Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), previously called chronic fatigue syndrome, is a chronic, multisystem disease defined by profound, unexplained fatigue that causes substantial impairment in daily function and persists beyond 6 months.^[1] Its defining, cardinal feature is post-exertional malaise (PEM) — a delayed, disproportionate worsening of symptoms after physical, cognitive, or even emotional exertion that would have been trivial before the illness, often described by patients as a crash. PEM, not fatigue alone, is what separates ME/CFS from ordinary tiredness or deconditioning, and recognizing it is the single most important diagnostic and counselling principle in this chapter.^[2] The underlying mechanism is incompletely understood, but converging evidence points to chronic immune activation resembling a persistent infection-like sickness response, disrupted energy metabolism, and autonomic nervous system dysregulation, often following a viral or other infectious trigger — a pattern now recognized on a large scale with post-COVID-19/long-COVID illness.^[3] No laboratory test or imaging study confirms the diagnosis, and no medication is approved to treat or cure it: care is symptom-targeted, individualized, and organized first and foremost around protecting the patient from PEM.

Post-exertional malaise (PEM)

PEM is an exacerbation of some or all of a patient's baseline symptoms — flu-like malaise, pain, cognitive dysfunction, nausea or gastrointestinal discomfort, weakness or instability, lightheadedness, sensory sensitivity, and worsened sleep — triggered by physical, cognitive, or emotional exertion that would not have caused a problem before the illness. It is typically delayed (onset hours to a day or more after the triggering activity), disproportionate to the activity itself, and can last hours to days, weeks, or even months to resolve. A patient does not need to undergo formal exercise testing to establish PEM — the history alone, ideally supported by a symptom or activity diary, is sufficient and preferred, since exercise testing can itself trigger a severe, prolonged crash.^[1]

Core symptom domains

• Profound fatigue that is new or has a definite onset (not lifelong), is not the result of ongoing excessive exertion, and is not substantially relieved by rest.
• Unrefreshing sleep — among the most consistently reported symptoms; patients wake unrefreshed regardless of how many hours they slept.
• Cognitive impairment (often called brain fog) — slowed information processing, short-term memory difficulty, trouble concentrating or finding words, and difficulty performing simple multi-step tasks.
• Orthostatic intolerance — lightheadedness, palpitations, nausea, or worsened cognitive symptoms that appear or worsen with standing or sitting upright and improve with lying down; a subset of patients meet formal criteria for postural orthostatic tachycardia syndrome (POTS).
• Widespread pain — myalgia, arthralgia without joint swelling, headache, and heightened sensitivity to touch, light, sound, or temperature.
• Sore throat, tender cervical or axillary lymph nodes, and feverishness can accompany PEM flares even without evidence of active infection.

• A preceding infection is the most commonly recognized trigger — implicated pathogens include Epstein-Barr virus, enteroviruses, human cytomegalovirus, human herpesvirus-6, and SARS-CoV-2 (COVID-19); ME/CFS-like illness following COVID-19 is now a well-recognized presentation of long COVID.^[3]
• A major physical stressor, surgery, or significant psychologically stressful life event preceding symptom onset in some patients.
• Family history of ME/CFS or related conditions, consistent with a proposed genetic contribution to immune regulation and stress response.
• Environmental exposure to toxins, chemicals, pesticides, or heavy metals has been proposed as a contributing factor in some cases.
• Diagnosed more frequently in women than men, though ME/CFS occurs across all ages, sexes, and socioeconomic groups and is substantially underdiagnosed, particularly in men.
• Presence of a hypermobility spectrum disorder or joint hypermobility (e.g., Ehlers-Danlos syndrome) is associated with a higher likelihood of comorbid ME/CFS and orthostatic intolerance.

ME/CFS is a clinical diagnosis made from the history and examination, supported by targeted investigations used to exclude other chronic active illnesses that can cause similar fatigue — it is not confirmed by any single test. A confident diagnosis cannot be made before symptoms have been present for 6 months, but evaluation to rule out treatable causes should proceed during that interval rather than being deferred, and a well-controlled comorbid medical condition does not by itself preclude a diagnosis of ME/CFS if the patient otherwise meets the criteria.^[1]

2015 National Academy of Medicine (Institute of Medicine) core diagnostic criteria

This is the most widely used diagnostic framework; the 2003 Canadian Consensus Criteria are a more detailed alternative sometimes preferred in research settings for their greater specificity against primary psychiatric conditions, but both frameworks share the same core requirement — PEM must be present.

