What's myalgic encephalomyelitis/chronic fatigue syndrome?
Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic, disabling illness defined clinically by a substantial limitation of activity with fatigue, post-exertional malaise, unrefreshing sleep, problems with memory and concentration, and orthostatic intolerance. It is a recognized, serious physical illness; what remains unsettled is its mechanism, not its reality.
| Also indexed as | MONDO:0005404 |
|---|---|
| Features mapped | 5 |
| Treatments mapped | 1 |
| Published sources | 7 |
| Last reviewed | 2026-08-04 |
Signs and symptoms
Post-exertional malaise
Post-exertional malaise (PEM) is the defining feature: a disproportionate worsening of symptoms after even minor physical or mental exertion, often delayed by hours or a day and slow to recover. It is the reason that pushing through activity tends to make the illness worse.
Fatigue
Profound fatigue that lasts at least six months and is not relieved by rest, substantially reducing what a person can do.
Cognitive impairment
Problems with memory and concentration, often described as brain fog, are common and can fluctuate with exertion.
Unrefreshing sleep
Sleep that does not restore energy, so a person wakes feeling unrefreshed regardless of how long they slept.
Orthostatic intolerance
Orthostatic intolerance, symptoms such as lightheadedness or a racing heart on standing, is part of the picture for many people.
How it is diagnosed
myalgic encephalomyelitis/chronic fatigue syndrome
Diagnosed using: 2015 National Academy of Medicine (IOM) diagnostic criteria.
“In 2015, the US National Academy of Medicine published new evidence-based clinical diagnostic criteria that have been adopted by the US Centers for Disease Control and Prevention.”
Treatment and management
What the research describes, not a recommendation. Treatment decisions belong with your clinician.
This covers treatments that appear in the published research mapped here. Investigational and experimental therapies are not included, so their absence is a boundary of this map, not a sign they do not exist.
activity pacing
There is no curative treatment, so care focuses on managing symptoms. Activity pacing, staying within one's available energy to avoid triggering post-exertional malaise, is the main recommended strategy for energy management. What helps any individual is worked out with their care team.
Used to help with: myalgic encephalomyelitis/chronic fatigue syndrome.
“A key challenge for individuals with ME/CFS is energy management and to date, the only recommended strategy…”
What changes how it shows up
The diagnosis is not the whole story. The factors and open questions below are described in the research mapped here as shaping whether, or how strongly, the condition shows up, or as points the field has not yet settled. They are not, on their own, its cause or its cure.
competing diagnostic criteria
There is no single agreed definition of ME/CFS. Several sets of diagnostic criteria are in use, including the Fukuda (1994), Canadian Consensus (2003), International Consensus (2011), and Institute of Medicine (2015) criteria, and they differ in what they require, most notably whether post-exertional malaise is mandatory. This guide does not pick one as correct; the divergence is an open question in the field, not a settled point.
Described as modulating: myalgic encephalomyelitis/chronic fatigue syndrome.
“ME/CFS is a chronic illness with no consensus regarding case diagnostic criteria, which creates difficulty for patients seeking assistance and disability benefits.”
genetic susceptibility (DecodeME GWAS loci)
DecodeME, a genome-wide association study (preprint, 2025), reported eight genome-wide significant loci associated with the risk of developing ME/CFS, implicating genes active in the immune response to infection and in the nervous system. These are common-variant susceptibility (risk) factors of small effect, not a single causal gene: ME/CFS is not a monogenic disease. This is preprint evidence that has not yet been peer-reviewed.
Described as modulating: myalgic encephalomyelitis/chronic fatigue syndrome.
“DecodeME has provided the first robust evidence that genetic variation contributes to the risk of developing…”
No longer recommended
Treatments earlier research once supported, that later and stronger evidence or guidance has overturned. Both the original and the superseding source are shown, so the change is visible rather than hidden.
graded exercise therapy
Once described as helping with myalgic encephalomyelitis/chronic fatigue syndrome, this is no longer recommended.
GET endorsed in the PACE-trial era (PMID:21334061), overturned by NICE NG206 (2021): graded exercise therapy should not be offered for ME/CFS. Prior claim preserved.
Overturned by: NICE:NG206.
“CBT and GET can safely be added to SMC to moderately improve outcomes for chronic fatigue…”
How to read the evidence labels
Where this comes from
This guide is built from 7 published source(s). Every claim above links back to one of them. Click any source ID to read the original on PubMed.
Take it further
Printed, source-linked documents built from this condition's graph — ready to bring to an appointment or attach to a coverage request. Every claim carries its published source, the same as this guide.