A lazy eye is an eye that wanders in toward the nose or out toward the ear. This laziness can occur constantly, but more often it happens intermittently. In fact, the most common time you will notice it is when your child is tired.
A lazy eye is usually the result of weakness in one of the muscles that control eye movement. There are 6 muscles attached to each eyeball — so 12 in all — that must coordinate to make the two eyes move in sync. If one of those muscles is either too strong or too weak, it will pull (or fail to pull) the eye in a certain direction. This is called strabismus. Although laziness could certainly happen in both eyes at the same time, it is far more likely that it will occur in just one eye.
When both eyes seem to point in toward the nose, a child will appear cross-eyed. This can be a result of unbalanced muscles in both eyes, but more often it is simply an optical illusion. The illusion may be caused by a wide space between the eyes, a flat nasal bridge, or a thickening of the upper eyelids where they meet the skin near the nose (called the epicanthal folds). In each of these circumstances, the child may appear to have crossed eyes because the whites of the eyes closest to the nose seem smaller than the whites of the eyes closest to the ears. This phenomenon is called pseudostrabismus. It is normal in children of many ethnic backgrounds, especially Asians.
The best way to tell whether your child has true strabismus or pseudostrabismus is to look at the way light reflects on the eyes. When your child is looking straight ahead and the light shines in the same place on both eyes, the eyes are aligned. When it reflects in different spots on the two eyes, the eyes are not aligned. Even though this test looks for how light “reflects,” doctors refer to it as the light “reflex”.
What are the possible complications?
The worst complication of untreated strabismus is amblyopia, which means functional blindness in the lazy eye. This happens because the brain is designed to integrate the images from both eyes “in stereo”. If one eye is misaligned or wandering randomly, the brain cannot integrate the input from both eyes, and it will categorically reject or suppress the input from the misaligned eye. If the brain accepts the input only from the dominant eye, the nondominant eye will behave as if it were blind. Even though the eye itself is normal, the brain will treat it as though it does not exist.
Amblyopia can be caused by strabismus, but it can also result from a number of other sources. If something blocks or blurs the vision from one eye for a long enough time, amblyopia can result. Examples include cataracts or improperly formed eyelids, both of which can obstruct a child’s view. Whatever the underlying cause, when the brain cannot integrate the input from one eye with the input from the other, the eventual result is functional blindness in one eye.
Sometimes amblyopia can be treated by patching the dominant eye, which encourage use of the nondominant eye. But if it is not treated in early childhood, even glasses and patching will not improve the condition.
Whereas most doctors and parents can easily detect strabismus, only an eye doctor comfortable examining young children can diagnose amblyopia without strabismus.