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.)
Palsy refers to paralysis, which can range from partial to complete.
Often, the cause of 4th cranial nerve palsy cannot be identified. The most common identified cause is
A head injury, often due to a motorcycle accident but sometimes even relatively minor head trauma
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.
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.