Q. Several months ago I had what I thought was a migraine headache. When it didn't go away, I went to see my doctor. I was also feeling tired, like I was coming down with the flu. But when I started to develop some blurred vision, I really became alarmed. It turns out I have something called temporal arteritis. What causes this problem? What are my options for treatment? Where can I get more information about it?

A. Temporal arteritis is an unusual condition that causes inflammation of arteries in your body. For unknown reasons, the arteries most often affected are the ones running up along the temples of your head, the temporal arteries. Because of this location, temporal arteritis can cause headaches on either side of your head. But because it can also affect the tiny arteries going to your eyes, it can lead to blurred vision and sudden, permanent blindness in the affected eye. (The condition usually only affects one eye.) Fortunately, treatment can often relieve the symptoms and help prevent blindness.

Temporal arteritis is also known as giant cell arteritis because it produces large cells on biopsy specimens that are viewed under a microscope. The condition is also called cranial arteritis because the arteries most often affected are those in the head, or cranium.

The cause of giant cell arteritis is unknown, but it seems to stem from a problem with the immune system. Something--perhaps an infection--appears to trigger the inflammation in arteries of the head, neck and arms. Giant cell arteritis generally doesn't start until after age 50; the average age of those affected is around 70. The condition affects twice as many women as men.

Giant cell arteritis often causes a headache, usually unlike any you've had before. Although the pain can be on both sides of your head, it typically affects one side, along the temple area. This area may also be tender, and you may even notice that your temporal artery is swollen. You may also develop a low-grade fever, fatigue, weight loss and muscle aches. A less common symptom is pain in your jaw when chewing. When it affects your eyes, giant cell arteritis can lead to blurred vision or partial loss of vision. This is a serious warning sign that you may suddenly develop complete, permanent blindness.

To help diagnose this condition, a doctor will first order a blood test known as a sed rate or ESR (erythrocyte sedimentation rate). Readings for this test, which measures the rate at which red blood cells settle in a tiny tube, are almost always very high in people with giant cell arteritis. To confirm that a patient has the condition, a doctor may order a temporal artery biopsy. In this outpatient procedure, a surgeon will take a small piece of your temporal artery. Although it's not foolproof, it will usually be positive in people with giant cell arteritis.

To treat the condition, your doctor will usually use high-dose steroids such as prednisone. These can have some serious side effects, but up to 15 percent of people who aren't treated will develop blindness. Your doctor will periodically check your sed rate to see how well your treatment is working. With giant cell arteritis, you may need at least two years of steroid therapy. Some people require them indefinitely.

Giant cell arteritis is closely related to a condition called polymyalgia rheumatica, which I discussed in this column last month [Consultation, October 19, 1999]. Polymyalgia rheumatica causes severe muscle aches and stiffness in the neck, shoulders, upper arms, hips and thighs. At least half of people with giant cell arteritis will also have polymyalgia rheumatica, and up to 33 percent of people with polymyalgia rheumatica will develop giant cell arteritis.

You can get more information about both giant cell arteritis and polymyalgia rheumatica from the National Institute of Arthritis and Musculoskeletal and Skin Diseases Web site: http://www.nih.gov/niams/healthinfo/pmrgcafs.htm.

Jay Siwek, chairman of the department of family medicine at Georgetown University Medical Center, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, DC 20071. Questions cannot be answered personally.