Merck Manual

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(Lazy Eye)


Leila M. Khazaeni

, MD, Loma Linda University School of Medicine

Reviewed/Revised Feb 2022 | Modified Sep 2022

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.

  • If diagnosed and treated early, amblyopia can be corrected.

  • Treatment includes eyeglasses or contact lenses, an eye patch, eye drops, or a combination.

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 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 Strabismus Strabismus is an intermittent or constant misalignment of an eye so that its line of vision is not pointed at the same object as the other eye. If untreated, strabismus can cause amblyopia ... read more Strabismus ) can 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 Double Vision Double vision is seeing 2 images of one object. Double vision may occur when only one eye is open (monocular diplopia) or, more commonly, when both eyes are open (binocular diplopia). Binocular... read more (diplopia) rather than in loss of vision.

Amblyopia caused by refractive errors

Amblyopia may be caused by an unequal refractive error Refractive Disorders in Children In refractive disorders, the eye is not able to properly focus images on the retina, causing blurred vision. Refractive disorders result in blurring of vision. Children may be unable to make... read more , 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 two eyes. These errors can develop in one or both eyes.

Amblyopia caused by blockage or decrease of vision

Did You Know...

  • Sometimes a teacher or school nurse is the first to notice a child has an eye disorder.

Symptoms of 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 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 Preventive Health Care Visits in Children Scheduled visits to the doctor (also called well-child visits) provide parents with information about their child's growth and development. Such visits also give parents an opportunity to ask... read more Preventive Health Care Visits in Children 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 practitioners. 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 practitioner who specializes in the diagnosis and treatment of vision or refractive problems).

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. 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.

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 Eyeglasses Refractive errors can be corrected with glass or plastic lenses mounted in a frame (eyeglasses) or with a small lens made of plastic floating or resting on the cornea (contact lens). Good vision... read more or contact lenses Contact Lenses Refractive errors can be corrected with glass or plastic lenses mounted in a frame (eyeglasses) or with a small lens made of plastic floating or resting on the cornea (contact lens). Good vision... read more and remove any cataracts. Once vision cannot be further improved with eyeglasses or contact lenses, doctors force 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.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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