Glaucoma is an eye disease where the pressure of the fluid in the interior of the eye is consistently too high. Several things happen if you have it for long enough: a decrease in peripheral vision, damage to the optic nerve, and pain are common in the advanced version. My father, for example, has had glaucoma for more than 30 years and is now effectively blind. (A stroke complicated things, but the glaucoma is what sealed the damage to the optic nerve.)
If caught in the early stages there are medications that can help and laser surgery can lower pressures almost immediately. My Dad had a rather invasive procedure: they poked a hole in the side of his eye and put a "tube" in to keep the hole open to bleed the excess pressure off. (Don't worry, you can't see it.) It's too late for his sight, but at least the glaucoma isn't hurting him.
Just thought I'd chip in the information.