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Amblyopia
is functional reduction in visual acuity of an eye caused by disuse
during visual development. Blindness can occur in the affected eye if
amblyopia is not detected and treated before age 8 yr. Diagnosis
is based on detecting a difference in visual acuity between the
two eyes. Treatment depends on the cause.
Amblyopia affects about 2 to 3% of children and almost always develops before age 2.
The brain must simultaneously receive a clear, focused, properly aligned, overlapping image from each eye for the visual system to develop properly. This development takes place mainly in the first 3 yr of life but is not complete until about 8 yr of age. Amblyopia results when there is persistent interference with the image from one eye but not the other. The visual cortex suppresses the image from the affected eye. If suppression persists long enough, vision loss can be permanent.
Etiology
There are 3 causes. Strabismus (see Eye Defects and Conditions in Children: Strabismus) can cause amblyopia because misalignment of the eyes results in different retinal images being sent to the visual cortex. Because the visual pathways are developed in adults, presentation of two different images results in diplopia rather than suppression of one image.
Similarly, anisometropia (inequality of refraction in the two eyes, most often from astigmatism, myopia, or hyperopia) results in different focus of the retinal images, with the image from the eye with the greater refractive error being less well focused.
Obstruction of the visual axis at some point between the surface of the eye and the retina (eg, by a cataract) interferes with or completely prevents formation of a retinal image in the affected eye.
Symptoms and Signs
Children rarely complain of unilateral vision loss, although they may squint or cover one eye. Very young children either do not notice or are unable to express awareness that their vision differs in one eye compared with the other. Some older children may report impaired vision in the affected eye or exhibit poor depth perception. When strabismus is the cause, deviation of gaze may be noticeable to others. A cataract causing occlusion of the visual axis may go unnoticed.
Diagnosis
Screening for amblyopia (and strabismus) is recommended for all children before starting school, optimally around age 3 yr. Photoscreening is one approach for screening very young children and children with learning and developmental disorders who are unable to undergo subjective testing. Photoscreening involves use of a camera to record images of pupillary reflexes during fixation on a visual target and red reflexes in response to light; the images are then compared for symmetry. Screening in older children consists of acuity testing with figures (eg, tumbling E figures, Allen cards, or HOTV figures or characters) or Snellen eye charts.
Identifying the underlying cause requires additional testing. Strabismus can be confirmed with the cover test or the cover-uncover test (see Eye Defects and Conditions in Children: Diagnosis). Ophthalmologists can confirm anisometropia by performing a refraction to assess the refractive power of each eye. Obstruction of the visual axis can be confirmed by ophthalmoscopy or slit-lamp examination.
Prognosis
and Treatment
Amblyopia may become irreversible if not diagnosed and treated before age 8, at which time the visual system has matured. Most of those identified and treated before age 5 have some vision improvement. Earlier treatment increases the likelihood of complete vision recovery. Recurrence (recidivism) is possible in certain cases until the visual system matures. Some patients have a small decrease in visual acuity of a line or two even after visual maturity has occurred.
Treatment should be directed by an ophthalmologist. Any underlying causes must be treated (eg, eyeglasses or contact lenses to correct refractive error, removal of a cataract). Use of the amblyopic eye is then encouraged by patching the better eye or by administering atropine drops into the better eye to provide a visual advantage to the amblyopic eye. Adherence to treatment is better with drop therapy. Maintenance treatment for prevention of recurrences may be recommended after improvement has stabilized, until a child is about 8 to 10.
Last full review/revision June 2007 by Albert W. Biglan, MD
Content last modified June 2007
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