Anatomical Variations of the External Branch of the Superior Laryngeal Nerve and its Correlations with the Superior Thyroid Artery and the Upper Pole of the Thyroid Gland: A Review of the Literature

Injury to the external branch of the superior laryngeal nerve (EBSLN) following thyroid or parathyroid surgery often results in devastating voice changes especially noticed by patients who use their voice professionally. The external branch arises from the vagus nerve which provides the motor supply to the cricothyroid muscle, an exclusive tensor of the vocal cords. The course of the EBSLN is subject to great anatomic variability. Damage to the nerve occurs during improper ligation of superior thyroid vessels close to the upper pole of thyroid gland in up to 58% of patients undergoing thyroid surgery. Thus, a thorough knowledge of the anatomy of the EBSLN and its variations, is essential requirement to minimize iatrogenic complications during surgery of the neck. Additionally, nerve monitoring methods have been developed to facilitate the localisation of the nerve, decreasing the possibility of nerve injury during thyroid surgery. The aim of this study is to analyze and review the anatomical variations of the EBSLN and its correlations with the superior thyroid artery and the upper pole of the thyroid gland. The literature search was performed during the period from February 1992 through November 2021. The position of the EBSLN was classified according to the Cernea and Kierner classification. Based on this classification, type 2a variant of EBSLN was the commonest followed by type 1. In addition, neuromonitoring allowed a higher identification rate than the visual identification.