Amblyopia

When you think of "going blind" you usually think of something in the eye being damaged. The eyeball itself, the optic nerve , the retina, whatever. But it's possible to be blind without any of that happening at all...such as with Amblyopia:

Short 'n Sweet:
A 'mild' defect with one of the six muscles responsible for moving the eyeball around in its socket causes a child to have decreased functioning of the eye (or even blindness) without physically damaging the eye at all!


Background Anatomy & Physiology:
As we all know, we are each capable of moving our eyeball around (left, right, up, down. . . and other stranger motions). This is achieved by a set of 6 muscles that attach to the eyeball from the left, right, top, and bottom and then move the eyeball in those directions. Muscle fibers are long sets of organic filaments with one special property: they can shorten themselves. Picture a rope attached between two blocks, if it were to shorten itself, the two blocks would be *pulled* together; but when it lengthens back to normal, they won't get pushed back apart. You can pull, but not push - for that you'd need a second rope on the other side to pull it back the other way. Well, the body recognizes this and so virtually all of the body's moving parts have at least two muscles. It takes the *pair* to achieve useful motion, one to pull it in the direction you want to go, and the other to pull it back later. The eye's extra-ocular muscles (outside of the eyeball itself) are no different and so there's the left/right and up/down pair.

So that's how we *move* the eye, but to appreciate Amblyopia, you need to know a bit about how we actually use our eyes to see. To achieve vision, we recieve light photons (the discrete particle that makes up light) through our pupils (the black hole in the center of your eye). These photons travel through the eyeball and strike the retina, a group of specialized cells which can react with a light photon to make a different chemical. These chemical messages of the light we've seen are then turned into electrical signals and transmitted up to the brain via the optic nerve. Here the brain interprets the signals and then merges the two of them (one from each eye) into one, providing a single useful picture and depth perception.
(Again, this is a gross simplification, but for the time being it's enough information)


How does it go bad?:
So let's say - for arguments sake, there's damage to the Lateral rectus muscle (mucle that pulls straight 'rectus' to the side) in the left eye. This means you can't turn your left eye left as much/ as well as you used to(damage doesn't mean it's absent!). What would happen if you looked to your left? Your right eye would go all the way to the left no problem (its muscle is fine), but your left eye would only be able to go part of the way (if at all). This would have your two eyes looking at two seaparate images rather than both looking at the same spot. The brain would see that and would give you 2x vision (since it knows it can't merge the two). It's the same effect as you get when you cross your eyes.

That's all good for us, because we're not little kids anymore. Childrens' brains are not as well developed (I'm only speaking medically here ;)) and are much more moldable at that stage of development, so for them things happen a little differently. Say you've got this 6 year old kid, and he develops this same muscle weakness; his eyes are *chronically* (frequently and over a long period of time) giving him mixed signals. When he's looking straight ahead or to the right he's fine, but when he looks to the left, he gets a mixed signal in his brain. Unlike us, at this point his brain has not yet molded itself to definitively recognize both signals! his mind is still trying to work out how to integrate two normal images and such, and so when it gets this double-signal it assumes that something is wrong:

In other words his brain *mistrusts* the signal!

After all, the left eye's signal is frequently bad, so why should it trust it? And so, deep within the brain, the pathway for interpreting and responding to the defective eye gets downgraded further and further. If left untreated, the brain will eventually shutdown that pathway altogether. . . and the eye will be perfectly healthy, but the patient will be quite blind in that eye for life. It's so cool -  the eyeball is *perfectly* functional and healthy, sending high-quality color images of everything it sees just fine! No cataracts, no retinal issues, no cell death, etc. . . but the brain refuses to acknowledge it and so you go blind in that eye!



Treatment (also cool):
So we can't do high-tech neurosurgery (brain surgery) to correct the brain's misperception - it's not like there's something in the brain that's sick and we can take out or anything. We *can* do surgery to repair the muscle defect (depending on what caused it) and fix the underlying problem. The issue is that doing that surgery will not reverse the remodeling of the brain pathways.

So here's the treatment: You tell the kid to wear an eyepatch over his good eye! For a week or so, you totally block the "healthy" signal so the brain is forced to rely on (and thus upgrade the pathways of) the unhealthy eye. After all, it's not like the unhealthy eye isn't capable of sight, or anything. If you wait too long to treat, this isn't possible as they're physiologically (roughly means 'functionally') blind in that eye (though not physically). Like the boy who cried wolf - that eye sent a bad signal so long that the brain just stopped believing it altogether :). After that week or so, you can then correct the muscle problem and job's done.
Luckily, this disease is usually caught and treated in time. Our 5-6 year old child feels gets the muscle weakness and complains of lazy-eye or double vision almost as soon as it starts. Mom is then quick to bring him into the doctor's office, and from there the problem is easily solved :).quick to bring him into the doctor's office.




And THAT's Amblyopia!