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The exact cause of primary open-angle glaucoma is unknown.

In cases where primary open-angle glaucoma is associated with increased intraocular pressure, the cause for the high pressure is generally accepted to be because of an imbalance in the production and drainage of fluid in the eye (aqueous humor). The channels that normally drain the fluid from inside the eye do not function properly. More fluid is continually being produced but cannot be drained because of the improperly functioning drainage channels (called trabecular meshwork). This results in an increased amount of fluid inside the eye, thereby raising the pressure.

Another way to think of high pressure inside the eye is to imagine a water balloon. The more water that is put into the balloon, the higher the pressure inside the balloon. The same situation exists with too much fluid inside the eye—the more fluid, the higher the pressure. Also, just like a water balloon can burst if too much water is put into it, the optic nerve in the eye can be damaged by too high of a pressure. See Media files 1-2.

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