Eye pressure, the normal tension in the eye, is regulated by the aqueous fluid produced by the ciliary body. Normal eye pressure is 10 to 21 millimeters of mercury, or mmHg. The pressure in the eye rises if the amount of aqueous fluid increases, either because too much fluid is being manufactured or because the fluid's exit from the eye is blocked.
Anatomy
Aqueous fluid delivers nutrients to the lens and the cornea, which have no blood supply, and carries off waste material. Aqueous fluid is created in the ciliary body, found behind the colored part of the eye, the iris, and flows past the lens, through the pupil, the hole in the center of the iris. The fluid then passes through the trabecular meshwork, which is located in the angle formed by the iris and cornea, according to optometric physician Ted Montgomery.
The trabecular meshwork contains a series of canals that carry the fluid around the edge of the iris, according to the Glaucoma Research Foundation. The fluid then exits the eye through the canal of Schlemm.
Measurement
Intraocular pressure, or IOP, is measured by a tonometer, a device that tests the tension of the eyeball. Average eye pressure is 16 mmHg, according to Montgomery. An ophthalmologist can also examine the eye through a microscope or through a special lens called a gonioscopy lens to check for damage to the optic disc, the area around the optic nerve, is visible, according to "A Patient's Guide to Glaucoma."
Causes
Intraocular pressure rises if the production of aqueous fluid increases or if the exit of fluid from the eye is blocked. This can happen if the aqueous fluid flows too slowly through the trabecular meshwork, which happens as many people age, because the meshwork gets clogged with cellular debris, or the canals shrink. Certain diseases such as diabetes can also slow the flow of fluid through the trabecular meshwork. If the angle at the iris and cornea becomes closed off, no fluid can pass through and pressure rises quickly.
Damage Done
High pressure in the eye damages the lamina cribrosa, which is cartilage found at the back of the sclera, the white part of the eye. The optic nerve passes through the lamina cribrosa, and when it's deformed, it pinches the nerve fibers and destroys them, according to Montgomery.
Types of Increased IOP
Ocular hypertension is an increase in intraocular pressure without damage to the optic nerve. Ocular hypertension has no symptoms. Ocular hypertension that progresses to where damage to the optic nerve occurs is called glaucoma. Glaucoma is not a single disease, but a symptom of many diseases, according to the Mayo Clinic. There are two types of glaucoma, open angle and closed angle. Open angle glaucoma is the most common type of glaucoma, occurring in 80 percent of all glaucoma cases, and is caused most often by slow drainage of fluid from the eye.
Closed angle glaucoma occurs because the angle between the cornea and iris is narrowed or blocked. If the angle closes completely, severe pain results. Narrow angle glaucoma is a medical emergency because vision can be completely lost from damage to the optic nerve if the problem isn't corrected quickly.
