Abducens Nerve Palsy

What is the abducens nerve?

The abducens nerve is the sixth cranial nerve. It carries signals to the lateral rectus muscle, a small muscle attached to the outer side of the eye. When this muscle contracts, the eye turns outwards, away from the nose. Each eye has its own lateral rectus muscle, supplied by its own sixth nerve.

The sixth nerve emerges from the lower part of the brain and travels a long way before reaching the lateral rectus. Damage at any point along this path can stop the nerve from working properly, or at all. Because the lateral rectus can no longer contract as it should, the eye drifts inwards, towards the nose.

Sometimes sixth nerve palsy occurs without any other symptoms; this is known as isolated sixth nerve palsy. At other times it is accompanied by additional symptoms, in which case it is called non-isolated sixth nerve palsy.

Sixth nerve palsy is sometimes present from birth, but it can also develop later in life as a result of another condition. In children, injury is a leading cause; in adults, stroke is one of the most common. Overall, the condition is relatively rare.

What causes sixth nerve palsy?

Many conditions can disrupt the function of the sixth cranial nerve and cause a palsy. Possible causes include:

• Injury (especially when a skull fracture is present)
• Stroke
• Infection (for example, Lyme disease or a viral infection)
• Brain tumour
• Inflammation of the nerve, for example after a vaccination
• Multiple sclerosis
• Raised pressure inside the brain (for example, from meningitis)

In congenital sixth nerve palsy, the problem with the sixth cranial nerve is present from birth, sometimes as a result of injury during delivery. In some cases, no cause can be identified. Sixth nerve palsy that occurs without additional symptoms is usually due to one of the following:

• Injury
• A congenital cause
• Viral illness
• High blood pressure
• Diabetes

What are the symptoms of sixth nerve palsy?

Sixth nerve palsy may affect one or both eyes, depending on its cause.

The most common symptom is double vision when both eyes are open. It tends to be more noticeable when looking into the distance or towards the side of the affected eye, although not everyone with sixth nerve palsy experiences it.

The eyes may also become misaligned, a condition known as strabismus. The affected eye may drift towards the midline. Early on, this may only be apparent when looking towards the affected side (for example, looking to the right with a right sixth nerve palsy). If the palsy worsens, the affected eye may drift towards the midline even when looking straight ahead.

In non-isolated sixth nerve palsy, other symptoms may be present as well. Depending on which other structures are affected, these can include:

• Hearing loss
• Facial weakness
• Reduced facial sensation
• A drooping eyelid
• Fever
• Headache
• Nausea and vomiting

How is sixth nerve palsy diagnosed?

Diagnosis begins with a thorough medical history and physical examination, including a detailed neurological assessment to identify any other affected parts of the brain and nervous system. A neuro-ophthalmologist may examine you first, and will also try to identify the root cause of the palsy.

Your neuro-ophthalmologist will usually order brain imaging, because the nerve often becomes compressed as it leaves the brain. Imaging is also important to make sure that a brain tumour is not causing your symptoms. Possible tests include computed tomography (CT) and magnetic resonance imaging (MRI). MRI provides more information, but it is often harder to arrange as quickly as a CT scan. A CT may be needed if your neuro-ophthalmologist suspects raised pressure inside the brain.

If a specific cause is suspected, further tests may be arranged. For example, you might need blood tests and a lumbar puncture if meningitis is suspected.

How is sixth nerve palsy treated?

Treatment depends on the underlying cause. Possible treatments include:

• Antibiotics, where the palsy is due to a bacterial infection
• Corticosteroids, where it is due to inflammation
• Surgery or chemotherapy, where it is due to a tumour

Sometimes there is no direct treatment available for the underlying cause.

Your neuro-ophthalmologist may prefer to wait several months before starting any further treatment, because the symptoms of sixth nerve palsy often improve on their own. A palsy that follows a viral illness frequently resolves completely within a few months. Symptoms following trauma may also improve over several months, although they are less likely to disappear entirely. Complete recovery is more likely when the palsy is isolated.

If symptoms are still present after six months or so, your neuro-ophthalmologist may recommend further treatment. Options include:

• Alternate patching of each eye to eliminate double vision
• Special prism glasses to help align the eyes
• Botulinum toxin to temporarily paralyse the muscle on the opposite side of the eye and improve alignment
• Strabismus surgery

What are the possible complications of sixth nerve palsy?

Isolated sixth nerve palsy does not usually cause complications in itself, but many of its possible underlying causes can. And, like any procedure, surgery for sixth nerve palsy carries some risk of complications.

When should I call my eye specialist?

Call your ophthalmologist if you develop any sudden, severe symptoms, such as loss of vision or difficulty moving an arm or a leg.

A note from Athens EyeCare Clinic

Sixth nerve palsy occurs when the sixth cranial nerve is damaged or stops working properly, causing problems with eye movement: the affected eye may be unable to turn outwards normally.

• Sometimes only the sixth cranial nerve is affected, but problems can occur in other parts of the body too.
• Sixth nerve palsy has many possible causes. In children, trauma is one of the most common.
• Treatment depends on the underlying cause.
• Symptoms often improve or resolve within several months.
• If they do not resolve completely, further treatment, possibly including strabismus surgery, may be needed.