context: Picture this: You look through a peephole and stare straight ahead. You see everything above, below, and to your sides. Suddenly, the peephole gets smaller and smaller. You see everything in front of you, but everything above, below, and around you goes black. It's like you're looking through a narrow tube or a tunnel. This is how it feels to have âtunnel visionâ -- a loss of your peripheral vision. It's what allows you to see objects all around you without turning your head or moving your eyes. It helps you to sense motion and walk without crashing into things. It's what you use to see something "out of the corner of your eye." Most often, it's a side effect of other medical conditions. Two of these, glaucoma and retinitis pigmentosa, are among the most common. Glaucoma: This disease is caused by the buildup of fluid and pressure in the eye. It can damage the nerve that carries information from the eye to the brain. When this happens, you may lose your peripheral vision. Over time, you could lose all of your eyesight. Luckily, doctors can prevent vision loss if they find your glaucoma early. Retinitis Pigmentosa ( RP): This genetic disorder damages the retina, the part of the eye that senses light. Night blindness is one of the first symptoms. You might also have a hard time telling different colors apart. Over time, you'll notice changes in your peripheral vision. You can get this condition at any age, but it usually strikes teens and young adults. Most people who have it are legally blind by age 40. Your eye doctor will give you a visual field test to check for blank spots in your vision -- spots you might not even notice yet. He'll place a bowl-shaped device in front of your face. You'll wear a patch over one eye so each can be tested separately. While you look straight ahead, lights flash at different points around the bowl. You'll press a button when you see lights, without turning your head from side to side. If you have an eye disease, your doctor might repeat this test every 6 to 12 months to measure changes in your vision. People who have a higher risk for glaucoma should also be tested regularly. There's no research to suggest it. But you can take control of some of the conditions that put you at risk. For example, glaucoma can strike anyone. If you're African- American, over age 60, or have a family history of glaucoma, you have an even greater chance of getting it. But you can lower your chances: See your doctor for a complete eye exam every 2 to 4 years, beginning at age 40. If you play sports or work around the house, wear protective glasses or goggles to protect your eyes. Eye injuries can cause glaucoma. Studies show that regular exercise can help reduce eye pressure, the main cause of this eye disorder. If you work out, you can also lower your high blood pressure, cutting your risk even more. If you lose your peripheral vision because of glaucoma or RP, you can't reverse it. But you can be proactive and slow down or stop the damage. For instance, if you do yoga, avoid a pose makes you be upside down because that has been shown to increase eye pressure. If your doctor finds and treats glaucoma early, he can give you medications to reduce the eye pressure that causes it. If that doesn't work, he might recommend surgery. Studies show that vitamin A may slow down vision loss caused by RP. Your doctor can help you find ways to cope with poor eyesight and may be able to slow or stop the damage.