Strabismus, (also called crossed-eyes), is a vision problem - the two eyes can not focus on the same point. This problem normally begins in early childhood, and it has no known cause, although it tends to be genetic.

Strabismus occurs because the eye muscles do not co-ordinate properly. When the eye muscles can not move correctly, the eyes become misaligned, and the brain will not be able to process the two images.

In addition, eye injury, blood vessel damage, brain tumor, nerve disorders may also cause adult strabismus. Strabismus symptoms often include: blurred vision, double vision, tired eyes, and light sensitivity.