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Glaucoma

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of glaucoma.

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. In understanding of 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.) 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.

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.

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 mm Hg. Measurements above this, however, do not necessarily predict glaucoma. For example, only about 10% of people with IOP levels between 21 - 30 mm Hg 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:

• 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 then occurs as fluid continues to be produced but does not drain out efficiently. Experts have still not definitely determined the precise area in the pathway where the blockage is most likely to occur. (In rare instances the pressure is high because the eye produces too much aqueous humor.)
• The fluid in the eye’s anterior chamber builds up and increases pressure within the eye. This is called intraocular pressure (IOP).
• 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 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. In about half of patients the damage in the eye is diffuse, that is the nerve damage is generalized. 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 (between 12 - 22 mmHg) in about 25 - 30% of U.S. glaucoma cases, a condition known as normal-tension glaucoma. (In Japan, the rates may be as high as 70%.) Other factors are present that cause optic nerve damage but do not affect IOP.

Closed-Angle Glaucoma

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

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 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) opens and closes like a camera shutter 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 cable, 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: 3/3/2007
• Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.

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