Double vision, also called diplopia, causes a person to see two images of a single object. There are two types of double vision: monocular and binocular.
Monocular diplopia is double vision in only one eye. The double vision continues even when the other eye is covered. The doubling does not go away when you look in different directions. Monocular diplopia can be caused by:
Astigmatism — This is an abnormal curvature of the front surface of the cornea.
Keratoconus — The cornea gradually becomes thin and cone-shaped.
Pterygium — This is a thickening of the conjunctiva, the thin mucous membrane that lines the inner surface of the eyelids and the whites of the eyes. The thickening extends on the cornea, the clear part of the surface of the eye.
Cataracts — The lens gradually becomes less transparent. Risk factors include being older than 65, having eye trauma or long-term diabetes, smoking, using steroid medications or having radiation treatments.
A dislocated lens — The ligaments that hold the lens in place are broken, and the lens moves out of place or wiggles. This can be caused by trauma to the eye or a condition known as Marfan's syndrome.
A mass or swelling in the eyelid — This condition can press on the front of the eye.
Dry eye — Your eyes do not produce enough tears.
Some retinal problems — Double vision can happen when the surface of the retina is not perfectly smooth, which can have a variety of causes.
Binocular diplopia is double vision related to a misalignment of the eyes. The double vision stops if either eye is covered. Any problem that affects one or more of the muscles around the eyeball that control the direction of the gaze can cause binocular diplopia. These are called extraocular muscles. Such problems include:
Strabismus — This is a childhood misalignment of the eyes that affects about 4% of children younger than age 6.
Damage to nerves controlling the extraocular muscles — Nerves can be injured by brain damage caused by infection, multiple sclerosis, stroke, head trauma or a brain tumor, especially a tumor located at the lower back portion of the brain. A tumor growing inside the eye socket or trauma to the eye socket can damage a nerve anywhere along its route to the eye muscles.
Diabetes — This disease can lead to problems with the nerves that control eye muscle movements. Sometimes this can happen before the person is aware that he or she has diabetes.
Myasthenia gravis — This is a neuromuscular illness that causes the body's muscles to tire easily and become weak. It occurs because the body's immune system mistakenly attacks the places where nerves transmit impulses to muscles, telling the muscles to contract.
Graves' disease — This is the most common cause of an overactive thyroid (hyperthyroidism). Some people with Graves' disease develop double vision due to swelling and thickening of the muscles that move the eyes within the eye socket.
Trauma to the eye muscles — The muscles of the eye socket can be injured by facial trauma, especially by a fracture of the thin bones of the eye socket.