The Anatomy of the Vestibular Nerve

The vestibular nerve, located in each inner ear, is essential for regulating balance and sensing motion of the body. This paired nerve is part of the pathway that connects sensory fibers in the vestibular apparatus—three tiny structures called the semicircular canals, plus the saccule and utricle—to the brainstem (cerebellum).

Along with the cochlear nerve, which is involved in hearing, it’s a branch of the vestibulocochlear nerve and an essential part of the vestibular system.

Given this critical role, damage to the vestibular nerve, as in Ménière’s disease, vestibular neuritis, encephalitis. and some other conditions leads to vertigo, involuntary eye movements, and inability to maintain posture, among other issues.   

Anatomy

Structure and Location

One of 12 pairs of cranial nerves, the vestibulocochlear nerve, of which the vestibular nerve is a branch, emerges from the pons and the medulla, two parts of the brainstem.

Traveling towards the front of the body along a canal in the temporal bone—a paired bone on each side of the skull—it passes through a juncture called the pontocerebellar cistern into the inner ear. It’s here that the vestibulocochlear nerve splits into the cochlear nerve (involved in hearing) and the vestibular nerve.

The vestibular nerve connects a bundle of nerves in the inner ear called the vestibular ganglion to four discrete areas in the brainstem: the superior, lateral, inferior, and medial vestibular nuclei.

This nerve bundle helps process direct signaling from the semi-circular canals, the utricle, and the saccule, structures that make up what’s called the bony labyrinth of the inner ear.

Vestibular nerve anatomy can be divided into three interconnected components:

• Superior division: The uppermost of the branches emanating from the vestibular ganglion, this division conveys information from the anterior and lateral semicircular canals as well as the utricle. It terminates in the superior and lateral vestibular nuclei
• Inferior division: Connected to the inferior and medial vestibular nuclei, the inferior division conveys information from the saccule and the posterior semicircular canal.
• Peripheral division: Collecting information from the underside of the semi-circular canal, this division passes to the brain via a small channel in the temporal bone called the foramen singular.

Anatomical Variations

The structure of the vestibular nerve is relatively consistent, and there are no normal variations to its course. However, its structure can be impacted when parts of the vestibulocochlear nerve are malformed at birth. These congenital defects include:

• Michael aplasia: This rare genetic abnormality leads to an absence or severe under-development of structures in the inner ear. In some of these cases, the vestibular nerve, among others, is absent.
• Common cavity: In cases of this birth defect, the cochlea and the vestibule (the central portion of the bony labyrinth, which contains the utricle and saccule) are undifferentiated.  
• Abnormal semicircular canals: Overdevelopment or underdevelopment of the semicircular canals can also occur, although rarely. Rarer still are those who are born with absent semicircular canals.

Function

Sensory Function

Given its role in processing information about body position, posture, and motion, the vestibular nerve is primarily associated with its sensory function. The saccule, the utricle and semicircular canals that comprise the vestibular apparatus all provide different kinds of information to this nerve. Here’s a quick breakdown:

Head Position

Located just behind the semicircular canals in the inner ear, the saccule is a small sac filled with fluid and calcium crystals. It also has vestibular hairs, which are nerve fibers connected to the vestibular nerve.

The fluid acts as a sort of level, remaining parallel to the ground as the head changes position, and the hairs are stimulated accordingly. The saccule works to sense the vertical position of the head.

Orientation and Tilt

From vestibular hairs in the utricle, the other fluid-filled sac of the vestibule, the vestibular nerve conveys information about head orientation and tilt to the brainstem. In a similar fashion to the saccule, this organ is sensitive to horizontal movements. 

Head Rotations

The three semicircular canals, just above the vestibule, are able to sense rotation, either in response to your own movement or to external motion (as in a car or roller coaster). They consist of three fluid-filled ducts aligned at right angles from each other.

Vestibular hairs inside of them are stimulated as the head changes position and the fluid moves in response. This signal is then transmitted to the brainstem along the vestibular nerve.

Motor Function

Since the vestibular nerve processes information about body position and sense of motion, it also helps coordinate some aspects of body movement. Here’s a quick breakdown:

• Regulating eye movements: The medial nuclei of the vestibular nerve in the pons and medulla of the brainstem connect to nerves regulating eye motion. Information about the position and angle of the head, as well as motion, is used to allow for coordinated eye movements. This way you’re able to keep a steady gaze while your head moves.
• Maintaining posture: In the brainstem, the vestibular nerve also connects with those regulating back and neck muscles to maintain posture. This system is also involved in quickly processing spatial and body position information to aid in keeping balance and ensuring the body is moving properly in space.  

Associated Conditions

Damage to the vestibular nerve can have serious implications, given its central role in sensing space and body position. Largely, this impact is experienced as vertigo (a sensation of rocking at all), dizziness, nausea, and vomiting. Conditions associated with vestibular nerve damage include:

• Nystagmus: This is a disorder of ocular motion, in which eye movements are jerky and unsteady; they may be slow in one direction, but fast in the opposite direction and tend to “jump.” This may lead to problems with vision and dizziness.
• Vestibular neuritis: Inflammation of the vestibulocochlear nerve can lead to the onset of this condition, characterized by severe vertigo, spinning, dizziness, loss of balance, nausea, and vomiting. Exact causes of this condition are unknown; however, it may be linked to viral infections, such as chickenpox, shingles, measles, hepatitis, and polio, among others.
• Ménière's disease: Excessive fluid buildup in the labyrinth, of which the vestibular apparatus is a part, can result in severe vertigo, dizziness, tinnitus (ringing in the ears), and hearing loss. Exact causes are unknown. Some believe this condition arises due to autoimmune disorders or previous viral infections, while others speculate it has to do with constrictions in blood supply.
• Encephalitis: An inflammation of the brain tissues, encephalitis can cause serious dysfunction in the vestibular system. Symptoms are severe and include high fever, movement disorders, neck stiffness, sound and light sensitivity, and many others. This condition is either due to an autoimmune disorder or as a result of viral infections, such as the West Nile and Zika.
• Ototoxicity: An unintended side effect of some drugs, otoxicity is characterized by hearing loss (when it affects the cochlear nerve) and severe dizziness (when it strikes the vestibular nerve). Though many medications can potentially trigger it, it’s most often associated with certain types of antibiotics, as well as some chemotherapy drugs.

Rehabilitation

Treatment for conditions associated with damage to the vestibular nerve tends to emphasize management of the symptoms, as well as the underlying conditions that caused them. Problems with eye movement, as in nystagmus, are usually managed with glasses or contact lenses, though some may have surgery on the eye muscles.

There are also a range of approaches to medically managing the dizziness and vertigo associated with vestibular nerve problems. These include:

• Medications, such as Bonine (meclizinee), Valium (diazepam), Cuvposa or Robinul (glycopyrrol), and Ativan (lorazepam) may be prescribed.     
• Dietary changes, such as limiting salt intake and taking diuretics, have been known to help.
• Antibiotic injections of gentamicin into the middle ear help reduce vertigo, though there is a risk of associated hearing loss. Some doctors use corticosteroids instead of antibiotics.
• Surgery is typically a last resort; however, some procedures can target and attempt to repair the vestibular nerve.
• Pressure pulse therapy involves using a newer, FDA approved device that uses air pressure to influence fluid levels in the inner ear. 
• Anti-viral medications may be prescribed, if the underlying cause is viral.
• Physical therapy approaches, such as balance therapy, can help the vestibular system and the brain develop new pathways and regain balance.