Recurrent laryngeal nerve

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Recurrent laryngeal nerve
Recurrent laryngeal nerve.svg
Course of the left recurrent laryngeal nerve
Gray622.png
Posterior view of tracheal and bronchial lymph glands, with the left and right recurrent nerves visible on either side.
Details
From Vagus nerve
Innervates Larynx
posterior cricoarytenoid
lateral cricoarytenoid
arytenoid
thyroarytenoid
aryepiglottis
esophagus
heart
trachea
inferior pharyngeal constrictor muscle
Identifiers
Latin nervus laryngeus recurrens
MeSH D012009
TA98 A14.2.01.166
TA2 6344
FMA 6246
Anatomical terms of neuroanatomy

The recurrent laryngeal nerve (RLN) is a branch of the vagus nerve (cranial nerve X) that supplies all the intrinsic muscles of the larynx, with the exception of the cricothyroid muscles. There are two recurrent laryngeal nerves, right and left. The right and left nerves are not symmetrical, with the left nerve looping under the aortic arch, and the right nerve looping under the right subclavian artery, then traveling upwards. They both travel alongside the trachea. Additionally, the nerves are among the few nerves that follow a recurrent course, moving in the opposite direction to the nerve they branch from, a fact from which they gain their name.

Contents

The recurrent laryngeal nerves supply sensation to the larynx below the vocal cords, give cardiac branches to the deep cardiac plexus, and branch to the trachea, esophagus and the inferior constrictor muscles. The posterior cricoarytenoid muscles, the only muscles that can open the vocal folds, are innervated by this nerve.

The recurrent laryngeal nerves are the nerves of the sixth pharyngeal arch. The existence of the recurrent laryngeal nerve was first documented by the physician Galen.

Structure

Passing under the subclavian artery, the right recurrent laryngeal nerve has a much shorter course than the left which passes under the aortic arch and ligamentum arteriosum. Gray505.png
Passing under the subclavian artery, the right recurrent laryngeal nerve has a much shorter course than the left which passes under the aortic arch and ligamentum arteriosum.

The recurrent laryngeal nerves branch from the vagus nerve, relative to which they get their names; the term "recurrent" from Latin : re- (back) and currere (to run), [1] indicates they run in the opposite direction to the vagus nerves from which they branch. [2] The vagus nerves run down into the thorax, and the recurrent laryngeal nerves run up to the larynx. [3] : 930–931 

The vagus nerves, from which the recurrent laryngeal nerves branch, exit the skull at the jugular foramen and travel within the carotid sheath alongside the carotid arteries through the neck. The recurrent laryngeal nerves branch off the vagus, the left at the aortic arch, and the right at the right subclavian artery. The left RLN passes in front of the arch, and then wraps underneath and behind it. After branching, the nerves typically ascend in a groove at the junction of the trachea and esophagus. [4] : 1346–1347  They then pass behind the posterior, middle part of the outer lobes of the thyroid gland and enter the larynx underneath the inferior constrictor muscle, [3] : 918  passing into the larynx just posterior to the cricothyroid joint. [5] The terminal branch is called the inferior laryngeal nerve. [6] : 19 

Unlike the other nerves supplying the larynx, the right and left RLNs lack bilateral symmetry. [7] The left RLN is longer than the right, because it crosses under the arch of the aorta at the ligamentum arteriosum. [4] : 1346–1347 

Nucleus

The somatic motor fibers that innervate the laryngeal muscles, and pharyngeal muscles are located in the nucleus ambiguus and emerge from the medulla in the cranial root of the accessory nerve. Fibers cross over to and join the vagus nerve in the jugular foramen. [8] : 86–88  Sensory cell bodies are located in the inferior jugular ganglion, [9] and the fibers terminate in the solitary nucleus. [8] : 86–88  Parasympathetic fibers to segments of the trachea and esophagus in the neck originate in the dorsal nucleus of the vagus nerve. [9]

Development

During human and all vertebrate development, a series of pharyngeal arch pairs form in the developing embryo. These project forward from the back of the embryo towards the front of the face and neck. Each arch develops its own artery, nerve that controls a distinct muscle group, and skeletal tissue. The arches are numbered from 1 to 6, with 1 being the arch closest to the head of the embryo, and the fifth arch only existing transiently. [10] : 318–323 

Arches 4 and 6 produce the laryngeal cartilages. The nerve of the sixth arch becomes the recurrent laryngeal nerve. The nerve of the fourth arch gives rise to the superior laryngeal nerve. The arteries of the fourth arch, which project between the nerves of the fourth and sixth arches, become the left-sided arch of the aorta and the right subclavian artery. The arteries of the sixth arch persist as the ductus arteriosus on the left, and are obliterated on the right. [10] : 318–323 

