Review|
doi: 10.1111/j.1365-2796.2011.02428.x

Myalgic encephalomyelitis: International Consensus Criteria
B. M. Carruthers1, M. I. van de Sande2, K. L. De Meirleir3, N. G. Klimas4, G. Broderick5, T. Mitchell6, D. Staines7,8,
A. C. P. Powles9, N. Speight10, R. Vallings11, L. Bateman12,13, B. Baumgarten-Austrheim14, D. S. Bell15, N. Carlo-Stella16,
J. Chia17,18, A. Darragh19, D. Jo20, D. Lewis21, A. R. Light22, S. Marshall-Gradisbik8, I. Mena23, J. A. Mikovits24, K. Miwa25,
M. Murovska26, M. L. Pall27 & S. Stevens28
From the 1Independent, Vancouver, BC, Canada; 2Independent, Calgary, AB, Canada; 3Department of Physiology and Medicine, Vrije University ofBrussels, Himmunitas Foundation, Brussels, Belgium; 4Department of Medicine, University of Miami Miller School of Medicine and Miami Veterans
Affairs Medical Center, Miami, FL, USA; 5Department of Medicine, University of Alberta, Edmonton, AB, Canada; 6Honorary Consultant for NHS at
Peterborough ⁄ Cambridge, Lowestoft, Suffolk, UK; 7Gold Coast Public Health Unit, Southport, Queensland; 8Health Sciences and Medicine, Bond
University, Robina, Queensland, Australia; 9Faculty of Health Sciences, McMaster University and St. Joseph’s Healthcare Hamilton, Hamilton, ON,
Canada; 10Independent, Durham, UK; 11Howick Health and Medical Centre, Howick, New Zealand; 12Fatigue Consultation Clinic, Salt Lake
Regional Medical Center; 13Internal Medicine, Family Practice, University of Utah, Salt Lake City, UT, USA; 14ME ⁄ CFS Center, Oslo University
Hospital HF, Norway; 15Department of Paediatrics, State University of New York, Buffalo, NY, USA; 16Independent, Pavia, Italy; 17Harbor-UCLA
Medical Center, University of California, Los Angeles, CA; 18EV Med Research, Lomita, CA, USA; 19University of Limerick, Limerick, Ireland; 20Pain
Clinic, Konyang University Hospital, Daejeon, Korea; 21Donvale Specialist Medical Centre, Donvale, Victoria, Australia; 22Departments ofAnesthesiology, Neurobiology and Anatomy, University of Utah, Salt Lake City, UT, USA; 23Department of Medicina Nuclear, Clinica Las Condes,
Santiago, Chile; 24Whittemore Peterson Institute, University of Nevada, Reno, NV, USA; 25Miwa Naika Clinic, Toyama, Japan; 26A. Kirchenstein
Institute of Microbiology and Virology, Riga Stradins University, Riga, Latvia; 27Department of Biochemistry & Basic Medical Sciences, Washington
State University, Portland, OR; and 28Department of Sports Sciences, University of the Pacific, Stockton, CA USA

Abstract. Carruthers BM, van de Sande MI, De
Meirleir KL, Klimas NG, Broderick G, Mitchell T,
Staines D, Powles ACP, Speight N, Vallings R, Bateman L, Baumgarten-Austrheim B, Bell DS, CarloStella N, Chia J, Darragh A, Jo D, Lewis D, LightAR, Marshall-Gradisbik S, Mena I, Mikovits JA,
Murovska M, Pall ML, Stevens S (Independent,
Vancouver, BC, Canada; Independent, Calgary, AB,
Canada; Department of Physiology and Medicine,
Vrije University of Brussels, Himmunitas Foundation, Brussels, Belgium; Department of Medicine,University of Miami Miller School of Medicine
and Miami Veterans Affairs Medical Center, Miami,
FL, USA; Department of Medicine, University ofAlberta, Edmonton, AB, Canada; Honorary Consultant for NHS at Peterborough ⁄ Cambridge, Lowestoft, Suffolk, UK; Gold Coast Public Health Unit,
Southport, Queensland; Health Sciences and Medicine, Bond University, Robina, Queensland,
Australia; Faculty of Health Sciences, McMaster
University and St Joseph’s Healthcare Hamilton,
Hamilton, ON, Canada; Independent, Durham, UK;
Howick Health and Medical Centre, Howick, New
Zealand; Fatigue Consultation Clinic, Salt Lake
Regional Medical Center; Internal Medicine, Family
Practice, University of Utah, Salt Lake City, UT,
USA; ME ⁄ CFS Center, Oslo University Hospital HF,
Norway; Department of Paediatrics, State University of New York, Buffalo, NY; Independent, Pavia,
Italy; Harbor-UCLA Medical Center, University ofCalifornia, Los Angeles, CA; EV Med Research,
Lomita, CA, USA; University of Limerick, Limerick,Ireland; Pain Clinic, Konyang University Hospital,
Daejeon, Korea; Donvale Specialist Medical Centre,

