chronic fatigue syndrome chronic fatigue syndrome chronic fatigue syndrome (cfs), also called myalgic encephalomyelitis (me) or me/cfs, is a complex, debilitating, long-term medical condition. the root cause(s) of the disease are unknown and the mechanisms are not fully understood. distinguishing core symptoms are lengthy exacerbations or flare-ups of the illness following ordinary minor physical or mental activity, known as post-exertional malaise (pem); greatly diminished capacity to accomplish tasks that were routine before the illness; and sleep disturbances.: 7 the center for disease control and prevention’s (cdc) diagnostic criteria also require at least one of the following: (1) orthostatic intolerance (difficulty sitting and standing upright) or (2) impaired memory or attention. frequently and variably, other common symptoms occur involving numerous body systems, and chronic pain is very common. the often incapacitating fatigue in cfs is different from that caused by normal strenuous ongoing exertion, is not significantly relieved by rest, and is not due to a previous medical condition. diagnosis is based on the person's symptoms because no confirmed diagnostic test is available. not to be confused with chronic fatigue, which is the symptom experienced in many chronic illnesses; including idiopathic chronic fatigue and fibromyalgia. chronic fatigue syndromeother namesmyalgic encephalomyelitis/ chronic fatigue syndrome (me/cfs), myalgic encephalomyelitis (me), post-viral fatigue syndrome (pvfs), chronic fatigue immune dysfunction syndrome (cfids), systemic exertion intolerance disease (seid), others: 20 chart of the symptoms of cfs according to various definitionsspecialtyrheumatology, rehabilitation medicine, endocrinology, infectious disease, neurology, immunology, internal medicine, paediatrics, other specialists in me/cfssymptomsworsening of symptoms with activity, long-term fatigue, othersusual onset10 to 30 years olddurationoften for yearscausesunknownrisk factorsfemale sex, virus and bacterial infections, blood relatives with the illness, major injury, bodily response to severe stress and others: 1–2 diagnostic methodbased on symptomstreatmentsymptomaticprevalenceabout 0.68 to 1% globally proposed mechanisms include biological, genetic, epigenetic, infectious, and physical or psychological stress affecting the biochemistry of the body. persons with cfs may recover or improve over time, but some will become severely affected and disabled for an extended period. no therapies or medications are approved to treat the cause of the illness; treatment is aimed at alleviation of symptoms. the cdc recommends pacing (personal activity management) to keep mental and physical activity from making symptoms worse. limited evidence suggests that counseling, personalized activity management, and the use of rintatolimod help improve some patients' functional abilities. about 1% of primary-care patients have cfs; estimates of incidence vary widely because various epidemiological studies have used dissimilar definitions. it has been estimated that 836,000 to 2.5 million americans and 250,000 to 1,250,000 people in the united kingdom have cfs. cfs occurs 1.5 to 2 times as often in women as in men. it most commonly affects adults between ages 40 and 60 years; it can occur at other ages, including childhood. other studies suggest that about 0.5% of children have cfs, and that it is more common in adolescents than in younger children.: 182 chronic fatigue syndrome is a major cause of school absence.: 183 cfs significantly reduces health, happiness, productivity, and can also cause socio-emotional disruptions such as loneliness and alienation. there is controversy over many aspects of the disorder. various physicians, researchers, and patient advocates promote different names and diagnostic criteria. results of studies of proposed causes and treatments are often poor or contradictory. signs and symptoms the united states centers for disease control and prevention (cdc) recommends these criteria for diagnosis: greatly lowered ability to do activities that were usual before the illness. this drop in activity level occurs along with fatigue and must last six months or longer. worsening of symptoms after physical or mental activity that would not have caused a problem before the illness. the amount of activity that might aggravate the illness is difficult for a person to predict, and the decline often presents 12 to 48 hours after the activity. the 'relapse', or 'crash', may last days, weeks or longer. this is known as post-exertional malaise (pem). sleep problems; people may still feel weary after full nights of sleep, or may struggle to stay awake, fall asleep or stay asleep. additionally, one of the following symptoms must be present: problems with thinking and memory (cognitive dysfunction, sometimes described as "brain fog") while standing or sitting upright; lightheadedness, dizziness, weakness, fainting or vision changes may occur (orthostatic intolerance) other common symptoms many, but not all people with me/cfs report: muscle pain, joint pain without swelling or redness, and headache tender lymph nodes in the neck or armpits sore throat irritable bowel syndrome chills and night sweats allergies and sensitivities to foods, odors, chemicals, lights, or noise shortness of breath irregular heartbeat increased sensitivity to sensory stimuli and pain have also been observed in cfs. the cdc recommends that people with symptoms of cfs consult a physician to rule out other illnesses, which may be treatable. onset the onset of cfs may be gradual or sudden. when it begins suddenly, it often follows an episode of infectious-like symptoms or a known infection, and between 20 and 80% of patients report an infectious-like onset.: 158 when gradual, the illness may begin over the course of months or years with no apparent trigger. studies disagree as to which pattern is more common.: 158 : 181 cfs may also occur after physical trauma such as a car accident or surgery. physical functioning results of a study on quality of life of people with cfs, showing it to be lower than in 20 other chronic conditions cfs often causes significant disability, but the degree can vary greatly. some people with mild cfs may lead relatively normal lives with vigilant energy management, while moderately affected patients may be unable to work or spend much time upright. people with severe cfs are generally housebound or bedbound, and may be unable to care for themselves. the majority of people with cfs have significant difficulty engaging in work, school, and family activities for extended periods of time. an estimated 75% are unable to work because of their illness, and about 25% are housebound or bedridden for long periods, often decades.: 32 in one review on employment