The cause of glaucoma generally is a failure of the eye to maintain an appropriate balance between the amount of internal (intraocular) fluid produced and the amount that drains away. Underlying reasons for this imbalance usually relate to the type of glaucoma you have. Just as a basketball or football requires air pressure to maintain its shape, the eyeball needs internal fluid pressure to retain its globe-like shape and ability to see. But when something affects the ability of internal eye structures to regulate intraocular pressure (IOP), eye pressure can rise to dangerously high levels causing glaucoma. Unlike a ball or balloon, the eye can't relieve pressure by springing a leak and "deflating" when pressure is too high. Instead, high eye pressure just keeps building and pushing against the optic nerve until nerve fibers are permanently damaged and vision is lost.