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 early in childhood. Diagnosis is based on detecting a difference in best corrected visual acuity between the 2 eyes that is not attributable to other pathology. Treatment depends on the cause.
Amblyopia affects about 2 to 3% (1) 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.
General reference
1. Hashemi H, Pakzad R, Yekta A, et al: Global and regional estimates of prevalence of amblyopia: A systematic review and meta-analysis. Strabismus 26(4):168–183, 2018. doi: 10.1080/09273972.2018.1500618
Etiology of Amblyopia
There are 3 causes:
Strabismus
Refractive errors
Obstruction of the visual axis
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 1 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 2 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 2 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. Ptosis also can block vision and cause amblyopia.
Symptoms and Signs of 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 complete cataract causing occlusion of the visual axis may cause leukocoria (a white reflex in the pupil, which may be seen in photos); however, a partial cataract may go unnoticed.
Diagnosis of Amblyopia
Early and periodic vision screenings
Photoscreening
Additional testing (eg, cover test, cover-uncover test, refraction, ophthalmoscopy, slit lamp)
Vision screening for amblyopia (and strabismus) begins immediately after birth with red reflex evaluation and is repeated at routine well-child checkups. Vision screening is most effective when age-appropriate screenings are done periodically throughout childhood. If a child is not able to do subjective vision testing with an eye chart by 3 or 4 years of age, referral to an eye care specialist is recommended (1).
Photoscreening is one approach for screening preverbal children and those who are unable to undergo subjective testing because of learning or developmental disorders. Photoscreening involves use of a special camera that analyzes red reflexes during fixation on a visual target to identify risk factors for amblyopia.
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 with 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.
Diagnosis reference
1. Loh AR, Chiang MF: Pediatric vision screening. Pediatr Rev 39(5):225–234, 2018. doi: 10.1542/pir.2016-0191
Treatment of Amblyopia
Eyeglasses or contact lenses
Cataract removal
Patching (occlusion therapy)
Treatment of strabismus if present
Treatment of amblyopia should be directed by an ophthalmologist experienced in managing eye disorders in children.
After correcting refraction (glasses or contact lenses) or clearing the visual axis (removal of a cataract), the mainstay of amblyopia treatment is occluding the better eye to force the brain to use the affected eye. In cases of refractive amblyopia, full-time wearing of glasses or contact lenses with close monitoring may be sufficient treatment, especially in cases of bilateral amblyopia.
Once improvement of visual acuity has plateaued with glasses wearing, patching (occlusion therapy) is begun (1). In the case of amblyopia caused by strabismus, occlusion therapy is done first, followed by treatment of the strabismus2).
Maintenance treatment for prevention of recurrences may be recommended for 1 to 2 years after improvement has stabilized.
Treatment references
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
2. Gunton KB: Advances in amblyopia: what have we learned from PEDIG trials? Pediatrics 131(3):540-547, 2013. doi: 10.1542/peds.2012-1622
Prognosis for Amblyopia
Amblyopia may result in permanent vision loss if not diagnosed and treated early in childhood, before the visual system has 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. The Amblyopia Treatment Studies have shown that amblyopia treatment may improve visual acuity if initiated even in early adolescence, up until 14 years of age (1).
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.
Prognosis reference
1. Scheiman M, Hertle R, Beck R, et al: Randomized trial of treatment of amblyopia in children aged 7 to 17 years. Arch Ophthalmol 123(4):437–447, 2005. doi: 10.1001/archopht.123.4.437
Key Points
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.
Early diagnosis and early initiation of treatment are crucial to a successful outcome.
