Anatomical study about the intra individual variability of the superior laryngeal nerve: What is the helpful surgical take-away?

The anatomical course of the external branch of the superior laryngeal nerve (EBSLN) is variable and the surgical importance of the EBSLN is due to the close relationship between the nerve and the superior thyroid vessels (STA). Intra individual variances, besides anatomical variances, could coexist between the left and right side. The goal of the anatomic study was to look for possible characteristics of the variability between sides to identify useful information for the surgeon. The data were collected from the dissection of 20 cadavers. As a result, 19 right EBSLN and 18 left EBSLN were found. Data were analyzed following Kierner’s classification. We looked for the external branch of the nerve, its crossing point with the STA and, in relation to the superior pole of the thyroidal gland, we measure the distance, in centimeters, to the right side and to the left side. From the collected and analyzed data, it is clear that the right-side EBSLN crosses in the majority of cases with the STA of a distance greater than 1 cm from the superior pole of the gland. Contrarily, the left-side EBSLN crosses with the STA just over the thyroidal gland or at a distance smaller than 1 cm from the superior pole. During a thyroidectomy, the superior laryngeal nerve is at risk of damage. Our study emphasized a greater risk of the left-side EBSLN in which, in almost 80% of cases, it crosses the STA at a distance smaller than 1 cm. Total thyroidectomy has been more frequently performed especially with its latest application also in nonmalignant thyroid gland disease.1 Among the many and most dangerous complications that could result from this practice are included iatrogenic injuries of nerves associated with phonation. Particular emphasis was put on the preservation of the inferior laryngeal nerve, given the dangerous outcomes that the aforementioned injuries could cause on vocal and respiratory functionality.2 Furthermore, injuries affecting the external branch of the superior laryngeal nerve (EBSLN) strongly jeopardize the quality of the voice by altering its timbre and, when fatigued and extensively used, reduce the patient’s capacity of producing certain sounds.3 For many years, the EBSLN reputation was diminished to the point that it was denominated by Lennquist et al.4 as “neglected nerve.” The EBSLN innervates the cricothyroid muscle and maintains strong anatomical relationships with the superior thyroid artery (STA) and the upper pole of the thyroid gland. The abovementioned relationships are in many ways classified according to the surrounding areas. Friedman suggests three anatomical variations of the EBSLN in relation to the inferior constrictor muscle of the pharynx5; conversely, Cernea ranks the EBSLN in relation to the STA according to the point where the nerve shifts from lateral to medial.6 Although the likelihood of detection varies from 10 to 93 per cent for many cases,7 the aim of these classifications is indeed to detect the nerve during a total thyroidectomy surgery to preserve it. A so-far-useful intraoperative monitoring of the nerve (IONM) was proposed to improve this percentage, even though it has not been standardized yet.8 Other anatomical studies highlight different variations for the nerve’s presence. According to Cernea, the Type 1, which is when the nerve crosses the STA more than 1 cm or slightly higher in relation to the upper pole of the thyroid gland, results in 68 per cent of the cases, while according to Kierner only in 42 per cent of the cases.9 At the same time, the percentage for the missed identification of the nerve ranges from a Cernea’s 7 per cent and a Friedman’s 20 per cent. Moreover, other studies present other variations characterized by the volume of the goiter altering the normal anatomical relationships, particularly when related to the upper pole.10 The preservation of the nerve is strictly linked to the knowledge of these different variations. This study shows side variability in the EBSLN course, with the aim of providing useful information for surgeons performing a total thyroidectomy surgery.