Surgical anatomy of the external branch of the superior laryngeal nerve.

Palsy of the external branch of the superior laryngeal nerve (EBSLN) during thyroid surgery is an important complication reported with varying frequency. This study was carried out to investigate the relationship between the EBSLN, the upper part of the thyroid gland and the inferior constrictor muscle of the pharynx (IC), and also to define consistent landmarks for identifying and preserving the EBSLN. Forty neck halves of 20 cadavers were dissected. Measurements were obtained between the crossing point of the EBSLN with the superior thyroid artery (STA) and the upper pole of the thyroid gland, the point where EBSLN penetrates the IC and the inferior thyroid tubercle, and the middle point of the oblique line of the thyroid cartilage, and the EBSLN. In 22.5%, the EBSLN crossed the STA more than 1 cm above the upper pole of the thyroid gland (Type I of Cernea et al. [1992a] Head Neck 14:380-383). In 60%, the EBSLN crossed the STA less than 1 cm above the upper pole of the thyroid gland (Type IIa of Cernea et al. [1992a] Head Neck 14:380-383). In 17.5%, the EBSLN crossed the STA under the upper pole of the thyroid gland (Type IIb of Cernea et al. [1992a], Head Neck 14:380-383). In 22.5%, the full course of the nerve was superficial to the IC (Type 1 of Friedman et al. [2002] Arch Otolaryngol Head Neck Surg 128:296-303). In 67.5%, the nerve penetrated the IC (Type 2 of Friedman et al. [2002] Arch Otolaryngol Head Neck Surg 128:296-303). In 10%, the nerve could not be identified at the lateral side of the IC (Type 3 of Friedman et al. [2002] Arch Otolaryngol Head Neck Surg 128:296-303). In conclusion, it is possible to identify the nerve superficial to the IC in 90% of specimens on average. Knowledge of the relationship between the EBSLN, IC, inferior thyroid tubercle, oblique line of the thyroid cartilage and the sternothyroid muscle will be useful for the surgeon in avoiding unexpected complications.