Some eye disorders are likely to happen more often than others. Like any body part, the human eye is prone to occasional malfunction based on usage or age.
Glaucoma is a form of damage to the optic nerve, damage that results from fluid buildup in the vitreous humour, which causes increased pressure in the eye that pushes on the optic nerve.
There are two kinds of glaucoma: primary open-angle glaucoma and closed-angle glaucoma.
Primary open-angle glaucoma is the most common glaucoma. Typically, the eye drains old fluids out at a slow and steady rate to maintain balance in the eye. In primary open-angle glaucoma, the eye does not drain as it should. This poor drainage causes fluid and pressure to build, which gradually damages the optic nerve. Open-angle glaucoma is generally painless at first and is relatively slow to develop.
Closed-angle glaucoma occurs when one's iris is too close to the "drainage angle" in the eye. The iris ends up blocking part of the drainage from the eye, much like an obstruction over a sink drain would. If the drainage becomes severe, it can cause an acute attack, where pressures in the eye rise rapidly. An acute episode is a true emergency. If you have sudden vision changes, severe eye pain, nausea and vomiting, headache, or seeing rainbow-colored halos around light sources, you should immediately go to the emergency room. All that aside, closed-angle glaucoma can also develop very slowly.