Glaucoma - Introduction

(Page 2)

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.

---

---

Optic nerve damage is the problem in all types of glaucoma. If left 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 nearly always preventable with early treatment.)

Primary open-angle glaucoma tends to start in one eye but eventually involves both eyes. 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.

Normal Tension Glaucoma

Intraocular eye pressure is normal (10 - 21 mmHg) in about 25 - 30% of U.S. glaucoma cases, a condition known as normal-tension glaucoma or low-pressure glaucoma. Factors are present that cause optic nerve damage without raising IOP.

Closed-Angle Glaucoma

Closed-angle glaucoma (also called angle-closure glaucoma) is responsible for about 15% of all glaucoma 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

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 is often an inherited condition and can usually be corrected with microsurgery.

The Eye

The Light-Processing Parts. To understand sight, one begins with light and its passage through the eye's sensitive camera-like structures:

• Light first passes through the cornea, a clear tissue at the front of the eye.
• Behind the cornea, the iris (the colored tissues of the eye) widens and narrows to regulate the passage of light.
• The lens, located behind the iris, focuses the light, which then hits the retina.
• The retina is an electric fragile membrane of nerve cells called photoreceptors that receive light and translate it into signals.
• A layer of cells, called the retinal ganglia, receive signals from the retina. These nerve cells are the front ends of the optic nerve, which, in turn, receive the signals.
• The optic nerve is actually a cable of about 1.2 million nerve fibers called axons. It carries the signals to the brain, which interprets them as images.
• They exit the eye through the optic disc, located in the back of the eye.

The Supportive Chambers. To help support and protect these sensitive structures, the eye contains two fluid-filled chambers:

• The posterior (rear) chamber is the large area behind the iris.
• Fluid passes from the posterior into the anterior (forward) chamber located in the bulging area between the iris and the front of the eye.

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).