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Amblyopia may be caused by any condition that affects normal visual development or use of the eyes. Amblyopia can be caused by strabismus, an imbalance in the positioning of the two eyes. Strabismus can cause the eyes to cross in (esotropia) or turn out (exotropia). Sometimes amblyopia is caused when one eye is more nearsighted, farsighted, or astigmatic than the other eye. Occasionally, amblyopia is caused by other eye conditions such as cataract.

Anything that interferes with clear vision in either eye during the critical period (birth to 6 years of age) can cause amblyopia. The most common causes of amblyopia are constant strabismus (constant turn of one eye), anisometropia (different vision/prescriptions in each eye), and/or blockage of an eye due to cataract, trauma, lid droop, etc.

Amblyopia is a neurologically active process. In other words, the loss of vision takes place in the brain. If one eye sees clearly and the other sees a blur, the brain can inhibit (block, ignore, suppress) the eye with the blur. The brain can also suppress one eye to avoid double vision. The inhibition process (suppression) can result in a permanent decrease in the vision in the blurry eye that can not be corrected with glasses, lenses, or lasik surgery.

There is a difference in the refractive state of the two eyes, such that one eye may be normal and not require a corrective lens, while the other eye is too nearsighted (myopic) or too farsighted (hyperopic). The condition is called Anisometropia, or difference in refractive error between the two eyes. As a consequence, the eye that needs a corrective lens will have a blurred image projected to the back of the eye (retina) and, in turn, the brain receives a blurred image from that eye. The eye (actually the brain) then becomes amblyopic. This condition is also called anisometropic amblyopia and some eye doctors may also call it refractive amblyopia. Anisometropic amblyopia is very insidious; the child looks perfectly normal but one eye is really very amblyopic. Treatment is often delayed because the parents think that the child is fine and that there is no reason to see an eye doctor for a comprehensive eye exam. As a consequence, children with anisometropic amblyopia are detected later and may have worse visual acuity and the child more difficult to treat and cure.

When an infant or young child has an uncorrected refractive error; this is, s/he needs glasses but no one knows s/he needs glasses, the blurred vision caused by the uncorrected refractive error can lead to amblyopia in one or both eyes. About 5% of amblyopic children have amblyopia due to an uncorrected refractive error. Again, this is why it is so important that infants and young children see an eye doctor for a comprehensive eye exam. This type of amblyopia is sometimes referred to as refractive amblyopia.

Diffractive errors like astigmatism, farsightedness and nearsightedness are the other major causes. The eye affected with any of the diffractive error stops working. Amblyopia caused by this type of conditions is hard to detect as one eye will function normally. Sometimes, this type of conditions can affect both the eyes. Occasionally, droopy eyelids, scar on the cornea and cataract leads to amblyopia. Trauma to the eye or optic nerve at any age can result in amblyopic condition.

Most often people confuse strabismus with amblyopia. Both are entirely different form of eye conditions. It must be noted that only constant strabismus can cause amblyopia. Strabismus can be easily detected but amblyopia cannot be detected easily.

Amblyopia occurs most commonly with misaligned or crossed eyes. The crossed eye “turns off” to avoid double vision, and the child uses only the better eye. The misaligned eye then fails to develop good vision.Refractive errors are eye conditions that are corrected by wearing eyeglasses. Amblyopia occurs when one eye is out of focus because it is more nearsighted, farsighted, or astigmatic than the other.

The unfocused (blurred) eye “turns off” and becomes amblyopic. The eyes can look normal, but one eye has poor vision. This is the most difficult type of amblyopia to detect since the child appears to have normal vision when both eyes are open. Amblyopia can also occur in both eyes if both eyes have very blurred vision. This can happen when there is a high degree of nearsightedness, farsightedness, or astigmatism.

An eye disease such as a cataract (a clouding of the eye’s naturally clear lens) may lead to amblyopia. Any factor that prevents a clear image from being focused on the retina at the back of the eye can lead to the development of amblyopia in a child. This is often the most severe form of amblyopia.