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Normal-Tension Glaucoma

Normal-Tension Glaucoma Overview

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.

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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.

  • Unusually fragile optic nerves may be inherited.

  • Reduced blood flow to the optic nerve can be due to disorders of the blood vessels (called vascular diseases), including vasospasms and ischemia.

    • Vasospasms are spasms or constrictions of the blood vessels.

    • Ischemia is reduced oxygen delivered to the tissue, in this case the optic nerve, because the blood vessels are either narrowed or obstructed.
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