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Mapping the Compound Muscle Action Potentials of Cricothyroid Muscle Using Electromyography in Thyroid Operations: A Novel Method to Clinically Type the External Branch of the Superior Laryngeal Nerve

Objective:Variations of the external branch of the superior laryngeal nerve (EBSLN) are well represented in literature but with conflicting reports about the types and variations. Using EMG, the cricothyroid compound muscle action potential (CMAP) was recorded, to identify the EBSLN and classified them according to clinical variation during routine thyroid operations. Summary Background Data:There is increasing awareness among patients and physicians of the consequences of injury to the external laryngeal nerve in thyroid surgeries. There are at least 4 types, which are described in the literature.1 The gold standard of nerve identification is EMG, which we used during operations in this study to identify all the EBSLN by demonstrating CMAP and reclassify them. Based on this principle, to avoid injury during thyroidectomy, we propose a new classification. Methods:This study was a prospective, descriptive study of the EBSLN done during elective thyroidectomy operations. CMAP was demonstrated and EMG was plotted to identify and type them according to clinical variations. The distance between the entry of superior thyroid vessels (STVs) to the lobe and the EBSLN, relationship with the cricothyroid muscle and inferior constrictor muscle were taken into consideration. Based on the possible risk to this during thyroidectomy, we divided them into 4 types. Results:A total of 70 external laryngeal nerves were dissected and confirmed by the CMAPs in 35 total thyroidectomy patients. Using CMAPs, all the nerves were identified. Ia.The nerve can be found within 1 cm from the entry of the vessels into the gland either anterior or intertwined to the STVs or within 3 cm from the cricoid cartilage. Ib.The nerve can be found posterior to the vessels but within 1 cm from the entry of the STV into the gland. This entry point was close to the anterior insertion line of the CT muscle onto cricoid cartilage. II.Nerve can be found within 1 to 3 cm from the entry of the vessels into the gland or within 3 to 5 cm from the cricoid cartilage. III.Nerve can be found between 3 and 5 cm from the entry of the vessels into the gland or more than 5 cm from the cricoid cartilage. Conclusion:New clinical typing of the EBSLN will give a better understanding of the EBSLN in the intraoperative period. EBSLN need not be identified in routine thyroidectomy operations. Even in large goiters, the incidence of nerve at risk does not increase significantly. Injury to EBSLN can be avoided if the principle of ligating the STVs individually away from the CT muscle and close to the capsule of the gland is followed.