Table of Contents
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 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. (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, which builds up 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. Increased IOP is present in most cases of glaucoma, but some patients have normal IOP, which is usually maintained at measurements of 10 - 21 mmHg. Higher IOP measurements increase the risk of glaucoma, but do not necessarily mean that the patient has glaucoma.
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:
- The drainage angle remains open, but tiny drainage channels in the trabecular meshwork pathway become clogged. This pathway is responsible for most aqueous humor fluid outflow. An imbalance occurs when fluid continues to be produced but does not drain out efficiently.
- The fluid in the eye’s anterior chamber builds up and increases pressure within the eye (intraocular pressure).
- The intraocular pressure exerts force on the optic nerve at the back of the eye.
- Over time, the persistent pressure or other factors irreversibly damages the delicate long fibers of the optic nerve, called axons, which convey images to the brain.
- As these axons die, the small cup-like head of the optic nerve may eventually collapse into an enlarged irregular shape.
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Review Date: 06/23/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine,
Harvard Medical School; Physician, Massachusetts General Hospital.
Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M.,
Inc.
A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)
