High pressure inside the eye can damage the optic nerve, causing loss of vision and eventually blindness. This condition is called glaucoma, and several specific types of glaucoma exist. Although all glaucoma results in optic nerve damages because of intraocular pressure, most forms of glaucoma cause a rise in pressure. Normal-tension glaucoma, however, occurs when the pressure inside the eyeball is within the normal range but still damages the optic nerve.
The causes of normal-tension glaucoma are unknown. Vision loss is directly related to optic nerve damage, but why the nerve is damaged when pressure is within the normal range is unclear. Normal intraocular pressure has a measurement of 10-20 millimeters of mercury. An estimated one-third of cases of glaucoma involve normal intraocular pressure.
This condition is characterized by progressive damage to the optic nerve. As the damage worsens, the field of vision becomes narrower. Normal-tension glaucoma is difficult to diagnose. Standard glaucoma tests rely on steadily increasing intraocular pressure. Diagnosis of normal-tension glaucoma depends on careful examination and visual acuity tests.
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Several theories have been proposed to explain why in some cases the optic nerve becomes damaged by normal pressure. One theory suggests that decreased blood flow causes cells in the optic nerve to die from a lack of oxygen and nutrients. Without the cells to carry the nerve impulses to the brain, vision suffers. An alternate theory cites weaknesses in the optic nerve that leaves the nerve particularly susceptible to pressure-related damage even when the pressure is within the normal range. Also, poor circulatory mechanisms might inhibit blood vessels from regulating pressure within the eye, causing damage.
Certain risk factors for normal-tension glaucoma have been identified. Anyone with a family history of any type of glaucoma is considered at risk for all types of glaucoma. Women are at a higher risk than men. The presence of a cardiovascular disease also raises a person’s risk. People of Japanese ancestry are particularly susceptible to the disease.
The optic nerve damage caused by any type of glaucoma is irreversible, so the goal of treatment is to prevent further damage. The treatment for normal-tension glaucoma is to keep the intraocular pressure as low as possible with medication. If medications are ineffective, laser or filtering surgery might be required. In some people, however, maintaining normal intraocular pressure is insufficient to prevent further damage. Damage to their optic nerves progresses regardless of low eye pressure.