Loss of vision in a weak or "lazy" eye

Commonly referred to as lazy eye, amblyopia is a loss of vision in one or both eyes. Amblyopia is not correctable with prescription lenses, and no detectable damage to the eye or visual system is present. Most commonly the result of poor vision development in children, amblyopia affects about 4% of the population.

What Causes Amblyopia?
Amblyopia is caused by an abnormality which interferes with the normal use of the eyes and visual development. Commonly, amblyopia is caused by strabismus, a condition in which the eyes are misaligned. With strabismus, often referred to as cross-eyes, the eyes do not work together and one or both eyes turn in, out, up or down.

Amblyopia can also be caused by vision problems such as nearsightedness, farsightedness, and astigmatism. When there is a large difference in visual acuity between the eyes, both eyes do not have equal focusing ability. One eye is out of focus because of a stronger degree of nearsightedness, farsightedness, or astigmatism. Therefore, this weaker eye does not develop properly. The eye with the better visual acuity does most or all of the visual work, allowing the other eye to become lazy or amblyopic.

More rarely, amblyopia is caused by an eye disease, such as cataracts, which interferes with proper visual development. In many cases, amblyopia is inherited. Whatever the underlying cause, amblyopia is the result of long-standing suppression of one eye. The child favors the eye which sees best and the other eye does not develop fully.

How Does Amblyopia Affect Vision?
In normal vision, both eyes look at an object at the same time and each sends a picture to the brain. These two pictures are blended into one three-dimensional image. When one eye is weaker than the other, both eyes do not focus on one object simultaneously. As a result, two different images are sent to the brain. The image sent by the stronger eye is clear, while the message sent by the weaker eye is blurry.

Since the brain is unable to blend the two images into one picture, a child quickly and unconsciously learns to ignore the image seen by the weaker eye. The normal eye takes over, while the weaker eye becomes lazy from lack of use. If the condition is left uncorrected, the weak or lazy eye will eventually develop amblyopia, or a loss of vision.

What Are the Symptoms of Amblyopia?
Because it usually causes no symptoms, amblyopia often goes undetected. Unless the child has a misaligned eye or other obvious abnormality, there is nothing to suggest the problem to even the most perceptive parents. The child accepts having one good eye and one poor eye and considers amblyopia to be the normal situation. In most cases, amblyopia must be detected by checking vision.

How Is Amblyopia Diagnosed?
Amblyopia is often detected when a substantial difference in vision between each eye is found. Vision may be tested with the E test, in which the child is asked to point in the same direction as the fingers of a capital E on the eye chart. A series of progressively smaller E's is used to determine how well the child can see. In very young children, visual acuity can be estimated by how well the child follows a small object with one eye covered. A normal eye will look directly at an object, while an amblyopic eye may look to the side.

How Is Amblyopia Treated?
Unfortunately the treatment of the cause of amblyopia does not cure amblyopia itself. After the causal problem is corrected, amblyopia must be treated separately. To correct amblyopia, the weak eye must be forced to work by patching the good eye or using drops to blur vision in the good eye. By impairing vision in the good eye, the weak eye is forced to work until the vision in both eyes becomes equal.

Amblyopia must be treated as soon as possible if vision is to be restored. The results of treatment depend on the duration and severity of amblyopia and the age at which treatment is begun. Close parental supervision of the child's treatment is necessary to ensure the return of good vision.