Amblyopia, a common cause of vision loss in children, is a decrease in vision that occurs because the brain ignores the image received from one eye. Vision loss may be permanent if the disorder is not diagnosed and treated before age 8.
Amblyopia affects about 2 to 3% of children and usually develops before age 2. However, any child under about age 8 can develop amblyopia.
A child's visual pathways are not fully developed at birth. The vision system and the brain need to be stimulated by clear, focused, properly aligned, overlapping images from both eyes in order to develop normally. This development takes place mainly in the first 3 years of life but is not complete until about 8 years of age. If the brain does not receive proper visual stimulation from an eye during the development period, it learns to ignore (suppress) the image from that eye, resulting in vision loss. If the suppression persists long enough, vision loss can be permanent. This permanent loss of vision is called amblyopia. There are several reasons for lack of proper visual stimulation, each of which can cause a type of amblyopia:
Amblyopia caused by refractive error:
Amblyopia may be caused by an unequal refractive error, usually farsightedness or astigmatism, that causes a large difference in focus between the two eyes.
Amblyopia caused by strabismus:
Misalignment of the eyes (strabismus) can also cause amblyopia. The eyes produce two images—one from each eye—that normally are fused or united into a single image in the brain and then integrated to produce three-dimensional images and high levels of depth perception. The ability to fuse images develops during early childhood. If the two images are so misaligned that they cannot be fused together, the brain suppresses an image, ignoring the input from that eye. The brain is unaware of the image from the affected eye even though the eye may be structurally normal. In adults, because the visual pathways are already developed, seeing two different images results in double vision (diplopia) rather than in loss of vision.
Amblyopia caused by blockage of vision:
A third type of amblyopia develops when a clouding or opacity of the lens of the eye (cataract) or of the cornea reduces or distorts the light entering an eye. A very droopy eyelid also can block vision and cause amblyopia.
Children with amblyopia may not notice their vision in one eye differs from the other or may be too young to describe symptoms. These children may squint, cover one eye, or have one eye that does not look in the same direction as the other, all of which may indicate a problem that requires examination. A cataract may go unnoticed. Some older children may report impaired vision in the affected eye or exhibit poor depth perception. Children, however, often do not appear to have a problem. If one eye sees well and the other does not, children compensate well and do not seem to function differently from their peers.
To detect problems in visual development, vision screening for all children should be started during early well-child examinations, preferably around age 3, and continued throughout childhood. In some areas, preschool children are screened by volunteers and local and regional agencies. Once children reach school age, screening is done in school by health practitioners. If a problem is found during screening, the child should see an eye doctor, either an ophthalmologist or an optometrist.
The sooner amblyopia or risk factors for amblyopia are detected, the more likely amblyopia can be prevented or corrected. Amblyopia may become irreversible if not diagnosed and treated before age 8, at which time the visual system has often matured. Most children who are identified and treated before age 5 have some vision improvement. Earlier treatment increases the likelihood that vision will be completely recovered. In certain circumstances, older children with amblyopia can still have vision improvement with treatment. Failure to effectively treat amblyopia may result in permanent visual impairment in the affected eye. For these reasons, vision screening programs for children should be supported by the community.
Treating amblyopia involves forcing the brain to use the visual images from the affected eye. Sometimes this is accomplished simply by correcting refractive errors with eyeglasses or contact lenses. Sometimes, doctors "handicap" the normal, stronger eye by putting a patch over it or using eye drops to blur its vision. If strabismus is the cause, it should be corrected with surgery (see see Strabismus) after vision has been equalized between the eyes. A cataract or other opacity in the eye may require surgical treatment.
Last full review/revision September 2013 by Christopher Fecarotta, MD; Wendy W. Huang, MD, PhD