Strabismus

What Is Strabismus?

Strabismus is a visual defect in which the eyes are misaligned and point in different directions. The misalignment may always be noticeable, or it may come and go. One eye may be directed straight ahead, while the other eye is turned inward, outward, upward or downward. The turned eye may straighten at times and the straight eye may turn.

Strabismus is a common condition among children, affecting about four percent, but can also occur later in life. It occurs equally in males and females and may run in families. However, many people with strabismus have no relatives with the problem.

Vision and the Brain

With normal binocular (two-eyed) vision, both eyes are aimed at the same target. The visual portion of the brain fuses the two pictures into a single 3 dimensional image.

When one eye turns, as in strabismus, two different pictures are sent to the brain. In a young child, the brain learns to ignore the image of the misaligned eye and sees only the image from the straight or best seeing eye. This causes loss of depth perception and binocular vision. Adults who develop strabismus often have double vision because the brain is already trained to receive images from both eyes and cannot ignore the image from the turned eye.

Amblyopia

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. This occurs in approximately half the children who have strabismus.

Amblyopia can be treated by patching the preferred or better seeing eye to strengthen and improve 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 later, amblyopia or reduced vision generally becomes permanent. As a rule, the earlier amblyopia is treated, the better the visual result.

Causes and Symptoms of Strabismus

The exact cause of the eye misalignment that leads to strabismus is not fully understood.

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.

The brain controls the eye muscles which explains why children with disorders that affect the brain, such as cerebral palsy, Down's syndrome, hydrocephalus, and brain tumors often have strabismus. A cataract or eye injury that affects vision can also cause strabismus.

The primary symptom of strabismus is an eye that is not straight. Sometimes a youngster will squint one eye in bright sunlight or tilt their head in a specific direction to use their eyes together. Signs of faulty depth perception may also be noticed. Adults who acquire strabismus will usually have double vision.

Detection and Diagnosis

Children should be examined by the family doctor, pediatrician, or ophthalmologist (a medical eye doctor) during infancy and preschool years to detect potential eye problems. This is particularly important if a relative has had strabismus or amblyopia.

In infants it is often difficult to determine the difference between eyes that appear to be crossed and true strabismus. Young children often have a wide, flat nose and a fold of skin at the inner eyelid that tends to hide the eye during side gaze, causing the eyes to appear crossed. This appearance of strabismus may improve as the child grows. True strabismus is not outgrown. An ophthalmologist can readily distinguish true from false strabismus.

Treatment

Treatment goals for strabismus are to preserve vision, to straighten the eyes, and to restore binocular vision. Depending on the cause of the strabismus, treatment may involve repositioning the unbalanced eye muscles, removing a cataract, or correcting other conditions which are causing the eyes to turn. After a complete eye examination, including a detailed study of the inner parts of the eye, an ophthalmologist can recommend appropriate optical, medical or surgical therapy. Covering or patching the good eye to improve vision in the amblyopic eye is often necessary.

Esotropia

The two most common types of strabismus are esotropia and exotropia. Esotropia describes an inward turning eye and is the most common type of strabismus in infants. Young children with esotropia do not use their eyes together. In most cases, early surgery to align the eyes is needed to obtain binocular vision and prevent permanent vision loss. Surgery at any age may result in improved side vision.

During surgery, the tension of the eye muscle in one or both eyes is adjusted. For example, in surgery for esotropia, the tight inner muscles may be removed from the wall of the eye and placed further back on the eye. This weakens their pull and allows the eyes to move outward. Sometimes the outer muscles are tightened by shortening the muscle length to allow the eyes to move outward.

Accommodative Esotropia

Accommodative esotropia is a common form of esotropia which occurs in farsighted children, usually two or older. When a child is young, they can focus their eyes to adjust for the farsightedness but the focusing effort (accommodation) required to see clearly stimulates the eyes to cross.

Glasses reduce the focusing effort and can straighten the eyes. Sometimes bifocals are necessary for close work. Eye drops, ointment or special lenses called prisms can also be used to straighten the eyes. Eye exercises occasionally help older children.

Exotropia

Exotropia, or an outward turning eye, is another common type of strabismus. This occurs most often when a child is focusing on distant objects. Often the exotropia will occur intermittently, particularly when the child is daydreaming, ill, or tired. Parents often notice that the child squints one eye in bright sunlight. Although glasses, exercises or prisms may reduce or help control the outward turning eye in some children, surgery is often needed.

Strabismus Surgery

The eyeball is never removed from the socket during any kind of eye surgery. Strabismus surgery involves making a small incision in the tissue covering the eye which allows the ophthalmologist access to the underlying eye muscles. Which eye muscles are repositioned during the surgery depends upon the direction the eye is turning. It may be necessary to perform surgery on one or both eyes.

When strabismus surgery is performed on children, a general anesthetic is required. Local anesthesia is an option for adults. Recovery time is rapid. People are usually able to resume their normal activities within a few days. After surgery, glasses or prisms may be useful. Over-or-under correction can occur and further surgery may be needed.