Donvale, Victoria, Australia; Departments orAnesthesiology, Neurobiology and Anatomy, University of Utah, Salt Lake City, Utah, USA; Health
Sciences and Medicine, Bond University, Robina,
Queensland, Australia; Department of Medicina
Nuclear, Clinica Las Condes, Santiago, Chile; Whittemore Peterson Institute, University of Nevada,Reno, NV, USA; Miwa Naika Clinic, Toyama, Japan;
A. Kirchenstein Institute of Microbiology and Virology, Riga Stradins University, Riga, Latvia; Department of Biochemistry & Basic Medical Sciences,
Washington State University, Portland, OR; Department of Sports Sciences, University of the Pacific,
Stockton, CA USA). Myalgic encephalomyelitis:International Consensus Criteria (Review). J Intern
Med 2011; 270: 327–338.
The label ‘chronic fatigue syndrome’ (CFS) has persisted for many years because of the lack of knowledge of the aetiological agents and the disease process. In view of more recent research and clinicalexperience 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 anunderlying pathophysiology. It is also consistent with
the neurological classification of ME in the World
Health Organization’s International Classification of
Diseases (ICD G93.3). Consequently, an International Consensus Panel consisting of clinicians,
researchers, teaching faculty and an independentpatient advocate was formed with the purpose ofdeveloping criteria based on current knowledge. Thirteen countries and a wide range of specialties wereª 2011 The Association for the Publication of the Journal of Internal Medicine

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represented. Collectively, members have approximately 400 years of both clinical and teaching experience, authored hundreds of peer-reviewed publications, diagnosed or treated approximately 50 000
patients with ME, and several members coauthoredprevious criteria. The expertise and experience of thepanel members as well as PubMed and other medical
sources were utilized in a progression of suggestions ⁄ drafts ⁄ reviews ⁄ revisions. The authors, free ofany sponsoring organization, achieved 100% consensus through a Delphi-type process. The scope of thispaper is limited to criteria of ME and their application.
Accordingly, the criteria reflect the complex symp-

tomatology. Operational notes enhance clarity andspecificity by providing guidance in the expression
and interpretation of symptoms. Clinical and research application guidelines promote optimal recognition of ME by primary physicians and other healthcare providers, improve the consistency of diagnoses
in adult and paediatric patients internationally andfacilitate clearer identification of patients for researchstudies.

Introduction

International Consensus Criteria

Myalgic encephalomyelitis (ME), also referred to in
the literature as chronic fatigue syndrome (CFS), is acomplex disease involving profound dysregulation of
the central nervous system (CNS) [1–3] and immune
system [4–8], dysfunction of cellular energy metabolism and ion transport [9–11] and cardiovascularabnormalities [12–14]. The underlying pathophysiology produces measurable abnormalities in physicaland cognitive function and provides a basis forunderstanding the symptomatology. Thus, the development of International Consensus Criteria thatincorporate current knowledge should advance theunderstanding of ME by health practitioners andbenefit both the physician and patient in the clinicalsetting as well as clinical researchers.

The Canadian Consensus Criteria were used as astarting point, but significant changes were made.
The 6-month waiting period before diagnosis is nolonger required. No other disease criteria requirethat diagnoses be withheld until after the patient has
suffered with the affliction for 6 months. Notwithstanding periods of clinical investigation will varyand may be prolonged, diagnosis should be madewhen the clinician is satisfied that the patient has
ME rather than having the diagnosis restricted by aspecified time factor. Early diagnoses may elicit new
insights into the early stages of pathogenesis;
prompt treatment may lessen the severity and impact.

