Glaucoma is a disease of the optic nerve, in which the nerve cells in the front of the optic nerve (the ganglion cells) die. The process is irreversible. Previously, it was believed that glaucoma was almost always due to increased intraocular pressure. However, glaucoma can occur in patients with normal and even low eye pressure, so damage to the optic nerve is now key for diagnosis.
The Aqueous Humor. To understand glaucoma, it is important to first consider aqueous humor, the clear, watery fluid that circulates continuously through the front (anterior) chamber of the healthy eye and is a primary focus of glaucoma research. (This fluid is not related to tears, nor is it the dense jelly-like substance called vitreous humor that is contained in the rear chamber.)
Aqueous humor serves two important functions in the eye:
Draining the Fluid and Intraocular Pressure. The aqueous fluid is continuously produced within the front of the eye, causing pressure known as intraocular pressure (IOP). To offset the in-flowing fluid and to maintain normal IOP, the fluid drains out between the iris and cornea (an area known as the drainage angle). It does so through two channels within this angle:
Intraocular Eye Pressure. Increased IOP is, indeed, present in most cases of glaucoma, but some patients have normal IOP, which is usually maintained at measurements of 10 - 20 mmHg. Higher IOP measurements increase the risk of glaucoma, but do not necessarily mean that the patient has glaucoma.
Most people with glaucoma have the form called primary-open-angle glaucoma (also called chronic open-angle glaucoma). Open-angle glaucoma is essentially a plumbing problem.
The disease process may occur as follows:
Optic nerve damage is the basic glaucoma condition. If it is untreated, eventually the nerve deteriorates until a person loses sight, first in the peripheral vision (the vision in the "corner of the eyes"). If it becomes severe, the person loses central vision (in the middle of the eyes), and may eventually become blind. (Blindness is fortunately nearly always preventable with early treatment.)
Primary open-angle glaucoma tends to start in one eye but eventually involves both. About half of patients have generalized (spread out) nerve damage. In the other half the disease is localized, causing wedge-shaped abnormalities in the nerve fiber layers of the retina.
Intraocular eye pressure is normal (12 - 22 mmHg) in about 25 - 30% of U.S. glaucoma cases, a condition known as normal-tension glaucoma. Other factors are present that cause optic nerve damage but do not affect IOP.
Closed-angle glaucoma (also called angle-closure glaucoma) is responsible for 15% of all cases. It is less common than open-angle glaucoma in the U.S., but it constitutes about half of the world's glaucoma cases because of its higher prevalence among Asians. The iris is pushed against the lens, sometimes sticking to it, closing off the drainage angle. This can occur very suddenly, resulting in an immediate rise in pressure. It often occurs in genetically susceptible people when the pupil shrinks suddenly. Closed-angle glaucoma can also be chronic and gradual, a less common condition.
Congenital glaucoma, in which the eye's drainage canals fail to develop correctly, is present from birth. It is very rare, occurring in about 1 in 10,000 newborns. This may be an inherited condition and often can be corrected with microsurgery.
The Light-Processing Parts. To understand sight, one begins with light and its passage through the eye's sensitive camera-like structures:
The Supportive Chambers. To help support and protect these sensitive structures, the eye contains two fluid-filled chambers:
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