Intraocular pressure refers to the pressure of the watery liquid, known as the aqueous humor, in the anterior chamber of the eye. The anterior chamber is the small space in the front of the eye between the clear cornea and the lens. Aqueous humor originates from blood that is filtered through capillaries, and it provides glucose and amino acids to tissues of the eye.
Production and flow of aqueous humor is continuous throughout the day and night. This fluid nourishes the tissues, and then drains out of the eye and back into the bloodstream. As the aqueous humor drains, it flushes away waste products. Overproduction of this fluid or any disruption in drainage of fluid can lead to an increase in intraocular pressure.
Normal intraocular pressure is between 10 and 20 millimeters of mercury (mm Hg) with the average being 15 to 16 mm Hg. Intraocular pressure is considered low if it falls below 5 mm Hg. Pressure can be low if there is a trauma to the eye, which would indicate there is a leakage of fluid out of the eye.
The pressure in the eye is considered elevated when it rises above 21 mm Hg. Elevated intraocular pressure is also known as ocular hypertension, and it is the primary risk factor for the eye disease glaucoma. Glaucoma can gradually damage the optic nerve and cause vision loss if left untreated. As it does run in families, those with a family history of glaucoma should routinely have their intraocular pressure measured by an eye doctor.
Intraocular pressure is measured with an instrument called a tonometer. The simplest tonometer measures pressure by blowing a puff of air against the cornea. If the pressure is elevated, the eye doctor may choose to use an applanation tonometer to re-check the pressure. This procedure is done while the eye doctor examines the cornea through a microscope. A small probe touches the cornea and measures pressure in the eye.
Keeping intraocular pressure within a normal range is the best way to prevent glaucoma. For those with elevated pressure, the eye doctor will most likely prescribe eye drops to use at home along with frequent visits to monitor eye pressure. There are a variety of medications, from beta blockers to prostaglandin-like compounds, which function by either reducing the production of aqueous humor or increasing the drainage. These medications have side effects, and a treatment plan should be carefully developed with the physician.