Sixth Nerve Palsy

Sixth nerve palsy is a condition where the sixth cranial nerve, which controls the muscle that moves your eye outward, is damaged. This damage prevents the affected eye from turning fully away from your nose. The main symptom is double vision (diplopia), especially when looking to the side, and the eye may turn inward.

What is Sixth Nerve Palsy?

Sixth nerve palsy, also known as abducens nerve palsy, occurs when the sixth cranial nerve is damaged. This nerve is crucial for controlling the lateral rectus muscle, which moves your eye outward, away from your nose. When this nerve doesn't work correctly, the affected eye cannot turn fully to the side, leading to double vision.

Your brain sends signals through twelve pairs of cranial nerves to control various body functions. The sixth cranial nerve is specifically responsible for moving one of your eye muscles. This muscle, called the lateral rectus, acts like a rope pulling your eye to the side. When the sixth nerve is damaged, the lateral rectus muscle becomes weak or paralyzed. This means the eye cannot move outward as it should. Instead, the eye might drift inward towards your nose, especially when you try to look to the side. This misalignment causes you to see two images instead of one.

Symptoms

The primary symptom of sixth nerve palsy is double vision (diplopia), where you see two images side-by-side. This double vision is often worse when you look toward the side of the affected eye. Your affected eye may also turn inward, and you might tilt or turn your head to try and see clearly.

Double vision, or diplopia, is the most common and noticeable symptom. Because the affected eye cannot move outward properly, it doesn't align with the other eye. This causes your brain to receive two slightly different images, which it interprets as seeing double. The double images usually appear next to each other. You might find that your double vision is more pronounced when you try to look in the direction of the affected eye. For example, if your right sixth nerve is affected, looking to your right will make the double vision worse. To compensate and try to merge the images, you might instinctively turn your head or tilt it to one side. The affected eye may also visibly turn inward (esotropia) when you look straight ahead or try to look to the side.

Causes & risk factors

Sixth nerve palsy can be caused by various underlying health conditions, including those that affect small blood vessels, such as diabetes and high blood pressure. Other causes include stroke, brain tumors, infections, or head injuries. Risk factors often overlap with these causes, particularly conditions like diabetes and hardening of the arteries.

Many cases of sixth nerve palsy, especially in older adults, are due to a lack of blood flow (ischemia) to the small blood vessels that supply the nerve. This is often linked to conditions like diabetes, high blood pressure (hypertension), and hardening of the arteries (atherosclerosis). These conditions damage blood vessels, making them less efficient at delivering oxygen and nutrients. Other potential causes include a stroke, a brain tumor, a brain aneurysm (a bulge in a blood vessel), or inflammation. Infections, such as Lyme disease or certain viral infections, can also affect the nerve. Head injuries, multiple sclerosis, and increased pressure in the brain (idiopathic intracranial hypertension or pseudotumor cerebri) are also known causes. Sometimes, despite thorough investigation, no specific cause is found; this is called idiopathic sixth nerve palsy. The main risk factors for developing sixth nerve palsy include having diabetes, high blood pressure, or atherosclerosis. Older age is also a risk factor, as these conditions are more common in older adults.

How it's diagnosed

Diagnosing sixth nerve palsy involves a thorough physical and neurological exam, along with a detailed eye exam to assess eye movement and alignment. Your doctor will also likely order imaging tests, such as an MRI or CT scan of your brain, and blood tests to identify any underlying causes like diabetes or infections.

When you see a doctor for double vision, they will perform a comprehensive physical and neurological examination. This includes checking your reflexes, balance, and coordination. A specialized eye exam will assess how your eyes move in all directions and look for any misalignment. The doctor will specifically check if your affected eye can turn outward. To determine the cause of the palsy, imaging tests are usually necessary. A magnetic resonance imaging (MRI) scan or a computed tomography (CT) scan of your brain and eye sockets (orbits) can help detect conditions like a stroke, brain tumor, or inflammation. Blood tests may also be ordered to check for conditions such as diabetes, inflammation, or specific infections like Lyme disease. In some cases, if increased pressure in the brain or an infection is suspected, a lumbar puncture (spinal tap) might be performed to analyze the fluid surrounding your brain and spinal cord.

Treatment options

Treatment for sixth nerve palsy often begins with addressing any underlying medical conditions, such as diabetes or high blood pressure. Many cases resolve on their own over time. To manage double vision, temporary solutions like an eye patch or prism glasses can be used. Other interventions, like botulinum toxin injections or surgery, are considered if symptoms persist.

