Introduction
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 has occurred in many 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:
- Nourishing the area around the colored iris and behind the cornea
- Exerting pressure to help maintain the eye's shape
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:
- The trabecular meshwork, a sponge-like, porous network, and its connecting passageways are referred to as the "conventional" outflow pathway. Most of the eye fluid outflow occurs in this region and flows from the trabecular meshwork to a group of vessels encircling the anterior chamber, called Schlemm's canal. From here, the fluid enters collection chambers and then flows out into the general blood circulatory system of the body.
- The uveoscleral pathway is located behind the trabecular meshwork and is called the "unconventional" pathway. Up to 30% of the fluid flows out through this channel.
Intraocular Eye Pressure. Previously, it was believed that glaucoma was almost always due to an abnormal rise in intraocular pressure.
Glaucoma is a term used to describe several types of eye conditions that affect the optic nerve. In most cases, damage to the optic nerve is caused by increased pressure in the eye, also known as intraocular pressure (IOP). Glaucoma can cause partial vision loss, with blindness as a possible eventual outcome.
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. Measurements above this, however, do not necessarily predict glaucoma. For example, only about 10% of people with IOP levels of 21 - 30 mmHg will actually develop glaucoma. This still puts such individuals at considerable risk for glaucoma, however.
Primary Open-Angle 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:
