Health Mechanisms

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What are myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and post-exertional malaise?

Understanding ME/CFS, post-exertional malaise, the published criteria (IOM/NAM, NICE) and what sets post-exertional malaise apart from ordinary fatigue.

  • 5 sources
  • 6 min read
  • Clinical review
  • Verified 9 November 2026

Key points

  • ME/CFS is a complex, chronic illness that affects several body systems. The 2015 IOM/NAM criteria (from the US Institute of Medicine, now the National Academy of Medicine) combine a substantial reduction in prior functioning accompanied by fatigue, post-exertional malaise (PEM) and unrefreshing sleep, together with cognitive difficulties and/or orthostatic intolerance [1, 2].
  • PEM can follow physical, cognitive, emotional or social exertion that is sometimes minimal; the worsening is often delayed, out of proportion and followed by prolonged recovery [1, 2].
  • PEM is not simply tiredness after exertion: its multi-symptom, delayed and prolonged nature is central to the clinical definitions [1, 2].
  • The NG206 guideline from NICE (the UK’s National Institute for Health and Care Excellence) states that programs based on fixed increases in activity, such as graded exercise therapy as it defines it, should not be offered; this document is not a French or US guideline [1].
  • In France, the Ameli information page updated on August 28, 2026 describes ME/CFS and PEM; it is not a guideline from the French National Authority for Health (HAS) [3].

Terminology and definition

Post-exertional malaise (PEM)

Fatigue, PEM and worsening after exertion: useful distinctions

Differences between the IOM/NAM and NICE criteria

The situation in France and the United States

What is still unknown

References and level of evidence

  1. NICE. Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management (NG206). NICE guideline 2021.(guideline or consensus)
    Limitations: UK guideline; not a French or US framework. Population: children, young people and adults with suspected or diagnosed ME/CFS. Read: online recommendations.
    Declared conflicts of interest: Institutional document; committee declarations not consulted
  2. CDC, based on the Institute of Medicine (National Academy of Medicine), 2015. IOM 2015 Diagnostic Criteria. CDC – ME/CFS (page updated May 10, 2024) 2015.(guideline or consensus)
    Limitations: CDC page summarizing the IOM/NAM criteria; the CDC states that it is not a federal guideline. The full IOM report was not read. Read: official page.
    Declared conflicts of interest: Not consulted (IOM report not read)
  3. Assurance Maladie. Un cas particulier : le syndrome de fatigue chronique ou encéphalomyélite myalgique. ameli.fr (updated August 28, 2026) 2026.(expert opinion)
    Limitations: General-public information page; it is neither a French National Authority for Health (HAS) guideline nor a French national diagnosis and care protocol (PNDS). Read: official page.
    Declared conflicts of interest: Institutional document
  4. Aoun Sebaiti M, Hainselin M, Gounden Y, Sirbu CA, Sekulic S, Lorusso L, Nacul L, Authier FJ. Systematic review and meta-analysis of cognitive impairment in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Scientific Reports, 12(1):2157 2022. DOI: 10.1038/s41598-021-04764-w. PMID: 35140252.(evidence synthesis)
    Limitations: Studies published from 1988 to 2019, with patients defined mainly by the 1994 CDC criteria; heterogeneous tests. Read: full text (PMC8828740).
    Declared conflicts of interest: The authors declare no conflicts of interest; funded by AFM and the COST program
  5. CDC. Diagnosing ME/CFS. CDC – ME/CFS (page shown as updated May 13, 2024) 2024.(expert opinion)
    Limitations: US clinical information page; the CDC states that it is not a federal guideline. Read: official page.
    Declared conflicts of interest: Institutional document
Written by
AI-assisted literature research (ChatGPT) and AI-assisted writing (Claude)
Editor
Jean Viry-Babel
Review status
Clinical review: Dr Françoise Charmoillaux (anesthesiologist)
Approved on
9 November 2026
Literature search
8 November 2026
Last verified
9 November 2026
Geographic scope
international, United States, France, United Kingdom
Audience
everyone
Conflicts of interest
No known financial ties. Separately, the publisher is developing a personal approach to these conditions under the name Connected Conditions, which is not cited here (see the legal notice).
Limitations of this page
Presents published definitions: cannot be used to make a diagnosis or to build an activity program. The full IOM report was not read (CDC summary only). Page not reviewed by an ME/CFS specialist.