Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is an umbrella term that includes both myalgic encephalomyelitis (ME) and chronic fatigue syndrome (CFS). It is a serious, chronic and complex disease that affects many systems of the body and can profoundly impact daily activities. The main symptom is disabling fatigue that does not improve with rest and lasts at least 6 months. Fatigue may get worse after activity, which is known as post-exertional malaise. Other signs and symptoms may include muscle pain, joint pain, concentration and memory problems, headaches, sleep problems, fever, sore throat, and/or tender lymph nodes.  ME/CFS generally occurs in young adults (20 to 40 years of age) and is twice as common in women.  The exact cause is not known, but symptoms may be triggered by an infection. Diagnosis can be difficult as there are no specific laboratory tests and the symptoms are common to a number of different illnesses. There is no cure or effective treatment for this condition but there are several clinical trials.  While the term ME/CFS is widely used, there is still a debate in the medical literature about the relationship between myalgic encephalomyelitis and chronic fatigue syndrome and there is no universally accepted definition.  The term “systemic exertion intolerance disease” has been proposed as new term for both diseases.

The cause of myalgic encephalomyelitis/chronic fatigue syndrome is not known. Some researchers have proposed that this condition is caused by viral infections or by immunological, hormonal or psychiatric problems. However, none of these possible explanations are proven. Some researchers believe that there may be a genetic predisposition for this condition and stress-related events act as triggers.

The Committee on the Diagnostic Criteria for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome, The Board of Select Populations, and the Institute of Medicine proposed diagnostic criteria which requires that the patient have the following three symptoms:

1. Chronic fatigue that interferes with daily activities and work, which is often profound, is of new or definite onset (not lifelong), is not the result of excessive exertion or other medical conditions, and is not greatly alleviated by rest.
2.  Post-exertional malaise. 
3.  Unrefreshing sleep. 

At least one of the two following symptoms is also required:

• Cognitive impairment (impairment of short-term memory or concentration).
• Orthostatic intolerance (Onset of symptoms when standing upright that are improved by lying back down).

Other  symptoms include post exertion illness lasting more than 24 hours, muscle pain, pain in the joints, headaches, tender lymph nodes, and sore throat.   These symptoms should have persisted or recurred during 6 or more consecutive months of illness and they cannot have first appeared before the fatigue. The diagnosis can only be made after 6 months because many other causes of similar fatigue do not last beyond 6 months. The patients should be asked questions about the frequency and severity of their symptoms.  Questionnaires or clinical observations that may help the diagnosis should also be used, such as the Wood Mental Fatigue Inventory. While there are no laboratory tests that can diagnose ME/CFS directly, the following tests, together with the symptoms, support the diagnosis (these tests are not routinely required, nor do negative results rule out the diagnosis):

• Two cardiopulmonary exercise tests (CPETs) separated by 24 hours that show inability to repeat the same maximal or anaerobic threshold measures on the second day; this mean that the values on the second CPET must be much lower than those on the first CPET (note that this test may worsen symptoms in these patients).
• Standing test or tilt test to evaluate for postural tachycardia syndrome, neurally mediated hypotension, and orthostatic hypotension.

Sleep studies do not seem to help to the diagnosis of this disorder. Neurological or psychological testing is not required for diagnosis. Other symptoms and testing that may support the diagnosis may include: 

• History of past infection from which patient never fully recovered with or without blood exams that show the presence of virus-specific immunoglobulin M (IgM), near the onset of illness.
• History of having repeated infections with or without exams showing an abnormal immune function, such as decreased function of natural killer cells in those with severe disease.

Many patients have other disorders as well, some of which—including fibromyalgia, irritable bowel syndrome, metabolic syndrome, sleep disorders, and depression—may have symptoms that overlap with those of CFS. The diagnosis and treatment of these conditions is necessary when caring for patients.