Superior Laryngeal Nerve Block

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58 Superior Laryngeal Nerve Block

The superior laryngeal nerve is a branch of the vagus nerve (cranial nerve X). At some variable point in the neck, it divides into internal and external branches. The internal branch provides sensory innervation of the larynx above the vocal cords. The external branch innervates the cricothyroid muscle, which is an adductor tensor of the vocal cords.

The superior laryngeal nerve lies close to the superior laryngeal artery. The internal branch of the superior laryngeal nerve lies on average 2.4 mm inferior to the greater horn of the hyoid bone.¹ The hyoid bone is U shaped in transverse scans of the neck. The superior laryngeal nerve enters the larynx through an aperture (ostium) in the thyrohyoid membrane together with the superior laryngeal artery and vein.

Block of the superior laryngeal nerve is sometimes used to facilitate awake fiberoptic intubation or transesophageal echocardiography.

Suggested Technique

A hockey-stick transducer with a small footprint can be used for imaging. A 27-gauge, 1.25-inch hypodermic needle can be used for the block. The patient is instructed not to swallow because this causes movement of the hyoid bone. The nerve is superficial, so an out-of-plane approach can be used to guide infiltration adjacent to the hyoid bone.

Acoustic standoff can be used to improve imaging of the superficial nerve. It addition, the conforming gel dissipates the transducer pressure over a larger area. This improves patient tolerance of transducer placement over a sensitive area of the neck.

Clinical Pearls

• The external branch of the superior laryngeal nerve lies near the superior pole of the thyroid gland. It can be difficult to identify, even with direct dissection and nerve stimulation.²

• The internal branch of the superior laryngeal nerve lies immediately inferior and deep to the greater cornu of the hyoid bone.³ This branch is about 2 mm in diameter in this location. The internal branch travels 7 mm before piercing the thyrohyoid membrane to supply sensory innervation to the larynx above the vocal cords.

• The superior laryngeal nerve lies superior to the superior laryngeal artery.⁴ The superior laryngeal nerve descends to join the superior laryngeal artery below the greater cornu (inferior horn) of the hyoid bone. Ultrasound can be used to guide injection of local anesthetic above the superior laryngeal artery.

• One concern is that aspiration may occur at any time throughout the duration of superior laryngeal nerve block.⁵ This block is best performed in patients who are nothing per mouth (NPO).

• Block assessment is performed by testing phonation (the external motor branch of the superior laryngeal nerve to the cricothyroid muscle) or by fiberoptic examination. If the external superior laryngeal nerve is blocked, the vocal cords will not be closed during fiberoptic intubation.

• The hyoid bone, and in particular the greater cornua, can be difficult to visualize with ultrasound imaging.

FIGURE 58-1 Nerves of the larynx.

(From netterimages.com, with permission.)

FIGURE 58-2 External photograph showing an approach to superior laryngeal nerve block with ultrasound imaging. An out-of-plane approach from the lateral aspect of the neck is shown (A). Corresponding sonograms for a short-axis view of the hyoid bone (B), at the level of the lesser cornua (C), and view with a broad linear probe (D). The hyoid bone can give a triangular acoustic shadow when viewed in short axis that can help identify its location. The hyoid bone has two horns, the lesser (superior) cornua and the greater (inferior) cornua. The superior cornua of the hyoid bone is highly echogenic. Despite its name, the greater (longer) horn of the hyoid bone is difficult to image because the hyoid bone narrows substantially in the posterior direction.

FIGURE 58-3 Long-axis view of the hyoid bone (A) shown with a sloping short-axis view of the lesser cornua (B). The hyoid is a small U-shaped bone as seen in this long-axis view. The internal branch of the superior laryngeal nerve lies immediately inferior and deep to the greater cornua of the hyoid bone.

FIGURE 58-4 Power Doppler imaging of the superior laryngeal artery as it descends into the larynx (A). The superior laryngeal nerve lies superior to the superior laryngeal artery. Sonogram showing the origin of the superior thyroid artery from the external carotid (B). The superior thyroid artery is the first branch of the external carotid artery. It gives rise to the superior laryngeal artery.

1 Furlan JC. Anatomical study applied to anesthetic block technique of the superior laryngeal nerve. Acta Anaesthesiol Scand. 2002;46:199–202.

2 Page C, Laude M, Legars D, et al. The external laryngeal nerve: surgical and anatomic considerations. Report of 50 total thyroidectomies. Surg Radiol Anat. 2004;26:182–185.

3 Stephens RE, Wendel KH, Addington WR. Anatomy of the internal branch of the superior laryngeal nerve. Clin Anat. 1999;12:79–83.

4 Furlan JC, Brandão LG, Ferraz AR, et al. Surgical anatomy of the extralaryngeal aspect of the superior laryngeal nerve. Arch Otolaryngol Head Neck Surg. 2003;129:79–82.

5 McLoughlin AL. Superior laryngeal nerve block as a supplement to total intravenous anesthesia for rigid laser bronchoscopy in a patient with myasthenic syndrome: risk of aspiration? Anesth Analg. 1993;76:903–904.

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