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In most forms of glaucoma, the pressure inside the eye—called intraocular pressure—rises because the eye's drainage canals become blocked or don't work properly. If the fluid cannot flow out of the eye, the pressure inside the eye rises. This increased pressure can damage the optic nerve and lead to vision loss and blindness.

The ocular fluid leaves the front part of the eye through the open angle where the cornea and iris meet. When the fluid reaches the angle, it flows through a spongy network, like a drain, the National Eye Institute (NEI) says. If the angle is not working properly, pressure inside the eye may rise and damage the optic nerve.

Increased pressure in the eye means you are at risk for glaucoma, the NEI says, but it doesn't mean you have glaucoma. Glaucoma is diagnosed when damage has occurred to the optic nerve. If you have increased eye pressure but no damage to the optic nerve, you do not have glaucoma, the NEI says. You are at risk, however. Not every person with increased pressure develops glaucoma, and the pressure that causes damage may be different for each person.

You can develop glaucoma even with normal eye pressure. This form of glaucoma is called low-tension or normal-tension glaucoma. This type of glaucoma reinforces why it is so important to have a complete eye exam to detect glaucoma. A pressure measurement alone is not enough.

Angle-closure glaucoma is a medical emergency. The fluid in the front area of the eye cannot reach the angle to drain out of the eye because it is blocked by part of the iris. Eye pressure increases suddenly, causing severe pain and nausea, redness of the eye, and blurred vision, the NEI says. Seek immediate medical help if you have any of these symptoms. If untreated, angle-closure glaucoma can cause blindness. It is more common in people of Asian descent and in people who are far-sighted, according to the Glaucoma Foundation.

Anyone can develop glaucoma, but some people are at higher risk than others, the NEI says. A few of the most important risk factors are increasing age, race, and family history.

People with early open-angle glaucoma do not have symptoms. That is why this type of glaucoma is sometimes called “the silent thief of vision.” Vision remains normal, and the eyes feel normal. As this form of glaucoma progresses, a person gradually loses peripheral vision until objects appear to be seen through a tunnel. Then that central vision decreases until it is gone. Glaucoma can develop in one or both eyes.

A thorough eye exam is important in detecting glaucoma. This exam includes dilating your eyes and looking at the optic nerve, which is damaged in glaucoma. A visual field test measures your peripheral vision, which is lost in glaucoma. Tonometry measures the pressure inside your eye. Pachymetry measures the thickness of your cornea, which can affect the eye pressure measurement.

Whatever the treatment, the earlier it is begun, the better the chance of retaining normal vision. Medication in the form of eye drops can lower the pressure in the eye or decrease the amount of fluid produced in the eye. One drawback to medications is that they may have unpleasant side effects, and, because glaucoma has no symptoms, patients may be tempted to stop taking the medication. Medication can be used to decrease fluid production or increase the rate at which fluid leaves the eye. Laser trabeculoplasty is laser surgery that helps fluid drain from the eye. If medication and laser surgery don't work, conventional surgery also may be used to create an opening for the fluid to drain.

Visit your eye doctor regularly for a dilated eye exam in order to detect the disease early. If you are diagnosed with glaucoma, take your medications and follow up with your eye doctor regularly. Have periodic checkups including pressure measurements, visual field tests, and examination of your optic nerve. These are necessary to make sure your treatment is working. If you have glaucoma, make sure your family members get checked. Family history is one of the biggest risk factors for developing glaucoma.