Double vision (diplopia)
When a person experiences double vision, or diplopia, they see two images of the same thing at the same time.
Double vision may be a long-term problem, or the symptoms may come and go.
Double vision may affect a person's ability to drive safely and the DVLA may need to be told about the condition.
What causes diplopia or double vision?
Opening your eyes and seeing a single clear image is something you probably take for granted. But that seemingly automatic process depends on the orchestration of multiple areas of the vision system. They all need to work together seamlessly:
- The cornea is the clear outermost disc covering the eye. It allows in light.
- The lens is behind the pupil. It focuses light onto the retina.
- Muscles of the eye, called extraocular muscles, perform the eye's precise movements.
- Nerves carry visual information from the eyes to the brain.
- The brain is where several areas process visual information from the eyes.
Problems with any part of the vision system can lead to diplopia. It makes sense to consider the causes of diplopia according to the part of the visual system that has the problem.
Cornea problems. Problems with the cornea often cause double vision in one eye only. Covering the affected eye makes the diplopia go away. The damaged surface of the eye distorts incoming light, causing double vision. Damage can happen in several ways:
- Infections of the cornea, such as shingles ( herpes zoster), can distort the cornea.
- An uncommon complication of LASIK surgery (laser eye surgery) can leave one cornea altered, creating unequal visual images.
Lens problems. Cataracts are the most common problem with the lens that causes double vision. If cataracts are present in both eyes, images from both eyes will be distorted. Cataracts are often correctable with surgery.
Muscle problems. If a muscle in one eye is weak, that eye can't move smoothly with the healthy eye. Gazing in directions controlled by the weak muscle causes double vision. Muscle problems can result from several causes:
- Myasthenia gravis is an autoimmune illness that blocks the stimulation of muscles by nerves inside the head. The earliest signs are often double vision and drooping eyelids (ptosis).
- Graves' disease is a thyroid condition that weakens the muscles of the eyes. Graves' disease commonly causes vertical diplopia. With vertical diplopia, one image is on top of the other.
Nerve problems. Several different conditions can damage the nerves and lead to double vision:
- Multiple sclerosis can affect nerves anywhere in the brain or spinal cord. If the nerves controlling the eyes are damaged, double vision can result.
- Guillain-Barre syndrome is a nerve condition that causes progressive weakness. Sometimes, the first symptoms occur in the eyes and cause double vision.
- Uncontrolled diabetes can lead to nerve damage in one of the eyes, causing eye weakness and diplopia.
Brain problems. The nerves controlling the eyes connect directly to the brain. Further visual processing takes place inside the brain. Many different causes for diplopia originate in the brain. They include:
- Increased pressure inside the brain from trauma, bleeding or infection
- Brain tumours
- Migraine headaches