Aging Eye

Chapter 5: Glaucoma: The Stealth Sight Stealer Like a thief in the night, glaucoma can snatch sight silently. Nearly four million people in the United States have a chronic form of the disorder, but at least half of those who have glaucoma don't know they have it. Like cataracts, glaucoma is usually painless, and progresses in the initial stages without symptoms. The National Glaucoma Research Foundation estimates that about 50 million people worldwide suffer from impaired vision and/or blindness from the disorder.Glaucomais actually a group of diseases characterized by excessive fluid pressure in the eye, which can damage theoptic nerve-- a bundle of more than one million nerve fibers that connects theretina,the light-sensitive layer of tissue at the back of the eye, with the brain. Good vision depends on a healthy optic nerve. In many cases, by the time vision loss from glaucoma is apparent, cells in the optic nerve have already been irreparably damaged and vision is gone forever.Glaucoma is a major cause of blindness, and threatens two percent of the population over age forty, and becomes even more common with age. However, if glaucoma is caught and treated early through regular routine ophthalmology checkups, vision can almost always be spared. Types of Glaucoma Although twenty-five to thirty types of glaucoma exist, those described here are the most common.Chronic Open-Angle GlaucomaChronic open-angle glaucoma,also calledchronic glaucoma,accounts for more than 90 percent of all glaucoma cases. According to the National Eye Institute, glaucoma affects about 3 million Americans; it's estimated that half of those afflicted with glaucoma don't know they have it, due to the lack of early symptoms. Glaucoma strikes African-Americans most frequently. Most prevalent in people over sixty, glaucoma also tends to run in families.According to the National Eye Institute, roughly 120,000 Americans are blind from this particular chronic form of the disorder. The namechronic open-angle glaucomacomes from the fact that the angle in the anterior (front) chamber of the eye remains open. For some reason that researchers don't fully understand, theaqueous humor-- the fluid in that front chamber -- drains too slowly. This leads to fluid backup and a gradual but persistent elevation in eye pressure, which can ultimately damage the optic nerve and cause vision loss if not caught in time and controlled by medication.More specifically, the aqueous humor circulates through the pupil into the front compartment of the eye, nourishing the lens and lining cells of the cornea, the clear outer covering of the eye. The aqueous humor then drains out of the eye through Schlemm's canal via thecircular trabecular meshwork,a sievelike drainage system of porous tissue, before being reabsorbed into surrounding blood vessels. As more aqueous humor is produced, excess fluid is eliminated through the trabecular meshwork to keep a healthy balance of pressure in the eye. The process works continuously as part of normal vision.In open-angle glaucoma, the drainage system breaks down, and the outgoing fluid flows too slowly through the meshwork, or not at all. Consequently, the fluid can't leave the eye as it should, and backs up like water in a clogged sink. As a result, the internal pressure in the eye rises. This, in turn, puts stress on the optic nerve, which is responsible for transmitting visual signals to the brain, which then translate into images. If the pressure continues, the nerve fibers that carry the optical messages die, and vision starts to fade. Nerve fibers on the outer edge are affected first, which is why those with glaucoma typically develop blind areas at the edges of their field of vision. If left untreated, your peripheral vision gradually closes in, until the cells supplying central vision are killedHarvard Medical School Staff is the author of 'Aging Eye' with ISBN 9780743215039 and ISBN 0743215036.