Diseases & Conditions
Amblyopia, a common cause of vision loss in children, is a decrease in vision that occurs because the brain ignores the image received from an eye. Vision loss may be irreversible if not diagnosed and treated before age 8.
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 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. 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. There are several reasons for lack of proper visual stimulation, each of which can produce a type of amblyopia.
Refractive Amblyopia: Amblyopia may be caused by an uncorrected or unequal refractive error, usually farsightedness or astigmatism, particularly when there is a large difference between the two eyes.
Strabismic Amblyopia: 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.
Deprivation Amblyopia: A third type of amblyopia develops when a clouding or opacity of the lens of the eye (cataract) or the cornea reduces or distorts the light entering an eye.
Symptoms and Diagnosis
Children with amblyopia 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. 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. Thus, to detect problems in visual development, vision screening for all children should be started during early well-child examinations 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 performed 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.
Treatment and Prognosis
Treating amblyopia involves forcing the brain to use the visual images from the problem eye. Sometimes this is accomplished simply by correcting refractive errors with eyeglasses. More often, doctors "handicap" the normal, stronger eye by putting a patch over it or using eye drops to blur vision, in the sound eye. If strabismus is the cause, it should be corrected (see Eye Disorders in Children: Strabismus ) after the vision has been equalized between the eyes. A cataract or other opacity in the eye may require surgical treatment
Treatment should be initiated promptly, preferably during the first 2 to 4 years of life. The earlier the treatment is initiated, the quicker the response will be. Amblyopia from any cause that has not been treated by age 8 usually cannot be fully reversed. Failure to effectively treat amblyopia may result in permanent blindness in the affected eye.
The sooner amblyopia or risk factors for amblyopia are detected, the more likely amblyopia can be prevented or corrected. For these reasons, vision screening programs for children should be supported by the community.
Last full review/revision April 2006 by Albert W. Biglan, MD
Source: The Merck Manual Home Edition