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What is Glaucoma?

"Glaucoma" describes a whole group of diseases affecting the eye, but all share the common fact that they cause the pressure within the eye (the intraocular pressure) to be at unhealthy levels for the affected person. Because many different types of glaucoma exist, treatment depends upon the type of glaucoma as well as a variety of other factors.

How does Glaucoma cause damage?

Normally, watery fluid (aqueous humor) constantly flows through the eye. This fluid keeps the eye firm and clear so the eyeball can function well visually. The relative state of inflow and outflow of aqueous humor partially determines how firm the eye is. If the outflow is blocked, pressure inside the eye builds up.

The pressure within the eye, the intraocular pressure, can directly damage the optic nerve, the nerve that carries the electrical impulses from the light-sensitive part of the eye to the brain, where the electrical impulses are put together to form a picture. Even when the intraocular pressure is not above average, it may still be high enough to cause optic nerve damage. Elevated intraocular pressure can damage other tissues as well, such as the cornea and the lens, and can squeeze out of the eye the blood needed to keep the nerves healthy, resulting in damage to the nerves or retina (the light-sensitive part of the eye).

Outflow of fluid, aqueous humor, can be blocked in different ways: The pupil, the hole through which the fluid flows as it passes from the back to the front of the iris (the colored part of the eye) can get blocked by adhesions or by a cataract. The sieve through which the fluid drains can become blocked by debris caused by inflammation, by deposits which are due to aging, by abnormal material which is sometimes the result of certain drugs, or by the iris itself. The veins into which the fluid flows when it leaves the eye can be partially blocked by other disease, or by pressure on the large veins in the orbit.

Average normal intraocular pressure in adults is 15 mm Hg (mercury). If the pressure is below 8 mm Hg or so, the eye may be too soft to function well. The actual upper limit of normal, however, is difficult to pinpoint.

If the pressure is consistently above 21 mm Hg, the chance of eye damage is probably around 10 percent. When the pressure inside the eye is above 26 mm Hg, the likelihood of eye damage increases to about 50 percent. When the intraocular pressure is above 30 mm Hg, the chance that damage will eventually develop may be close to 100 percent if the elevated pressure persists long enough.

What constitutes normal intraocular pressure is an individual matter for each person. For example, some persons with an intraocular pressure of 16 mm Hg may need surgery; others with a pressure of 30 mm Hg may not even need any treatment.