The problem with broadly inclusive criteria [15, 16] isthat they do not select homogeneous sets of patients.
The Centers for Disease Control prevalence estimates
increased tenfold from 0.24% using the Fukuda criteria [17] to 2.54% using the Reeves empirical criteria[16]. Jason et al. [18] suggest that there are flaws in
Reeves’ methodology because it is possible to meetthe empirical criteria for ME without having any
physical symptoms and it does not discriminate patients with ME ⁄ CFS from those with major depressive
disorder. Patient sets that include people who do nothave the disease lead to biased research findings,
inappropriate treatments and waste scarce researchfunds [19].
Some symptoms of the Fukuda criteria overlap withdepression, whereas the Canadian Consensus Criteria [20] differentiate patients with ME from those whoare depressed and identify patients who are more
physically debilitated and have greater physical andcognitive functional impairments [21].

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Keywords: chronic fatigue syndrome, criteria, definition, diagnosis, myalgic encephalomyelitis.

Using ‘fatigue’ as a name of a disease gives it exclusiveemphasis and has been the most confusing andmisused criterion. No other fatiguing disease has‘chronic fatigue’ attached to its name – e.g. cancer ⁄ chronic fatigue, multiple sclerosis ⁄ chronic fatigue – except ME ⁄ CFS. Fatigue in other conditions isusually proportional to effort or duration with a quick
recovery and will recur to the same extent with thesame effort or duration that same or next day. The
pathological low threshold of fatigability of ME described in the following criteria often occurs with minimal physical or mental exertion and with reduced
ability to undertake the same activity within the sameor several days.
The International Consensus Criteria (Table 1) identify the unique and distinctive characteristic patternsof symptom clusters of ME. The broad spectrum ofsymptoms alerts medical practitioners to areas ofpathology and may identify critical symptoms moreaccurately [18–20]. Operational notes following eachcriterion provide guidance in symptom expressionª 2011 The Association for the Publication of the Journal of Internal Medicine Journal of Internal Medicine 270; 327–338

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Table 1 Myalgic encephalomyelitis: international consensus criteria
Adult and paediatric • clinical and research
Myalgic encephalomyelitis is an acquired neurological disease with complex global dysfunctions. Pathological dysregulation of
the nervous, immune and endocrine systems, with impaired cellular energy metabolism and ion transport are prominent features.
Although signs and symptoms are dynamically interactive and causally connected, the criteria are grouped by regions of pathophysiology to provide general focus.
A patient will meet the criteria for postexertional neuroimmune exhaustion (A), at least one symptom from three neurologicalimpairment categories (B), at least one symptom from three immune ⁄ gastro-intestinal ⁄ genitourinary impairment categories(C), and at least one symptom from energy metabolism ⁄ transport impairments (D).
A. Postexertional neuroimmune exhaustion (PENE pen’-e): Compulsory
This cardinal feature is a pathological inability to produce sufficient energy on demand with prominent symptoms primarily inthe neuroimmune regions. Characteristics are as follows:
1. Marked, rapid physical and ⁄ or cognitive fatigability in response to exertion, which may be minimal such as activities
of daily living or simple mental tasks, can be debilitating and cause a relapse.
2. Postexertional symptom exacerbation: e.g. acute flu-like symptoms, pain and worsening of other symptoms.
3. Postexertional exhaustion may occur immediately after activity or be delayed by hours or days.
4. Recovery period is prolonged, usually taking 24 h or longer. A relapse can last days, weeks or longer.
5. Low threshold of physical and mental fatigability (lack of stamina) results in a substantial reduction in pre-illness
activity level.
Operational notes: For a diagnosis of ME, symptom severity must result in a significant reduction of a patient’s premorbid
activity level. Mild (an approximate 50% reduction in pre-illness activity level), moderate (mostly housebound), severe (mostly
bedridden) or very severe (totally bedridden and need help with basic functions). There may be marked fluctuation of symptom
severity and hierarchy from day to day or hour to hour. Consider activity, context and interactive effects. Recovery time: e.g.
Regardless of a patient’s recovery time from reading for ½ hour, it will take much longer to recover from grocery shopping for ½ hourand even longer if repeated the next day – if able. Those who rest before an activity or have adjusted their activity level to theirlimited energy may have shorter recovery periods than those who do not pace their activities adequately. Impact: e.g. An
outstanding athlete could have a 50% reduction in his ⁄ her pre-illness activity level and is still more active than a sedentary person.
B. Neurological impairments
At least one symptom from three of the following four symptom categories
1. Neurocognitive impairments
a. Difficulty processing information: slowed thought, impaired concentration e.g. confusion, disorientation, cognitive
overload, difficulty with making decisions, slowed speech, acquired or exertional dyslexia
b. Short-term memory loss: e.g. difficulty remembering what one wanted to say, what one was saying, retrieving words,
recalling information, poor working memory
2. Paina. Headaches: e.g. chronic, generalized headaches often involve aching of the eyes, behind the eyes or back of the head that
may be associated with cervical muscle tension; migraine; tension headaches
b. Significant pain can be experienced in muscles, muscle-tendon junctions, joints, abdomen or chest. It is noninflammatory in nature and often migrates. e.g. generalized hyperalgesia, widespread pain (may meet fibromyalgia criteria), myofascial or radiating pain
3. Sleep disturbancea. Disturbed sleep patterns: e.g. insomnia, prolonged sleep including naps, sleeping most of the day and being awake mostof the night, frequent awakenings, awaking much earlier than before illness onset, vivid dreams ⁄ nightmaresb. Unrefreshed sleep: e.g. awaken feeling exhausted regardless of duration of sleep, day-time sleepinessª 2011 The Association for the Publication of the Journal of Internal Medicine Journal of Internal Medicine 270; 327–338

