Glaucoma (glah-CO-muh) is an eye disease characterized by higher-than-normal pressure inside your eye. The pressure can increase gradually or suddenly, slightly or dramatically, and for various reasons, depending on the type of glaucoma. Of all the possible causes of blindness, glaucoma is among the most common, but it's also the easiest to prevent. Acute glaucoma is a type of glaucoma characterized by a sudden and severe increase in pressure within the eye. Only about one percent of all glaucomas can be classified as acute. Acute glaucoma can occur for two reasons. One is that a drainage angle in your eye that's always been abnormally narrow suddenly becomes completely blocked. In this case, the resulting disease is known as acute simple glaucoma. Another type of acute glaucoma, known as acute secondary glaucoma, is actually a complication of one of several other eye conditions, such as an infection, allergic reaction, trauma, or cataract. While the pressure increase in chronic glaucoma is so slight that it can be detected only with special instruments, the increased pressure in acute glaucoma can often be felt by merely touching the front of your eye with your fingers. The pressure is so great, it can cause severe pain and damage to your whole eye. Your cornea can become clouded, causing blurred vision. The blood vessels in your eyeball become red and swollen, and the nerves surrounding them respond to the pressure, leading to the pain. Additional symptoms can include headache, nausea, vomiting, and abdominal pain. Unless you undergo immediate treatment in the form of surgery, your optic nerve can be destroyed.