What causes glaucoma?

Damage to the optic nerve is thought to be caused by increased pressure in the eye (intraocular pressure, or IOP). This may result from excess fluid, called aqueous humor, building up in the eye because the eye produces too much or drains too little of the fluid. However, many cases of glaucoma develop without increased IOP. In these cases, decreased blood flow to the optic nerve may cause the damage.
Glaucoma may develop after an eye injury, after eye surgery, from the growth of an eye tumor, or as a complication of a medical condition such as diabetes. Certain medications (corticosteroids) may cause glaucoma when they are used to treat eye inflammation or other diseases. Glaucoma that develops as a result of another condition is called secondary glaucom