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Table 1 Continued
Adult and paediatric • clinical and research
4. Neurosensory, perceptual and motor disturbances
a. Neurosensory and perceptual: e.g. inability to focus vision, sensitivity to light, noise, vibration, odour, taste and touch;impaired depth perception
b. Motor: e.g. muscle weakness, twitching, poor coordination, feeling unsteady on feet, ataxia
Notes: Neurocognitive impairments, reported or observed, become more pronounced with fatigue. Overload phenomena may
be evident when two tasks are performed simultaneously. Abnormal accommodation responses of the pupils are common.
Sleep disturbances are typically expressed by prolonged sleep, sometimes extreme, in the acute phase and often evolve into
marked sleep reversal in the chronic stage. Motor disturbances may not be evident in mild or moderate cases but abnormal tandem gait and positive Romberg test may be observed in severe cases.
C. Immune, gastro-intestinal and genitourinary Impairments
At least one symptom from three of the following five symptom categories
1. Flu-like symptoms may be recurrent or chronic and typically activate or worsen with exertion. e.g. sore throat, sinusitis, cervical and ⁄ or axillary lymph nodes may enlarge or be tender on palpitation
2. Susceptibility to viral infections with prolonged recovery periods
3. Gastro-intestinal tract: e.g. nausea, abdominal pain, bloating, irritable bowel syndrome
4. Genitourinary: e.g. urinary urgency or frequency, nocturia
5. Sensitivities to food, medications, odours or chemicals
Notes: Sore throat, tender lymph nodes, and flu-like symptoms obviously are not specific to ME but their activation in reaction toexertion is abnormal. The throat may feel sore, dry and scratchy. Faucial injection and crimson crescents may be seen in the tonsillar fossae, which are an indication of immune activation.
D. Energy production ⁄ transportation impairments: At least one symptom
1. Cardiovascular: e.g. inability to tolerate an upright position - orthostatic intolerance, neurally mediated hypotension, postural orthostatic tachycardia syndrome, palpitations with or without cardiac arrhythmias, light-headedness ⁄ dizziness
2. Respiratory: e.g. air hunger, laboured breathing, fatigue of chest wall muscles
3. Loss of thermostatic stability: e.g. subnormal body temperature, marked diurnal fluctuations; sweating episodes, recurrent feelings of feverishness with or without low grade fever, cold extremities
4. Intolerance of extremes of temperature
Notes: Orthostatic intolerance may be delayed by several minutes. Patients who have orthostatic intolerance may exhibit mottlingof extremities, extreme pallor or Raynaud’s Phenomenon. In the chronic phase, moons of finger nails may recede.
Paediatric considerations
Symptoms may progress more slowly in children than in teenagers or adults. In addition to postexertional neuroimmune exhaustion, the most prominent symptoms tend to be neurological: headaches, cognitive impairments, and sleep disturbances.
1. Headaches: Severe or chronic headaches are often debilitating. Migraine may be accompanied by a rapid drop in temperature, shaking, vomiting, diarrhoea and severe weakness.
2. Neurocognitive impairments: Difficulty focusing eyes and reading are common. Children may become dyslexic, which
may only be evident when fatigued. Slow processing of information makes it difficult to follow auditory instructions or takenotes. All cognitive impairments worsen with physical or mental exertion. Young people will not be able to maintain a fullschool programme.
3. Pain may seem erratic and migrate quickly. Joint hypermobility is common.
Notes: Fluctuation and severity hierarchy of numerous prominent symptoms tend to vary more rapidly and dramatically than in
adults.

