The Anatomy of the Recurrent Laryngeal Nerve

This duo of nerves is critical for speech

In This Article

The recurrent laryngeal nerve (RLN) branches off the vagus nerve and supplies function to some muscles of the larynx (voice box). You have two, one on each side, as you do with other nerves. However, the two RLNs are different from other nerves in that their courses are not symmetrical. The RLN plays an important role in your ability to speak and damage to it can result in speech problems.

Anatomy

A major feature of nerves is that they branch off in order to connect with different structures all over your body. While most of your nerves branch off from the spinal cord, the 12 cranial nerves originate in your brain.

The vagus nerve is the 10th cranial nerve. It emerges from the brain, exits the skull, then runs down your neck along the carotid arteries.

The vagus nerve deals with many of the body's automatic functions, playing a role in the function of the heart, lungs, and digestive system. Through its branches, it performs many other functions, as well. The branches of the vagus nerve include several nerves that are involved in speech, including:

  • Pharyngeal branches, which provide motor function to the soft palate and part of the throat
  • Superior laryngeal nerve, which innervates parts of the throat and larynx, including the cricothyroid muscles
  • Recurrent laryngeal nerve, which supplies nerve function to all of the muscles that are intrinsic to (contained fully within) the larynx with the exception of the cricothyroid muscles

"Recurrent" is part of the name because of the RLN's unusual course. Rather than running in the same direction as the vagus nerve as it descends through your neck and abdomen, the RLN curves to run in the opposite direction—back up your neck. Not many nerves do this, which is why it's worth noting it in the name.

Additionally, the RLN is unusual because the left and right nerves follow different courses from each other, while most nerves follow the same path on each side.

Structure

After the RLN branches off from the vagus nerve, it continues branching out. Its major branches are:

  • Inferior laryngeal branch, which serves most of the intrinsic muscles of the larynx
  • Visceral sensory fibers from the area below the larynx
  • Motor branches to some of the muscles in the throat

It also has myriad smaller branches all along its course.

Location

The right and left RLN have non-symmetrical courses because they emerge from the vagus nerve near the heart, which is off to the left of your chest rather than centered.

The left RLN splits off just above the heart, near the arch of the aorta (an artery). It passes down in front of the aorta, then loops around underneath and behind it. The right RLN branches off at the right subclavian artery and then makes a loop around it before returning to the throat. However, the subclavian artery is slightly higher and significantly thinner than the aorta, so the right nerve doesn't have to descend nearly as far into the chest. This makes the left RLN significantly longer than the right RLN.

This feature has sometimes been called "evidence of poor design" by scientists who study evolution because the left RLN is seven times longer than it would be if it traveled a direct course from the head to the neck.

On the other hand, some scientists point out that the RLN supplies numerous autonomic and sensory nerves as it travels back up toward the larynx, so the unusual U-turn, therefore, serves an important function.

Once it turns back upward, the RLN travels through a groove where the trachea (windpipe) and esophagus meet, pass behind part of the thyroid gland, then enter the larynx underneath a throat muscle called the inferior constrictor.

Anatomical Variations

In some people, one side of the RLN isn't actually recurrent.

This variation is more common with the right RLN. Instead of branching off down near the heart, it leaves the vagus nerve around the cricoid ring of the trachea, which is situated just below the larynx.

This variation is believed to be present in between 0.5% and 1% of people. Most of the time, this variation in RLN course goes along with a variation in how the major arteries in the chest are arranged.

Less often, the aortic arch is on the right side of the chest instead of the left, so the left RLN has a direct course rather than a recurrent one.

Experts suspect there's some variability in the course and configuration of some of the branches of the RLN, including those that go to the trachea, esophagus, cardiac plexus, and inferior pharyngeal constrictor muscle.

Function

The recurrent laryngeal nerve serves both motor and sensory functions. "Motor" has to do with movement and "sensory" deals with the senses, such as touch and temperature information.

