Last week, this column dealt with what doesn't cause chronic fatigue (namely Epstein Barr virus). Today, I'm going to discuss what does cause chronic fatigue -- or at least a few of its causes. Unfortunately, some people suffer from chronic fatigue, and no exact cause can be found. Yet even for them, there's still hope.

Chronic fatigue is a symptom of something wrong, not a disease in itself. Many physical and emotional problems can make you feel run-down, listless and tired. Your doctor's job is to try to pinpoint the cause and to decide whether it's serious.

Some people, however, have chronic fatigue without any apparent cause. A small number of them have what's known as chronic fatigue syndrome. Until recently, doctors linked CFS to Epstein Barr virus infection. But EBV turns out not to be the cause.

CFS is a special type of chronic fatigue that appears to be associated with malfunction of the immune system. How do you know if you have this condition? Well, you have CFS if you experience:

Newly developed fatigue severe enough to interfere with your normal activities that has lasted for at least six months.

No physical or emotional problem that would otherwise explain your fatigue.

At least eight of the following symptoms, which must persist over the course of illness: 1) low-grade fever, 2) chills, 3) sore throat, 4) swollen glands in the neck, 5) muscle pains or weakness, 6) exhaustion for at least 24 hours after exercise that you previously tolerated, 7) new headaches unlike any in the past, 8) joint pains, 9) mental difficulties, such as forgetfulness, irritability, confusion, inability to concentrate, depression or being bothered by bright lights, 10) sleeping problems (sleeping too much or too little), and finally, 11) the fatigue and related symptoms should have come on rather suddenly and unexpectedly, over a few hours or days rather than gradually over weeks or months.

Only a small number of people with fatigue have CFS. Researchers at the National Institutes of Health are currently studying this unusual syndrome, but so far no new leads about its cause or treatment have turned up. (The NIH no longer accepts new patients into its CFS study).

If most people who consult their doctor because of being tired don't have CFS, what do they have? Sometimes, they have an underlying medical condition, such as an infection (for example, tuberculosis), cancer or diabetes. In most cases, your doctor will be able to discover one of these physical causes by taking a history, doing a physical examination and ordering some blood tests and X-rays.

But much of the time, people with fatigue don't have an underlying physical ailment. Instead, many are suffering from depression, stress or anxiety, conditions that might not be readily apparent to either the patient or the physician.

Although many people don't like the thought of having a psychological problem, I'm usually relieved to discover that an unsuspected depression is the real culprit. Depression is, at least, something that can be diagnosed and, more important, treated.

Although some people feel ashamed when told they're suffering from anxiety or depression, they shouldn't. These are physical as well as emotional disorders and more a sign of some imbalance in brain chemistry than a defect in character.

Too often, however, no obvious explanation for fatigue turns up after a complete evaluation. If nothing shows up at first, it's important to stay under the care of a concerned physician. You may also benefit from counseling on how to cope with a potentially debilitating condition.

In addition to physical and emotional illnesses, sometimes you can be fatigued because you're just plain tired. I've had patients ask me why they're tired all the time, only to discover that they're the mother of a newborn who's up all night and who also has a toddler at home, or a shift worker holding down two jobs to make ends meet.

But for those who don't get a good explanation for their chronic fatigue, especially those suffering from CFS, here are support groups and sources for further information: 1) Chronic Fatigue and Immune Dysfunction Syndrome Society, P.O. Box 230108, Portland, Ore. 97223; 2) Chronic Fatigue and Immune Dysfunction Syndrome Association, P.O. Box 220398, Charlotte, N.C. 28222, 704-362-2343; 3) National Chronic Fatigue Syndrome Association, 919 Scott Ave., Kansas City, Kan. 66105, 913-321-2278, or locally, 4) The Northern Virginia Chronic Fatigue Support Group, 11253 Ramrod Rd., Woodbridge, Va. 22192, 703-590-9404.

For a packet of information about CFS, send a self-addressed stamped business-size envelope to any of the above national organizations or to Box CFS, National Institute of Allergy and Infectious Diseases, Bldg. 31, Rm 7A32, NIH, Bethesda, Md. 20892.

Jay Siwek, a family physician from Georgetown University, 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.

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