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Table 1 Continued
Adult and paediatric • clinical and researchClassification——— Myalgic encephalomyelitis——— Atypical myalgic encephalomyelitis: meets criteria for postexertional neuroimmune exhaustion but has a limit of twoless than required of the remaining criterial symptoms. Pain or sleep disturbance may be absent in rare cases.
Exclusions: As in all diagnoses, exclusion of alternate explanatory diagnoses is achieved by the patient’s history, physical examination, and laboratory ⁄ biomarker testing as indicated. It is possible to have more than one disease but it is important that each
one is identified and treated. Primary psychiatric disorders, somatoform disorder and substance abuse are excluded. Paediatric: ‘primary’ school phobia.
Comorbid entities: Fibromyalgia, myofascial pain syndrome, temporomandibular joint syndrome, irritable bowel syndrome,interstitial cystitis, Raynaud’s phenomenon, prolapsed mitral valve, migraines, allergies, multiple chemical sensitivities, Hashimoto’s thyroiditis, Sicca syndrome, reactive depression. Migraine and irritable bowel syndrome may precede ME but then become
associated with it. Fibromyalgia overlaps.

and contextual interpretation. This will assist the primary clinician in identifying and treating patientswith ME in the primary care setting.
Criteria are supported by research
Criterial symptoms are supported by a study of morethan 2500 patients that determined which symptoms
had the greatest efficacy to identify patients with ME[22]. Investigations into gene expression [23–27] andstructure further support the criteria at a molecularlevel including anomalies of increased oxidative
stress [4, 28], altered immune and adrenergic signalling [29, 30] and altered oestrogen receptor expression [31]. In addition, evidence supporting a genetic
predisposition to ME points to modifications in serotonin transporter genes [32, 33], the glucocorticoid
receptor gene [34], as well as HLA class II involvement[35]. The potential combinatorial effects of these modifications have received limited attention [33, 36].
Some early broad-based studies show a lack of objective findings such as no association with HLA genotype [37]. A study of patients from a twin registry suggested that environmental factors may outweigh any
genetic predisposition in broader patient populations[38].
Underlying problems of inconsistent findings in research studies have been identified [39, 40] and include a need for studies to be based on larger sample
sizes with a more clearly defined phenotype, in particular one that recognizes the likely existence of significant subgroups within the patient population. In a
study of the Reeves empirical criteria [16], Jason et al.
[18] reported that 38% of patients diagnosed withmajor depressive disorder were misclassified as having CFS and only 10% of patients identified as having
CFS actually had ME. Accordingly, the primary goalof this consensus report is to establish a more selective set of clinical criteria that would identify patients
who have neuroimmune exhaustion with a pathological low threshold of fatigability and symptom flare in
response to exertion. This will enable patients to bediagnosed and enrolled in research studies internationally under a case definition that is acceptable tophysicians and researchers around the world.
Postexertional neuroimmune exhaustion (PENE pen¢-e)‘Malaise’ – a vague feeling of discomfort or fatigue [41]– is an inaccurate and inadequate word for the pathological low-threshold fatigability and postexertional
symptom flare. Pain and fatigue are crucial bioalarm
signals that instruct patients to modify what they are
doing in order to protect the body and prevent further
damage. Postexertional neuroimmune exhaustion ispart of the body’s global protection response and isassociated with dysfunction in the regulatory balance
within and between the nervous, immune and endocrine systems, and cellular metabolism and ion
transport [42–46]. The normal activity ⁄ rest cycle,
which involves performing an activity, becoming fatigued and taking a rest whereby energy is restored,
becomes dysfunctional.
Numerous papers document abnormal biological responses to exertion, such as loss of the invigorating
effects of exercise [20], decreased pain threshold[47–49], decreased cerebral oxygen and blood volume ⁄ flow [50–53], decreased maximum heart rate

