The most common form of glaucoma is primary open-angle glaucoma. This condition occurs as a result of pressure on the eye, more formally known as intraocular pressure.
To understand how this pressure builds up, we need a little background on the workings of the eye. Our eyes have their own mini windshield-wiping system built in, and just like in automobiles, this system ensures we can see. At the front of the eye, clear fluid known as aqueous humor is released from the anterior chamber. The fluid travels to nearby eye tissue, nourishing it and enabling it to do its job. After feeding the eye tissue, the fluid exits the eye through a drain known as the trabecular meshwork. Never noticed that drain in your eyes? Well, it's only one-fiftieth of an inch wide, located at an angle where the cornea and iris meet [source: Glaucoma Foundation].
Sometimes though, that fluid can't get through the drain. Doctors don't know exactly why this happens; it may have something to do with the aging of cells in that area, a structural defect or a change in the immune system. With nowhere to go, the fluid builds up and raises intraocular pressure. We all need some eye pressure; that's what helps the eye keep its shape. How much is too much, though, varies from person to person. In the case of too much pressure, cells start to die at the weak point, which unfortunately is the all-important point where the optic nerve leaves the eye on its journey to the brain.
There are no symptoms to alert you to the fact that the optic nerve is being damaged by intraocular pressure. The first thing to go is peripheral vision, which can be an easy loss to overlook, because people just turn their heads. Once vision is lost, though, there's no way to restore it.
One common misconception about glaucoma is that the condition is merely the intraocular pressure, but a person doesn't actually have glaucoma until there's damage to the optic nerve. Not everyone with increased eye pressure will develop glaucoma, and in one instance of glaucoma, increased eye pressure isn't even present beforehand. Normal-tension glaucoma occurs without the buildup of intraocular pressure and is believed to be related to reduced blood flow in the area of the optic nerve. The lack of blood flow is what ends up doing the damage to the optic nerve.
There's one other instance in which the intraocular pressure doesn't cause damage to the optic nerve over time; rather, the intraocular pressure comes on fast and causes an emergency situation. Find out about this type of glaucoma on the next page.