Introduction

Glaucoma is defined as 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 been observed in many patients with normal and even low eye pressure, so the definition now rests on the damage to the optic nerve.

The Aqueous Humor. For some understanding of glaucoma, it is important to first consider aqueous humor, the clear, watery fluid that circulates continuously through the 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.) It serves two important functions in the eye:

• It nourishes the area around the colored iris and behind the cornea.
• It exerts pressure to help maintain the eye’s shape.

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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.