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|[54], impaired oxygen delivery to muscles [55], elevated levels of nitric oxide metabolites [56] and worsening of other symptoms [57]. Patients reach theanaerobic threshold and maximal exercise at amuch lower oxygen consumption level [58]. Reported prolonged effects of exertion include elevatedsensory signalling to the brain [59] that is interpreted as pain and fatigue [29], elevated cytokine
activity [60], delay in symptom activation [61] and arecovery period of at least 48 h [57]. When an exercise test was given on two consecutive days, somepatients experienced up to a 50% drop in their ability
to produce energy on the second evaluation [62].
Both submaximal and self-paced physiologicallylimited exercise resulted in postexertional malaise[48].
Neurological impairments
Some viruses and bacteria can infect immune andneural cells and cause chronic inflammation. Structural and functional pathological abnormalities [3]within the brain and spinal cord suggest dysregulation of the CNS control system and communication
network [62], which play crucial roles in cognitive
impairment and neurological symptoms [20]. Neuroinflammation of the dorsal root ganglia, gatekeepersof peripheral sensory information travelling to thebrain, has been observed in spinal autopsies (Chaudhuri A. Royal Society of Medicine Meeting 2009).
Identified cerebrospinal fluid proteomes distinguishpatients from healthy controls and post-treatment
Lyme disease [63]. Neuroimaging studies report irreversible punctuate lesions [64], an approximate 10%
reduction in grey matter volume [65, 66], hypoperfusion [50, 67–71] and brain stem hypometabolism [1].
Elevated levels of lateral ventricular lactate are consistent with decreased cortical blood flow, mitochondrial dysfunction and oxidative stress [72]. Researchsuggests that dysregulation of the CNS and autonomic nervous system alters the processing of pain
and sensory input [29, 47, 73, 74]. Patients’ perception that simple mental tasks require substantial effort is supported by brain scan studies that indicate
greater source activity and more regions of the brain
are utilized when processing auditory and spatial
cognitive information [75–77]. Poor attentional
capacity and working memory are prominent disabling symptoms [20, 75, 78].
Immune impairments
Most patients have an acute infectious onset with flulike and ⁄ or respiratory symptoms. A wide range of

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infectious agents have been reported in the subsets ofpatients, including xenotropic murine leukaemia
virus-related virus (XMRV) [79] and other murine leukaemia virus (MLV)-related viruses [80], enterovirus[81–83], Epstein–Barr virus [84], human herpes virus
6 and 7 [85–87], Chlamydia [88], cytomegalovirus[89], parvovirus B19 [90] and Coxiella burnetti [84].
Chronic enterovirus infection of the stomach and altered levels of D Lactic acid-producing bacteria in thegastrointestinal tract have been investigated [82, 91].
Possibly, the initial infection damages part of the CNS
and immune system causing profound deregulation
and abnormal responses to infections [4]. Publications describe decreased natural killer cell signallingand function, abnormal growth factor profiles, decreased neutrophil respiratory bursts and Th1, witha shift towards a Th2 profile [4–8, 92, 93]. Chronic immune activation [27], increases in inflammatory cytokines, pro-inflammatory alleles [4–8, 94–96], chemokines and T lymphocytes and dysregulation of theantiviral ribonuclease L (RNase L) pathway [62, 97–100] may play a role in causing flu-like symptoms,
which aberrantly flare in response to exertion [5, 92].

Energy production ⁄ transport impairments
The consistent clinical picture of profound energy
impairment suggests dysregulation of the mitochondria and cellular energy metabolism and ion
transport and channelopathy [9–11, 100, 101]. A
biochemical positive feedback cycle called the ‘NO ⁄ONOO- cycle’ may play a role in maintaining thechronic nature of ME, the presence of oxidative
stress [102–104], inflammatory cytokine elevation[94–96] and mitochondrial dysfunction [105–108]
and result in reduced blood flow and vasculopathy[106, 107].
Findings of ‘small heart’ with small left ventricularchamber and poor cardiac performance in patient
subsets [109, 110] support previous reports of cardiac and left ventricular dysfunction [13, 111, 112],
which predispose to orthostatic intolerance [14, 113].
Low blood pressure and exaggerated diurnal variation may be due to abnormal blood pressure regulation [114]. Altered control and reduced cortisol production during and following exercise may be
involved. Orthostatic intolerance is associated withfunctional impairment and symptom severity [115].
Measurable vascular abnormalities suggest that thebrain is not receiving sufficient circulating blood volume in an upright position [12, 113], which is intensified when standing in one place such as a groceryª 2011 The Association for the Publication of the Journal of Internal Medicine Journal of Internal Medicine 270; 327–338

