Glaucoma is sometimes called the silent thief, slowly stealing your sight before you realize anything's wrong. The most common form of glaucoma develops gradually, giving no warning signs. Many people aren't even aware they have an eye problem until their vision is extensively damaged. Glaucoma is the second most common cause of vision loss in the United States. It affects approximately 3 million Americans.
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Actually, glaucoma is not just one disease but a group of them. The common feature of these diseases is that abnormally high pressure inside the eyeball damages the optic nerve. The optic nerve is a bundle of more than a million nerve fibers at the back of the eye. It's like a big electric cable made up of thousands of individual wires carrying the images you see from the retina to the brain.
Blind spots develop in your visual field when the optic nerve deteriorates, starting with your peripheral (side) vision. If left untreated glaucoma may lead to blindness in both eyes.
Fortunately, only a small percentage of people with the disease ever lose their sight. Recent medical advances have made it easier to diagnose and treat glaucoma. And if detected and treated early, glaucoma need not cause even moderate vision loss. But it does require regular monitoring and treatment for the rest of your life.
Understanding eye pressure
Internal pressure in your eye, called intraocular pressure (IOP), allows the eye to hold its shape and function properly. IOP is like air in a balloon - too much pressure inside the balloon affects its shape and may even cause it to pop. In the case of your eye, too much pressure can damage the optic nerve.
Fluids inside the eye help maintain the IOP. These fluids are the vitreous, which fills the vitreous cavity, at the back of the eye, and the aqueous humor, which fills the anterior chamber, at the front of the eye. Aqueous humor is continuously produced and circulated through the anterior chamber before draining out of the eye. This continuous flow of fluid nourishes the lens and the cornea and removes unwanted debris. A healthy eye produces aqueous humor at the same rate that it drains fluid, thus maintaining a normal pressure.
Aqueous humor exits the eye through a drainage system located at the angle formed where the iris and the cornea meet. Here it passes through a sievelike system of spongy tissue called the trabecular meshwork and drains into a channel called Schlemm's canal. The fluid then merges into the body's bloodstream.
When the drainage system doesn't function properly - for example, if the trabecular meshwork gets clogged - the aqueous humor can't flow at its normal rate and pressure builds within the eye. For reasons not completely understood, the increased eye pressure gradually damages the nerve fibers that make up the optic nerve.
Types
There are several types of glaucoma. The differences have to do with what's causing the fluid blockage that builds pressure in the eye.
Primary open-angle
Primary open-angle glaucoma, also called chronic open-angle glaucoma, accounts for most cases of the disease. Although the drainage angle formed by the cornea and the iris remains open, the aqueous humor drains too slowly. This leads to fluid backup and a gradual buildup of pressure within the eye. Damage to the optic nerve is so slow and painless that a large portion of your vision can be lost before you're even aware of a problem.
The cause of primary open-angle glaucoma remains unknown. It may be that the aqueous humor drains or is absorbed less efficiently with age, but then not all older adults get this form of glaucoma.
Angle-closure
Angle-closure glaucoma, also called closed-angle glaucoma, is a less common form of the disease. It occurs when the drainage angle formed by the cornea and the iris closes or becomes blocked. The aqueous humor can't exit through the trabecular meshwork, resulting in an increase in eye pressure. Angle-closure glaucoma can be chronic (progressing gradually) or acute (coming on suddenly).
Most people with this type of glaucoma have a very narrow drainage angle, which may be an abnormality from birth. Angle-closure glaucoma is more common among farsighted people, who tend to have smaller eyes that can narrow the angle. Normal aging also may cause angle blockage. As you get older, your lens becomes larger, pushing your iris forward and narrowing the space between the iris and the cornea.
If you have a narrow drainage angle and your pupils become widely dilated, the angle may close and cause a sudden increase in eye pressure. This attack of acute angle-closure glaucoma requires immediate treatment. Although an acute attack often affects only one eye, the other eye is at risk of an attack as well. Several factors can cause your pupils to dilate:
- Darkness or dim light
- Stress or excitement
- Certain medications, including antihistamines, tricyclic antidepressants and eyedrops used to dilate your pupils, which may not cause the angle to close until several hours after the drops are put in
Secondary
Both open-angle and angle-closure glaucoma can be primary or secondary conditions. They're called primary when the cause of the condition is unknown. They're called secondary when the condition can be traced to a known cause, such as an injury or an eye disease. Secondary glaucoma may be caused by a variety of medical conditions, medications, physical injuries, and eye abnormalities or deformities. Infrequently eye surgery can cause secondary glaucoma.
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Low-tension
Low-tension glaucoma is an unusual and poorly understood form of the disease. In this form, eye pressure remains within a normal range but the optic nerve is damaged nevertheless. Why this happens is unknown, although some experts believe that people with low-tension glaucoma may have an abnormally fragile optic nerve or a reduced blood supply to the optic nerve, caused by a condition such as closed arteries (atherosclerosis). Under these circumstances even normal pressure on the optic nerve is enough to cause damage. To find out more, you can check out Natural Vision Improvement.