More than 3 million Americans have glaucoma. But, experts think that half of them don't know that they have it. And yet, finding and treating glaucoma early—before it causes major vision loss—is the best way to control the disease, according to the National Eye Institute (NEI).

"The No. 1 risk for blindness is not getting an eye exam," says Dr. Steven L. Mansberger, director of clinical trials, Devers Eye Institute at Legacy Health System.

Learning more about glaucoma and how it's detected and treated can help you avoid vision damage or blindness.

What is glaucoma?

Glaucoma is a group of eye diseases that can cause damage to your optic nerve. That nerve is the part of your eye that sends images to your brain. If you don't notice that you have glaucoma and it's not treated, it will gradually rob you of your peripheral vision and eventually blind you.

Glaucoma is often caused by the pressure inside your eye building up too high. This pressure depends on a balance between how fast you make fluid in your eye and how fast the fluid drains out. In most cases, high pressure occurs because the fluid is blocked from draining away.

"We don't have a clear understanding of what causes glaucoma," says Dr. John Morrison, director of glaucoma services, Casey Eye Institute at Oregon Health & Science University. "Many labs, including mine, are trying to understand how increased intraocular [internal eye] pressure damages the optic nerve so that we can develop new, more effective treatments for glaucoma."

Most common form of glaucoma

Primary open-angle glaucoma is the most common type of glaucoma in the U.S., affecting 80 to 90 percent of those with the disease. In its early stages, you don't see or feel any symptoms.

"Glaucoma is known as the sneak thief of sight," explains Dr. Mansberger. "Because it affects your peripheral vision, you don't notice it until the disease has progressed very far, stealing your sight until it seems as if you're seeing things through a keyhole."

Who's at risk?

Everyone can get glaucoma. But you are more likely to get glaucoma if you:

  • Are age 60 or older
  • Are African-American (if so, you also have a greater risk of getting glaucoma at a younger age)
  • Have a family history of glaucoma or a close relative with glaucoma (parents or siblings)
  • Have a history of high internal eye pressure

Severe nearsightedness, health conditions such as diabetes, steroid use and eye injuries add to the chance that you might get glaucoma.

How is it diagnosed?

The National Eye Institute advises people who are part of a high-risk group to have a dilated eye exam every two years. Check with your doctor to find out when and how often you should have a complete eye exam.

But don't put off that trip to the eye doctor! If your optic nerve is damaged or you are blinded by glaucoma, it can't be reversed. If you get treated immediately when you are in the early stages of open-angle glaucoma, you can slow the damage to your eyes. With the proper treatment, you can keep your sight.

Getting checked for glaucoma is easy and painless. Doctors use:

  • A visual acuity test—that good old eye chart test you've seen since grade school
  • Pupil dilation for closer examination of your eye
  • A visual field test to measure your peripheral vision
  • Tonometry—a test to measure the fluid pressure inside your eye

"Because about 50 percent of people with glaucoma may not have elevated pressure, it's very important for doctors to examine the optic nerve as well as check eye pressure," explains Dr. Morrison. "The key is to look at the optic disk for characteristic signs" of damage caused by glaucoma.

How is it treated?

Although glaucoma can't be cured, it can be treated and controlled. In the U.S., the first line of treatment is medication—generally eye drops. "Four main classes of medication are primarily used now; all [are] designed to lower intraocular pressure," says Dr. Morrison.

If medications don't work, then doctors often recommend laser surgery on the part of the eye that drains fluid. If other treatments don't work, you may need conventional surgery to make new pathways so the fluid can drain more easily.

"Many advances have been made in the past decade, and a lot of cutting-edge research is in progress," says Dr. Mansberger. "In 10 years, the way we test and treat glaucoma may be very different, with new ways to lower eye pressure and protect the optic nerve."

The good news is that highly successful glaucoma treatments are available. It's up to you to find out if you're at risk or have the disease so that you can get treatment to protect your vision and prevent blindness.

Published on March 6, 2009; updated on May 30, 2014.