Most people know amblyopia by its unflattering nickname, "lazy eye." (The term "lazy eye" can also refer to a specific type of amblyopia called strabismus.)
Amblyopia is a disorder in which the eyes are unable to coordinate their alignment or to maintain parallel focus. Simply put, the eyes don't want to play nice with each other. The disorder begins in childhood. Since infection, injury, or other illness is usually not present, the disorder is said to be mechanical (related to a structural mechanism), rather than pathological (related to a disease process). In rare cases, amblyopia is brought on by childhood cataracts, nerve degeneration, or other conditions.
Amblyopia causes conflicting signals to be transmitted from the optic nerve to the brain. Unable to make sense of them, the brain begins to pay selective attention to visual messages from the dominant eye. This filtering process, called suppression, describes what happened to Kim R., whose right eye didn’t work as well as her left when she was a child: "When I was 5 years old, my brain was having such a hard time focusing my eyes together that it had all but stopped trying to interpret the images from the weaker eye. My parents had my eyes checked early, though, and the problem was diagnosed in time for it to be corrected."
Usually only one eye is affected by amblyopia. A key symptom is reduced depth perception. Because the eyes are operating independently of one another, the brain has trouble fusing the visual impressions it receives from each eye into a single three-dimensional image. Other symptoms include the following:
- An eye that appears to turn in or out. (Doctors call this deviation, but most people call it a "wandering eye.")
- Significant differences in visual ability (acuity) between the right and left eyes
- Squinting, eye strain, and resulting headaches
- A habit of closing or covering one eye to see, especially while reading
About 2% to 5% of children have amblyopia. The condition seems to have a genetic component, since it tends to run in families. Infants born prematurely are more likely to develop the disorder. The condition is more common among children with developmental delays, but it's unclear whether the two simply tend to occur in tandem or whether the amblyopia actually causes learning difficulties that lead to developmental delays.
As in Kim's case, early detection is important. A thorough eye examination is all that's needed to diagnose this condition. The provider will perform a visual acuity check in both eyes, using a standard eye chart to determine whether the child's vision is normal. He or she will also ask the child to follow a moving object, such as a pen or finger, in order to observe how well the child's eyes track together.
If amblyopia is not treated in childhood, the resulting visual impairment can't be corrected in adulthood. Remember, the eyes are only half of the equation that adds up to sharp vision. The other half is the brain, which must interpret the signals that travel to it from the eyes via the optic nerve (see How the Eye Works). Children's brains are like wet modeling clay--with a little effort and a light touch, they can be formed and molded quite easily. Treatment of amblyopia is thought to be most effective up to about ages 6 to 9, while the visual pathways in the brain are still actively developing.
After age 17, the brain is more like a clay pitcher that has dried and is ready to be fired in a kiln. In other words, while the adult brain isn't totally inflexible, its contours can no longer be reshaped. People with severe amblyopia who do not receive treatment before age 17 may have irreversible blindness or low vision in the affected eye.
Treatment centers on forcing the brain to accept visual signals from the debilitated eye so that the eyes can work together. This is accomplished by covering, or occluding, the stronger eye with a patch to obscure vision for several weeks or months, obliging the eye to yield the upper hand to its less assertive counterpart. This role reversal can also be accomplished by intentionally blurring the vision of the dominant eye using eye drops or a special contact lens.
Performing surgery to correct a muscle imbalance is sometimes necessary. In addition, many children benefit from wearing eyeglasses to improve and even out their vision. All of these treatments are more successful the earlier they are initiated, although they may be useful into the late teen years, until the brain has fully matured (from a physiological standpoint, that is).
Your child's provider may also prescribe exercises to teach the eyes to cooperate with each other--team-building activities, you might call them. Kim recalls slogging through such exercises. "I had to wear a patch all the time and do all these exercises at home after school every day. For one exercise, I had search through the newspaper and circle every instance of the word "the" I could find. For another, I'd hold a string that had five beads on it, spaced a foot apart. The string was attached to a doorknob. The task was to hold the string to my nose and look down its length, using both eyes to focus on each successive bead, one at a time. For quite some time, I kinda hated my parents for making me do all these exercises."
Kim is grateful now, though, that her folks made sure she stuck to the program. "I realize how lucky I am that my parents were aware enough to get my eyes checked early and to make sure I did my eye care 'homework' every day. Besides," she adds, "things have improved for children with amblyopia since I was a kid. They have way cooler patches now, in fun colors or printed with Barbie or GI Joe! And they stick to your face just like a Band-Aid! It makes me a little jealous. When I was 5, I looked like a pirate!"