Motor Function

The intrinsic muscles of the larynx that the RLN innervates (supplies nerve function to) are responsible for opening, closing, and changing the tension of your vocal cords. This includes the:

  • Posterior cricoarytenoid muscles, which are the sole muscles to open the vocal cords
  • Interarytenoid muscle, which, unlike most muscles, is innervated from both sides instead of by either the right or left nerve

Without the RLN and the muscles it serves, you wouldn't be able to speak. The RLN also sends motor and secretory fibers to the segments of the esophagus and the trachea that are in the throat, where they play a role in swallowing and secreting saliva.

Sensory Function

The RLN carries sensory information to the brain from mucous membranes that lie beneath the lower surface of the larynx's vocal fold. It also sends sensory fibers, along with the motor and secretory fibers, to the esophagus and trachea.

Associated Conditions

Problems with the RLN can be caused by:

  • Injury
  • Surgery
  • Tumors
  • Disease

Injury

Trauma to the throat or anywhere along its course can cause damage to the RLN. Injury can lead to:

  • Dysphonia (weakened or hoarse voice)
  • Aphonia (loss of voice)
  • Respiratory tract dysfunction
  • Paralysis of the posterior cricoarytenoid muscle on the same side as the damaged nerve

Because the posterior cricoarytenoid muscle acts alone to open the vocal cords, severe damage or damage to both sides of the RLN may result in a complete loss of the ability to speak. It can also cause breathing problems during physical activity, which is called dyspnea.

As the RLN heals from injury, you may experience uncoordinated movements of the vocal cord that will go away with further healing.

Surgery

The most common surgeries to damage the RLN are thyroid and parathyroid surgery, because of how close the nerve is to the thyroid gland, which sits at the front of the throat. In fact, the RLN can pass in front of, behind, or between branches of the right inferior thyroid artery.

This problem is rare, though, with permanent damage occurring in less than 3% of thyroid surgeries. Even so, because the impairment or loss of speech has a significant impact on your life, it's one of the leading causes of lawsuits against surgeons.

RLN damage can be assessed by laryngoscopy, in which a special light confirms that there's no movement in the vocal cords on the damaged side, or by electromyography (EMG), a test that looks at nerve function.

Symptoms of RLN damage due to thyroid surgery depend on whether the damage is confined to one side or impacts both sides.

When one side is damaged, the voice may be normal right after surgery and then change over the next several days or even weeks to become hoarse or breathy. That's because the paralyzed vocal fold starts out in a somewhat normal position but then atrophies over time. This can also cause:

  • Loss of voice
  • Inability to raise the volume of your voice
  • Choking and sputtering while drinking
  • Difficulty drawing a breath

With bilateral (both side) paralysis of the vocal fold, which is most common after a total thyroidectomy, symptoms typically show up right away. The airway is often partially obstructed, and the patient may be in respiratory distress.

A condition called biphasic stridor is also possible. It results from turbulent airflow through the throat and results in a harsh, vibrating, and variably pitched voice.

In some cases of bilateral paralysis, breathing problems and/or exertion-related stridor may not be apparent until later.

Tumors

In some lung cancer cases, the tumor(s) compresses the recurrent laryngeal nerve, more frequently on the left than on the right. This can cause hoarseness, but it also may be a sign that the tumor is advanced and inoperable. In some extreme cases, the surgeon may intentionally sever the RLN in order to remove a tumor.

Tumors in the neck may also compress or damage the RLN.

Other Disease

Other diseases that may cause problems with the RLN include:

  • Ortner's syndrome (also called cardiovocal syndrome) which can cause RLN palsy
  • Expansion of structures inside the heart or major blood vessels, which can cause nerve impingement

In these cases, symptoms are similar to that of RLN injury.

Rehabilitation

Recurrent laryngeal nerve reinnervation is a surgery that can help alleviate hoarseness after damage to one side of the RLN results in vocal cord paralysis.

This outpatient procedure generally takes between two and three hours. The surgeon makes a small incision and inserts a plumping material that temporarily improves the voice while the reinnervation takes place. It allows the functioning nerve to send signals to the injured nerve. Over time, the nerve signals should improve, and the vocal cord will function properly again.

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Article Sources

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