Amblyopia, sometimes called "lazy eye," is a visual impairment that results when vision is much stronger in one eye than the other for any one of several reasons. If the condition goes uncorrected, the brain fails to form the appropriate connections with the weaker eye, essentially ignoring its input so that the disability becomes permanent.
Strabismus
According to the Mayo Clinic, strabismus is the most common cause of amblyopia. Strabismus refers to a muscle weakness that allows one eye to wander in relation to the other, so that it turns outward (extropia) or inward (estropia) independently. The brain essentially turns off input from this wandering eye because it cannot reconcile the misaligned images that it receives.
Structure
Often the eyes look completely normal in cases of amblyopia. In other cases, there might be an obvious structural abnormality. One eye might be significantly larger than the other, or it might have an abnormal shape.
Cataract
Cataract is the medical term for clouding over of the normally clear lens of the eye. The clouding scatters light as it comes through the lens so that the retina cannot produce a sharp, clear image to send to the brain. Usually associated with aging, cataracts can also be present from birth, especially in babies born to mothers with certain types of infections, or those with a genetic syndrome that produces cataracts. If the cataract is not surgically removed, amblyopia can result. Amblyopia due to cataract is classified as deprivation amblyopia because the eye itself does not receive adequate input.
Refractive
Refractive amblyopia occurs when the problem lies with the retina, the focal point of the eye. In cases of a severe imbalance in the level of astigmatism, near- or far-sightedness between the two eyes, the brain will shut down the connections with the more affected eye. Although glasses can help with this type of amblyopia, often corrective treatment such as patching the dominant eye becomes necessary, too. With refractive amblyopia, the eyes usually appear normal, and the problem surfaces only upon a standard vision test.
Toxic
Toxic amblyopia occurs due to damage to the portion of the optic nerve near the eyeball. The damage can result from severe malnutrition, often seen in alcoholics, according to the Merck Manual, or from chemicals such as lead, methanol and chloramphenicol. If the source of the damage is rapidly identified and removed, vision can return to normal, but if the optic nerve atrophies, the loss of vision will be permanent.
