Glaucoma is sometimes called the silent thief, slowly stealing your sight before you realize anything's wrong. The most common form of glaucoma develops gradually, giving no warning signs. Many people aren't even aware they have an eye problem until their vision is extensively damaged.

Glaucoma is not just one disease but is a group of them. The common feature of these diseases is that abnormally high pressure inside the eyeball damages the optic nerve. The optic nerve is a bundle of more than a million nerve fibers at the back of your eye. It's like a big electric cable made up of thousands of individual wires carrying the images you see from the retina to your brain. Blind spots develop in your visual field when the optic nerve deteriorates, starting with your peripheral (side) vision. If left untreated glaucoma may lead to blindness in both eyes.

Fortunately, only a small percentage of people with the disease lose their sight. Recent medical advances have made it easier to diagnose and treat glaucoma. And if detected and treated early, glaucoma need not cause even moderate vision loss. But it does require regular monitoring and treatment for the rest of your life.

Signs and symptoms

Glaucoma occurs in several types, and signs and symptoms vary depending on the type of glaucoma you have.

Primary open-angle glaucoma progresses with few or no symptoms until the condition reaches an advanced stage. As increased eye pressure continues to damage your optic nerve, you lose more and more of your peripheral vision. If glaucoma is left untreated you can develop tunnel vision, and eventually lose all sight. Open-angle glaucoma usually affects both eyes, although at first you may have vision loss in just one eye.

Acute angle-closure glaucoma develops suddenly in response to a rapid rise in eye pressure. Permanent vision loss can occur within hours of the attack, so it requires immediate medical attention. An attack often happens in the evening when the light is dim and your pupils have become relatively dilated. Pain may be severe. Signs and symptoms include:

• Blurred vision

• Halos around lights

• Reddening of the eye

• Severe eye pain

• Nausea and vomiting

Both open-angle and angle-closure glaucoma can be primary or secondary conditions. They're called primary when the cause is unknown. They're called secondary when the condition can be traced to a known cause, such as an injury or an eye disease. Signs and symptoms of secondary glaucoma vary and depend on what's causing the glaucoma.

• Nausea

• vomiting

Causes

Internal pressure in your eye, called intraocular pressure, allows your eye to hold its shape and function properly. Intraocular pressure is like air in a balloon — too much pressure inside the balloon affects its shape and may even cause it to pop. In the case of your eye, too much pressure can damage the optic nerve.

Fluids inside your eye help maintain the intraocular pressure. These fluids are the vitreous, which fills the vitreous cavity at the back of your eye, and the aqueous humor, which fills the anterior chamber at the front of your eye. Aqueous humor is continuously produced and circulated through the anterior chamber before draining out of your eye. This continuous flow of fluid nourishes the lens and the cornea and removes unwanted debris. A healthy eye produces aqueous humor at the same rate that it drains fluid, thus maintaining a normal pressure.

Aqueous humor exits your eye through a drainage system located at the angle formed where the iris and the cornea meet. Here it passes through a sieve-like system of spongy tissue called the trabecular meshwork and drains into a channel called Schlemm's canal. The fluid then merges into your bloodstream.

When the drainage system doesn't function properly — for example, if the trabecular meshwork becomes clogged — the aqueous humor can't filter out of the eye at its normal rate, and pressure builds within your eye. For reasons that doctors don't completely understand, the increased eye pressure gradually is associated with damage to the nerve fibers that make up the optic nerve.

Types of glaucoma

• Primary open-angle glaucoma. This form, also called chronic open-angle glaucoma, accounts for most cases of the disease. Although the drainage angle formed by the cornea and the iris remains open, the aqueous humor drains too slowly. This leads to fluid backup and a gradual buildup of pressure within your eye. Damage to the optic nerve is so slow and painless that a large portion of your vision can be lost before you're even aware of a problem. The cause of primary open-angle glaucoma remains unknown. It may be that the aqueous humor drains or is absorbed less efficiently with age, but not all older adults get this form of glaucoma.

• Angle-closure. Angle-closure glaucoma, also called closed-angle glaucoma, is a less common form of the disease. It occurs when the drainage angle formed by the cornea and the iris closes or becomes blocked.Most people with this type of glaucoma have a very narrow drainage angle, which may be an abnormality from birth. As you get older, your lens becomes larger, pushing your iris forward and narrowing the space between the iris and the cornea. Whether the narrow drainage angle is an abnormality from birth or a result of aging, as this angle narrows, the iris gets closer to the trabecular meshwork. If it gets too close, the aqueous humor can't exit through the trabecular meshwork, resulting in a buildup of fluid and an increase in eye pressure. Angle-closure glaucoma can be chronic (progressing gradually) or acute (appearing suddenly). The acute form occurs when the iris is forced up against the trabecular meshwork and completely blocks the drainage of the aqueous humor.

• Angle-closure glaucoma is more common among farsighted people, who tend to have smaller eyes that can narrow the angle. Normal aging also may cause angle blockage.

• If you have a narrow drainage angle and your pupils become dilated, the angle may close and cause a sudden increase in eye pressure. This attack of acute angle-closure glaucoma requires immediate treatment. Although an acute attack often affects only one eye, the other eye is at risk of an attack as well. Several factors can cause your pupils to dilate, including darkness or dim light, stress or excitement, and certain medications. These medications include antihistamines, tricyclic antidepressants and eyedrops used to dilate your pupils. However, dilating eyedrops may not cause the angle to close until several hours after the drops are put in your eyes. Acute angle-closure glaucoma is a medical emergency that can cause vision loss within hours of its onset.

• Secondary. Both open-angle and angle-closure glaucoma can be primary or secondary conditions. They're called primary when the cause of the condition is unknown. They're called secondary when the condition can be traced to a known cause, such as an injury or an eye disease. Secondary glaucoma may be caused by a variety of medical conditions, medications, physical injuries, and eye abnormalities or deformities. Infrequently eye surgery can be associated with secondary glaucoma.

• Low-tension. Low-tension glaucoma is a poorly understood form of the disease. In this form, eye pressure remains within what is ordinarily thought to be the normal range, but the optic nerve is damaged nevertheless. Why this happens is unknown. Some experts believe that people with low-tension glaucoma may have an abnormally sensitive optic nerve or a reduced blood supply to the optic nerve caused by a condition such as atherosclerosis, a hardening of the arteries caused by accumulation of fatty deposits (plaques) and other substances. Under these circumstances even normal pressure on the optic nerve is enough to cause damage.

• atherosclerosis

Doctors don't completely understand the underlying causes of glaucoma. Open-angle glaucoma may have a genetic link. That is, a defect in one or more genes may cause certain individuals to be more susceptible to the disease. People with a family history of glaucoma are more likely to develop it themselves. However, the exact genetic defects responsible for its occurrence haven't been identified.

Other factors appear to contribute to the disease, but doctors don't know for sure what these factors are and the relationships among them. Although glaucoma is normally associated with increased eye pressure, people with normal or low eye pressure can experience vision loss. And people with higher-than-normal eye pressure may never experience optic nerve damage.

Doctors have debated for years how damage to the optic nerve occurs. One theory holds that the pressure of backed up aqueous humor causes structural damage and ultimately death to the nerve fibers. Another theory suggests that nerve fibers die when small blood vessels that feed the optic nerve become blocked or when the blood supply is disrupted.

The cause of decreased drainage through the trabecular meshwork also presents a puzzle. Decreased drainage may be a result of normal aging, yet not all older adults develop glaucoma.

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