağ

Objective: Meticulous anatomical studies of cadaver larynges revealed terminal branches of the external branch of the superior laryngeal nerve (EBSLN) reaching the anterior thyroarytenoid muscle (TA) region and communicating with branches of the recurrent laryngeal nerve (RLN) in 41% to 85% of cases. This nerve is called “human communicating nerve”. Although the visual identification of the RLN is accepted to be the gold standart to preserve the nerve, there is still not a standart technique to preserve the EBSLN. In this study; we aimed to evaluate the intraoperative neuromonitoring’s (IONM) contribution to visual and functional identification of EBSLN, and the nerve’s contribution to the TA muscle function electrophysiologically.

Material and Methods: The prospective data of 50 consecutive (42F, 8M) patients, who underwent thyroid surgery with IONM for the exploration of EBSLN, were evaluated. The surface endotracheal tube-based Medtronic NIM3 (Medtronic, Jacksonwille, FL) IONM device was used. The function of EBSLN was evaluated by cricothyroid muscle twitch. Additionally, EBSLN’s contribution to the vocal cord adduction was evaluated by the electromyographic records.

Results: Fourty one patients were bilaterally and 9 were unilaterally intervented. Eighty four (92.3%) of 91 EBSLNs at risk (43 left, 48 right) were identified. Fourty four (52.4%) of 84 EBSLNs were identified visually, before being stimulated with the probe. Additionally, 18 (21.4%) EBSLNs were identified visually, after being identified with the probe. Although 22 (26.2%) EBSLNs were identified with the probe, they were not able to be visualized. The IONM provided significant contribution to visual (p<0.05) and functional (p<0.001) identification of EBSLN. Positive EMG responses were obtained from 44 (52.4%) of 84 identified EBSLNs. Unilateral or bilateral positive EMG responses were achieved from the TA muscles in 29 (70.7%) out of bilaterally intervented 41 patients, with the stimulation of the EBSLNs. Positive EMG waveforms of TA muscles were detected in 13 (44.8%) patients bilaterally and 16 (55.2%) patients unilaterally.

Conclusion: IONM is superior to the only visual identification, because IONM improves the visual and functional identification rate of the EBSLN. We suggest that the IONM should be used to diminish the risk of EBSLN injury in the superior pole dissection routinely. Although the EBSLN’s main function is to innervate the cricothyroid muscle, it also contributes to the vocal cord’s adductor function unilaterally or bilaterally, in two third of the patients. This innervation may be related to the vocal cord’s functional and positional variabilities in the patients with RLN paralyses after thyroidectomies.

Keywords: Cricothyroid muscle, human communicating nerve, intraoperative neuromonitoring, the external branch of the superior laryngeal nerve, thyroarytenoid muscle

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