Often doctors cannot identify the cause, but when they can, the cause is usually a head injury, sometimes a minor one.
People see double images, but tilting the head to the side opposite the affected eye can eliminate them.
Doctors suspect palsy of the 4th cranial nerve based on the symptoms, but computed tomography or magnetic resonance imaging may be done.
The cause, if identified, is treated.
(See also Overview of the Cranial Nerves Overview of the Cranial Nerves Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing... read more .)
Palsy refers to paralysis, which can range from partial to complete.
Causes of Fourth Cranial Nerve Palsy
Often, the cause of 4th cranial nerve palsy cannot be identified. The most common identified cause is
A head injury Overview of Head Injuries Head injuries that involve the brain are particularly concerning. Common causes of head injuries include falls, motor vehicle crashes, assaults, and mishaps during sports and recreational activities... read more , often due to a motorcycle accident but sometimes even relatively minor head trauma
Occasionally, diabetes Blood vessel complications in diabetes People with diabetes mellitus have many serious long-term complications that affect many areas of the body, particularly the blood vessels, nerves, eyes, and kidneys. (See also Diabetes Mellitus... read more 
causes this palsy by damaging small blood vessels that carry blood to the nerve. Rarely, the cause is a tumor, a bulge (aneurysm Aneurysms of Arteries in the Arms, Legs, and Heart An aneurysm is a bulge (dilation) in the wall of an artery. (See also Aortic Branch Aneurysms and Brain Aneurysms.) Aneurysms may occur in any artery. Aneurysms are most common in the aorta... read more ) in an artery in the skull, or multiple sclerosis Multiple Sclerosis (MS) In multiple sclerosis, patches of myelin (the substance that covers most nerve fibers) and underlying nerve fibers in the brain, optic nerves, and spinal cord are damaged or destroyed. The cause... read more .
Symptoms of Fourth Cranial Nerve Palsy
One or both eyes may be affected. The affected eye cannot turn inward and down. As a result, people see double images, one above and slightly to the side of the other. Thus, going down stairs, which requires looking inward and down, is difficult. However, tilting the head to the side opposite the affected eye muscle can compensate and eliminate the double images. This position can eliminate the double images because people use eye muscles that are unaffected by the palsy to focus both eyes on an object.
Diagnosis of Fourth Cranial Nerve Palsy
Limited eye movement
Computed tomography or magnetic resonance imaging
Usually, 4th cranial nerve palsy is suspected if a person has characteristic limited eye movement. Computed tomography (CT) or magnetic resonance imaging (MRI) of the brain may be done to identify the cause.
Treatment of Fourth Cranial Nerve Palsy
Treatment of the cause, if identified
Eye exercises
Prism glasses
The disorder causing 4th cranial nerve palsy, if identified, is treated.
Eye exercises may help, as may wearing prism glasses. Prism glasses have lenses that are thinner at the top and thicker at the base. When light passes through the prism, it moves more slowly through the prism's base than the top. Thus, the prism bends the light and adjusts for the double vision caused by the palsy.
The palsy usually resolves over time, but sometimes surgery is eventually needed.
