Strabismus

Strabismus is a condition in which the eyes cross or do not work together normally, which may lead to permanent loss of vision in one eye.

When people cross their eyes, the world suddenly doubles. Images, like the words on this page, become blurred, and it appears as if there are two of everything. When the eyes function correctly, they work together to focus images and to allow the brain to develop a three-dimensional view of the world. But when the eyes cannot work together, as when people cross their eyes, the brain sees two of everything. The result is double vision.

Fortunately, both eyes work together for most people. But some people have an eye disorder that causes the eyes to fail to line up properly, resulting in blurred or double vision. This condition is called strabismus (stra-BIZ-mus), which comes from a Greek word that means squinting. Often in strabismus, one eye may remain straight and the other eye may turn in, as if the person is crossing one eye.

Why Do People Have Strabismus?

Strabismus usually develops during infancy or early childhood. In most cases, there is no known cause, although sometimes several members of the same family have the disorder. This may mean that in some cases strabismus is inherited, like eye color. Other possible causes include:

• Farsightedness, causing focus difficulties
• Damage to one eye or to the part of the brain that controls the muscles involved in eye movement
• Other disorders that affect the brain, including Down syndrome, cerebral palsy, and hydrocephalus
• Less commonly, vision is blocked by a tumor ^* or cataract that causes cloudiness in the normally clear lens of the eye

Strabismus affects about 3 to 5 percent of children in the United States. It occurs in boys and girls equally. Fortunately, if it is diagnosed and treated early, there is a good chance of saving or improving vision in the affected eye.

Some adults have strabismus. This may be because they were not treated for it as a child, or because the treatment was not effective. Other adults may develop strabismus when a disorder such as stroke ^* causes the eyes to cross or not work together normally.

How Do Six Muscles in Each Eye Work as One?

The eyes and the nerves that connect them to the brain work like the two lenses of binoculars. They merge the images seen by each eye into one image. Six muscles are attached to each eye, and they control how the eyeball moves left and right or up and down. To make it possible for the brain to develop a single three-dimensional image, the muscles must work together to focus, just as the two lenses of binoculars must be aligned to focus together.

People with strabismus have trouble with one or more of the muscles in an eye. Instead of working together, one eye is out of step. Sometimes strabismus seems to come and go, depending on how tired the eyes are, and sometimes the eyes remain out of step. There are different forms of strabismus:

• When one eye points inward toward the nose, which makes the person look cross-eyed, the condition is called "esotropia" (es-o-TRO-pe-a).
• When one eye points away from the nose, as if looking to the outside, the condition is called "exotropia" (ek-so-TRO-pe-a) or "walleye."
• When the brain turns off the vision in the turned eye in favor of the vision in the straight eye, the condition is called "strabismic amblyopia" (stra-BIZ-mik am-blee-O-pee-a) or "lazy eye."

Amblyopia does not mean that the eye is lazy. Instead, the brain turns off the image coming from the optic nerve in that eye so the person sees only one clear image of the world instead of having blurred or double vision.

A Little Pirate

Mrs. Apple noticed that the eyes of her baby Chloe often did not work together. She had read in a book how babies sometimes appear cross-eyed or how it seems one eye is looking off in another direction from the other. This can be normal for a very young baby. But when Chloe was about four months old, Mrs. Apple became worried. Chloe's left eye seemed to be looking at her nose when Mrs. Apple moved her face close, and the right eye seemed to be looking straight ahead. Mrs. Apple took the baby to an ophthalmologist ^* for an eye exam and was told that Chloe likely had strabismus.

It is usually a parent who first notices the signs of strabismus when children are infants or preschoolers. The children are too young to complain about double or poor vision. If Mrs. Apple had not taken action because of her worries, the strabismus might have developed into ambly-opia, leaving Chloe without vision in the crossed eye. Without treatment, amblyopia may become permanent.

The ophthalmologist recommended that Chloe wear an eye patch over her normal eye. The doctor explained that this could force the weaker eye to develop vision more properly.

How Do Doctors Diagnose Strabismus and Amblyopia?

Diagnosis

Doctors use a variety of methods to diagnose strabismus and amblyopia. Most involve observation of how the child looks at objects, since most children are too young to recognize the letters on a standard eye chart. The doctor will cover one eye and then the other, holding and moving objects, and watching to see if the child squints or tries to cover or close one eye in favor of the other. The doctor also will check the alignment of the eyes by shining a light in both eyes to see if the reflection falls in the same place in the pupils (the black spot in the center of the eye) of both eyes.

Many children do not like to have their eyes covered during these exams. Some are frightened of the equipment that may be held close to their faces. New techniques under development use computers to track eye movements from a distance, sometimes while the child is watching a cartoon.

Treatment

The most common treatment of strabismus involves wearing a patch like Chloe's over the stronger eye. The brain now starts to try to send and receive signals from the weaker eye, and the muscles that control it try to bring the eye back to a normal focus. The same result often is achieved with eye drops that blur the vision in the normal eye to make the other eye work harder. Doctors also may prescribe special eyeglasses for some children with strabismus. Some of these eyeglasses use prisms that change how the image is sent into the eye.

New techniques involve disabling or weakening one or more of the muscles in the eye. This is done to force the other muscles to work harder to bring the affected eye into focus with the normal eye. This can be done surgically by repositioning the eye muscles of one or both eyes. The operations can leave the eyes straight and vision normal, although sometimes the eyes appear straight but people still need eyeglasses to achieve good vision. Sometimes, injections are used to disable one or more eye muscles for a period of time. This may achieve similar results to surgery.

Treatment is most effective when children are young, which is why vision testing and early diagnosis are important. Strabismus and ambly-opia do not simply go away, as some people believe. With treatment, children like Chloe can have almost normal vision and no restrictions on activities as they grow up.