Fibromyalgia, also called fibrositis, is a complex, disabling and chronic condition characterized by widespread long-term pain, fatigue and stiffness in the muscles, ligaments and tendons. People with fibromyalgia have multiple tender points on the body where slight pressure causes pain. In addition to muscular pain and fatigue, fibromyalgia also may be associated with sleep problems, depression and an inability to think clearly.

Although its symptoms are similar to other joint diseases, such as arthritis, fibromyalgia is not truly a form of arthritis because it does not cause inflammation or damage to the joints, muscles or other tissues. Like arthritis, however, fibromyalgia is considered a rheumatic condition, a medical condition that impairs the joints or soft tissues and causes chronic pain.

The cause of fibromyalgia is unknown. However, researchers believe the condition may be caused by a central nervous system malfunction that amplifies the detection and sensation of pain. This theory suggests that patients with fibromyalgia may have a lower pain threshold because of increased sensitivity in the brain to pain signals. Abnormal pain processing also may be responsible for symptoms of several chronic pain disorders that many fibromyalgia patients also experience, including irritable bowel syndrome (IBS), temporomandibular joint disorder (TMJ), chronic low back pain and other chronic pain disorders.

Several factors, including sleep disorders; psychological stress; family history of fibromyalgia; infection; injury; depression; immune, endocrine (hormonal), or biochemical abnormalities; and changes in muscle metabolism may lead to the development of fibromyalgia. These events, however, may or may not be present in individuals diagnosed with fibromyalgia.

The disease is fairly common, affecting approximately 2 to 4 percent of the U.S. population, mostly women of childbearing age.