Glaucoma is a condition of the optic nerve that causes progressive loss of vision. Most people with glaucoma have high pressure inside the eye that damages the optic nerve. Normal-tension glaucoma (also called low-tension glaucoma) is a unique condition in which optic nerve damage and vision loss have occurred despite a normal pressure inside the eye.
Eye pressure, called intraocular pressure (IOP), is measured in millimeters of mercury (mm Hg). Normal eye pressure ranges from 10-21 mm Hg. Most people with glaucoma have IOP of greater than 21 mm Hg; however, in normal-tension glaucoma, people have IOP within the normal range.
Treatment of open-angle glaucoma -- the most common form of the disease -- requires lowering the eye's pressure by increasing the drainage of aqueous humor or decreasing the production of the fluid. Medications can accomplish both of these goals. Surgery and laser treatments are directed at improving the eye's aqueous drainage.
If not diagnosed early, open-angle glaucoma may significantly damage vision and even cause blindness. That is why it's so important to have your eye doctor test you regularly...
By definition, people with normal-tension glaucoma have open, normal-appearing angles. In fact, the features of normal-tension glaucoma are similar to primary open-angle glaucoma (POAG), the most common form of glaucoma (see Primary Open-Angle Glaucoma).
Although the occurrence of normal-tension glaucoma varies worldwide, it is very prevalent in Japan.
In the United States, up to 15-25% of people with open-angle glaucoma experience normal-tension glaucoma.
According to the Baltimore Eye Study, 50% of individuals with changes in their optic disc (the front surface of the optic nerve) and in their visual field had an IOP of less than 21 mm Hg on a single visit, and 33% had an IOP of less than 21 mm Hg on 2 measurements.
Normal-tension glaucoma is more common in women than in men.
Normal-tension glaucoma affects adults who are an average age of 60.
Normal-Tension Glaucoma Causes
Although its cause is not completely understood, normal-tension glaucoma is generally believed to occur either because of an unusually fragile optic nerve that can be damaged despite a normal pressure inside the eye or because of reduced blood flow to the optic nerve.