status, more than half of cfs patients were on disability benefits or temporary sick leave, and less than a fifth worked full-time. in children, cfs is a major cause of school absence.: 183 according to a 1996 study by mccully et al, people with cfs report critical reductions in levels of physical activity, and according to a 2009 study, patients exhibit a reduction in the complexity of their activities. many people with cfs also experience strongly disabling chronic pain. symptoms can fluctuate over time, making the condition difficult to manage. persons who feel better for a period may overextend their activities, triggering post-exertional malaise and a worsening of symptoms. severity may also change over time, with periods of worsening, improvement or remission sometimes occurring. people with cfs have decreased quality of life according to the sf-36 questionnaire, especially in the domains of vitality, physical functioning, general health, physical role, and social functioning. however, their scores in the "role emotional" and mental health domains were not substantially lower than healthy controls. a 2015 study found that people with cfs had lower health-related quality of life than 20 other chronic conditions, including multiple sclerosis, kidney failure, and lung cancer. cognitive functioning cognitive dysfunction is one of the most disabling aspects of cfs due to its negative impact on occupational and social functioning. 50 to 80% of people with cfs are estimated to have serious problems with cognition. cognitive symptoms are mainly due to deficits in attention, memory, and reaction time. measured cognitive abilities are found to be below projected normal values and likely to affect day-to-day activities, causing increases in common mistakes, forgetting scheduled tasks, or having difficulty responding when spoken to. simple and complex information-processing speed and functions entailing working memory over long time periods are moderately to extensively impaired. these deficits are generally consistent with the patient's perceptions. perceptual abilities, motor speed, language, reasoning, and intelligence do not appear to be significantly altered. patients who report poorer health status tend to also report more severe cognitive trouble, and better physical functioning is associated with less visuoperceptual difficulty and fewer language-processing complaints.: 24 inconsistencies of subjective and observed values of cognitive dysfunction reported across multiple studies are likely caused by a number of factors. differences of research participants' cognitive abilities pre- and post-illness onset are naturally variable and are difficult to measure because of a lack of specialized analytical tools that can consistently quantify the specific cognitive difficulties in cfs. cause the cause of cfs is unknown. both genetic and environmental factors are believed to contribute, but the genetic component is unlikely to be a single gene. problems with the nervous and immune systems, and energy metabolism, may be factors. cfs is a biological disease, not a psychiatric or psychological condition, and is not caused by deconditioning. however, the biological abnormalities found in research are not sensitive or specific enough for diagnosis.: 1437 because the illness often follows a known or apparent viral illness, various infectious agents have been proposed, but a single cause has not been found. hypothesized infections include mononucleosis, chlamydophila pneumoniae, human herpesvirus 6, and lyme disease. inflammation may be involved. cfs has been observed to develop after mononucleosis and gastroenteritis. risk factors all ages, ethnic groups, and income levels are susceptible to the illness. the cdc states that while caucasians may be diagnosed more frequently than other races in america, the illness is at least as prevalent among african americans and hispanics. a 2009 meta-analysis found that asian americans (odds ratio: 0.097; 95%; confidence interval: 0.004 - 0.65) have a much lower risk of cfs than white americans, while native americans (or: 11.5; 95% ci: 1.08 - 56.4) have a higher (probably a much higher) risk and african americans (or: 2.95; 95% ci: 0.69 - 10.4) probably have a higher risk. the review acknowledged that studies and data were limited. more women than men get cfs. a large 2020 meta-analysis estimated that between 1.5 and 2.0 times more cases are women. the review acknowledged that different case definitions and diagnostic methods within datasets yielded a wide range of prevalence rates. the cdc estimates cfs occurs up to four times more often in women than in men. the illness can occur at any age, but has the highest prevalence in people aged 40 to 60. cfs is less prevalent among children and adolescents than among adults. people with affected relatives appear to be more likely to get me/cfs, implying the existence of genetic risk factors. results of genetic studies have been largely contradictory or unreplicated. one study found an association with mildly deleterious mitochondrial dna variants, and another found an association with certain variants of human leukocyte antigen genes. viral and other infections post-viral fatigue syndrome (pvfs) or post-viral syndrome describes a type of chronic fatigue syndrome that occurs following a viral infection. one review found higher epstein–barr virus (ebv) antibody activity in patients with cfs, and that a subset were likely to have increased ebv activity compared to controls. viral infection is a significant risk factor for cfs, with one study finding 22% of people with ebv experience fatigue six months later, and 9% having strictly defined cfs. a systematic review found that fatigue severity was the main predictor of prognosis in cfs, and did not identify psychological factors linked to prognosis. a review of long covid research found half of those affected met diagnostic criteria for me/cfs. another review found that risk factors for developing cfs after mononucleosis, dengue fever, or q-fever included longer bed-rest during the illness, poorer pre-illness physical fitness, attributing symptoms to physical illness, belief that a long recovery time is needed, as well as pre-infection distress and fatigue. the same review found biological factors such as cd4 and cd8 activation and liver inflammation are predictors of sub-acute fatigue but not cfs. a study analyzing the relationship between diagnostic labels and prognosis found that patients diagnosed with me had the worst prognosis, and that patients with pvfs had the best. according to a review, it is unclear whether this was because people labeled with me had a more severe or persistent illness, or because being labelled with me adversely affects prognosis. the national academy of medicine report says it is