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

Background: The variations in the anatomy of the external branch of the superior laryngeal nerve (EBSLN) are generally classified according to the relationship of the nerve to the superior thyroid artery, or the superior pole of the thyroid. Both artery and superior pole are themselves variable landmarks, and therefore are not consistent between subjects. We sought to examine EBSLN anatomy in relation to alternate, more consistent surgical landmarks.

Methods: Fifteen hemi-larynges from 20 embalmed human cadavers were dissected anatomically. Each nerve was categorized using established classification systems, and terminal branching patterns were also noted. Nerve location was recorded in relation to three different constant anatomical structures: the laryngeal prominence, midline junction of the cricothyroid muscles and ipsilateral cricothyroid joint.

Results: All cadavers were of European descent. The EBSLN had two branches to the cricothyroid muscle in 34% of cases. The EBSLN anatomical types found were mainly Cernea type 1 (80%), with 7% type 2a and 13% type Ni. An EBSLN was more likely to lie in an 'at risk' location if the subject was less than 160 cm tall. The EBSLN entered the crico-thyroid muscle at a median distance of 14 mm lateral from the laryngeal prominence and 8 mm inferiorly. The median distance from the medial-most point of the cricothyroid muscle junction was 14 mm laterally and 14 mm superiorly, and from the cricothyroid joint the entry lay a median distance of 10 mm superiorly and 11 mm medially.

Conclusions: The variability of EBSLN anatomy is again evident, as is the need for careful and knowledgeable surgical technique. New surgical landmarks for the relations of the insertion of the EBSLN reveal a deployment range for each, but one of more of these landmarks may prove useful in thyroid surgery.