By 'double vision', people usually mean seeing two visual images of an object rather than one. This tends to be like seeing a 'shadow' around every image rather than seeing two totally separate objects, but a few people with severe double vision do see two full and separated objects. The medical term for double vision is diplopia.

Medical attention

It's essential you see your GP to establish why you have double vision. While it usually develops as a result of a squint, more worrying conditions, such as tumours, can also be the cause.

When double vision develops in an adult who has never had it before, there's particular cause for concern.

Squints

A squint is the most common cause of double vision, especially in children. In a squint the two eyes don't look in exactly the same direction because some of the muscles are weak or paralysed.

As each eye sees an object from a slightly different angle, this may result in a double image of the object (although what actually happens is a bit more complex than this).

Not all squints cause double vision, but paralytic squints (where there's muscle paralysis) generally do.

Squints can be corrected by surgery if it's performed early in life, and this should 'cure' the double vision. It's much more difficult to correct double vision from a squint in adult life.

Cosmetic surgery can cure the look of the squint, but by then the brain may have learned to see separately with the two eyes and can no longer fuse their two pictures into one.

Fortunately, the brain usually learns to suppress the second image and the view from one eye dominates.

Serious causes

Other causes for double vision include:

• A tumour or cancer, in or behind the eye, which distorts the visual image. This will need very specialised treatment.
• A blood clot or tumour behind the eye which prevents normal movement of one of the eyes. Again urgent specialist treatment is essential.
• A condition called exopthalmos, where the eyeballs protrude and the eye muscles swell, disrupting the alignment and movement of the eyes. This is usually caused by problems with the thyroid gland, which needs medical treatment (but treating the thyroid doesn't always cure the eye problem).
• Abnormalities within the eye. Very occasionally double vision occurs because of problems such as dislocation of the lens in the eye, which needs specialist medical treatment.

This article was last medically reviewed by Dr Trisha Macnair in January 2008