Glaucoma happens when the aqueous humor is prevented from draining properly. A rise in intraocular pressure can deform the optic nerve head and may eventually lead to the destruction of the optic nerve.
Spencer Sutton/Science Source
Glaucoma (glaw-KO-ma) is a group of disorders that cause fluid pressure to increase inside the eye, which may result in vision loss.
If a balloon is slowly filled with water, eventually it bursts. But if the same balloon has several pin-sized holes at one end, then it becomes possible to continue adding water to the balloon to maintain its round shape, without breaking the balloon, as long as the amount of water being added is equal to the amount escaping through the pinholes.
The eye has a similar system of liquid that continuously flows in and out of a small chamber at the front of the eyeball. The problem for people with glaucoma is that the drainage out of the eye is blocked or not working properly. It is similar to the balloon that is filling with water. Without a way to make room for more liquid, pressure builds up. Although a pressure buildup will not cause the eye to burst, it may damage nerves at the rear of the eyeball that carry images to the brain.
Glaucoma is one of the leading causes of blindness in the United States. It affects more than three million people, especially the elderly and people of African ancestry. The disease is also hard to diagnose. Pressure can grow in the eye for years before the effects on vision are noticed. By then, often the damage has been done.
The eye is about the same size as a ping-pong ball and is divided into two compartments. The larger compartment at the rear of the eye contains a gel-like substance called vitreous (VIT-re-us) humor, which helps to maintain the eyeball's shape and to transmit light. The front compartment, or anterior chamber, is smaller and is filled with a watery liquid called aqueous (AY-kwee-us) humor. This clear liquid brings in nutrients vital to the eye's health and carries out waste that can damage it.
The aqueous humor flows from behind the front colored portion of the eye, which is called the iris. It moves through the pupil, the opening in the center of the iris, into the front chamber of the eye. The liquid flows through the chamber and out a tiny drainage canal that has a fine, meshlike covering. The small hole on the canal rests at an angle where the colored iris meets the cornea, the clear cup-shaped disk at the front of the eyeball.
The cause of most cases of glaucoma is not understood, although people of African ancestry, people who have diabetes or other family members with glaucoma, or those who have suffered eye injuries are at greater risk. Aging is another risk factor for glaucoma.
Except for the rare cases of acute glaucoma that develop suddenly, most people do not realize they have glaucoma. As pressure is building in the eye, many of the millions of nerve cells at the rear of the eye are destroyed. The nerves that die first affect peripheral (pe-RIF-er-al) vision, the ability to see the sides of a normal field of vision. When the loss of vision becomes severe enough for a person to notice, the damage is so great that little can be done.
The best way to diagnose glaucoma is through an eye exam that uses an instrument called a tonometer (to-NOM-e-ter) to measure the pressure in the eye. One type of tonometer registers eye pressure by lightly touching the eye's surface. Eye drops are used to make this procedure painless. Another tonometer uses a puff of air to measure eye pressure. The doctor or eye specialist (ophthalmologist or optometrist) may also use a scope that shines light in the eye to look for damage to the optic nerve. Peripheral vision can be checked as part of the eye exam.
Early diagnosis of glaucoma is the key to preventing vision loss. If glaucoma is discovered before the increased eye pressure has destroyed many nerves, vision can be saved in many cases. Routine eye exams, including tests for glaucoma, are important, especially for adults older than 35 years. Such eye exams are especially important for people at greatest risk for glaucoma, including people of African ancestry, people with relatives who have glaucoma, people with diabetes, and people with previous eye injuries.
The most common treatment involves eye drops that reduce pressure. Sometimes surgery is necessary either to open the drainage canal or to create a new one.
See also Aging • Blindness • Cataracts • Eye Disorders: Overview
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Eye Smart. “What Is Glaucoma?” American Academy of Ophthalmology. http://www.geteyesmart.org/eyesmart/diseases/glaucoma/ (accessed October 21, 2015).
American Optometric Association. 1505 Prince St., Suite 300, Alexandria, VA 22314-2874. Toll-free: 800-365-2219. Website: http://www.aoa.org (accessed October 21, 2015).
Glaucoma Research Foundation. 251 Post St., Suite 600, San Francisco, CA 94108. Toll-free: 800-826-6693. Website: http://www.glaucoma.org (accessed April 21, 2016).
National Eye Institute. 2020 Vision Pl., Bethesda, MD 20892-3655. Telephone: 301-496-5248. Website: http://www.nei.nih.gov (accessed April 21, 2016).
* acute describes an infection or other illness that comes on suddenly and usually does not last very long.