History and examination

• Characterize fatigue onset (gradual versus sudden, especially following an identifiable infection), duration, and its relationship to activity — specifically screen for PEM using open-ended questions and, where useful, a symptom or activity diary rather than a single yes/no question.
• Screen every core symptom domain directly: unrefreshing sleep, cognitive impairment, orthostatic symptoms, and pain.
• Full physical examination focused on excluding an alternative explanation — thyroid exam, lymph node exam, cardiopulmonary exam, and a joint/skin exam for signs of an inflammatory or connective tissue disorder.

Investigations

Laboratory testing in ME/CFS is expected to be normal — its purpose is to exclude other explanations for chronic fatigue, not to confirm the diagnosis. A reasonable baseline panel includes complete blood count with differential, erythrocyte sedimentation rate or C-reactive protein, thyroid-stimulating hormone, fasting glucose, electrolytes, creatinine, urea, calcium, phosphorus, total protein, albumin, liver enzymes, anti-tissue transglutaminase (celiac screen), and urinalysis.^[2] Viral titers and neuroimaging are not required and not generally helpful in routine practice and should not be ordered reflexively — reserve more extensive or specialist-directed testing (e.g., autoimmune serologies, sleep study, tilt-table testing) for when specific history or exam findings point toward it.

Orthostatic intolerance assessment

A simple in-office passive standing test (supine vital signs, then repeated at 1, 3, 5, and 10 minutes of quiet standing) or the NASA 10-minute lean test can identify orthostatic intolerance without needing formal tilt-table testing in most patients; a sustained heart rate increase of 30 beats per minute or more (40 or more in adolescents) without a drop in blood pressure within 10 minutes of standing supports a diagnosis of POTS. Refer for formal tilt-table testing or cardiology assessment when the bedside test is inconclusive or symptoms are severe.^[4]

Comorbidities and treatable mimics to screen for

These conditions are common in patients with ME/CFS, can meaningfully worsen the overall symptom burden, and are often independently treatable — actively screen for them rather than attributing every symptom to ME/CFS itself.

• Fibromyalgia — widespread pain with characteristic tender points; frequently coexists with ME/CFS.
• Mast cell activation syndrome — flushing, hives, gastrointestinal symptoms, and reactions to foods, medications, or temperature change; consider a trial of antihistamines/mast cell stabilizers when suggestive symptoms are present.
• Hypermobility spectrum disorders, including Ehlers-Danlos syndrome — screen with the Beighton hypermobility score; associated with a higher rate of orthostatic intolerance.
• Obstructive sleep apnea, restless legs syndrome, and circadian/sleep-phase disorders — refer for a sleep study when suggestive features (snoring, witnessed apneas, an urge to move the legs at rest, or persistent excessive daytime sleepiness out of proportion to reported nighttime sleep) are present.
• Irritable bowel syndrome — a common comorbid gastrointestinal diagnosis.
• Hypothyroidism and other treatable endocrine or hematologic causes of fatigue, excluded by the baseline panel above.
• Depression and anxiety — common and important to identify and treat in their own right as comorbidities, but are not the cause of ME/CFS and should not be treated as a substitute diagnosis.

• Relieve the patient's most disruptive symptoms first, prioritized according to the patient's own goals.
• Prevent and minimize post-exertional malaise by teaching pacing within a sustainable personal energy envelope.
• Improve sleep quality and restore some sense of restorative sleep.
• Reduce orthostatic intolerance symptoms and the associated fall/syncope risk.
• Control pain to a tolerable level using the lowest effective intervention.
• Support cognitive function within the limits of the disease, without provoking cognitive PEM.
• Validate the patient's experience, maintain a supportive therapeutic relationship, and support overall quality of life and psychosocial function.
• Avoid deconditioning from excessive inactivity while never pushing exertion past the level that triggers PEM — both extremes are harmful.

Pacing and the energy envelope

Pacing means distributing activity throughout the day in small, manageable amounts separated by rest, and staying within the amount of energy the patient's body can currently tolerate (the energy envelope) rather than doing as much as possible on a good day. Patients who are able to stay within their personal energy envelope show meaningful improvements in physical functioning and fatigue severity.^[5] Explicitly counsel patients to avoid the push-crash cycle — overexerting on a good day, crashing, resting until they feel better, then overexerting again — since this cycle perpetuates rather than resolves PEM. Cognitive activity should be paced the same way as physical activity, since cognitive exertion can trigger PEM just as physical exertion can. Occupational therapy or physiotherapy input from a clinician familiar with ME/CFS-specific pacing (not standard reconditioning) can help patients build and adjust an individualized activity plan.^[5]

Why graded exercise therapy is avoided

Older guidance recommended graded exercise therapy — a fixed, progressive increase in activity regardless of symptoms — on the assumption that deconditioning, rather than organic exertion intolerance, explained the fatigue. Current CDC and NICE guidance no longer recommends this approach: patients with genuine PEM do not tolerate standard progressive-exercise prescriptions the way patients with other chronic illnesses do, and pushing through symptoms in this way can cause a severe, sometimes prolonged relapse rather than gradual improvement.^[2][6] This is a genuine, significant change from older teaching and needs to be communicated clearly, since some patients will have previously been told to push through their symptoms.

