The recurrent laryngeal nerve (also called the ” lower laryngeal nerve “) is a branch of the vagus nerve (X cranial nerve, ie the tenth cranial nerve), which innervates all the intrinsic muscles of the larynx, with the exception of cricothyroid muscles (innervated by the superior laryngeal nerve). In the human organism the laryngeal nerve is equal, that is, there are two, one right and one left, both traveling parallel and alongside the trachea, however they are not symmetrical between them:
- the left recurrent laryngeal nerve surrounds the arch of the aorta;
- the right recurrent laryngeal nerve comes to surround the right subclavian artery, then moves upwards.
A characteristic of both nerves is their tendency to follow a recurrent course , that is, moving in the opposite direction to the nerve from which they depart (the vagus): the term “recurrent” derives from this characteristic. The recurrent laryngeal nerve innervates the muscles that are on the same side of the body , with the exception of the interarithenoid muscle which receives nerve fibers from both sides.
The left and right recurrent laryngeal nerves:
- innervate and allow the sensitivity of the larynx, below the vocal cords;
- damage nerve fibers (cardiac branches) to the deep cardiac plexus;
- they branch to the trachea, esophagus and lower constrictor muscle.
The recurrent laryngeal nerve controls all the intrinsic muscles of the larynx, with the exception of the cricothyroid muscle. Then innervate the following muscles:
- posterior cricoaritenoid muscle
- lateral cricoaritenoidea muscle
- transverse arytenoid muscle
- oblique arytenoid muscle
- thyroid arthenoid muscle.
These muscles act synergistically to open, close and regulate the tension of the vocal cords and include the posterior cricoaritenoid muscle, the only muscle with the function of opening the vocal cords. The recurrent laryngeal nerve also carries sensory information from the mucous membranes of the larynx , below the lower surface of the vocal cord, and sensory, secretory and motor fibers to the cervical segments of the esophagus and trachea.
Airway phonation and patency
The posterior cricoaritenoid muscles, the only muscles that can move the vocal cords, are innervated by this nerve. The correct functioning of the vocal cords allows phonation (i.e. the emission of voice) and ensures correct swallowing and patency of the airways , which is why an abnormal function of a rope (due to the injury of a recurrent nerve) causes dysphonia and the simultaneous malfunction of both vocal cords (due to the injury of both recurrent nerves) is a potentially fatal event due to the possibility of suffocation .
The vocal cords during breathing and phonation
The vagus nerve, from which the recurrent laryngeal nerves depart, leaves the skull at the jugular foramen and travels inside the carotid sheath, next to the carotid arteries, through the neck. The recurrent laryngeal nerves depart from the vagus, the left from the arch of the aorta and the right from the right subclavian artery. The left recurrent laryngeal nerve passes in front of the aortic arch and therefore surrounds it. After branching, the nerve typically goes up in a groove to the junction of the trachea and esophagus. It then runs posteriorly to the external lobes of the thyroid gland and enters the larynx, below the lower area of the constricting muscle of the pharynx, passing into the larynx just posteriorly to the cricothyroid joint. The terminal branch is called the lower laryngeal nerve. Unlike the other nerves that supply the larynx, the recurrent right and left laryngeal nerves lack bilateral symmetry. The left recurrent laryngeal nerve is longer than the right, because it crosses the arterial ligament under the arch of the aorta.
The somatic motor fibers that innervate the laryngeal and pharyngeal muscles are located in the ambiguous nucleus and emerge from the spinal cord in the cranial root of the accessory nerve. The fibers cross and join the vagus nerve in the jugular foramen. Sensory cell bodies are located in the lower jugular ganglion, and the fibers end in the solitary nucleus. The parasympathetic fibers to the trachea and esophagus segments in the neck originate in the dorsal nucleus of the vagus nerve.
In about 1 in 100-200 people, the right lower laryngeal nerve is not recurrent, in fact it branches out from the vagus nerve approximately at the level of the cricoid cartilage. Typically, this anatomical variant is also accompanied by other variants, and in particular in the arrangement of the main arteries in the thorax; more often, the right subclavian artery originates from the left side of the aorta and crosses the esophagus. A non-recurring lower laryngeal nerve, but on the left is even rarer, since it requires that the aortic arch be on the right side, and is associated with an arterial variant that prevents the nerve from being stretched in the thorax by the left subclavian artery. In about 4 out of 5 people, there is a branch connecting the lower laryngeal nerve (a branch of the recurrent laryngeal nerve) and the internal laryngeal nerve (a branch of the upper laryngeal nerve). This variant is commonly called Galen’s anastomosis or galeni loop and is one of several documented anastomoses between the two nerves. While the recurrent laryngeal nerve surrounds the subclavian artery or aorta, it emits several nerve branches. There are several variables in the configuration of these branches directed to the cardiac plexus, trachea, esophagus and the lower pharyngeal constrictor muscle.
Trauma of the recurrent laryngeal nerve
The recurrent laryngeal nerve can be injured as a result of trauma, during surgery, due to the local spread of a tumor or in other ways. An injury to the recurrent laryngeal nerve can cause a weakening of the voice (hoarseness), dysphonia , or complete loss of the voice (aphonia; sometimes respiratory tract problems are recorded. The nerve injury can paralyze the posterior cricoaritenoid muscle from the same side. This muscle it is solely responsible for opening the vocal cords and paralysis can cause difficulty in breathing ( dyspnoea), most often during physical activity. Injury to both recurrent left and right laryngeal nerves can result in more serious damage, such as an inability to speak. Further problems can emerge during healing, since the nerve fibers that re-anastomize, can cause alteration of the movement of the vocal cords and uncoordinated movements of the vocal cord.
The nerve receives a lot of attention from surgeons, because during neck surgery, especially the thyroid and parathyroid glands, the nerve is at high risk of injury. Nerve damage can be assessed by laryngoscopy: during this procedure an otolaryngologist using a light source (for example a stroboscopic light) confirms the absence of movement of the vocal cords on the affected side. The right recurrent laryngeal nerve is more susceptible to damage during thyroid surgery, because it is located very close to the bifurcation of the lower right thyroid artery, and runs variable in front, behind or between its branches. The nerve becomes permanently damaged in 0.3-3% of thyroid surgery; transient damage occurs in 3-8% of surgeries. This complication is a major cause of medico-legal problems for surgeons
The recurrent laryngeal nerve can be compressed by tumor masses. Studies have shown that 2-18% of lung cancer patients develop hoarseness due to compression of the laryngeal nerve, usually on the left. The appearance of hoarseness is associated with a worse prognosis and, if it turns out to be the presenting symptom, it often indicates inoperable tumors. The nerve can be intentionally interrupted during lung cancer surgery in order to completely remove the tumor mass.