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store check-out line. Significant reduction in heart
rate variability during sleep is associated with poor
sleep quality and suggests a pervasive state of nocturnal sympathetic hypervigilance [116].
Application of criteria
Diagnostic criteria serve two necessary but divergent
functions – the first is diagnosing individuals in a clinical setting and the second is identifying patient sets
for research studies.
Clinical application
General considerations
1 Determine whether symptom cluster patterns arecongruent with those expected from dysfunction of anunderlying causal system.
2 Symptoms interact dynamically within a stablecluster because they share the same deep causalroots. Patients’ contextual observations are essential
in determining the expression of interaction of symptom patterns and severity of their impact.
3 Symptom severity impact must result in a 50% or
greater reduction in a patient’s premorbid activity
level for a diagnosis of ME. Mild: approximately 50%
reduction in activity, moderate: mostly housebound,severe: mostly bedbound and very severe: bedboundand dependent on help for physical functions.
4 Symptom severity hierarchy should be determined
periodically to help orient and monitor treatment.
5 Criterial subgroups: Postexertional neuroimmune
exhaustion is the hallmark feature. It may be helpful
to subgroup according to which of the other diagnostic criterial patterns best represent a patient’s clusterof most severe symptoms: neurological, immune,energy metabolism ⁄ transport or eclectic (symptoms
widely distributed amongst subgroups).
6 Separate primary symptoms from secondary symptoms and aggravators. Distinguish primary symptom
complexes formed by a disease process from secondary effects of coping with the disease, such as anxiety
about finances. Determine the effects and burden of
aggravators and stress enhancers such as fast paced
environments and exposure to toxins.
7 Determine total illness burden by assessing symptom severity, interaction and overall impact. Consider
all aspects of the patient’s life – physical, occupa-

tional, educational, social and personal activities ofdaily living. Patients who prioritize their activities
may be able to do one important activity by eliminating or severely reducing activities in other aspects of
their life.
8 The International Symptom Scale should not be
part of the initial clinical interview because it may disturb the weighting and significance of results obtained for an individual patient. When used periodically, it can help position the patient within thegroup, orient the treatment programme and monitor
its effectiveness.
Paediatric considerations
1 If possible, interview a young person with both parents because each may remember different symptoms or interactive events that may help determine
onset and when the illness began to interfere withdaily function.
2 Children cannot be expected to judge pre-illness
function with current function. Assess impact bycomparing hobbies, educational, social and sport
activities the child participated in before illness withpresent activity level.
3 Children may appear irritable when they are asked
to do something when they feel exhausted. On theother hand, they are often able to accommodate fatigue by resting, which may be inappropriately interpreted as being lazy.
4 School Phobia: Young patients spend most of
their out-of-school hours resting, whereas children
with school phobia will be socializing and participating in activities. However, it is possible that
school phobia may become a secondary symptom
because of bullying or academic difficulties owing
to having ME.
5 Natural Course: Children can be very severely afflicted but those whose symptoms are of mild to moderate severity generally are more likely to have themgo into remission than adults. Prognosis cannot be
predicted with certainty.
Research application
A clinical diagnosis must be confirmed before apatient can provide useful general knowledge about
the disease. The data obtained from patients allow
controlled and meaningful observations and suggest
hypotheses to be tested and confirmed or refuted.

