When one eye develops good vision while the other does not, the eye with the poorer vision is called amblyopic. Usually, only one eye is affected by amblyopia, but it is possible for both eyes to be "lazy." The condition is common; approximately two or three out of every 100 people has amblyopia.
Amblyopia is caused by any condition that affects normal use of the eyes and visual development. In many cases, amblyopia may be hereditary, or passed down through the family. Amblyopia has three major causes: (1) Strabismus (misaligned eyes) - Amblyopia occurs most commonly with strabismus, or 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. (2) Unequal focus and refractive errors - Refractive errors are eye conditions that are corrected by wearing eyeglasses or contact lenses. Amblyopia occurs when one eye is out of focus because it is more nearsighted, farsighted, or astigmatic than the other. When this is the case, the brain will use the better-seeing eye and essentially "turn off" vision from the weaker eye. (3) Cloudiness in the normally clear eye tissues - 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. Cloudiness of normally clear eye tissues is often the most severe form of amblyopia.
In order to have normal vision, it is important that both eyes develop equal vision. If a child has amblyopia and cannot use his or her eyes together normally, vision does not develop properly and may even decrease. After the first nine years of life, the visual system is normally fully developed and usually cannot be changed. The treatment for amblyopia depends on the underlying cause, but the best time to correct amblyopia is during infancy or early childhood.