Babies are not born with perfect vision. Normal vision develops slowly over the first several years of life. The eye and the brain together learn how to see over the first few years of life. Like a muscle, vision only grows if it is used. If for any reason the brain does not receive a clear image through an eye, that eye will never learn to see clearly. If sight does not develop normally, this is called amblyopia, or lazy eye.
A number of common childhood eye conditions can cause lazy eye. If children are extremely near-sighted or far-sighted, both eyes will be blurred constantly, and both may become lazy. Some children have one eye that is more near-sighted or far-sighted than the other, and can develop amblyopia in just the one eye.
Another common childhood eye condition that can cause lazy eye is ocular misalignment, also called strabismus. This is a condition in which the two eyes are looking in different directions. This may look like crossed eyes, if the misaligned eye points inward, or wall-eyes, if the misaligned eye points outward. In either case, only one eye at a time is looking where the child wants to look, and to avoid double vision, the brain ignores the vision coming through the misaligned eye. If this vision is ignored long enough, lazy eye can develop in the misaligned eye.
Some uncommon causes of lazy eye are conditions that block vision from entering the eye, such as a droopy eyelid or a cataract. These are uncommon in children, but do happen to some children.
A lazy eye can be treated to restore normal vision. However, it has to be diagnosed in order to be treated, and timing is very important. If a lazy eye is not diagnosed and treated by the age of nine or ten, the vision loss becomes permanent. All children should have their vision measured before starting school and at least once or twice while in elementary school to make sure their vision is developing normally in both eyes. Any abnormal vision measurement, whether in one or both eyes, should be evaluated by an eye specialist to make sure amblyopia is not present.
Treating lazy eye depends in part on the cause of the amblyopia, and the first step is to fix the cause. Droopy eyelids can be tucked, and cataracts can be removed. For more common causes, glasses can correct near-sightedness or far-sightedness, and eye muscle surgery can realign misaligned eyes.
Once the cause of amblyopia is fixed, the vision lost must be regained. This can be accomplished by patching the good eye for brief periods or by blurring the vision in the good eye with special eye drops. Patching or blurring the good eye forces the lazy eye to work harder so that vision in the lazy eye catches up to vision in the good eye. It is important to follow the doctor’s instructions carefully when blurring the good eye with a patch or eye drops, because too much blurring of the good eye can make it become lazy.