Superior Laryngeal Nerve External Branch

ramus externus nervi laryngei superioris

The external branch of the superior laryngeal nerve (eSLN) descends alongside the superior thyroid artery to supply the cricothyroid muscle, the only laryngeal muscle supplied by the SLN rather than the recurrent laryngeal nerve. It also sends sensory fibres to the subglottic laryngeal mucosa. The cricothyroid is the principal tensor of the vocal fold for high-pitched phonation.

Region: Neck

Clinical Relevance

Clinical Notes

The eSLN is the nerve most commonly injured in thyroid surgery, producing a subtle but significant change in voice quality — specifically loss of high-frequency pitch control and vocal fatigue, detectable by laryngoscopy showing cricothyroid asymmetry. The Cernea classification describes the variable eSLN anatomy relative to the superior thyroid artery: Type 1 crosses above the upper pole vessels (safest); Type 2a crosses near the artery; Type 2b crosses below the upper pole level within 1 cm of the superior pole (highest risk). Individual vessel ligation rather than mass ligation of the upper pole protects the eSLN.

Pathology

Common Injuries & Conditions

External SLN Injury in Upper Thyroid Pole Ligation

Mass ligation of the superior thyroid vessels without identifying the external branch of the superior laryngeal nerve risks the eSLN passing between or below the vessels; injury produces cricothyroid paresis with loss of the highest vocal range and pitch fatigue particularly affecting professional singers and voice users.