Crossed eyes, also referred to as strabismus, is caused by a variety of factors that include congenital tumors, nerve disorders and weak eye muscles. According to WebMD, most individuals with strabismus are born with the disorder, and in many cases there is no underlying cause. When strabismus occurs in individuals who were not born with the disorder, it can indicate a stroke or serious medical issue.
In addition to eyes that turn towards the nose, strabismus can also cause one eye to look out to one side, look up or turn down. Children who are born with crossed eyes often favor one eye to avoid double vision. This can lead to amblyopia, or "lazy eye." Strabismus also affects depth perception and peripheral vision. It is treated with prescription eyeglasses and eye patches to properly align the eyes.
According to the American Optometric Association, misaligned eyes send two separate images to the brain, causing double vision. Eventually, the brain trains itself to ignore one of the images to eliminate double vision. There are four types of strabismus that are classified by the way the eyes turn. When the eyes turn inward, it is referred to as esotropia. When the eyes turn outward, it is called exotropia. Upward turning eyes are referred to as hypertropia. Eyes that turn downward are classified as hypotropia.