The Anatomy of the Abducens Nerve

The abducens nerve, sometimes called the abducent nerve, is responsible for the movement of the lateral rectus muscle, which allows your eye to rotate away from the center of your body and look to the left or right. The abducens is the sixth cranial nerve (CN VI). This nerve has only a motor function and is lacking a sensory function.

The abducens is considered an extraocular nerve, which literally means “outside of the eye.” Along with the oculomotor nerve (CN III) and the trochlear nerve (CN IV), it provides movement to the muscles around the eyeball rather than attaching to the eye itself.

Anatomy

Humans have 12 cranial nerves that exist in generally symmetrical pairs, one on each side of your head. Unless it’s necessary to distinguish one side from the other (such as if one of them is damaged), each pair is most often referred to as a single nerve.

The cranial nerves are different from the rest of your nerves, which originate in the spinal cord. Cranial nerves originate from the brain and brainstem and perform functions in your face and throat.

Structure and Location

The abducens nerve emerges from the brainstem, which sits low in the back of your brain and connects to the spinal column. The specific area the nerve comes from is called the abducens nucleus.

It leaves the brainstem and passes through an area of the brain called the subarachnoid space. From there, it travels upward and pierces a tough membrane, called the dura mater, which envelops the brain and spinal cord.

It then runs between the dura and the skull through a space called the Dorello canal and takes a sharp turn toward your face to move into the cavernous sinus. There, it follows the internal carotid artery to where it can enter the orbit (the eye socket) through the superior orbital fissure.

Finally, it connects to the lateral rectus muscle of the eye. The lateral rectus muscle connects to the outside of your eye, right in line with the pupil.

A small branch of the abducens nerve connects to the contralateral medial rectus muscle.

Anatomical Variations

Many nerves have known anatomical variations. These are important for doctors to know so they can make proper diagnoses and especially during surgery, so they can avoid damaging a nerve with an irregular course.

The abducens nerve has a few known variations. In some people, the nerve may:

• Send out abnormal branches.
• Take an unusual course from the brainstem to the lateral rectus muscle.
• Be doubled or even tripled in sections or along its entire course.

Function

The function of the abducens nerve is quite simple and straightforward:

• It moves the eye outward (abduction) so you can look to the side.
• Via the contralateral medial rectus muscle, it coordinates the simultaneous side-to-side movement of your eyes.

Associated Conditions

The abducens nerve travels farther through the skull than any other cranial nerve. This leaves it vulnerable to injury at multiple points. Damage to this nerve is called abducens nerve palsy or sixth cranial nerve palsy.

Downward pressure on the brainstem is a common cause of abducens damage. Other causes include:

• Trauma
• Aneurysm
• Ischemia (loss of blood flow)
• Infection
• Inflammation due to injury or illness
• Stroke
• Metabolic disease (i.e., Wernicke disease)
• Demyelinating lesions
• Carcinomatous meningitis
• Basal skull fracture
• Damage to the orbit (eye socket)
• Damage from surgery or other medical procedures, especially involving the cavernous sinus

In people with diabetes, poorly controlled blood sugars are a significant risk factor for abducens nerve palsy, as are certain problems in the cavernous sinus. (However, diabetic neuropathy and cavernous sinus problems are likely to affect many nerves beyond the abducens.)

In many cases, possibly as many as 30%, the cause is never identified.

The primary symptom of abducens nerve palsy is an inability to abduct the eye, which can cause it to droop inward, as if “crossed.” This can result in diplopia, which is the clinical term for double vision, because the eyes aren’t looking in the same direction.

Rehabilitation

The treatment and management of abducens nerve palsy is different for children than it is for adults.

Treatment in Children

In children, treatment focuses on training the eye to restore proper movement. This can be accomplished in several ways, including:

• Patching one eye at a time and periodically alternating, which also helps prevent “lazy eye” in the unaffected side
• Prism therapy to prevent double vision
• Botulism injections
• If other treatments fail, surgery

Treatment in Adults

The first-line treatment for abducens nerve palsy in adults is, most often, wait and see. Most cases resolve on their own.

When that doesn’t happen, the treatment is determined based on what caused the palsy. Treatment may include:

• Steroids to relieve inflammation
• Surgery or lumbar puncture to relieve pressure
• Treatments similar to those for children, with the exception of alternate patching