Orthostatic intolerance measures

• Increase fluid intake to roughly 2 to 3 litres per day, spread across the day.
• Increase dietary sodium intake (discuss an appropriate target with the patient's prescriber, particularly if there is coexisting hypertension or renal or cardiac disease).
• Waist-high compression garments or an abdominal binder to reduce venous pooling.
• Elevate the legs when sitting, avoid prolonged standing and rapid position changes, and avoid hot environments and alcohol, which worsen orthostatic symptoms.
• Small, more frequent meals may reduce post-meal (postprandial) worsening of orthostatic symptoms.

Sleep

Address sleep problems early — they can worsen fatigue, memory, headache, and pain. Encourage consistent sleep-wake timing, a dark, cool bedroom, and avoiding screens before bed. Refer for a sleep study when there are features suggesting sleep apnea, restless legs syndrome, a circadian/sleep-phase disorder, or persistent excessive daytime sleepiness that is out of proportion to reported nighttime sleep.

Nutrition

No specific diet or supplement regimen has been shown to broadly benefit ME/CFS. Encourage a balanced diet with adequate protein, vegetables, and fruit; patients with significant functional limitation may need practical help with shopping, meal preparation, or access to prepared foods. Correct any specific nutritional deficiency that has developed because fatigue has limited a patient's ability to shop for or prepare balanced meals, and consider a general multivitamin for patients whose diet is not balanced. Vitamin D deficiency is common, related to reduced sun exposure in patients who are housebound or bedbound — check levels and supplement appropriately.^[2]

Cognitive behavioural therapy

CBT may be offered to patients who want support coping with the impact of a chronic illness, but must never be positioned as a treatment that reverses or cures ME/CFS — doing so reinforces the historically harmful framing of this as a primarily psychological condition.^[2]

Validating the patient's experience

ME/CFS has a long history of being dismissed, minimized, or attributed purely to psychological causes by clinicians. Actively acknowledging the reality and severity of the illness, providing a clear and reasonable explanation of the diagnosis, and creating a supportive environment where the patient feels believed are core, evidence-informed parts of management, not just bedside manner. Suggest patient support groups or peer communities for self-management support and validation from others living with the condition.

General counselling points

• Avoid polypharmacy — patients with ME/CFS frequently see multiple providers and can accumulate overlapping medications; review the full medication list at every visit.
• Consider that a medication's adverse effect can mimic or worsen an ME/CFS symptom (e.g., sedation, orthostatic hypotension, cognitive slowing) — evaluate new or worsening symptoms for a drug cause before assuming disease progression.

Breastfeeding

Continue effective non-pharmacologic measures. Sedating antihistamines, low-dose beta-blockers, and pyridostigmine are generally considered compatible with breastfeeding; reassess any other medication for infant exposure and reevaluate the overall regimen postpartum, since symptom severity commonly changes after delivery.

Pediatric and adolescent patients

ME/CFS occurs in children and adolescents and the same core diagnostic criteria apply, but presentation and impact differ — school attendance and participation are often the most affected domains, and pacing needs to explicitly account for the physical and cognitive demands of a school day. Involve the school in accommodation planning (reduced course load, rest breaks, flexible attendance) as a core part of management, and favour a multidisciplinary pediatric team for diagnosis and ongoing care given the developmental stakes of a prolonged, severe illness during childhood or adolescence.

Renal, hepatic, and drug-sensitive patients

Patients with ME/CFS frequently report unusual sensitivity to medications generally, independent of renal or hepatic function — start every new medication, for ME/CFS or any other indication, at a low dose and titrate slowly.^[2] Adjust renally cleared agents (pyridostigmine, midodrine) for reduced kidney function, and use caution with hepatically metabolized agents (fludrocortisone, tricyclic antidepressants) in significant hepatic impairment.

• Reassess the patient's own priority symptom(s) at each visit and track functional status over time, not just symptom presence or absence — a structured symptom or activity diary is useful for both PEM frequency/severity and overall trend.
• Ask specifically about medication tolerability at every visit given the frequent drug sensitivity in this population, and stop any medication not providing clear benefit.
• For patients on fludrocortisone or midodrine: monitor blood pressure (including for supine hypertension) and serum potassium periodically.
• For patients on a beta-blocker or ivabradine: monitor heart rate and blood pressure.
• Recheck vitamin D level after supplementation is started and periodically thereafter in housebound or bedbound patients.
• Screen for mood symptoms and suicidal ideation at intervals given the significant chronic disease burden, not only at diagnosis.
• Reassess comorbidities (fibromyalgia, mast cell activation syndrome, sleep apnea, hypermobility spectrum disorder, IBS) periodically, since these can independently change and affect overall symptom control.
• Review the complete medication list periodically to identify and address polypharmacy.

