Strabismus is a visual defect in which the eyes are misaligned (not straight) and points in different directions. Usually, one eye can look straight-ahead while the other eye turns inward (towards the nose), outward (away from the nose), upward, or downward. At times, the turned eye may straighten out while the straight eye becomes turned. Strabismus tends to be common, occurring in about 4% of children. Strabismus can run in families or it may be sporadic in occurrence.
A person who has straight eyes can aim their eyes at the same target. This straight-eyed person generally is capable of superimposing the image from each eye into one, single 3-dimensional image. We call this binocular vision. A person who has strabismus, however, has one eye aimed at one target and the other eye looking at another target. In a young child, the brain ignores (suppresses) the image from the turned eye and sees only the image from the turned eye, and so the brain receives 2 different images, one from the straight eye and one from the turned eye. When this happens, the person sees double (diplopia).
Normal alignment of both eyes during childhood allows good vision to develop in each eye. Abnormal alignment, as in strabismus, may cause reduced vision or amblyopia. The brain will recognize the image of the better-seeing eye and ignore the image of the weaker or amblyopic eye. Amblyopia can be treated by aligning the eyes and by patching the preferred or better-seeing eye to straighten and improve the vision in the weaker eye. If amblyopia is detected in the first few years of life, treatment is often successful. If adequate treatment is delayed until late, amblyopia or reduced vision generally becomes permanent. As a rule, the earlier amblyopia is treated, the better the visual result.
There are many different causes of strabismus. Six eye muscles, controlling eye movement, are attached to the outside of each eye. In each eye, two muscles move the eye right or left. The other four muscles move it up or down and control tilting movements. To line up and focus both eyes on a single target, all eye muscles of each eye must be balanced and working together with the corresponding muscles of the opposite eye. It is the brain which controls these eye muscles and so very often strabismus is caused by a mis-wiring, or bad connection, from the brain to the eyes. Poor vision in one eye, such as a cataract, also may cause strabismus. Another common cause of strabismus is hyperopia (farsightedness) which may cause the eyes to turn inward (crossed-eyes).
Esotropia means that the misaligned eye is turned inward toward the nose(crossed eyes). If a child is very farsighted (hyperopia), this may cause the eye to turn inward and we call this accommodative esotropia. Usually, glasses will help to straighten the eyes. If a child develops esotropia shortly after birth, we term this infantile esotropia and usually surgery is necessary to properly align the eyes. Exotropia means that the misaligned eye is turned outward, away from the nose. Glasses and patching can be used to treat exotropia, but often surgery is needed.
Treatment for strabismus depends on the type of strabismus a person has. Whenever possible, medical treatment consisting of glasses, patching, drops, and sometimes exercise is used. Amblyopia is also treated, if present. Surgery is used as a last resort if medical treatment will not work. The goals of treatment are to achieve adequate alignment that allows for the development of binocular vision and improved side vision.