The external branch of the superior laryngeal nerve is the smaller motor branch that descends on the outer surface of the inferior pharyngeal constrictor to supply the cricothyroid muscle, the only intrinsic laryngeal muscle not supplied by the recurrent laryngeal nerve. The cricothyroid is the tensor of the vocal folds and its paralysis reduces high-pitched voice and vocal fold elongation. The nerve runs close to the superior thyroid artery pedicle.
| Origin | Superior laryngeal nerve (CN X) |
|---|---|
| Course | Descends on the inferior pharyngeal constrictor close to the superior thyroid artery and passes deep to the upper pole of the thyroid gland to the cricothyroid |
| Motor |
The external branch is at risk during thyroidectomy when the superior thyroid pedicle is tied near the upper pole. The Cernea classification grades this risk: type 1 crosses the superior thyroid vessels more than 1 cm above the upper pole (lowest risk), type 2a crosses within 1 cm above the pole, and type 2b crosses below the upper border of the pole (highest risk). Injury causes a subtle but important voice change, with loss of high notes, reduced range and vocal fatigue, which matters most to singers and professional voice users. The nerve is protected by ligating the superior thyroid vessel branches individually and close to the thyroid capsule rather than by mass ligation of the pedicle.
Ligation of the superior thyroid pedicle close to the upper thyroid pole risks including the external SLN with the artery, producing unilateral cricothyroid paralysis with loss of high-pitched phonation, reduced vocal range, and voice fatigue that is particularly debilitating for professional voice users; protected by meticulous superior pedicle dissection at the thyroid capsule.
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