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Exertion intolerance in ME is qualitatively and quantitatively different from normal fatigue, is disproportionate to energy expended, and is not relieved by rest or sleep. This is accompanied by multi-system dysfunction, including neurological, cognitive, cardiac, respiratory, gastrointestinal, and sensory symptoms
- adapted from the International Consensus Criteria Carruthers et al., 2011 and related ME literature
Introduction
Understanding ME and CFS - distinguishing Myalgic Encephalomyelitis (ME) from Chronic Fatigue Syndrome (CFS)
Myalgic Encephalomyelitis (ME) and Chronic Fatigue Syndrome (CFS) are often used interchangeably, but they are not the same. ME has a clearly defined disease process, while CFS, by definition, has historically been described as a syndrome.
Much of the confusion between ME and CFS arises from differences in research and clinical definitions. One aim of this resource is to clearly explain those differences.
A defining feature of ME is acquired dysfunction of the central nervous system (CNS), with widespread systemic effects. By contrast, CFS has historically been defined primarily by the presence of chronic fatigue and related symptoms. It is important for both clinicians and patients to understand that CFS is a syndrome (a collection of symptoms), whereas ME is a distinct neurological disease.
About Myalgic Encephalomyelitis (ME)
Myalgic Encephalomyelitis was named based on clinical observation and pathological findings seen in patients, including autopsy evidence. These findings included inflammation of the brain and spinal cord, deterioration of the dorsal root ganglia, and other neurological abnormalities. The term was coined in the UK in 1956, broadly meaning muscle pain with inflammation of the brain and spinal cord.
In 1969, the World Health Organization (WHO) formally recognised Myalgic Encephalomyelitis as a neurological disease and classified it under ICD code G93.3, where it remains today.
Meaning of the term
My = muscle
Algic = pain
Encephalo = brain
Mye = spinal cord
Itis = inflammation
What is ME?
Myalgic Encephalomyelitis is a chronic, fluctuating, multisystem neurological disease that affects multiple organs and body systems. It is characterised by dysfunction in neurological, immune, cardiac, vascular, metabolic, and energy production pathways.
The hallmark feature of ME is exertion intolerance, in which even minimal physical, cognitive, sensory, or orthostatic exertion leads to a disproportionate and prolonged worsening of symptoms. This worsening is known as Post-Exertional Neuroimmune Exhaustion (PENE).
Disease process and progression
ME represents a major disruption of the central nervous system, often associated with immune system dysfunction. In many cases, the illness follows a viral or infectious trigger and has a rapid (acute) onset, though gradual onset can also occur.
ME affects the body systemically. Activity or stimulation beyond an individual’s post-illness limits can result in a significant worsening of symptoms that may persist for days, weeks, months, or longer. In addition to relapses, repeated or severe overexertion may contribute to long-term deterioration, disease progression, or, in rare cases, death.
Severity and disability
ME is a lifelong illness that can range from mild to profound in severity. Levels of severity include mild, moderate, severe, very severe, and profound ME.
Mild ME may still significantly limit daily functioning and require careful pacing.
Moderate ME often results in reduced mobility and difficulty maintaining work or education.
Severe to very severe ME can leave individuals housebound or bedbound, requiring assistance with basic daily needs.
Profound ME may involve complete immobility, extreme hypersensitivity to light, sound, and touch, and an inability to communicate.
It is estimated that over 25% of people with ME are severely affected, with many unable to leave their homes and some permanently confined to bed.
Symptoms
People with ME experience a wide range of symptoms affecting multiple systems. While individuals may have different combinations of symptoms, the core pattern of illness is remarkably consistent.
Common symptoms include:
Post-exertional neuroimmune exhaustion (PENE)
Debilitating exhaustion and weakness not relieved by rest
Cognitive dysfunction ("brain fog")
Pain (muscle, joint, nerve, and headaches)
Autonomic dysfunction (including orthostatic intolerance)
Sleep disturbance
Hypersensitivity to light, sound, touch, and chemicals
More than 60 neurological, immunological, cardiovascular, metabolic, and musculoskeletal symptoms have been documented in ME.
Exertion intolerance (PENE)
PENE is a cardinal feature of ME, as defined in the International Consensus Criteria (ICC). It refers to the body and brain’s inability to recover normally after even small amounts of exertion.
Worsening of symptoms may be delayed, often peaking 24–72 hours or more after exertion. Physical, cognitive, sensory, emotional, or orthostatic stress can all trigger PENE. In some cases, pushing beyond limits has resulted in long-term deterioration and permanent loss of function.
Energy Dysfunction
Energy is essential for all cellular and organ functions. In ME, research indicates abnormalities in cellular energy production, including impaired production of adenosine triphosphate (ATP), the body’s primary energy currency.
When energy production is impaired, organs such as the brain may be unable to function normally. This can manifest as cognitive dysfunction, memory problems, difficulty processing information, and impaired communication. Ongoing energy disruption is associated with progressive functional impairment.
Similar abnormalities in energy metabolism have been observed in other neurological diseases, including multiple sclerosis.