In many cases, especially when the palsy is due to microvascular ischemia in older adults, sixth nerve palsy resolves spontaneously within a few weeks to months without specific treatment. However, the most important step is to identify and treat any underlying cause. For example, if diabetes or high blood pressure is the cause, managing these conditions effectively is crucial for recovery. While waiting for the nerve to recover, or if it doesn't recover fully, several options can help manage the double vision. Wearing an eye patch over one eye can eliminate the double image, though it reduces your peripheral vision. Prism glasses, which have special lenses (Fresnel prisms), can help merge the two images into one by bending the light before it reaches your eyes. For persistent symptoms, other interventions may be considered. Botulinum toxin (Botox) injections into the eye muscles can temporarily weaken the stronger muscle, helping to balance eye alignment. Eye muscle surgery is typically a last resort, considered only if the condition has been stable for at least 6 to 12 months and other treatments have not been successful. It's important to note that the evidence for the effectiveness of these interventions is limited, and your doctor will discuss the best approach for your specific situation.

Recovery & outlook

The recovery and outlook for sixth nerve palsy vary significantly depending on its underlying cause. Many cases, particularly those linked to microvascular ischemia (lack of blood flow), resolve completely on their own within several weeks to months. For other causes, recovery might be slower or incomplete, and some people may require ongoing management for double vision.

The prognosis for sixth nerve palsy is generally good, especially for cases caused by microvascular ischemia. In these situations, the nerve often heals itself, and eye movement returns to normal. The recovery period can range from a few weeks to several months. If the palsy is due to other causes, such as a brain tumor or a severe head injury, the recovery process might be more complex and depend on the successful treatment of the primary condition. In some instances, full recovery may not occur, and some degree of double vision or eye misalignment might persist. Your doctor will monitor your progress closely. If the condition doesn't improve or worsens, further investigations may be needed to re-evaluate the cause. Even if full recovery of eye movement isn't achieved, options like prism glasses or, in some cases, surgery can help improve vision and quality of life.

When to see a doctor

You should see a doctor immediately if you experience sudden double vision or any new, unexplained changes in your vision. It is especially important to seek urgent medical attention if double vision occurs alongside other neurological symptoms, such as weakness, numbness, difficulty speaking, or a severe headache, as these could indicate a serious underlying condition.

Sudden onset of double vision is a symptom that always warrants a prompt medical evaluation. While many cases of sixth nerve palsy are not life-threatening, double vision can also be a sign of more serious conditions like a stroke, brain tumor, or aneurysm. Do not delay seeking care if your double vision is accompanied by other concerning symptoms. These might include weakness or numbness on one side of your body, problems with balance or coordination, slurred speech, difficulty swallowing, or a sudden, severe headache. These combined symptoms could point to an emergency and require immediate medical attention.

Frequently asked questions

Can sixth nerve palsy go away on its own?

Yes, many cases of sixth nerve palsy, especially those caused by a lack of blood flow to the nerve (microvascular ischemia), resolve completely on their own. This spontaneous recovery usually happens within a few weeks to several months, particularly in older adults.

Is sixth nerve palsy a sign of a stroke?

Sixth nerve palsy can sometimes be a symptom of a stroke, especially if it appears suddenly and is accompanied by other neurological signs like weakness or numbness. However, it has many other causes, including diabetes, high blood pressure, and infections. A doctor's evaluation is needed to determine the cause.

How do prism glasses help with double vision?

Prism glasses help with double vision by bending the light before it enters your eyes. This optical effect shifts the image from the misaligned eye so that it lines up with the image from the other eye. This allows your brain to combine the two images into a single, clear view.

Can children get sixth nerve palsy?

Yes, children can get sixth nerve palsy. In children, it is often linked to viral infections or increased pressure within the brain (idiopathic intracranial hypertension). As with adults, a thorough medical evaluation is important to identify the specific cause and guide appropriate treatment.

What is the difference between an eye patch and prism glasses for double vision?

An eye patch works by blocking vision in one eye, completely eliminating the double image. However, it also removes depth perception and peripheral vision. Prism glasses, on the other hand, allow both eyes to see but optically merge the two images, preserving more of your visual field and depth perception.

How long does recovery from sixth nerve palsy usually take?

Recovery time for sixth nerve palsy varies. For cases due to microvascular ischemia, improvement often begins within weeks, with full recovery typically occurring within three to six months. If the cause is more complex, such as a brain injury or tumor, recovery can take longer or may not be complete.

Sources

• MedlinePlus — Sixth Nerve Palsy
• Mayo Clinic — Sixth Nerve Palsy
• Cochrane Library — Sixth Nerve Palsy