Fourth nerve palsy is a problem that happens when a certain muscle in your eye is paralyzed. Three nerves control how your eyes move, where your eyelids are, and how large your pupils are. These three nerves are:

• Third cranial nerve (oculomotor nerve).
• Fourth cranial nerve (trochlear nerve).
• Sixth cranial nerve (abducens nerve).

The fourth cranial nerve controls the actions of one of the external eye muscles, the superior oblique muscle. This muscle runs from the back of the eye socket to the top of the eye. It passes through a loop of tissue near the nose known as the trochlea. It turns the eye inward and downward.

The fourth cranial nerve is the only cranial nerve that starts at the back of the brain. It has a longer path through the skull than any other cranial nerve. It enters the eye socket through an opening at the back and then travels to the superior oblique muscle.

Diseases or injuries to the fourth cranial nerve can cause the superior oblique muscle to be paralyzed. The name for this condition is fourth nerve palsy. Other names for it are superior oblique palsy and trochlear nerve palsy.

You may have fourth nerve palsy from birth, or you may develop it later. It usually happens in only one eye, but it can occur in both.

Most children with fourth nerve palsy have it from birth (congenital fourth nerve palsy). The child may have other health problems along with congenital fourth nerve palsy.

In adults, congenital causes are still the most common cause of fourth nerve palsy, even though symptoms are first noticed as an adult. Head injury, including whiplash and concussions, is another common cause of fourth nerve palsy. Poor blood flow related to diabetes also can cause it. Fourth nerve palsy that is caused by injury may not go away.

Many times, the cause of fourth nerve palsy is not clear. The name for this is idiopathic fourth nerve palsy. In adults, many cases of fourth nerve palsy that are not caused by injury are idiopathic. This type of fourth nerve palsy often goes away on its own.

Less common causes of fourth nerve palsy include:

• A bulging area of an artery (aneurysm). This can press on the nerve or burst and decrease blood supply to the nerve.
• Increase in pressure inside the skull (increased intracranial pressure). This can press on the nerve.
• Infection.

Double vision (diplopia) is a common symptom of fourth nerve palsy. It occurs only when both eyes are open. With one eye shut, you see one image. But when you open both eyes, you may see two images. One of the images may appear to be above the other, or above and to the side of the other. For some people, one image may appear slightly turned compared with the other. If there is little separation between images, the overall image may appear blurry instead of double.

Other symptoms may include:

• One iris higher than the other. The iris is the colored part of your eye.
• Holding your head at a tilt. This is to help with the vision problem.
• Pain, usually above the eyebrow. This can happen if the fourth nerve palsy is idiopathic or related to diabetes.

If you have had the palsy for a while, your brain may have started to ignore images from that eye. If this is the case, you may not have double vision.

Your doctor will probably take a health history, asking you about your recent symptoms and your past health information. They can check your cranial nerves by doing an exam. They will look at the position of your eyes at rest and then have you follow an object with your eyes. The doctor may also check how your pupils react to light, measure the pressure in your eye, and look at the backs of your eyes. They may ask to look at old photos of you to try to find out when the problem started.

Several other conditions can cause double vision. These include myasthenia gravis, Grave disease, Wernicke syndrome, and some types of migraines. Myasthenia gravis is an autoimmune disease that weakens muscles. Sometimes myasthenia gravis affects only the muscles of the eyes. Grave disease is an autoimmune disease that causes the thyroid gland to be overactive. It can also affect the eyes. One symptom is bulging eyes due to inflammation and extra material in the eye socket. Wernicke syndrome is a brain condition caused by long-term alcohol drinking and vitamin deficiency. It can cause paralysis of the eye muscles.

Your doctor may order tests to help tell fourth nerve palsy from other conditions. These may include:

• Blood tests, to look for autoimmune diseases and thyroid hormone levels.
• CT scan or MRI, to look at your brain and cranial nerves.
• Ultrasound, to look at the muscles of the eye.
• Spinal tap (lumbar puncture), to look for causes of increased intracranial pressure.
• Nerve stimulation tests.

Your doctor may send you to an eye care provider (ophthalmologist) for diagnosis and to manage your condition. You may even need to see a neuro-ophthalmologist. This is an eye doctor with special training to treat nerve problems of the eye.

Treatment of fourth nerve palsy depends on its cause. Idiopathic fourth nerve palsies tend to go away on their own. Palsies caused by injury can also get better with time. If something is pressing on the fourth cranial nerve, you may need surgery to ease the pressure.

Possible treatment of fourth nerve palsies include:

• Over-the-counter pain medicines.
• Prism glasses. These can bring the double images together as one image.
• Eye patch. You switch this from one eye to the other so that one eye doesn't get weak or lazy.
• Surgery to realign the eyes.

The goal of surgery is to get rid of the double vision. This can also correct the tendency to tilt your head. You may need surgery on both eyes if the eye problem is severe.

Fourth nerve palsies that don't go away on their own cause a change in how your eyes work with one another. This means that the other eye moves with the affected eye. That way, the image separation you see stays the same no matter which way you are looking.

Children with fourth nerve palsy may develop a change in how their face looks (asymmetry), especially if they tend to keep their head tilted. The muscles on one side of the face do not develop the same as those on the other side, so the two sides start to appear different over time.

See your doctor if you develop double vision. This could be a symptom of a fourth nerve palsy or another serious condition.

Having double vision and a sudden severe headache could be a symptom of a stroke. Call 911 right away if you think you may be having a stroke. It could be from a blood clot (ischemic stroke) or a ruptured blood vessel (hemorrhagic stroke). Other symptoms of stroke include trouble speaking, weakness, numbness, or confusion.

• Fourth nerve palsy means that a certain muscle in your eye is paralyzed.
• It is caused by disease or injury to the fourth cranial nerve. In children, it is most often present at birth (congenital). In adults, it is most often caused by injury.
• Many cases of fourth nerve palsy are idiopathic. This means the cause is unknown.
• People with fourth nerve palsy often have one iris that is higher than the other, tilt their head, and have double vision.
• Some types of fourth nerve palsy may go away on their own. You may need surgery if the palsy does not go away.

Tips to help you get the most from a visit to your doctor:

• Know the reason for your visit and what you want to happen.
• Before your visit, write down questions you want answered.
• Bring someone with you to help you ask questions and remember what your provider tells you.
• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you.
• Know why a new medicine or treatment is prescribed and how it will help you. Also know what the side effects are.
• Ask if your condition can be treated in other ways.
• Know why a test or procedure is recommended and what the results could mean.
• Know what to expect if you do not take the medicine or have the test or procedure.
• If you have a follow-up appointment, write down the date, time, and purpose for that visit.
• Know how you can contact your doctor if you have questions.