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General considerations
1 Patients should meet the full criteria for epidemiological studies. If specific subgroups or atypical ME
are included in a research study, that should beclearly indicated.
2 Specificity: Because critical symptoms are compulsory, it ensures proper selection of patients. Key operational guidelines enhance clarity and specificity.
Ranking the hierarchy of the most troublesome
symptoms may be helpful in some studies.
3 Reliability: Symptoms must not be viewed as anominal checklist. The International Consensus Criteria focus on symptom patterns, which increase reliability. The International Symptom Scale ensuresconsistency in the way questions are asked and further increases the reliability of data collected in different locations. Patients should complete the International Symptom Scale prior to entering a researchstudy.
Optional considerations
Classifying patients by subgroups to enable the comparison of patients within the diagnosis of ME may be
helpful in some studies.
1 Onset: acute infectious or gradual.
2 Onset severity may be a good predictor of severityin the chronic phase.
3 Symptom severity: mild, moderate, severe, very
severe.
4 Criterial subgroups: neurological, immune, energy
metabolism ⁄ transport or eclectic.
(See clinical application for symptom severity and criterial subgroups.)
Conclusions
The International Consensus Criteria provide aframework for the diagnosis of ME that is consistent with the patterns of pathophysiological dysfunction emerging from published research findings and clinical experience. Symptom patterns
interact dynamically because they are causallyconnected. This has been formally addressed bysome investigators who have used well-establishedmultivariate statistical techniques, such as common factor or principal component analyses to
identify symptom constructs [117, 118]. Others
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the analysis of gene expression profiles [28] and todelineate patient subgroups [119]. Consistent withthis approach, the panel is developing an International Consensus Symptom Scale (ICSS) that willbuild on these underlying interactions. However, anecessary first step in establishing a quantitativescore for any diagnostic instrument is the specification of measurable factors that are most relevant to
the illness. Establishing such criteria was the primary objective of this work, and we believe theInternational Consensus Criteria will help clarify
the unique signature of ME.
It is important to note that the current emphasis
must primarily remain a clinical assessment, withselection of research subjects coming later. For thisreason, the panel is developing Physicians’ Guidelines, which will include diagnostic protocol basedon the International Consensus Criteria and treatment guidelines that reflect current knowledge.
Individuals meeting the International Consensus
Criteria have myalgic encephalomyelitis and should
be removed from the Reeves empirical criteria andthe National Institute for Clinical Excellence (NICE)
criteria for chronic fatigue syndrome. These guidelines are designed specifically for use by the primarycare physician in the hope of improving rapid diagnosis and treatment by first-line medical care providers. This may result in the development of anadditional short-form version that would build on
the relationships linking symptoms to formulate anabbreviated screening protocol. For the first time,clinical, paediatric and research applications areprovided, which will advance the understanding of
myalgic encephalomyelitis and enhance the consistency of diagnoses internationally. The compulsory
critical criteria allow comparable data to be collected in various locations and may assist in developing consistent biomarkers and further insights
into the mechanism and aetiology of myalgic
encephalomyelitis.
Funding
This Consensus paper is free of sponsorship. All
authors contributed their time and expertise on a volunteer basis and no one received any payments orhonorariums.
Conflict of interest statement
All authors have disclosed potential conflicts of interest, and all members declare that they have no competing interests.

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Review: ME: Intl. Consensus Criteria

Acknowledgements
The panel would like to gratefully acknowledge theparticipation and support of the patients and theirfamilies in the research described herein, and uponwhich, these guidelines are based.

8

Author contributions

10

Coeditors – conception, drafting of paper andrevisions: BM Carruthers, MI van de Sande. Initialsuggestions and subsequent critical reviews: KL De
Meirleir, NG Klimas, G Broderick, T Mitchell,
D Staines, ACP Powles, N Speight, R Vallings, L
Bateman, B Baumgarten-Austrheim, DS Bell, N Carlo-Stella, J Chia, A Darragh, D Jo, D Lewis, AR Light,
S Marshall-Gradisbik, I Mena, JA Mikovits, K Miwa,
M Murovska, ML Pall, S Stevens.
Final approval and consensus

9

11

12

13

14

There was 100% consensus by the authors on the
final consensus paper. BM Carruthers, MI van deSande, KL De Meirleir, NG Klimas, G Broderick,T Mitchell, D Staines, ACP Powles, N Speight, R
Vallings, L Bateman, B Baumgarten-Austrheim, DS
Bell, N Carlo-Stella, J Chia, A Darragh, D Jo, D Lewis,
AR Light, S Marshall-Gradisbik, I Mena, JA Mikovits,
K Miwa, M Murovska, ML Pall, S Stevens.

15

16

17

18

Consensus coordinator
M van de Sande.
19

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Correspondence: Dr Bruce Carruthers, 4607 Blenheim St., Vancouver, British Columbia V6L 3A3, Canada.
(fax: +1 604 263 9059; e-mail: bcarruth@telus.net); andDr. Gordon Broderick, Division of Pulmonary Medicine, Departmentof Medicine, University of Alberta, WMC 2E4.41 WC Mackenzie
Health Sciences Bldg, 8440 – 112 Street, Edmonton AB T6G 2R7,
Canada.
(fax: +1 780 407 6384; e-mail: gordon.broderick@ualberta.ca).

ª 2011 The Association for the Publication of the Journal of Internal Medicine Journal of Internal Medicine 270; 327–338