Diagnosis and Misconceptions
ME is not a psychological disorder. People with ME are not malingering and are not seeking secondary gain. Psychological support may help individuals cope with the burden of chronic illness, but it is not a treatment or cure for ME.
Normal routine test results do not mean that nothing is wrong; rather, they reflect the lack of appropriate diagnostic tests.
ME is not simply a diagnosis of exclusion. While other conditions must be ruled out, exertion intolerance with PENE is a defining diagnostic feature.
Clear diagnostic criteria exist, including the International Consensus Criteria (ICC) and International Consensus Primer (ICP), which clinicians can use to diagnose ME and guide management.
Impact on quality of life
ME causes profound disability and significantly reduced quality of life. Studies consistently show that people with ME score lower on health-related quality-of-life measures than individuals with many other serious chronic illnesses, including heart disease, cancer, HIV, and multiple sclerosis.
ME affects mobility, self-care, nutrition, communication, social interaction, education, employment, and family life. Those with severe and very severe ME are particularly vulnerable and often require extensive medical, social, and practical support.
Prevalence
ME affects people of all ages, including children. One prevalence estimates range, i.e. from 0.4% to 1% of the population, equates to:
An estimated 10,000–21,000 people in Ireland
There are no large, study-based prevalence data available specifically for Ireland using confirmed epidemiological research.Based on extrapolation from international and UK figures, patient organisations in Ireland estimate ~21,000
up to ~2% (broader)
equates to ~100,000 people
Approximately 403,900 people in the UK
Recent UK research (2025) suggests that around 0.6% of the UK population may have ME/ME-CFS, which equates to roughly 400,000 people overall. This figure comes from a study analysing NHS data, with prevalence estimates as high as ~0.92% in women and ~0.25% in men - averaging about 0.6% or around 403,900 people across the UK.(the figure is a 62% increase from historic estimates of 250,000)
Based on UK population figures, that prevalence roughly breaks down to:
England: ~341,350 peopleNorthern Ireland: ~11,350 peopleScotland: ~32,475 peopleWales: ~18,740 people
UK charities feel the estimated prevalence of people with a potential diagnosis of ME following Covid infection, based on 2023 estimates from the Office for National Statistics (ONS), could be as high as 1.35million in the UK (1.14million in England, 38,000 in Northern Ireland, 109,000 in Scotland, and 63,000 in Wales), representing a considerable health burden on the NHS and social care services and reinforcing the need for good quality care and support.
up to ~2% (broader)
equates to ~1,350,000 people
We can only make an estimate of the likely numbers affected based on good quality epidemiological research. There are no large population-wide epidemiological studies that provide an accurate prevalence estimate for ME in Ireland. The above UK statistics were provided in April 2025 by Professor Chris Ponting and Gemma Samms (DecodeME) from the University of Edinburgh, in a study that was funded by ME Research UK.
Management and Prognosis
There is currently no cure for ME and no single pathway to recovery. Management focuses on risk minimisation, avoidance of overexertion, and treatment of individual symptoms where possible.
Some people may experience partial recovery or remission, though relapse can occur. Many remain ill for years. People who are able to avoid repeated overexertion tend to have better long-term outcomes, though those with very severe or profound ME may have no safe activity limits.
Final Notes
Myalgic Encephalomyelitis is a distinct, serious neurological disease recognised by the World Health Organization since 1969. It is a biological illness with complex multisystem involvement and devastating impact.
ME requires recognition, education, and appropriate care pathways to reduce harm and improve quality of life for those affected and their families.
About Chronic Fatigue Syndrome (CFS)
Chronic Fatigue Syndrome (CFS) is a term that has been widely used in research and clinical practice since the late 1980s. However, the terminology and diagnostic frameworks associated with CFS have been a persistent source of confusion, misunderstanding, and harm for people with Myalgic Encephalomyelitis (ME).
As the authors of the 2011 International Consensus Criteria noted, the use of the word fatigue as the name of a disease is inherently misleading. Fatigue is a common symptom in many illnesses, including cancer, multiple sclerosis, heart disease, and autoimmune conditions, yet none of these are labelled as “chronic fatigue” disorders. In most illnesses, fatigue is proportional to effort and improves with rest. This is not the case in ME.
In ME, even minimal physical or cognitive exertion can trigger a severe and prolonged worsening of multiple symptoms, with recovery taking days, weeks, or longer, and sometimes resulting in lasting deterioration.
Origins of the CFS label
The term Chronic Fatigue Syndrome was first introduced by the U.S. Centers for Disease Control and Prevention (CDC) in 1988 following outbreaks of an ME-like illness, including the well-known Lake Tahoe outbreak. The initial CDC definition (often referred to as the “Holmes” criteria) placed disproportionate emphasis on fatigue and did not adequately reflect the neurological and systemic features characteristic of ME.
Subsequent definitions, including the 1994 Fukuda criteria, the 2005 Reeves criteria, and the 2015 Institute of Medicine (now National Academy of Medicine) report, continued to prioritise fatigue as the core feature. Other definitions, such as the 1991 UK Oxford criteria and the 2003 Canadian Consensus Criteria (CCC), further contributed to inconsistency in research and clinical practice.
