Amblyopia is functional reduction in visual acuity of an eye caused by disuse during visual development. Severe loss of vision can occur in the affected eye if amblyopia is not detected and treated before age 8. Diagnosis is based on detecting a difference in best corrected visual acuity between the two eyes that is not attributable to other pathology. Treatment depends on the cause.
Amblyopia affects about 2 to 3% of children and usually develops before age 2; however, any child under about age 8 can develop amblyopia.
The brain must simultaneously receive a clear, focused, properly aligned 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.
There are 3 causes:
Strabismus can cause amblyopia because misalignment of the eyes results in different retinal images being sent to the visual cortex. When this misalignment occurs, a child's brain can pay attention to only one eye at a time, and the input from the other eye is suppressed. Because the visual pathways are already fully developed in adults, presentation of 2 different images results in diplopia rather than suppression of one image.
Anisometropia (inequality of refraction in the 2 eyes due to astigmatism, myopia, or hyperopia) can also cause amblyopia because it 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 congenital cataract) interferes with or completely prevents formation of a retinal image in the affected eye. This obstruction can cause amblyopia.
Amblyopia is often asymptomatic and is commonly uncovered only on routine vision screening. Children rarely complain of unilateral vision loss, although they may squint or cover one eye. Very young children 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.
Screening for amblyopia (and strabismus) is recommended for all children before starting school, optimally around age 3.
Photoscreening is one approach for screening very young children who are unable to undergo subjective testing because of learning or developmental disorders. 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, HOTV figures or characters) or Snellen eye charts.
Identifying the underlying cause requires additional testing. Strabismus can be confirmed with the alternate cover test or the cover-uncover test (see diagnosis of strabismus). Ophthalmologists can confirm anisometropia by doing a refraction on each eye. Obstruction of the visual axis can be confirmed by ophthalmoscopy or slit-lamp examination.
Amblyopia may become irreversible if not diagnosed and treated before age 8, at which time the visual system has often matured. Most children identified and treated before age 5 have some vision improvement. Earlier treatment increases the likelihood of complete vision recovery. In certain circumstances, older children with amblyopia can still have vision improvement with treatment. Recurrence (recidivism) is possible in certain cases until the visual system matures. Some patients have a small decrease in visual acuity even after visual maturity has occurred.
Treatment of amblyopia should be directed by an ophthalmologist experienced in managing eye disorders in children. Any underlying causes must be treated (eg, eyeglasses or contact lenses to correct refractive error, removal of a cataract, treatment of strabismus). 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 age 8 to 10.
Amblyopia is visual loss in one eye caused by lack of clearly focused, properly aligned input to the visual cortex from each eye during early childhood prior to maturation of the visual pathways.
Diagnosis is mainly by screening tests, which should be done at about age 3 yr.
Treatment is directed at the cause (eg, correcting refractive error, removing cataracts, treatment of strabismus) followed by patching or administering atropine drops into the better eye.