Binocular Vision Orthoptics Strabismus

Binocular vision refers to the ability of the two eyes to work together in a coordinated way. Perfect binocular vision requires the two eyes to be aligned precisely (to within about 0.03°) in their fixation of the object of regard. Strabismus (synonyms: heterotropia, squint, cast, turning eye) describes the condition where the eyes manifestly fail to maintain alignment. Strabismus affects about 2.5% of the population. In young children, the image in the eye that turns will be at least partially suppressed to prevent double vision. This almost invariably results in a permanent visual loss (amblyopia, or "lazy eye") in that eye. This visual loss can be largely prevented or treated in young children (up to about 7 years) by patching the non-strabismic (normal) eye. The strabismus may be of a small angle and is often undetected by parents, and sometimes is not discovered by school screening tests.

Most types of strabismus are not usually thought to be associated with reading difficulties, and some authorities have suggested that a stable unilateral strabismus might actually be associated with better than average reading because the ocular dominance is well established. For this reason, it has been suggested, controversially, that excessive patching to treat amblyopia in early life could cause problems with unstable ocular dominance in the school years (Fowler & Stein, 1983). Although most cases of strabismus are unilateral, a few patients alternate freely from using one eye to the other. If this alternation occurs during reading, then this could in some cases cause a confused perception of text.

Eye care practitioners assess strabismus with a variety of tests, but the most important is the cover test (Evans, 2002). In this test, each eye is covered in turn while the patient fixes a target and the movement of the uncovered eye is observed. If, for example, the right eye is covered, and the left is seen to move in to take up fixation, then it can be deduced that there is a left divergent strabismus (the left eye is deviated outward). The cover test requires very little cooperation and no verbal response from the child.

It is perhaps surprising in most cases of strabismus, the eye muscles work perfectly. The misalignment of the eyes usually results from a problem with the brain's control of the eye muscles, rather than from a problem with the muscles themselves. Not all cases of strabismus require treatment, but those that do are variously treated with spectacles, eye exercises, or surgery, in addition to the patching described above.

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