The amount of light entering the eye (right) is controlled by the pupil, which dilates and contracts accordingly. The cornea and lens, whose shape is adjusted by the ciliary body, focus the light on the retina, where receptors convert it into nerve signals that pass to the brain. The front portion of the eye is filled with a watery substance called the aqueous humor, while the rear portion of the eye is filled with jellylike vitreous humor. When aqueous humor does not drain from the eye properly, glaucoma results. Lacrimal glands (left) secrete tears that wash foreign bodies out of the eye and keep the cornea from drying out. Blinking compresses and releases the lacrimal sac, creating a suction that pulls excess moisture from the eye’s surface.
The front part of the eye is filled with a watery fluid, known as the aqueous humor. This fluid helps the eye maintain its shape and delivers oxygen and nutrients to the cornea and the lens, the eye structure that refracts light to form images. The aqueous humor is produced by the ciliary body, a small gland located just behind the lens. The fluid percolates through the pupil and circulates through the front chamber of the eye. It then drains away through a network of tiny channels, called the trabecular meshwork, located at the front of the eye where the cornea and iris meet.
When the aqueous humor does not drain properly, the fluid backs up, causing the pressure inside the eye to increase, and glaucoma develops. The increased pressure inside the eye compresses and damages the optic nerve, the bundle of nerve cells that transmit visual information from the eye to the brain. This damage to the optic nerve results in vision loss. In chronic simple glaucoma, the aqueous humor drains through the trabecular meshwork more slowly than normal, much as a sink empties more slowly when the drain is clogged.
Age is a primary risk factor for glaucoma: the condition strikes 1.5 percent of people over age 40, and 15 to 20 percent of people over age 70. The disease tends to run in families, although it is not inherited in a regular, predictable pattern. Glaucoma affects three times as many blacks as whites, and blacks may develop glaucoma at a younger age and suffer more severe vision loss than whites. Other factors that increase the risk of glaucoma include diabetes, nearsightedness, high blood pressure, and long-term use of cortisone or other steroid medication. Although risk factors for the disease are well understood, the cause of glaucoma remains unknown.