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.
The iris is a circular, pigmented membrane that provides the eye its color and the opening in the center is the pupil of the eye.
The iris is made up of muscular fibers that control the amount of light entering the pupil so that you can see clearly. The iris accomplishes this task by making the pupil smaller in bright light and larger in dim light.
In some people, the iris can become inflamed. This is termed iritis.
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.
Normal-Tension Glaucoma Symptoms
Regular eye examinations with an ophthalmologist (a medical doctor who specializes in eye care and surgery) are very important, because people with normal-tension glaucoma do not experience any early symptoms of it.
Because of its silent nature, people do not usually have any visual complaints until late in the course of the disease. By the time a person with normal-tension glaucoma would notice vision loss, significant amounts of optic nerve damage and vision loss would have already occurred. The optic nerve damage and vision loss are permanent.
When to Seek Medical Care
Regular eye examinations with an ophthalmologist are important to screen for optic nerve damage and vision loss, despite a normal eye pressure.