After birth, the ductus arteriosus regresses to form the ligamentum arteriosum. During growth, these arteries descend into their ultimate positions in the chest, creating the elongated recurrent paths. [10] : 318–323 

Variation

In roughly 1 out of every 100–200 people, the right inferior laryngeal nerve is nonrecurrent, branching off the vagus nerve around the level of the cricoid cartilage. Typically, such a configuration is accompanied by variation in the arrangement of the major arteries in the chest; most commonly, the right subclavian artery arises from the left side of the aorta and crosses behind the esophagus. A left nonrecurrent inferior laryngeal nerve is even more uncommon, requiring the aortic arch be on the right side, accompanied by an arterial variant which prevents the nerve from being drawn into the chest by the left subclavian. [11] : 10, 48 

In about four people out of five, there is a connecting branch between the inferior laryngeal nerve, a branch of the RLN, and the internal laryngeal nerve, a branch of the superior laryngeal nerve. This is commonly called the anastomosis of Galen (Latin : ansa galeni), even though anastomosis usually refers to a blood vessel, [12] [13] : 35  and is one of several documented anastomoses between the two nerves. [14]

As the recurrent nerve hooks around the subclavian artery or aorta, it gives off several branches. There is suspected variability in the configuration of these branches to the cardiac plexus, trachea, esophagus and inferior pharyngeal constrictor muscle. [15]

Function

The recurrent laryngeal nerves control all intrinsic muscles of the larynx except for the cricothyroid muscle. [15] [a] These muscles act to open, close, and adjust the tension of the vocal cords, and include the posterior cricoarytenoid muscles, the only muscle to open the vocal cords. [16] : 10–11  The nerves supply muscles on the same side of the body, with the exception of the interarytenoid muscle, which is innervated from both sides. [15]

The nerves also carry sensory information from the mucous membranes of the larynx below the lower surface of the vocal fold, [17] : 847–9  as well as sensory, secretory and motor fibres to the cervical segments of the esophagus and the trachea. [8] : 142–144 

Clinical significance

Injury

Recurrent laryngeal nerve visible during resection of a goitre Rekurrens.png
Recurrent laryngeal nerve visible during resection of a goitre

The recurrent laryngeal nerves may be injured as a result of trauma, during surgery, as a result of tumour spread, or due to other means. [16] : 12  Injury to the recurrent laryngeal nerves can result in a weakened voice (hoarseness) or loss of voice (aphonia) and cause problems in the respiratory tract. [18] [16] : 11–12  Injury to the nerve may paralyze the posterior cricoarytenoid muscle on the same side. This is the sole muscle responsible for opening the vocal cords, and paralysis may cause difficulty breathing (dyspnea) during physical activity. [19] Injury to both the right and left nerve may result in more serious damage, such as the inability to speak. Additional problems may emerge during healing, as nerve fibres that re-anastamose may result in vocal cord motion impairment, uncoordinated movements of the vocal cord. [16] : 12–13 

Surgery

The nerve receives close attention from surgeons because the nerve is at risk for injury during neck surgery, especially thyroid and parathyroid surgery; as well as esophagectomy. [20] [4] Nerve damage can be assessed by laryngoscopy, during which a stroboscopic light confirms the absence of movement in the affected side of the vocal cords. The right recurrent laryngeal nerve is more susceptible to damage during thyroid surgery because it is close to the bifurcation of the right inferior thyroid artery, variably passing in front of, behind, or between the branches. [17] : 820–1  Similarly, thermal injury can occur with the use of radio frequency ablation to remove thyroid nodules. [21] [22] [23] The nerve is permanently damaged in 0.3–3% of thyroid surgeries, and transient paralysis occurs in 3–8% of surgeries; accordingly, recurrent laryngeal nerve damage is one of the leading causes of medicolegal issues for surgeons. [24] A 2019 systematic review concluded that the available evidence shows no difference between visually identifying the nerve or utilizing intraoperative neuroimaging during surgery, when trying to prevent injury to recurrent laryngeal nerve during surgery. [25]

Tumors

The RLN may be compressed by tumors. Studies have shown that 2–18% of lung cancer patients develop hoarseness because of recurrent laryngeal nerve compression, usually left-sided. [26] This is associated with worse outcomes, and when found as a presenting symptom, often indicates inoperable tumors. The nerve may be severed intentionally during lung cancer surgery in order to fully remove a tumor. [27] : 330  The RLN may also be damaged by tumors in the neck, especially with malignant lymph nodes with extra-capsular extension of tumor beyond the capsule of the nodes, which may invade the area that carries the ascending nerve on the right or left.

