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 permanent if the disorder is not diagnosed and treated early in childhood.
Amblyopia can be caused by focusing problems (refractive errors), misalignment of the eyes (strabismus), glaucoma, cataracts, or other eye problems.
Children can have no symptoms, or symptoms that include squinting, covering one eye, or having one eye that does not look in the same direction as the other.
The diagnosis is based on the results of vision testing.
Treatment includes eyeglasses or contact lenses, an eye patch, eye drops, or a combination.
If diagnosed and treated early, amblyopia can be corrected.
Amblyopia affects about 2 to 3% of children and usually develops before age 2. However, any child under about age 8 can develop amblyopia.
Causes of Amblyopia
A child's visual pathways (see figure Tracing the 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 lasts 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:
Misalignment of the eyes (strabismus)
Focusing problems (refractive errors)
Blockage of vision
Amblyopia caused by strabismus
Misalignment of the eyes (strabismus) can cause amblyopia. The eyes produce 2 images—1 from each eye—that normally are fused or united into a single image in the brain and then integrated to produce 3-dimensional images and high levels of depth perception. The ability to fuse images develops during early childhood. If the 2 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 2 different images results in double vision (diplopia) rather than in loss of vision.
Amblyopia caused by refractive errors
Amblyopia may be caused by an unequal refractive error, usually farsightedness (inability to see close objects clearly), nearsightedness (inability to see distant objects clearly), or astigmatism (an irregular curvature of the focusing surfaces of the eye).
A refractive error causes blurring of the image or images reaching the brain, which results in a large difference in focus between the 2 eyes. These errors can develop in one or both eyes.
Amblyopia caused by blockage or decrease of vision
A third type of amblyopia develops when a clouding or opacity of the lens of the eye (such as from a congenital cataract) or of the cornea reduces or distorts the light entering an eye or when another cause (such as glaucoma) leads to decreased vision in an eye. A very droopy eyelid also can block vision and cause amblyopia.
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Symptoms of Amblyopia
Amblyopia often does not cause symptoms. It is commonly found only during a routine vision examination.
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 complete cataract may cause a white pupil (leukocoria) that may be seen in photos, but a partial cataract may go unnoticed. Some older children may report impaired vision in the affected eye or exhibit poor depth perception. Often, however, children 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.
Diagnosis of Amblyopia
Early and periodic vision screenings
To detect problems in visual development, vision screening for all children is done immediately after birth and is repeated at well-child examinations throughout childhood. In some areas, preschool children are screened by volunteers and local and regional agencies. If a child is not able to do a vision test with an eye chart that has pictures, figures, or letters by 3 or 4 years of age, the child should be evaluated by an eye care specialist.
Once children reach school age, screening is also done in school by health care professionals. If a problem is found during screening, the child should see an eye doctor, either an ophthalmologist (a medical doctor who specializes in the evaluation and treatment of all types of eye disorders) or an optometrist (a health care professional who specializes in the diagnosis and treatment of vision or refractive problems).
Treatment of Amblyopia
Eyeglasses or contact lenses
Removal of cataracts
Patching or eye drops
To treat amblyopia, doctors first correct refractive errors by having the child wear eyeglasses or contact lenses and remove any cataracts. Once vision cannot be further improved with eyeglasses or contact lenses, doctors encourage the child to use the weaker eye by putting a patch over the better eye (patching) or using eye drops to blur the vision in the better eye. Patching or using eye drops in the better eye allows the weaker eye to get stronger.
If strabismus is the cause, doctors first try to equalize vision with patching or eye drops before correcting the strabismus with surgery.
Prognosis for Amblyopia
The sooner amblyopia or risk factors for amblyopia are detected, the more likely amblyopia can be prevented or corrected. Amblyopia may result in permanent vision loss if not diagnosed and treated early in childhood, at which time the visual system has often matured. Early diagnosis and treatment increase the likelihood that vision will be completely recovered. In certain circumstances, older children with amblyopia can still have vision improvement with treatment.
If amblyopia is not effectively treated, vision in the affected eye may be permanently impaired. For these reasons, vision screening programs for children should be supported by the community.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Children's Eye Foundation of AAPOS: Practical information about prevention, detection, research, and education to protect the vision of children
