CFS) !

A long-lasting illness with disabling fatigue, unrefreshing sleep and worsening of all symptoms after even small physical or mental effort.

Updated 2026-11-01

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What it is

A chronic, multisystem illness defined by a substantial drop in activity lasting more than 6 months with profound fatigue that rest does not relieve, post-exertional malaise (a delayed worsening of symptoms after exertion) and unrefreshing sleep, plus cognitive impairment or orthostatic intolerance. It often starts after an infection. It is a recognised medical condition, not a psychiatric diagnosis, although depression may coexist.

Who gets it

Estimated prevalence is about 0.2 to 1% of adults, with women affected 2 to 3 times more than men and a peak between 40 and 60 years; adolescents can also be affected. Most patients are never diagnosed. After the COVID-19 pandemic, a proportion of people with post-COVID condition meet ME/CFS criteria, increasing its relevance in Latin America, where awareness and specialist services remain scarce.

Causes

Cause unknown; likely heterogeneous
Frequently triggered by infections: Epstein-Barr virus (infectious mononucleosis), SARS-CoV-2, enteroviruses and others
Proposed mechanisms: immune dysregulation, abnormal cellular energy metabolism, autonomic dysfunction and neuroinflammation
Possible genetic predisposition; physical or emotional stress may precede onset

Risk factors

Female sex
Age 40 to 60 years (and adolescence)
Preceding viral infection, including COVID-19 and infectious mononucleosis
Family history of ME/CFS

Pathophysiology

The mechanism is not fully understood. Studies show altered immune responses (e.g. abnormal natural killer cell function and cytokines), impaired energy production in cells after exertion, reduced cerebral blood flow on standing and autonomic dysfunction with orthostatic intolerance. Post-exertional malaise, the core feature, reflects an abnormal recovery from exertion: in repeated exercise tests oxygen uptake at the second test falls instead of staying stable. Hypothalamic-pituitary-adrenal axis and central nervous system changes may explain sleep and cognitive symptoms.

Classification

By severity: mild, moderate, severe (mostly housebound) and very severe (bedbound and dependent)
Post-infectious onset versus gradual onset
Diagnostic criteria sets: 2015 US National Academy of Medicine criteria, Canadian Consensus Criteria, International Consensus Criteria

Symptoms and signs

Profound, persistent fatigue that is new, not due to excessive effort and not relieved by rest, with a large drop in activity
Post-exertional malaise: worsening of symptoms 12 to 48 hours after minor physical, mental or emotional effort, lasting days or weeks
Unrefreshing sleep and other sleep disturbances
Cognitive impairment ("brain fog"): slow thinking, poor memory and concentration
Orthostatic intolerance: dizziness, palpitations or fainting when standing (sometimes postural tachycardia)
Muscle and joint pain, headaches, sore throat, tender lymph nodes and sensitivity to light, noise or chemicals

Diagnosis

Clinical: Clinical diagnosis after excluding other causes: at least 6 months of reduced function with fatigue, post-exertional malaise and unrefreshing sleep, plus either cognitive impairment or orthostatic intolerance (2015 National Academy of Medicine criteria). Symptoms must be present at least half the time with moderate or severe intensity. Ask specifically about post-exertional malaise, since it separates ME/CFS from other fatigue states.
Tests: No diagnostic test exists. Basic tests exclude other causes: complete blood count, ESR or CRP, metabolic panel, glucose, renal and liver function, TSH, ferritin, creatine kinase, urinalysis, and, guided by history, celiac serology, HIV or hepatitis tests and sleep studies. A 10-minute standing test or tilt test documents orthostatic intolerance.
Imaging: Not required; imaging is used only to investigate red flags or alternative diagnoses.

Differential diagnosis

Hypothyroidism, anaemia, diabetes and adrenal insufficiency
Major depressive disorder and anxiety disorders
Obstructive sleep apnoea and other sleep disorders
Fibromyalgia (often coexists)
Autoimmune diseases, multiple sclerosis and cancer
Medication side effects and substance use

Goal of treatment

There is no cure or approved disease-specific treatment; care aims to help the patient stay within their energy limits to avoid crashes, treat the symptoms that impair quality of life most and maintain function and social support.

Treatment

Lifestyle: Pacing (energy management): planning activity and rest to stay within the energy envelope and avoid post-exertional malaise; graded exercise that ignores symptoms is no longer recommended because it can worsen the illness.
Medical: Symptom-targeted treatment: sleep hygiene and drugs for sleep disturbance, analgesics or neuropathic pain agents, treatment of coexisting depression or anxiety, and drugs for orthostatic intolerance when needed.
Conservative: Orthostatic measures (adequate fluids and salt, compression garments), psychological support and counselling for coping, and occupational or school adjustments.
Follow-up: Regular review of function, mood, nutrition and new symptoms; care plans for severe and very severe patients at home or in hospital (low light and noise, help with feeding and pressure-area care).

Complications

Loss of work, schooling and social life
Depression and social isolation
Deconditioning and, in very severe cases, malnutrition and pressure injuries
Harm from unproven treatments or from pushing exercise beyond limits

Prognosis

Most adults improve only partly and full recovery is uncommon; symptoms usually fluctuate with relapses. Adolescents have a better chance of recovery. About a quarter of patients are housebound or bedbound at some point.

Test yourself

Think of the answer, then tap the question to see it.

What is the cardinal symptom that distinguishes ME/CFS from other causes of fatigue?

Post-exertional malaise: delayed worsening of symptoms after physical, mental or emotional exertion.

How long must symptoms last for a diagnosis of ME/CFS in adults?

At least 6 months.

Which activity-management strategy is recommended?

Pacing, staying within the energy envelope; graded exercise therapy is no longer recommended.

Which infections commonly precede ME/CFS?

Viral infections such as Epstein-Barr virus (infectious mononucleosis) and SARS-CoV-2.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: ME/CFS Basics | ME/CFS | CDC (Centers for Disease Control and Prevention (CDC), 2026); Myalgic Encephalomyelitis/Chronic Fatigue Syndrome | MedlinePlus (MedlinePlus (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.

Hypothalamus highlighted on the Durgical 3D model