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Strabismus

Strabismus results from eye misalignment, which produces nonparallel, uncoordinated eye movement that impairs vision. It's also known as heterotropia or slang terms, such as squint, cross-eye, or walleye.

Strabismus is the medical term referring conditions in which one or both eyes are not aligned properly. There are three common types of strabismus:

  1. Esotropia or in-turning of the eyes. This is often referred to as crossed eyes or a squint.
  2. Exotropia or out-turning eyes. This is also known as walleye, or divergent eyes.
  3. Finally, in Hypertropia there is vertical strabismus, meaning the eyes are out of alignment vertically.

Causes

Strabismus may be caused by unequal pulling of muscles on one side of the eye or a paralysis of the ocular muscles.

Occasionally, when a farsighted child tries to focus to compensate for the farsightedness, he or she will develop accommodative strabismus. This condition usually appears before two years of age, and can occur as late as six.

Signs and Symptoms

Adults are much more likely to be bothered by symptoms from strabismus than young children. It is unusual for a child to complain of double vision.  Children should undergo vision screening exams to detect problems early.  The younger the child is when strabismus is detected and treated, the better the chance of normal vision.  The following are common signs and symptoms:

  • Turned or crossed eye
  • Head tilt or turn
  • Squinting
  • Double vision (in some cases)

Diagnostic tests

If the onset of strabismus is sudden or if the CNS is involved, neurologic examination may be used to determine the origin (muscular or neurologic) of the condition.

Other diagnostic procedures are used to determine the best treatment. A visual acuity test is used to evaluate macular vision by determining the patient's vision from a 20' (6-m) distance. Hirschberg's method is used to measure the degree of deviation as the patient gazes at a light about 13" (33 cm) away the examiner notes how the cornea reflects light.

Retinoscopy determines refractive error (usually with the pupils dilated), the Maddox rod test is used to assess specific muscle involvement, a convergence test shows distance at which convergence is sustained and a duction test reveals eye movement limitatior's.

The cover-uncover test demonstrates eye deviation and the rate of recovery to original alignment. An alternate cover test shows intermittent or latem deviation.

Treatment

Therapy depends on the type of strabismus. For amblyopia, it includes patching the normal eye and prescribing corrective glasses to keep the eye straight and to counteract farsightedness (as in accommodative esotropia).

Surgery may correct strabismus related to basic esotropia or residual accommodative esotropia after correction with glasses. This may be done to correct misalignment and improve the patient's appearance..

The timing of surgery varies. For example, at age 6 months, an infant with equal visual acuity and obvious esotropia will have the deviation corrected surgically. But a child with unequal visual acuity and an acquired deviation will have the affected eye patched until visual acuity is equal. Then he may undergo surgery.

Surgical correction includes recession (moving the muscle posteriorly from its original insertion) or resection (shortening the muscle). In some patients combination surgery, such as the resection of one muscle and the recession of its antagonist, is required.

Other procedures involve transplanting a muscle to improve rotation of a paralyzed muscle or using adustable sutures. Complications may include over correction, undercorrection, slipped muscle, and globe perforation.

Postoperative therapy may include patching the affected eye and applying combination antibiotic and corticosteroid eyedrops. Corrective glasses may still be necessary, and surgery may have to be repeated.

The use of botulin neurotoxin A (Botox-A) may be used in conjunction with surgery. The toxin is injected into the extraocular muscle and interferes with the release of acetylcholine at the neuromuscular junction. The antagonist muscle is strengthened over a period of months while the injected muscle is weakened.

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