In Spain, chronic Fatigue Syndrome is managed by general medicines. Chronic fatigue syndrome — formally myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) — is a chronic, complex, multisystem disease characterized by profound, disabling fatigue that does not improve with rest and is made dramatically worse by physical, cognitive, or emotional exertion. Post-exertional malaise (PEM) — the worsening of symptoms hours to days after even modest activity — is the cardinal feature that separates ME/CFS from other forms of chronic tiredness.
Chronic fatigue syndrome — also called myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS), systemic exertion intolerance disease (SEID), and post-viral fatigue syndrome — is a chronic, complex, multisystem disease characterized by profound disabling fatigue and post-exertional malaise. ICD-10 codes include G93.32 (myalgic encephalomyelitis) and R53.82 (chronic fatigue, unspecified); G93.3 (postviral fatigue syndrome) and U09.9 (post-COVID condition unspecified) also apply for specific subgroups. The Institute of Medicine 2015 report defined the criteria as: substantial reduction or impairment in the ability to engage in pre-illness levels of activity for more than 6 months, accompanied by fatigue that is profound, new onset, not the result of ongoing excessive exertion, and not substantially alleviated by rest; post-exertional malaise; unrefreshing sleep; AND at least one of cognitive impairment or orthostatic intolerance. Multiple alternative criteria sets exist (Fukuda 1994, Canadian Consensus Criteria 2003, International Consensus Criteria 2011), and clinicians select based on context.
The key symptoms of Chronic Fatigue Syndrome are: Substantial reduction in pre-illness activity (work, school, social, recreational) lasting more than 6 months, with fatigue that is profound, new onset, and not substantially alleviated by rest., Post-exertional malaise (PEM): worsening of symptoms after physical, cognitive, or emotional exertion, often delayed by 12-72 hours and lasting hours to weeks; the cardinal symptom required for diagnosis., Unrefreshing sleep — waking exhausted despite apparently adequate sleep duration; difficulty staying asleep and disturbed sleep architecture are common., Cognitive impairment ('brain fog') with difficulties in attention, concentration, working memory, word finding, and processing speed., Orthostatic intolerance — symptoms (lightheadedness, palpitations, fatigue) worse on standing or sitting upright; postural orthostatic tachycardia syndrome (POTS) and orthostatic hypotension are common., Widespread musculoskeletal pain (myalgia and arthralgia) without inflammation; often fluctuating., Persistent headaches of new pattern (often migraine-like) since illness onset..
Diagnosis of ME/CFS is clinical, based on validated criteria and exclusion of medical conditions that could explain the symptom complex. The Institute of Medicine 2015 SEID criteria are commonly used: (1) substantial reduction or impairment in the ability to engage in pre-illness levels of activity for more than 6 months, accompanied by profound new-onset fatigue not substantially alleviated by rest; (2) post-exertional malaise; (3) unrefreshing sleep; (4) AND at least one of cognitive impairment or orthostatic intolerance. International Consensus Criteria 2011 add post-exertional neuroimmune exhaustion (PENEM) and broader symptom categories. There is no diagnostic biomarker; investigations rule out alternative explanations. Baseline workup includes complete blood count, ESR, CRP, comprehensive metabolic panel (renal and liver function, electrolytes, calcium, magnesium, phosphate), thyroid function (TSH, free T4), ferritin, B12 and folate, vitamin D, HbA1c, urinalysis, ANA, anti-CCP, rheumatoid factor, creatine kinase, IgG/IgA/IgM, EBV serology, HIV, hepatitis B and C, Lyme serology if exposure risk, and cortisol if features suggest adrenal insufficiency. Additional studies based on clinical features may include sleep study (for unrefreshing sleep), tilt-table or active-stand test (for orthostatic intolerance), 2-day cardiopulmonary exercise testing (CPET — can document characteristic decline in oxygen uptake on day 2), and brain MRI if focal neurology. Once medical exclusions are made and IOM criteria met, the diagnosis is established. Psychiatric assessment for depression, anxiety, and PTSD is important — these often coexist but do not by themselves account for ME/CFS. Specialist referral to an ME/CFS clinic, neurologist, or autonomic specialist is recommended for severe cases and POTS workup.
Outcome in ME/CFS is highly variable. Approximately 5-30% of patients recover fully or substantially over years; another 30-50% have meaningful improvement with consistent symptom management; the remainder have persistent or progressive disability. Recovery rates are higher in adolescents (around 40-50%), in post-EBV illness, and with early diagnosis and rigorous pacing. Severe and very severe ME/CFS (25% of all cases) tends to be more persistent, although individual cases of significant improvement are documented. Mortality from ME/CFS is low overall, but severe disease is associated with cardiovascular morbidity, suicide, and complications of immobility. Long COVID-related ME/CFS has been newly described and longer-term outcomes are being studied. Quality of life in ME/CFS is profoundly impaired — comparable in many studies to multiple sclerosis, chronic kidney disease on dialysis, or congestive heart failure. Access to disability accommodations, supportive family and social networks, and rigorous pacing are the most important modifiable factors for long-term wellbeing. Research into biomarkers, immunomodulators (rituximab failed in phase 3 but other agents are being investigated), antiviral and metabolic agents, and long COVID treatments is intensifying and offers cautious hope for future disease-modifying therapy.
ME/CFS is best managed by primary care clinicians with chronic disease expertise, supported by specialist input from ME/CFS clinics, neurology, autonomic dysfunction services, and rehabilitation medicine for severe cases. Patients with severe orthostatic symptoms benefit from autonomic specialist evaluation. Patients with severe ME/CFS need home-based multidisciplinary care.
Find specialists →ME/CFS is a chronic condition without acute resolution. Recovery in the 5-30% who improve substantially typically occurs over 1-5 years. Pacing benefits become apparent over weeks to months. Symptomatic treatments (orthostatic medications, sleep aids, pain management) take days to weeks to show effect. Patients in long COVID-related ME/CFS may have somewhat different timelines being established by ongoing research.
Conventional graded exercise therapy is not recommended (NICE NG206, 2021/2024). Activity must be paced to stay within the individual's energy envelope and avoid post-exertional malaise. Movement that does not provoke PEM (e.g., gentle stretching, brief walking with rests) can be helpful. Heart-rate monitoring below the anaerobic threshold helps prevent crashes. Rehabilitation services with ME/CFS expertise can support individualized activity management.
Look for a clinician who recognizes ME/CFS as a biomedical illness, uses current diagnostic criteria, supports pacing-based management, and does not recommend graded exercise therapy. Avoid clinicians who promote ME/CFS as a primarily psychological condition. In the UK, NHS ME/CFS clinics are commissioned in many regions. In the US, specialist centres include the Bateman Horne Center, Stanford ME/CFS Initiative, and clinics at major academic centres.
Medically reviewed by AIHealz Medical Editorial Board · May 13, 2026
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