Glaucoma can quietly take your side vision before you notice anything is wrong. The National Eye Institute says the symptoms can start so slowly that you may not catch them, and the only way to find out if you have it is a comprehensive dilated eye exam.

Glaucoma is a group of eye diseases that damage the optic nerve, a nerve in the back of the eye, according to the institute. At first it usually has no symptoms, which is why the institute says half of people with glaucoma do not know they have it. Over time, vision loss usually begins with side vision, especially the part closest to your nose, and without treatment it can lead to blindness.

The exam is simple and painless, the institute says. Your doctor gives you eye drops to widen your pupil, checks your eyes for glaucoma and other problems, and runs a visual field test to check your side vision. Anyone can get glaucoma, but some people are at higher risk, and the institute says those people need a dilated exam every 1 to 2 years.

There is no cure, but the institute says early treatment can often stop the damage and protect your vision. Doctors use medicines, laser treatment and surgery, and many of these work by lowering eye pressure. Since glaucoma can run in families, the institute suggests encouraging relatives to get checked, and asking your own doctor about your risk and how often you should be seen.