Other disease

In Ortner's syndrome or cardiovocal syndrome, a rare cause of left recurrent laryngeal nerve palsy, expansion of structures within the heart or major blood vessels impinges upon the nerve, causing symptoms of unilateral nerve injury. [28]

Other animals

Horses are subject to equine recurrent laryngeal neuropathy, a disease of the axons of the recurrent laryngeal nerves. The cause is not known, although a genetic predisposition is suspected. The length of the nerve is a factor since it is more common in larger horses, and the left side is affected almost exclusively. As the nerve cells die, there is a progressive paralysis of the larynx, causing the airway to collapse. The common presentation is a sound, ranging from a musical whistle to a harsh roar or heaving gasping noise (stertorous), accompanied by worsening performance. The condition is incurable, but surgery can keep the airway open. Experiments with nerve grafts have been tried. [29] : 421–426 

Although uncommon in dogs, bilateral recurrent laryngeal nerve disease may be the cause of wheezing (stridor) when middle-aged dogs inhale. [30] : 771 

In sauropod dinosaurs, the vertebrates with the longest necks, the total length of the vagus nerve and recurrent laryngeal nerve would have been up to 28 metres (92 ft) long in Supersaurus , but these would not be the longest neurons that ever existed: the neurons reaching the tip of the tail would have exceeded 30 metres (98 ft). [31]

Evidence of evolution

The extreme detour of the recurrent laryngeal nerves, about 4.6 metres (15 ft) in the case of giraffes, [32] : 74–75  is cited as evidence of evolution, as opposed to intelligent design. The nerve's route would have been direct in the fish-like ancestors of modern tetrapods, traveling from the brain, past the heart, to the gills (as it does in modern fish). Over the course of evolution, as the neck extended and the heart became lower in the body, the laryngeal nerve remained in its original course.

History

Ancient Greek physician Galen demonstrated the nerve course and the clinical syndrome of recurrent laryngeal nerve paralysis, noting that pigs with the nerve severed were unable to squeal. Galen named the nerve the recurrent nerve, and described the same effect in two human infants who had undergone surgery for goiter. [16] : 7–8  [33] In 1838, five years before he would introduce the concept of homology to biology, anatomist Richard Owen reported upon the dissection of three giraffes, including a description of the full course of the left recurrent laryngeal nerve. [34] [35] Anatomists Andreas Vesalius and Thomas Willis described the nerve in what is now regarded as an anatomically standard description, and doctor Frank Lahey documented a way for its interoperative identification during thyroid operations. [36]

Notes

Related Research Articles

<span class="mw-page-title-main">Vagus nerve</span> Main nerve of the parasympathetic nervous system

The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that carries sensory and motor fibers. It creates a pathway that interfaces with the parasympathetic control of the heart, lungs, and digestive tract.

<span class="mw-page-title-main">Larynx</span> Voice box, an organ in the neck of amphibians, reptiles, and mammals

The larynx, commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about 4–5 centimeters in diameter. The larynx houses the vocal cords, and manipulates pitch and volume, which is essential for phonation. It is situated just below where the tract of the pharynx splits into the trachea and the esophagus. The word 'larynx' comes from the Ancient Greek word lárunx ʻlarynx, gullet, throatʼ.

<span class="mw-page-title-main">Trachea</span> Cartilaginous tube that connects the pharynx and larynx to the lungs

The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all animals lungs. The trachea extends from the larynx and branches into the two primary bronchi. At the top of the trachea, the cricoid cartilage attaches it to the larynx. The trachea is formed by a number of horseshoe-shaped rings, joined together vertically by overlying ligaments, and by the trachealis muscle at their ends. The epiglottis closes the opening to the larynx during swallowing.

<span class="mw-page-title-main">Subclavian artery</span> Major arteries of the upper thorax, below the clavicle

In human anatomy, the subclavian arteries are paired major arteries of the upper thorax, below the clavicle. They receive blood from the aortic arch. The left subclavian artery supplies blood to the left arm and the right subclavian artery supplies blood to the right arm, with some branches supplying the head and thorax. On the left side of the body, the subclavian comes directly off the aortic arch, while on the right side it arises from the relatively short brachiocephalic artery when it bifurcates into the subclavian and the right common carotid artery.

<span class="mw-page-title-main">Posterior cricoarytenoid muscle</span> Muscle of the larynx

The posterior cricoarytenoid muscle is a intrinsic muscle of the larynx. It arises from the cricoid cartilage; it inserts onto the arytenoid cartilage of the same side. It is innervated by the recurrent laryngeal nerve. Each acts to open the vocal folds by pulling the vocal fold of the same side laterally. It participates in the production of sounds.

<span class="mw-page-title-main">Common carotid artery</span> One of the two arteries that supply the head and neck with blood

In anatomy, the left and right common carotid arteries (carotids) are arteries that supply the head and neck with oxygenated blood; they divide in the neck to form the external and internal carotid arteries.

