There are two different definitions for the term wall eye. In one definition of wall eye, one eye has a very light iris, not corresponding to the other eye’s color. Horses with one very light eye may be said to be wall-eyed.
More commonly, when people use the term wall eye, they’re referring to a type of strabismus. This is a condition that can be treated, especially when caught early, and means that the eyes don’t focus in the same direction. A person with wall eye has one eye with an iris that doesn’t quite point in the same direction. The eye may look off to the side, upward or downward and isn't coordinated with the focus and movement of the unaffected eye. This may also be called squint eyes or lazy eyes. Typically only one eye is affected, and the other eye, if the wall eye is covered, tends to function well and normally.
Wall eye, like all forms of strabismus can affect depth perception in a most serious way, and when possible it should be treated when children are still toddlers. This is done usually through a combination of surgery, then eye exercises, special glasses, and medications. Treatment is most effective in children under 6, though children over 6 may still have good to excellent results if they were not diagnosed earlier.
A person with wall eye may have eyes that appear somewhat bulging or at least protrude a little bit more than is commonly seen, though this is not always present. In most cases, the lack of coordinated focus between the two pupils is easy to recognize and thus gets early treatment. The specific cause of the condition is unknown, though it appears that most forms of strabismus occur while an unborn child is still developing, and that what occurs is failure of appropriate development in the eye muscles. As yet there is no specific gene linked to this disorder and it may be more of a genetic mutation rather than an actual gene that might fail to develop the eye muscles properly.
The condition should not be confused with crossed eyes, which usually manifest as the eyes appearing to focus on the nose. Crossed eye can affect only one eye, but may also affect both. Cases of wall eye occurring in both eyes are extremely rare. Usually one eye works very well, while the other fails to provide the coordination needed for bifocal vision that accurately perceives depth.