Intraocular (say "in-truh-AW-kyuh-ler") pressure is the pressure caused by the fluid inside the eye that helps maintain the shape of the eye. The level of pressure inside the eyes depends on:

• How much fluid is produced inside the eye.
• Whether fluid can travel normally through the eye.
• How well the fluid drains from the eye.

The pressure within the eyes varies during the day. Normally, the pressure inside the eye ranges from 10 millimeters of mercury (mm Hg) to 21 mm Hg.

• Intraocular pressure (IOP) can vary from person to person. The higher the pressure in the eye, the greater the chance of damage to the optic nerve. Optic nerve damage leads to the eye disease glaucoma. Some people who have higher-than-normal pressure in their eyes don't develop glaucoma.
• Not all people with glaucoma have increased pressure inside the eye.
• Many people who have intraocular pressures consistently above 27 mm Hg develop optic nerve damage unless the pressure is lowered by medicine.
• Some people have intraocular pressures consistently higher than 21 mm Hg but do not develop optic nerve damage. This condition is called ocular hypertension.

Doctors measure IOP with a tonometry test. This is one of several tests that helps determine your risk for glaucoma.