Strabismus

Definition

Strabismus is a condition in which the eyes do not point in the same direction. It can also be referred to as a tropia or squint.

Description

Strabismus occurs in 2-5% of all children. About half are born with the condition, which causes one or both eyes to turn:

• Inward (esotropia or "crossed eyes")
• Outward (exotropia or "wall eyes")
• Upward (hypertropia)
• Downward (hypotropia).

Strabismus is equally common in boys and girls. It sometimes runs in families.

Types of strabismus

Esotropia is the most common type of strabismus in infants. Accommodative esotropia develops in children under age two who cross their eyes when focusing on objects nearby. This usually occurs in children who are moderately to highly farsighted (hyperopic).

Another common form of strabismus, exotropia, may only be noticeable when a child looks at far-away objects, daydreams, or is tired or sick.

Sometimes the eye turn is always in the same eye; however sometimes the turn alternates from one eye to the other.

Most children with strabismus have comitant strabismus. No matter where they look, the degree of deviation does not change. In incomitant strabismus, the amount of misalignment depends upon which direction the eyes are pointed.

False strabismus (pseudostrabismus)

A child may appear to have a turned eye, however this appearance may actually be due to:

• Extra skin that covers the inner corner of the eye
• A broad, flat nose
• Eyes set unusually close together or far apart.

This condition, false strabismus, usually disappears as the child's face grows. An eye doctor needs to determine whether the eyeturn is true or pseudostrabismus.

With normal vision, both eyes send the brain the same message. This binocular fixation (both eyes looking directly at the same object) is necessary to see three-dimensionally and to aid in depth perception. When an eye is misaligned, the brain receives two different images. Young children learn to ignore distorted messages from a misaligned eye, but adults with strabismus often develop double vision (diplopia).

A baby's eyes should be straight and parallel by three or four months of age. A child who develops strabismus after the age of eight or nine years is said to have adult-onset strabismus.

Causes & symptoms

Strabismus can be caused by a defect in muscles or the part of the brain that controls eye movement. It is especially common in children who have:

• Brain tumors
• Cerebral palsy
• Down syndrome
• Hydrocephalus
• Other disorders that affect the brain.

Diseases that cause partial or total blindness can cause strabismus. So can extreme farsightedness, cataracts, eye injury, or having much better vision in one eye than the other.

In adults, strabismus is usually caused by:

• Diabetes
• Head trauma
• Stroke
• Brain tumor
• Other diseases affecting nerves that control eye muscles.

The most obvious symptom of strabismus is an eye that isn't always straight. The deviation can vary from day to day or during the day. People who have strabismus often squint in bright sunlight or tilt their heads to focus their eyes.

Diagnosis

Every baby's eyes should be examined by the age of six months. A baby whose eyes have not straightened by the age of four months should be examined to rule out serious disease.

A pediatrician, family doctor, ophthalmologist, or optometrist licensed to use diagnostic drugs uses drops that dilate the pupils and temporarily paralyze eye-focusing muscles to evaluate visual status and ocular health. Early diagnosis is important. Some eye turns may be a result of a tumor. Untreated strabismus can damage vision in the unused eye and possibly result in lazy eye (amblyopia).

Treatment

Preserving or restoring vision and improving appearance may involve one or more of the following:

• Glasses to aid in focusing and straighten the eye(s)
• Patching to force infants and young children to use and straighten the weaker eye
• Eye drops or ointments as a substitute for patching or glasses, or to make glasses more effective
• Surgery to tighten, relax, or reposition eye muscles
• Medication injected into an overactive eye muscle to allow the opposite muscle to straighten the eye
• Vision training (also called eye exercises).

Prognosis

Early consistent treatment usually improves vision and appearance. The most satisfactory results are achieved if the condition is corrected before the age of seven years old.

Further Reading

For Your Information

Organizations

• American Academy of Ophthalmology. P.O. Box 7424, San Francisco, CA 94120-7424. (415) 561-8500. http://www.eyenet.org/
• American Academy of Pediatric Ophthalmology and Strabismus (AAPOS). http://med-aapos.bu/edu/
• American Optometric Association. 243 North Lindbergh Blvd., St. Louis, MO 63141. (314) 991-4100. http://www.aoanet.org

  Other

• Crossed Eyes (Strabismus). http://www.webxpress.com/vhsc/ces.html (8 May 1998)
• Strabismus. http://www.eyenet.org/public/faqs/strabismus_faq.html (7 May 1998)
• Strabismus (Crossed Eyes) http://www.theeyestation.com/other/strabism.htm (8 May 1998)

Gale Encyclopedia of Medicine. Gale Research, 1999.