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 Encyclopedia - Intraocular Pressure (IOP)

What causes intraocular pressure?
How is intraocular pressure measured?

Intraocular pressure is a measurement of the fluid pressure inside the eye. This fluid, or aqueous humor, nourishes the cornea, iris, and lens, and it helps the eye maintain its globular shape. The typical eye produces about 4 c.c. of fluid a day, which is circulated and then drains out of the eye.

What causes intraocular pressure?

If the drainage system becomes clogged or if too much fluid is produced, pressure inside the eye can build up. The reasons for buildup are not fully understood.

Increased intraocular pressure can also result from the use of topical steroid eye drops such as those used in the treatment of an inflammation within the eye. Although these medications may be necessary in the treatment of the disease, they can cause a temporary elevation of intraocular pressure.

Normal eye pressure, as measured by an eye doctor, usually ranges between 10 and 21 mm of mercury, with an average of 16. Physical activity, stress, rapid fluid intake, and caffeine can account for a small plus or minus change in an intraocular pressure reading. Pressure that is consistently above 21 indicates ocular hypertension. The condition can develop into glaucoma, a serious disease that causes damage to the optic nerve. Because glaucoma is the leading cause of blindness among adults in the United States, early detection and treatment are key to preventing serious loss of vision or blindness.

How is intraocular pressure measured?

Intraocular pressure is measured with an instrument called a tonometer. There are two types of tonometers, but the most accurate is considered to be the applanation tonometer, an instrument that looks like a pen. After numbing eye drops are administered, the instrument is applied gently to the front surface of the eye and provides a pressure reading. The other type of tonometer is a noncontact tonometer, which directs a warm puff of air toward the eye without touching it.

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