<span class="mw-page-title-main">Superior thoracic aperture</span> Opening at the top of the thoracic cavity (area enclosed by the ribcage)

The superior thoracic aperture, also known as the thoracic outlet, or thoracic inlet refers to the opening at the top of the thoracic cavity. It is also clinically referred to as the thoracic outlet, in the case of thoracic outlet syndrome. A lower thoracic opening is the inferior thoracic aperture.

<span class="mw-page-title-main">Inferior pharyngeal constrictor muscle</span> Skeletal muscle of the pharynx

The inferior pharyngeal constrictor muscle is a skeletal muscle of the neck. It is the thickest of the three outer pharyngeal muscles. It arises from the sides of the cricoid cartilage and the thyroid cartilage. It is supplied by the vagus nerve. It is active during swallowing, and partially during breathing and speech. It may be affected by Zenker's diverticulum.

<span class="mw-page-title-main">Stylopharyngeus muscle</span> Muscle

The stylopharyngeus muscle is a muscle in the head. It originates from the temporal styloid process. Some of its fibres insert onto the thyroid cartilage, while others end by intermingling with proximal structures. It is innervated by the glossopharyngeal nerve. It acts to elevate the larynx and pharynx, and dilate the pharynx, thus facilitating swallowing.

<span class="mw-page-title-main">Pharyngeal arch</span> Embryonic precursor structures in vertebrates

The pharyngeal arches, also known as visceral arches, are transient structures seen in the embryonic development of humans and other vertebrates, that are recognisable precursors for many structures. In fish, the arches support the gills and are known as the branchial arches, or gill arches.

<span class="mw-page-title-main">Aortic arch</span> Part of the aorta

The aortic arch, arch of the aorta, or transverse aortic arch is the part of the aorta between the ascending and descending aorta. The arch travels backward, so that it ultimately runs to the left of the trachea.

<span class="mw-page-title-main">Arytenoid muscle</span> Muscle of the larynx

The arytenoid muscle or interarytenoid muscle is a composite intrinsic muscle of the larynx, consisting of a transverse part and an oblique part - the two parts may be considered as separate muscles: an unpaired transverse arytenoid muscle, and a bilaterally paired oblique arytenoid muscle.

<span class="mw-page-title-main">Superior thyroid artery</span> Blood vessel

The superior thyroid artery arises from the external carotid artery just below the level of the greater cornu of the hyoid bone and ends in the thyroid gland.

<span class="mw-page-title-main">Inferior thyroid artery</span> Artery of the neck

The inferior thyroid artery is an artery in the neck. It arises from the thyrocervical trunk and passes upward, in front of the vertebral artery and longus colli muscle. It then turns medially behind the carotid sheath and its contents, and also behind the sympathetic trunk, the middle cervical ganglion resting upon the vessel.

<span class="mw-page-title-main">Superior laryngeal nerve</span> Branch of the vagus nerve

The superior laryngeal nerve is a branch of the vagus nerve. It arises from the middle of the inferior ganglion of the vagus nerve and additionally receives a sympathetic branch from the superior cervical ganglion.

<span class="mw-page-title-main">Muscular triangle</span>

The inferior carotid triangle, is bounded, in front, by the median line of the neck from the hyoid bone to the sternum; behind, by the anterior margin of the sternocleidomastoid; above, by the superior belly of the omohyoid.

<span class="mw-page-title-main">Superior cardiac nerve</span>

The superior cardiac nerve arises by two or more branches from the superior cervical ganglion, and occasionally receives a filament from the trunk between the first and second cervical ganglia. It runs down the neck behind the common carotid artery, and in front of the Longus colli muscle; and crosses in front of the inferior thyroid artery, and recurrent nerve. The course of the nerves on the two sides then differs.

Vocal cord paresis, also known as recurrent laryngeal nerve paralysis or vocal fold paralysis, is an injury to one or both recurrent laryngeal nerves (RLNs), which control all intrinsic muscles of the larynx except for the cricothyroid muscle. The RLN is important for speaking, breathing and swallowing.

<span class="mw-page-title-main">Pharynx</span> Part of the throat that is behind the mouth and nasal cavity

The pharynx is the part of the throat behind the mouth and nasal cavity, and above the esophagus and trachea. It is found in vertebrates and invertebrates, though its structure varies across species. The pharynx carries food to the esophagus and air to the larynx. The flap of cartilage called the epiglottis stops food from entering the larynx.

The cardiac branches of the vagus nerve are two sets of nerves found in the upper torso, in close proximity to the larynx. The specific branches are the cervical cardiac branches of vagus nerve and the thoracic cardiac branches of vagus nerve.

References

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