STRABISMUS ("Crossed Eyes")
Excerpted from Triad's Eye Care Notes © 1989-2005 by Triad Publishing Co.

Strabismus (struh-BIZ-muss) is a term that describes eyes that are not properly aligned or do not move together as they should. One eye may look straight ahead, while the other eye turns inward, outward, upward, or downward.

For the two eyes to remain aligned, they need to have similar vision and focusing ability, and the muscles that move them need to work together. Only then can a person have binocular vision and depth perception, meaning that the images from each eye are fused (blended) by the brain into a single image that appears three-dimensional. If one eye does not look in the same direction as the other, binocular vision cannot exist.

In a young child, the deviating eye may eventually lose its ability to see clearly. This is called amblyopia, or "lazy eye." Strabismus affects about four percent of all children, boys and girls equally, and tends to run in families.

What Causes Strabismus?

Most often, there is no identifiable cause -- the child is simply born with a misalignment or develops it early in childhood. But there are also many known causes: for example, one eye that is blind or has defective vision from birth (as from a congenital cataract); one eye that is extremely nearsighted, farsighted or astigmatic, or the amount of eyeglass correction required by the two eyes is vastly different; one or more absent, injured or defective nerves to the eye muscles, causing the muscles controlled by the nerve to function improperly; damage to a part of the brain dealing with eye movement or eye muscle control; injury from trauma that damages any eye muscles or nerves; blindness from disease or injury. Intentionally crossing the eyes never causes strabismus; the eyes cannot get "stuck" in a crossed position.

Types of Strabismus

The most common type, in which one eye turns inward (crossed eyes), is esotropia. With exotropia (wall eyes), one eye turns out. Less common are hypertropia (one eye turns upward) and hypotropia (one eye turns downward). In some people it is always the same eye that deviates. In others the deviation shifts from one eye to the other; this is called "alternating."

Two other distinctions are important. A misalignment may be constant or intermittent. And it may be comitant or incomitant. Comitant strabismus, the type usually seen in children, means that no matter which way the eyes look, the amount of misalignment is the same. This is in contrast to incomitant strabismus, in which the amount of deviation is constantly changing, depending on which direction you look.

"Adult-onset strabismus" is any misalignment that comes on after normal binocular vision has developed, usually by age 8. Unlike childhood strabismus, the adult type usually creates symptoms, such as double vision, which may be accompanied by nausea.

What Is a Phoria?

"Tropia" is another word for strabismus (as in esoTROPIA). "Phoria" is a related and much more common condition in which the misalignment is only a tendency. The eyes appear aligned and work together normally because the phoria is kept under control, but it can be unmasked by covering either eye.

Phorias are named in the same way as tropias: esoPHORIA (tendency for one eye to turn in), exophoria (out), hyperphoria (up), and hypophoria (down). If a phoria is large, much (unconscious) effort may be needed to keep the eyes aligned and working together to avoid seeing double, and this effort may cause eyestrain and headache. But usually a phoria causes no symptoms at all.

"Intermittent" is another descriptive term related to strabismus. When strabismus is intermittent, the eyes are aligned and appear straight some of the time, but lapse into strabismus at other times. These lapses are more common with exo (outward) deviations than with other types. At the times when the eyes are straight, only an exophoria (a tendency toward exotropia) is present; if one eye suddenly turns out, the exophoria has become an exotropia. The tropia is more likely to appear late in the day, in the bright outdoors, or when you are ill. As the years go on, intermittent strabismus tends to become more constant and less intermittent.

Examination

The eyes should be examined as soon as you even suspect that they might be crossing or wandering, no matter how small the misalignment might be. No child is too young to be seen, and early care can prevent later heartache. The sooner treatment is begun, the better your child's chance for achieving normal vision in each eye and good binocular depth perception. Correction after the age of 6 or 7 is more difficult and the result less satisfactory.

A complete eye examination and refraction (measurement of vision and a check for glasses) involves the use of eyedrops to dilate the pupils and temporarily paralyze the focusing mechanism. Eye movements, quality and degree of stereopsis (3-D vision), and the ability to recognize double vision will all be checked, depending on the age and cooperation of the patient. Determination of the cause may involve referral to other types of specialists.

Treatment

For children, we would like to achieve normal appearance, good vision in each eye (with or without glasses), binocular vision, and depth perception. In adults, the goals are binocular vision (which eliminates double vision) and relief of any discomfort. If an adult has a childhood strabismus that was never treated, it is too late to improve any amblyopia or depth perception, so the goal may be simply cosmetic -- to make the eyes appear to be aligned -- though sometimes treatment does enlarge the extent of side vision.

Treatment may consist of eyeglasses, patching, eye coordination exercises (called orthoptics) and/or surgery on the eye muscles.

-- Eyeglasses, with or without patching, are often tried first and can usually reduce the amount of deviation. This is especially true for accommodative esotropia, a type of strabismus in which farsightedness is a major part of the problem. (Eyeglasses can be worn by infants as young as a few months.) The glasses must usually be worn constantly, often for life. If surgery is thought necessary, it is designed to correct only the deviation that remains with the glasses on.

-- Patching is the main treatment for infants and young children who have amblyopia (lazy eye). A patch is placed over the normal (preferred) eye, to force the use of the amblyopic eye until vision improves and equalizes. Generally, surgery is postponed until that happens. In adults, a patch over one eye is one method of eliminating any double vision. Prisms incorporated into the eyeglasses is another.

-- Orthoptic exercises may be useful when the deviation is slight or intermittent, and then only in very specific circumstances. Used inappropriately, orthoptics can be wasteful and can lead to delay in starting proper treatment.

-- Surgery consists of tightening some eye muscles and loosening others, to change their pull on the eyeball and bring the eyes into alignment. (Occasionally, a loosening effect can be accomplished without surgery by injecting a paralyzing medication, called Botox, directly into the muscle. The effect of this treatment does not last and may need to be repeated every few months.) Surgery is sometimes performed on infants as young as a few months of age when there is a good chance of obtaining binocular vision. During the first month or two following surgery, orthoptic exercises may be designed to develop the ability to use both eyes together normally. Many times, more than one operation is necessary to obtain good eye alignment. Glasses may also be required after surgery for the best possible visual result.

Prognosis

The outcome of treatment is dependent on many factors, such as the type of strabismus, age of onset, and visual acuity of each eye. It often involves years of commitment and care. Most patients can obtain comfort and a highly acceptable appearance with good eye alignment; some also gain fully normal function, with coordinated use of both eyes (binocular fusion and depth perception). Each patient's potential for a good result is different. This fact must be well understood to avoid disappointment.

Excerpted from Triad's Eye Care Notes © 1989-2005 by Triad Publishing Co.

Patients: for more information about your eyes, see: Taking Care of Your Eyes: A Collection of the Patient Education Handouts Used by America's Leading Eye Doctors