The human eye has a clear liquid called the aqueous humor that circulates inside the front portion of the eye. Your eye constantly produces a small amount of the fluid to maintain a constant, healthy eye pressure. The fluid is constantly being replenished, and the old fluid flows out of the eye through a microscopic drainage system.
But sometimes the aqueous humor may not flow through the drainage system properly, and there is an imbalance between producing and draining the fluid in the eye. The drainage channels are blocked or do not function properly. This increases pressure inside the eye, which can press on the optic nerve in the back of the eye, damaging nerve fibers. The effect is similar to filling a balloon with water: the more water you put into the balloon, the higher the pressure. And just like a water balloon can burst if there is too much water put into it, the optic nerve in the eye can be damaged by pressure that's too high.
This condition is called glaucoma. Other common causes of glaucoma include trauma or chemical injury to the eye; severe eye infection; blockage of blood vessels in the eye; and inflammation of the eye.
Symptoms of glaucoma include seeing halos around lights; tunnel vision; vision loss, redness or pain in the eye, nausea or vomiting; and an eye that looks hazy. Those most at risk of developing glaucoma are adults over 40, or those with diabetes, or of African-American descent.