Intraocular pressure (IOP) 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 is removed 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 can vary from person to person.
• Not all people with open-angle glaucoma (OAG) have increased pressure inside the eye. As many as 40% to 50% of cases of OAG may occur without increased pressure inside the eye, and most people with elevated pressures will never get glaucoma.^{1, 2}