ME, CFS, and the hybrid term ME/CFS
ME is sometimes inaccurately referred to as “Chronic Fatigue Syndrome” or grouped under the hybrid term ME/CFS. The combined label emerged as an attempt to bridge historically divergent concepts of illness but has had unintended consequences.
The term ME/CFS has no independent classification within the World Health Organization (WHO) system. WHO classification rules do not permit a disease to be classified under more than one rubric, and there is no ICD code for “ME/CFS” as a distinct entity.
The use of fatigue-focused terminology has contributed to:
Trivialisation of a serious neurological disease
Misclassification of patients with diverse fatiguing illnesses under a single umbrella
Conflation of ME with conditions that have very different disease mechanisms
Within the Irish health system, including the HSE, diagnoses of CFS or ME/CFS are commonly applied, particularly in paediatric settings. This mixed use of terminology can create confusion for patients, families, and clinicians, and may result in inappropriate management.
Consequences of Conflation
CFS and ME/CFS function as umbrella terms encompassing a wide range of symptoms common to many different illnesses. As a result, ME symptoms have frequently been confused with those of neurasthenia, fibromyalgia, post-viral fatigue states, multiple chemical sensitivities, chronic infections, and other conditions.
Over time, the CFS label has increasingly been applied to people whose primary or sole symptom is fatigue. Many individuals diagnosed with CFS do not meet diagnostic criteria for ME. Conversely, many people with ME have received a CFS diagnosis that does not adequately reflect the severity or nature of their illness, leaving them without appropriate care and risk minimisation.
This conflation has contributed to decades of neglect of people with ME, particularly those with severe and very severe disease, whose complex neurological and systemic symptoms are frequently misunderstood as unexplained or psychosomatic fatigue.
Psychiatric Misattribution
The emphasis on fatigue has also facilitated inappropriate psychiatric interpretations of ME and CFS. In some settings, these conditions have been reframed using terms such as “functional” or “central sensitisation” syndromes, despite a lack of evidence that psychiatric interventions can treat or cure ME.
Psychiatric diagnoses are rarely supported by integrated physiological investigation of affected organ systems. More than seven decades after major ME outbreaks were first documented, no psychiatric treatment has been shown to restore people with ME to health.
Psychological support may help individuals cope with the experience of chronic illness, as it does for many medical conditions, but it should never be presented as curative or explanatory of ME.
Implications for research
In recent years, a growing number of biomedical researchers have adopted more stringent diagnostic criteria when studying ME. However, some studies continue to recruit participants using broad CFS criteria. This significantly limits the reliability and applicability of findings to people with ME.
Because CFS is not a single illness but a descriptive label applied to heterogeneous patient groups, it cannot be researched as a unified disease entity. This has led to wasted resources, inconsistent findings, and stalled progress toward understanding disease mechanisms and developing effective treatments.
Characteristics commonly associated with CFS diagnoses
Individuals diagnosed with CFS often present with one or more of the following features:
Gradual onset of symptoms
Onset following prolonged stress or overwork
Post-infectious fatigue following common viral illnesses
Fatigue or exhaustion as the dominant symptom
Absence of clear neurological or cardiac dysfunction
Attribution of illness persistence to psychological or personality factors
Mild or self-limiting illness, sometimes improving with rest, exercise programmes, or psychological interventions
Illness that is not considered life-threatening
These characteristics differ markedly from those seen in ME.
Key distinctions between ME and CFS
As noted by Dr Byron Hyde:
“Where the one essential characteristic of ME is acquired central nervous system dysfunction, that of CFS is primarily chronic fatigue.”
Key features that distinguish ME from CFS include:
Typically acute onset
Demonstrable organ pathology, particularly cardiac involvement
Neurological signs in acute and sometimes chronic phases
Specific involvement of the autonomic nervous system
Frequently subnormal body temperature
Chronic fatigue not being an essential defining feature of the chronic phase of ME
While most people with ME would meet broad CFS criteria, many people diagnosed with CFS do not meet criteria for ME. Conversely, a person with a confirmed diagnosis of ME may be excluded from some CFS definitions because of the presence of significant neurological disease.
Why terminology matters
Persisting with fatigue-focused terminology obscures the underlying disease processes in ME. Grouping diverse illnesses under the CFS umbrella confounds research findings, dilutes biological signals, and hampers progress toward effective treatments.
ME is recognised by the World Health Organization as a neurological disease with an as-yet-unknown cause. It is categorised separately from Chronic Fatigue Syndrome. Accurate terminology is essential to ensure appropriate diagnosis, research, care, and protection from harm.
Further resources
ME: The Illness and Common Misconceptions – Abuse, Neglect, Mental Incapacity by Jane Colby, Tymes Trust linked here
The Tangled Story of ME: a documentary exploring the history, controversy, and neglect surrounding ME, featuring commentary from Professor Leonard Jason linked here
These resources provide further historical and educational context for understanding the distinction between ME and CFS.
In view of more recent research and clinical experience that strongly point to widespread inflammation and multisystemic neuropathology, it is more appropriate and correct to use the term 'myalgic encephalomyelitis' (ME) because it indicates an underlying pathophysiology.
- Carruthers et al, Journal of Internal Medicine
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