Chronic fatigue syndrome is a condition of prolonged and severe tiredness or weariness (fatigue) that is not relieved by rest and is not directly caused by other conditions. To be diagnosed with this condition, your tiredness must be severe enough to decrease your ability to participate in ordinary activities by 50%.

See also: Fatigue

The exact cause of chronic fatigue syndrome (CFS) is unknown. Some researchers suspect it may be caused by a virus, such as Epstein-Barr virus or human herpes virus-6 (HHV-6). However, no distinct viral cause has been identified.

Recent studies have shown that chronic fatigue syndrome may be caused by inflammation of pathways in the nervous system, and that this inflammation may be some sort of immune response or process. CFS may occur when a viral illness is complicated by a problem with the body's immune response. Other factors such as age, prior illness, stress, environment, or genetics may also play a role. CFS most commonly occurs in women ages 30 to 50.

The Centers for Disease Control (CDC) describes CFS as a distinct disorder with specific symptoms and physical signs, based on ruling out other possible causes. The number of persons with CFS is unknown.

Symptoms of CFS are similar to those of most common viral infections (muscle aches, headache, and fatigue), often developing within a few hours or days and lasting for 6 months or more.

Main symptoms:

• Fatigue or tiredness, never experienced to this extent before (new onset), lasting at least 6 months and not relieved by bed rest
• Fatigue that is severe enough to restrict activity (serious fatigue develops with less than one-half of the exertion compared to before the illness)

Other symptoms:

• Fatigue lasting more than 24 hours after an amount of exercise that would normally be easily tolerated
• Mild fever (101 degrees F or less)
• Sore throat
• Lymph node tenderness in the neck or armpit
• Muscle weakness, all over or multiple locations, not explained by any known disorder
• Muscle aches (myalgias)
• Feeling unrefreshed after sleeping an adequate amount of time
• Headaches, different from previous headaches in quality, severity, or pattern
• Joint pain, often moving from joint to joint (migratory arthralgias), without joint swelling or redness
• Forgetfulness or other similar symptoms including difficulty concentrating, confusion, or irritability

Physical examination may confirm the fever, lymph node tenderness, lymph node swelling, or other symptoms. The throat may appear red without drainage or pus.

The health care provider can presume a diagnosis of chronic fatigue syndrome (CFS) only after ruling out all other known possible causes of fatigue, such as:

• Infections
• Immune or autoimmune disorders
• Tumors.
• Muscle or nerve diseases (such as multiple sclerosis)
• Endocrine diseases (such as hypothyroidism)
• Psychiatric or psychological illnesses, particularly depression (since CFS itself may be associated with depression, a diagnosis of depression does not rule out CFS but fatigue related to depression alone must be ruled out for CFS to be diagnosed)
• Drug dependence
• Other illnesses (such as heart, kidney, liver diseases)

A diagnosis of CFS must include:

• Extreme, prolonged fatigue
• Absence of other causes of chronic fatigue (excluding depression)
• At least 4 of the other symptoms listed

There are no specific tests to confirm the diagnosis of CFS, though a variety of tests are usually done to exclude other possible causes of the symptoms.

There are some typical findings on tests that, while not specific enough to diagnose CFS, are seen consistently in people who are eventually diagnosed with the disorder. These include:

• Higher levels of specific white blood cells (CD4 T cells) compared to other types of white blood cells (CD8 T cells)
• Brain MRI showing swelling in the brain or destruction of part of the nerve cells (demyelination)
• Specific white blood cells (lymphocytes) containing active forms of EBV or HHV-6