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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Chronic fatigue syndrome is a complicated illness characterized by at least six months of extreme fatigue that is not relieved by rest, and a group of additional symptoms that also are constant for at least six months. In many people with chronic fatigue syndrome, the disorder begins suddenly, often following a flulike infection or an episode of physical or psychological trauma, such as surgery, a traumatic accident or the death of a loved one. In other cases, chronic fatigue syndrome develops gradually. The illness lasts for many months or years, and only a small percentage of people recover full health.

Many people feel tired a lot of the time, and many seek help from their doctors. Most people who experience chronic (long-lasting) fatigue are not suffering from chronic fatigue syndrome. Depression and overwork are much more common causes of chronic fatigue.

The exact cause of chronic fatigue syndrome remains a mystery. The illness can follow a number of common infectious illnesses, such as Lyme disease or infectious mononucleosis, but not all cases are tied to infections. Testing has found that people with chronic fatigue syndrome have abnormalities in the brain, particularly in the hypothalamus (a part of the brain that regulates hormones and vital functions) and the pituitary gland. Testing also has found that patients have abnormalities in the part of the nervous system called the autonomic nervous system, which controls blood pressure, heart rate, body temperature and other vital body functions. For example, many patients with chronic fatigue syndrome have an unusually high heart rate and low blood pressure when they have been standing for a while.

The immune system remains activated for long periods in people with chronic fatigue syndrome. Many recent studies indicate that patients with chronic fatigue syndrome have defects in the ability of cells in their bodies to make energy. Some studies indicate that certain genes are built differently, and that the activity of genes in white blood cells is different, in patients with chronic fatigue syndrome.

Many of these abnormalities seem to come and go, and are not permanent conditions. Furthermore, not all of the abnormalities affect every patient with chronic fatigue syndrome.

In the United States, federal health authorities estimate that chronic fatigue syndrome affects 1 to 8 of every 1,000 Americans older than age 18. Women are affected about twice as often as men. Although the illness is most common in people 25 to 45 years old, chronic fatigue syndrome can attack people of all age groups, including children. The condition also is found in people of all racial, ethnic and economic backgrounds. It appears to be more common in African-Americans and Latinos, and in people in lower socioeconomic groups. It appears to be less common in Asian-Americans. Studies from the U.S. Centers for Disease Control and Prevention (CDC) and other research groups estimate that the United States loses between $9 to $25 billion each year in reduced productivity and medical expenses due to chronic fatigue syndrome.

Although most cases of chronic fatigue syndrome do not occur during epidemics, at least 30 outbreaks of chronic fatigue syndrome have been reported, during which many people in the same area suddenly developed the illness at the same time. However, health experts have failed to identify a cause for their chronic fatigue syndrome symptoms.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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