Laryngeal Nerve Anatomy: Introduction, Vagus Nerve (Cranial Nerve X), Superior Laryngeal Nerve (2024)

The recurrent laryngeal nerve (RLN) is a branch of the vagus.

The course of the recurrent laryngeal nerves on the right and left sides is different:

The right RLN leaves the right vagus nerve as it crosses the right subclavian artery and loops posteriorly under the artery. The right RLN initially traverses at an angle towards the tracheoesophageal groove and then runs parallel to it.

The left RLN originates from the left vagus nerve as it crosses the aortic arch. It then passes posteriorly under the arch and the ligamentum arteriosum. The left RNL travels in a course that is parallel and close to the tracheoesophageal groove. The lower origin and consequently, a longer course of the left RLN makes it more vulnerable to injury than the right RLN.

In the neck, both nerves follow the same course and pass superiorly accompanied by the inferior thyroid artery. As it approaches the thyroid gland, the RLN may pass anterior or posterior to the inferior thyroid artery or between its branches. ^[6] The RLN on both sides pass deep to the lower border of the inferior constrictor muscle and enter the larynx posterior to the cricothyroid articulation.

The RLN supplies four intrinsic muscles of the larynx (Lateral cricoarytenoid, posterior cricorytenoid, transverse and oblique interarytenoid and thyroarytenoid) but not the cricothyroid muscle. The interarytenoid muscle, the only unpaired muscle of the larynx, receives innervation from both RLNs. Before entering the larynx, the RLN also sends branches to the inferior constrictor muscle and cricopharyngeus muscle.

The RLN supplies the mucosa of the vocal cord and the subglottis.

Extralaryngeal branching can take place at any point along the course of the nerve but is uncommon inferior to the level of the inferior thyroid artery. ^[7] Studies have shown that much variability to the extralaryngeal and intralaryngeal branching exists from person to person, as well as from side to side. ^{[8, 9, 10, 11]}

Traditionally, the extralaryngeal branches were described as functionally discrete fibers, separated into the anterior and posterior branches, where the anterior branches solely innervate the adductor muscles (thyroarytenoid, interarytenoid, and lateral cricoarytenoid), whereas the posterior branches innervate the abductor muscles (posterior cricoarytenoid). However, other studies have described no consistent functional pattern of branching of the anterior and posterior laryngeal branches. ^[12]

The RLN is at high risk of injury during thyroid surgeries. Although controversy still surrounds whether the identification of the RLN during thyroidectomy will affect the incidence of it is damage or not, most surgeons advocate identification and dissection of the nerve during the procedure to reduce the risk of injury. ^[7]

Several surgical landmarks have been proposed to identify the RLN during surgery, including relation of the nerve to inferior thyroid artery, relation to tracheoesophageal nerve, relation to Berry’s ligament, and relation to Zuckerkandl’s tubercle.

Relationship of RLN to inferior thyroid artery

The relationship between the RLN and inferior thyroid artery varies. ^{[13, 14]} Steinberg reported that the RLN ascends in the neck between the branches of the inferior thyroid artery in about 6.5% of individuals, posterior to the inferior thyroid artery in 61.5%, and anterior to the inferior thyroid artery in 32.5%. ^[13] On the right side, the nerve may be in any of three locations in relation to the artery. On the left side, it is more likely to lie posterior to the artery.

Relationship of RLN to Berry’s ligament

The RLN is often in close proximity to Berry's ligament, with most nerves found within 3 mm from Berry's ligament. ^[15] Some authors report that the RLN penetrates through the Berry’s ligament. ^[16]

Relationship of RLN and tracheoesophageal groove

The distal end of the RLN was identified along the tracheoesophageal groove. The nerve was identified more consistently at the cricothyroid articulation. Shindo et aldescribe that most of the right RLNs course between 15-45º when entering the cricothyroid joint, whereas most of the left RLNs course between 0-30º. ^[6] This difference is due to the more angled path the right RLN takes when ascending in the neck.

Shindo et al recorded the angle that the RLN forms with the tracheoesophagealgroove. ^[6] The most common course of the right and left RLN was type II (15°-30°). The next common course on the right side was type III (30°-45°) and on the left was type I (0°-15°).

Relationship of RLN and tubercle of Zuckerkandl

The tubercle of Zuckerkandl represents a thickening where the ultimobranchial body fuses into the median thyroid process and can be enlarged into a nodular process. When enlarged, it is a consistent landmark for the RLN because the nerve almost always courses medial and deep to it.

Anastomosis of RLN

The anastomosis of Galen (also called the ramus anastomoticus or Ansa of Galen) occurs within the framework of the larynx and is an anastomosis between the ipsilateral recurrent laryngeal nerve and the internal branch of the superior laryngeal nerve.Generally, the posterior branch of the RLN contributes to the anastomosis; however, the anterior branch can also contribute to the anastomosis.Traditionally, theanastomosis of Galen has been described to provide purely sensory and autonomic innervation. More recent studies have shown that the anastomosis may also contain motor fibers.

The "human communicating nerve" is an anastomosis between the external branch of the SLN and the distal RLN. Approximately 70% of human larynges have this anastomosis.The human communicating nerve may contain both sensory innervation to the larynx and motor innervation to the thyroarytenoid muscle.

Variants of RLN

Rarely (0.5% to 1% of individuals), in presence of aberrant right subclavian artery, arising from the aorta after the left subclavian artery has given off, the right RLN passes directly from the vagus nerve in the neck towards the larynx and does not recur around subclavian artery. This uncommon anatomic variation of the RLN makes it highly susceptible to surgical injury and is known as right “nonrecurrent” laryngeal nerve.

A left nonrecurrent laryngeal nerve is extremely rare.

The RLN may be in an abnormal location if a largegoiter exists, especially if the goiter extends into the substernal or retroesophageal spaces, or when neoplastic changes in the neck have occurred. The nerve can be displaced in any direction and, more commonly, be fixated or splayed to the posterior aspect of the thyroid capsule.