What is amblyopia?

Normal vision develops with regular, equal use of the eyes. Amblyopia, commonly called "lazy eye," usually occurs when one eye is not used enough for the visual system in the brain to develop properly. The brain ignores the images from the weak eye and uses only those from the stronger eye, which leads to poor vision. Amblyopia usually affects only one eye, but it may occur in both eyes. Children can develop amblyopia between birth and about age 7.

A child with amblyopia may not even know that he or she is using only one eye. Ignoring the image from the weak eye is an unconscious response over which the child has no control.

What causes amblyopia?

Any condition that prevents a child's eyes from forming a clear, focused image or prevents the normal use of one or both eyes can result in amblyopia.

Many cases of amblyopia result from eyes that look at two different points in space at the same time, sending two different images to the brain. This condition is called strabismus. In a young child with strabismus, the brain learns to avoid the confusion of two images by ignoring (suppressing) the image from one eye. For more information, see the topic Strabismus.

Amblyopia may also develop if a child is much more nearsighted or farsighted in one eye than in the other. If one eye sees much more clearly than the other, the brain learns to ignore the blurry image from the weaker eye. Amblyopia can develop in both eyes if they are very nearsighted or very farsighted.

Conditions that prevent light from entering the eye for a long period of time can also cause amblyopia. A defect in the lens, such as a cataract, or in the clear "window" at the front of the eye (the cornea) may cause amblyopia in this way. Amblyopia from these causes is rare but serious. Without early treatment, the child may never develop normal vision in the affected eye.

What are the symptoms?

Some children with amblyopia may appear to have an eye that wanders or does not move with the other eye. But in most cases, amblyopia does not cause symptoms. Therefore, early detection is important.

Other symptoms of amblyopia include eyes that do not move in the same direction or fix on the same point, crying or complaining when one eye is covered, squinting or tilting the head to look at something, or an upper eyelid that droops.

What increases the risk of amblyopia?

Factors that put a child at higher risk of developing amblyopia include:

• Misaligned eyes (strabismus).
• Unequal vision in the eyes, such as one eye being much more nearsighted or farsighted than the other.
• Extreme nearsightedness or farsightedness in both eyes.
• Anything that prevents light from passing through the eye, such as a defect in the cornea or the lens (cataract) or, in rare cases, a droopy eyelid.
• A family history of amblyopia or strabismus.
• Low birth weight.
• Premature birth.

How is amblyopia diagnosed?