Intraoperative Neuromonitoring of the External Branch of the Superior Laryngeal Nerve in Thyroid and Parathyroid Surgery

The external branch of the superior laryngeal nerve (EBSLN) innervates the cricothyroid muscle (CTM) and plays a major role in producing high-frequency voice. The EBSLN usually travels along with the superior thyroid vessels and crosses them above the superior thyroid pole. However, anatomical variants exist, and some EBSLNs are more intimately associated with the superior pole of the thyroid, placing the nerve at high risk for injury during thyroid and parathyroid surgeries. Several classification schemes have been introduced in order to categorize EBSLNs according to their anatomy and risk for injury, including the Cernea Classification scheme. As with the recurrent laryngeal nerve (RLN), visual identification of the nerve is the gold standard for preventing EBSLN injury. However, despite sparse published evidence, intraoperative neuromonitoring (IONM) has surfaced as a valuable adjunct to visual inspection in identifying and preserving the EBSLN, especially for the nerves that are not readily visible. This review discusses the significance of EBSLN, the current evidence for use of IONM, and the techniques for neuromonitoring of EBSLN.