ME/CFS — Criteria, Epidemiology, Severity Levels, Overlap

4. ME/CFS — criteria, epidemiology, severity levels, overlap

Myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) is a severe, chronic multisystem disease which the WHO classifies as a neurological disorder (ICD-10 G93.3; ICD-11 8E49). It is accompanied by signs of neurological, immunological, autonomic and energy-metabolic disturbances; the underlying pathophysiology, however, is not yet conclusively clarified and is the subject of active research (the formerly common labelling as a purely "neuroimmunological" disease pre-empts the still-open question of mechanism). Its core symptom is post-exertional malaise (PEM) — a disproportionate worsening of symptoms after physical, mental or emotional effort that was previously managed without problems. Characteristic is the often delayed onset, typically around 12–48 hours (sometimes up to 72 hours) after the exertion, and a duration of days to weeks (a so-called "crash") [1][12]. PEM distinguishes ME/CFS from ordinary exhaustion and is the diagnostically decisive feature.

Diagnostic criteria

Several sets of criteria exist, some of them differing in strictness:

• IOM/NAM criteria 2015 (clinical, adopted in the USA by the CDC): for the diagnosis, all three core symptoms must be present — (1) a substantial reduction of activity over at least 6 months with profound fatigue not relieved by rest, (2) PEM and (3) unrefreshing sleep — plus at least one of two additional criteria: cognitive impairment or orthostatic intolerance [1][2]. Important is the threshold qualifier: the symptoms must be present at least half of the time at moderate, substantial or severe intensity [1].
• Canadian Consensus Criteria (CCC 2003): stricter and closer to research use. PEM with prolonged recovery (typically ≥24 h) is mandatory; in addition, sleep disturbances, pain, ≥2 neurological/cognitive manifestations and symptoms from autonomic, neuroendocrine and immunological categories are required. The required symptom duration in adults is ≥6 months (the statement of a shortened duration of ≥3 months in children is widespread, but partly goes back to supplementary paediatric criteria and should not be attributed unchecked to the CCC itself). The CCC select a smaller, on average more severely affected patient group [4].
• International Consensus Criteria (ICC 2011): use exclusively the term "myalgic encephalomyelitis". The mandatory criterion is post-exertional neuroimmune exhaustion (PENE); there is no 6-month waiting period, which is intended to enable an earlier diagnosis [5].
• NICE guideline NG206 (2021, UK): requires four mandatory symptoms — fatigue, PEM, unrefreshing/disturbed sleep and cognitive difficulties ("brain fog"). The diagnosis should be made in adults as well as in children and adolescents only when the symptoms persist for at least 3 months. The frequently cited period of 4 weeks refers exclusively to the earlier suspicion of ME/CFS in children and adolescents (with then earlier referral to a specialist unit) — not to making the diagnosis [3].

Severity levels

The NICE guideline defines four severity levels [3]:

• Mild: largely self-caring, light housework possible, usually still (reduced) in work; leisure activities have been given up.
• Moderate: restricted mobility, mostly no longer in work, regular rest periods needed.
• Severe: barely any self-care, often wheelchair-dependent, pronounced cognitive and sensory impairment.
• Very severe: bedridden all day, care-dependent, needing help with personal hygiene and eating, extreme sensitivity to light, noise and touch.

According to frequently cited estimates, roughly a quarter (~25 %) of all those affected are house- or bedbound, many of them fully in need of care; this order of magnitude, however, rests on a limited data basis and should be understood as a rough estimate [9].

Epidemiology

In the USA, the CDC/NCHS Data Brief no. 488 (NHIS 2021–2022) reports a prevalence of 1.3 % of adults — women 1.7 %, men 0.9 %, with a peak at 60–69 years (2.1 %) and higher values with low income and in rural regions [6]. Worldwide, a pre-pandemic estimate was around 0.89 % (~65–71 million people) [11]. In Germany the number of those affected — mostly on the basis of extrapolations — is put at a rise, driven by Long COVID, from about 400,000 to over 600,000 (figures for 2024 in part up to ~650,000), including estimates of about 80,000 children and adolescents; a substantial share (by some accounts around two thirds) is regarded as unable to work [9][10]. These German figures come mostly from professional-society and association estimates and carry marked uncertainty. Historically, 80–90 % of cases are considered undiagnosed [11].

Overlap with Long COVID

PEM is the shared diagnostic link between Long COVID and ME/CFS. A meta-analysis (Dehlia & Guthridge, Journal of Infection 2024) found that, pooled, 51 % of Long COVID patients meet the ME/CFS criteria (95 % CI 42–60 %; 13 studies, n=1,973) [7]. The large RECOVER adult cohort (published in the Journal of General Internal Medicine, 2025) showed that 4.5 % of those infected with SARS-CoV-2 met the ME/CFS criteria after ≥6 months compared with 0.6 % of the non-infected; the adjusted hazard ratio was 4.93 (95 % CI 3.62–6.71), i.e. an approximately fivefold increased risk (the "eightfold" increase named in press releases refers to the crude prevalence ratio, not to the adjusted analysis). PEM was the most common symptom [8]. A frequent co-condition is orthostatic intolerance or postural tachycardia syndrome (POTS), which is at the same time part of the orthostatic IOM additional criterion [1].

Assessment and uncertainties

The various sets of criteria identify patient groups of different sizes and different severity, which makes prevalence figures and comparisons between studies harder. The German case numbers rest partly on extrapolations and carry uncertainty.

One reason for the thin evidence base: in the US, ME/CFS received only around 7% of the NIH research funding that its disease burden would call for, according to a widely cited analysis (Mirin, Dimmock & Jason, Work 2020) – concretely, 15 million USD in 2017 against a disease-burden-adjusted 203 million USD (a gap of 188 million USD); a roughly 14-fold increase would be needed to match the disease burden. Comparable, more recent (2023–2026) figures for NIH RECOVER or EU funding for ME/CFS were not available to us [13].

On therapy, medications and supplements the following should be noted:

• There is so far no causal, approved therapy for ME/CFS. Treatments target individual symptoms (e.g. orthostatic intolerance, sleep, pain) and are largely off-label on a limited evidence base. Drug treatments should only be prescribed and monitored by a doctor; this text deliberately gives no dosages or intake recommendations.
• Safety note (important): unlike in many other states of exhaustion, escalating/graded physical training programmes (graded exercise therapy, GET) are not recommended for ME/CFS. The NICE guideline NG206 (2021) explicitly advises against GET, because fixed escalation schemes can trigger PEM and worsen the condition [3]. Likewise, cognitive behavioural therapy should be understood not as curative but at most as a supportive measure.
• A central component of management is pacing (activity management within one's individual exertion limit) in order to avoid PEM [12].

This text serves solely as information, does not replace medical advice or diagnosis and contains no individual treatment or healing instruction. In the event of worsening, very severe illness (e.g. being bedridden, swallowing/nutrition problems) or crises, medical or emergency help must be sought.