Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a disabling chronic illness. People with ME/CFS experience profound fatigue that does not go away with rest, as well as problems with sleep and memory or concentration. The hallmark symptom is post-exertional malaise (PEM), a worsening of the illness that can start immediately or up to days after even minor physical or mental activity. This "crash" can last hours, days or several months. Further common symptoms include orthostatic intolerance—dizziness or faintness when upright—and pain. The cause of the disease is unknown. ME/CFS often starts after an infection, such as infectious mononucleosis, and the illness has a genetic risk factor that can also run in families. Studies find impairment in the nervous and immune systems, as well as in energy production. A diagnosis is established with the presence of clinical symptoms and systematic differential diagnosis, as there is no verified diagnostic laboratory test or imaging procedure available. ME/CFS symptoms can improve or worsen over time, but a full recovery is uncommon. No therapies or medications are approved to treat the cause of the condition, and treatment or management is aimed at relieving symptoms. Pacing of activities can help avoid PEM which can worsen symptoms for extended periods, and counselling may help in coping with the illness. Before the COVID-19 pandemic, ME/CFS affected two to nine out of every 1,000 people, depending on the diagnostic criteria. Since the pandemic began, with the similarity of long COVID, many more people fit a diagnosis of ME/CFS. It occurs more often in women than in men and is more common in middle age, but can occur at all ages, including childhood. The societal and economic impact of the disease is large, and the symptoms can be socially isolating. About a quarter of those affected are unable to leave their bed or home. People with ME/CFS often face stigma in healthcare settings, and care is complicated by controversies around the cause and treatments of the illness. Doctors may be unfamiliar with diagnosing the illness, as it is often not fully covered in medical school. Historically, research funding for ME/CFS has been far below that of diseases with comparable impact.
Classification and terminology ME/CFS has been classified as a neurological disease by the World Health Organization (WHO) since 1969, initially under the name benign myalgic encephalomyelitis. The classification as a neurological disease is based on symptoms that indicate a central role of the nervous system. Alternatively, on the basis of abnormalities in immune cells, ME/CFS is labelled a neuroimmune condition. The disease can further be regarded as a post-acute infection syndrome (PAIS) or an infection-associated chronic condition. PAISes such as long COVID and post-treatment Lyme disease syndrome share many symptoms with ME/CFS and are suspected to have a similar cause. Many names for the illness have been proposed. The most commonly used are myalgic encephalomyelitis, chronic fatigue syndrome, and the combined umbrella term myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Agreement on a name has been difficult because the cause and pathology remain unknown. In the WHO's recent classification, the ICD-11, chronic fatigue syndrome and myalgic encephalomyelitis are named under post-viral fatigue syndrome. The term post-infectious fatigue syndrome was initially proposed as a subset of "chronic fatigue syndrome" with a documented triggering infection, but is also used as a synonym of ME/CFS, or for a broader set of fatigue conditions after infection. Most individuals with ME/CFS object to the name "chronic fatigue syndrome", which was formalised in 1988. They consider the term simplistic, and that it trivialises the debilitating symptoms, preventing the illness from being taken seriously. There are also issues with the previous term used since 1956, "myalgic encephalomyelitis", as there is limited evidence of brain inflammation (myalgia means 'muscle pain' and encephalomyelitis means 'brain and spinal cord inflammation'). The umbrella term ME/CFS retains the often used phrase CFS without trivialising the disease, but some object to this name too, because they believe previous dissimilar diagnostic criteria for ME and CFS identify separate illnesses. A 2015 report from the US Institute of Medicine recommended the illness be renamed systemic exertion intolerance disease (SEID) and suggested new diagnostic criteria. While the new name was not widely used, the diagnostic criteria was adopted by the Centers for Disease Control and Prevention (CDC). Like CFS, the name SEID only focuses on a single symptom, and opinion from those affected was generally negative.
Signs and symptoms ME/CFS causes debilitating fatigue, sleep problems, and post-exertional malaise (PEM, overall symptoms getting worse after mild activity). In addition, cognitive issues, orthostatic intolerance (dizziness or nausea when upright) or other physical symptoms may be present (see also § Diagnostic criteria). Symptoms significantly reduce the ability to function and typically last for three to six months before a diagnosis can be confirmed. ME/CFS usually starts after an infection. Onset can be sudden or more gradual over weeks to months.
Core symptoms People with ME/CFS experience persistent debilitating fatigue. It is made worse by normal physical, mental, emotional, and social activity, and is not a result of ongoing overexertion. Rest provides limited relief from fatigue. Particularly in the initial period of illness, this fatigue is described as "flu-like". Individuals may feel "physically drained" and unable to start or finish activities. They may also feel restless while fatigued, describing their experience as "wired but tired". When starting an activity, muscle strength may drop rapidly, which can lead to difficulty with coordination, clumsiness or sudden weakness. Mental fatigue may also make cognitive efforts difficult. The fatigue experienced in ME/CFS is of a longer duration and greater severity than in other conditions characterised by fatigue.
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