1. Confirm pacing is actually being implemented correctly before escalating drug therapy — inadequate pacing or an ongoing push-crash cycle is a common, often overlooked cause of apparent treatment failure.
2. Reassess for a missed or new comorbid/alternative diagnosis if the symptom pattern has changed, especially with new focal findings or symptoms outside the core ME/CFS domains.
3. If orthostatic intolerance persists despite non-pharmacologic measures and a first-line agent, escalate through the orthostatic intolerance ladder (see Drug Tables) and consider cardiology referral for further evaluation and treatment.
4. If pain remains poorly controlled with simple analgesics, trial low-dose naltrexone or a low-dose tricyclic antidepressant, and refer to a pain specialist if still inadequate.
5. If sleep remains disrupted despite hygiene measures and a short trial of a sedating agent, refer for a formal sleep study to exclude sleep apnea, restless legs syndrome, or a circadian/sleep-phase disorder.
6. For progressive functional decline toward the severe or very severe end of the spectrum despite optimized management, refer to a specialist ME/CFS or complex chronic disease clinic for multidisciplinary care.
7. At every step, review the full medication list for polypharmacy and confirm any drug being continued is providing a clear, ongoing benefit.

• Prescribing graded exercise therapy or advising a patient to push through fatigue — current evidence indicates this can provoke a severe, sometimes prolonged PEM crash rather than gradual improvement.
• Dismissing symptoms as purely psychological or attributing them only to deconditioning, which delays legitimate workup and damages the therapeutic relationship.
• Ordering extensive or exotic testing (broad autoimmune panels, viral titers, routine neuroimaging) without a specific clinical indication, instead of the focused baseline panel used to exclude common treatable causes.
• Diagnosing ME/CFS before adequately excluding treatable causes of chronic fatigue, or, conversely, delaying diagnosis and management for the full 6 months when the criteria are already clearly met.
• Starting any new medication at a standard adult starting dose without first adjusting downward, given how frequently patients with ME/CFS report drug sensitivity.
• Missing a treatable comorbidity — sleep apnea, hypothyroidism, mast cell activation syndrome, or a hypermobility spectrum disorder — that could meaningfully improve symptoms if addressed on its own.
• Positioning cognitive behavioural therapy as a cure rather than a coping-support adjunct.
• Overlooking suicidal ideation screening in a population with a significant chronic disease burden and a history of having their symptoms dismissed.
• Treating rituximab or other unproven immunomodulatory therapy as an established ME/CFS treatment — a large randomized trial found no benefit and an increased risk of adverse effects, despite earlier small studies suggesting promise.

How to use this table: Doses are adult doses and require patient-specific confirmation, including renal/hepatic adjustment. Patients with ME/CFS frequently report unusual drug sensitivity, so start low and titrate slowly. Select any drug name to open its complete Pharmasist Drug Database page.

Orthostatic intolerance / POTS agents

Start with non-pharmacologic measures for every patient (see Non-Pharmacologic Treatment). The Syncope chapter in this library covers the broader orthostatic-intolerance work-up in more depth and is a useful companion reference.

Evidence note: beta-blocker/pyridostigmine dosing and comparative efficacy is drawn from a randomized clinical trial in postural tachycardia syndrome^[7], pyridostigmine's specific benefit for post-exertional malaise/exercise intolerance is drawn from a randomized, placebo-controlled trial in ME/CFS^[8], and the overall approach is cross-checked against Canadian POTS management guidance^[4].

Sleep agents

Reserve medication for after sleep hygiene measures alone have been tried (see Non-Pharmacologic Treatment).

Pain agents

Start with simple analgesics; escalate only if inadequate.

Evidence note: low-dose naltrexone evidence in this table draws in part on interventional data from a long-COVID cohort^[9], given the overlapping patient population and limited ME/CFS-specific trial data.

Cognitive impairment and fatigue agents — occasional use only

Evidence does not support benefit with regular, routine use of a stimulant; reserve for occasional use ahead of a specific planned activity.

Adjunct for acute PEM or sensory overload

Off-label, short-term use only.

Nutritional supplementation

Not recommended — insufficient evidence: early small studies suggested possible benefit from rituximab, a B-cell-depleting monoclonal antibody, in ME/CFS. A subsequent large randomized, double-blind, placebo-controlled trial found no benefit and an increased risk of adverse effects. Do not offer rituximab for ME/CFS outside a formal research setting.^[2]