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 years of life but is not complete until about 8 years 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.
Refractive errors (astigmatism, myopia, or hyperopia) can cause amblyopia due to a blurring of the image or images reaching the brain. Anisometropic amblyopia occurs in cases of an inequality of refraction between the two eyes, resulting in a different focus of the retinal images, with the image from the eye with the greater refractive error being less well focused. Bilateral amblyopia may occur in cases of equally high refractive errors in both eyes, because the brain receives two blurred images.
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 before age 3.
Photoscreening is one approach for screening very young children and those 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, which do not require knowing the alphabet (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 refractive error 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. The earlier treatment is begun, the greater 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). In cases of refractive amblyopia, full-time glasses wearing with close monitoring may be sufficient treatment, especially in cases of bilateral amblyopia. Once improvement of visual acuity has plateaued with glasses wearing, occlusion therapy is begun (1). 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.
1. Writing Committee for the Pediatric Eye Disease Investigator Group, Cotter SA, Foster NC, et al: Optical treatment of strabismic and combined strabismic-anisometropic amblyopia. Ophthalmology 119(1):150–158, 2012. doi: 10.1016/j.ophtha.2011.06.043.
Amblyopia is visual loss in an 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, including photoscreening, and early diagnosis and early initiation of treatment are crucial to a successful outcome.
Treatment is directed at the cause (eg, correcting refractive error, removing cataracts, treating strabismus) followed by patching or administering atropine drops into the better eye to provide a visual advantage to the amblyopic eye.