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Amblyopia is a condition in which one eye signals to the brain less often, or less well, than the other eye does. The brain is fully using one eye while, for various reasons, not paying much attention to the other. The common name for it, lazy eye, is therefore misleading, as the brain is as much involved as the lesser used eye itself. In some cases it may be the brain's response to widely differing degrees of nearsightedness or farsightedness between the eyes. It can also be a response to the two eyes not being aligned together, a condition called strabismus. Because the brain is seeing two different images, it is clearer to concentrate on one and discount the other.

Amblyopia is found in young children, apparently there since birth but often not detected for several years. The child has no way of knowing their vision is not usual, of course, and furthermore the underused eye is physically normal. The favoring of one eye over the other often becomes more apparent when a child begins learning to read, as the child looks at words not straight-on, but cocking their head to one side slightly. The person may also have a tendency to bump into things on one side, namely that of the unused eye.

The most common treatment for amblyopia is simply for the child to wear a patch over the dominant eye for a few hours every day. The underused eye as well as the parts of the brain corresponding to it are suddenly forced to actively be engaged. Except in severe cases, the underused eye and it's representation in the brain are believed to have been working all along, but in a minimal way. Vision in that eye may initially not be as sharp as in the other, the muscles in it a little slow to respond to changes, but this is temporary. Even in more serious cases, six to eight hours of wearing the patch each day is as effective as wearing it constantly. Many children only have to wear it a couple hours each day – meaning they can do so at home and not worry about "what the other kids think".

For children who refuse to wear the patch at all, eye drops containing atropine can be used to somewhat blur the vision of the dominant eye – just enough to bring the other eye into greater use. It used to be worried that children not diagnosed and treated for amblyopia by a certain age, perhaps as young as seven years old, would miss a developmental window of opportunity beyond which therapy would not work. Fortunately, this has been found to be a fairly flexible limit, with adolescents as old as 17 still responding to patching therapy. However treatment can take longer when started later so, as with most health concerns, early detection is best.

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