Glaucoma can cause blindness if it is left untreated. Only about half of the three million Americans who have glaucoma are even aware that they have the condition.

When glaucoma develops, usually you don’t have any early symptoms and the disease progresses slowly. In this way, glaucoma can steal your sight very gradually. Fortunately, early detection and treatment can help preserve your vision.

There are several types of glaucoma:

Open-angle glaucoma
The most common form of glaucoma is called primary open-angle glaucoma. It occurs when the trabecular meshwork of the eye gradually becomes less efficient at draining fluid. As this happens, your eye pressure, called intraocular pressure (IOP), rises. Raised eye pressure leads to damage of the optic nerve. Damage to the optic nerve can occur at different eye pressures among different patients. Your ophthalmologist (Eye M.D.) establishes a target eye pressure for you that he or she predicts will protect your optic nerve from further damage. Different patients have different target pressures.

Typically, open-angle glaucoma has no symptoms in its early stages and your vision remains normal. As the optic nerve becomes more damaged, blank spots begin to appear in your field of vision. You usually won’t notice these blank spots in your day-to-day activities until the optic nerve is significantly damaged and these spots become large. If all of the optic nerve fibers die, blindness results.

Half of patients with glaucoma do not have high eye pressure when first examined. That is why it is essential that the optic nerve be examined by an ophthalmologist for proper diagnosis.

Normal-tension glaucoma
Eye pressure is expressed in millimeters of mercury (mm Hg), the same unit of measurement used in weather barometers.

Although normal eye pressure is considered a measurement less than 21 mm Hg, this can be misleading. Some people have a type of glaucoma called normal-tension, or low-tension glaucoma. Their eye pressure is consistently below 21 mm Hg, but optic nerve damage and visual field loss still occur. People with normal-tension glaucoma typically receive the same methods of treatment used for open-angle glaucoma.

Conversely, ocular hypertension is a condition where someone has higher eye pressure than normal, but does not have other signs of glaucoma, such as optic nerve damage or blank spots that show up in their peripheral (side) vision when tested. Someone with ocular hypertension is considered a “glaucoma suspect” because he or she may be at risk for developing glaucoma later. Just like people with glaucoma, people with ocular hypertension need to be closely monitored by an ophthalmologist to ensure they receive appropriate treatment.

Closed-angle glaucoma
A less common form of glaucoma is closed-angle, or narrow-angle, glaucoma. Closed-angle glaucoma occurs when the drainage angle of the eye becomes blocked. Unlike open-angle glaucoma, eye pressure usually goes up very fast. The pressure rises because the iris — the colored part of the eye — partially or completely blocks off the drainage angle. People of Asian descent and those with hyperopia (farsightedness) tend to be more at risk for developing this form of glaucoma.

If the drainage angle becomes completely blocked, eye pressure rises quickly resulting in a closed-angle glaucoma attack. Symptoms of an attack include:

• Severe eye or brow pain;
• Redness of the eye;
• Decreased or blurred vision;
• Seeing colored rainbows or halos;
• Headache;
• Nausea;
• Vomiting.

A closed-angle glaucoma attack is a medical emergency and must be treated immediately. Unfortunately, people at risk for developing closed-angle glaucoma often have few or no symptoms before the attack.

People at risk for closed-angle glaucoma should avoid over-the-counter decongestants and other medications where the packaging states not to use these products if you have glaucoma.

Congenital glaucoma
Congenital glaucoma is a rare type of glaucoma that develops in infants and young children. It happens when the eye’s drainage system doesn’t develop fully or correctly before birth. Congenital glaucoma can be inherited.

Secondary glaucoma
Glaucoma can often develop as the result of another eye condition or disease. This is known as secondary glaucoma. For example, someone who has a tumor or people undergoing long-term steroid therapy may develop secondary glaucoma. Other causes of secondary glaucoma include:

• Eye injury;
• Inflammation of the eye;
• Abnormal blood vessel formation from diabetes or retinal blood vessel blockage;
• Use of steroid-containing medications (pills, eyedrops, sprays);
• Pigment dispersion: tiny fragments or granules from the iris (the colored part of the eye) can circulate in the aqueous humor (the fluid within the front portion of the eye) and block the trabecular meshwork, the tiny drain for the eye’s aqueous humor.