Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Lazy Eye (Amblyopia)

What Is It?

Lazy eye, also called amblyopia, is an eye problem that can occur in growing children. In the typical child with lazy eye, the right and left eyes have significantly different qualities of vision, so that the images produced by one eye are weak or distorted compared with the images produced by the other eye. Because the weak eye sends poorly focused images to the brain, the brain learns to depend on the stronger eye for its visual information. If this situation is not corrected, the brain eventually chooses to accept images from the stronger eye alone and ignores images from the weak one. In other words, the weak eye doesn't learn to see.

The brain's choice usually is made early in childhood when the brain's visual pathways are still developing. This critical period begins at birth and probably ends sometime between ages 6 and 9. If lazy eye is not diagnosed and treated within this critical period, the brain may choose to ignore the weak eye permanently, causing a lifelong loss of vision on that side.

Lazy eye has several causes, including:

• Crossed eyes (strabismus) — Children with crossed eyes often have double vision (diplopia) when they use both eyes at the same time. To prevent this, a child may consistently focus with one eye more than the other.

• Problems related to severe nearsightedness or farsightedness — When a child has nearsightedness (distant objects look blurry) or farsightedness (nearby objects look blurry), the problem may not affect both eyes equally. For example, one eye may have perfectly normal vision, while the other is blurred; or both eyes may be blurred, but one is worse than the other. In either situation, the brain gradually learns to ignore visual images from the eye that has poorer vision.

• Structural problems — Sometimes, a growing child's vision is blocked by a structural problem of the eye or eyelid. Common examples include a congenital cataract (an opaque area that develops inside the lens of the eye before birth), a scar on the cornea or congenital ptosis (a drooping eyelid that is present at birth).

In the United States, lazy eye affects an estimated 1% to 2% of the population. In rare cases, the brain ignores both eyes because both produce blurry images. This can cause permanent blindness in both eyes.