A patient with ME/CFS will meet the criteria for fatigue, post-exertional malaise and/or fatigue, sleep dysfunction and pain; have two or more neurological/cognitive manifestations and one or more symptoms from two of the categories of (a) autonomic, (b) neuroendocrine and (c) immune manifestations; and adhere to item 7.

1. Fatigue (Required)

The patient must have a significant degree of new onset, unexplained, persistent, or recurrent physical and mental fatigue that substantially reduces activity level.

2. Post-Exertional Malaise and/or Post-Exertional Fatigue (Required)

There is an inappropriate loss of physical and mental stamina, rapid muscular and cognitive fatigability, post-exertional malaise and/or post-exertional fatigue and a tendency for other associated symptoms within the patient’s cluster of symptoms to worsen.

There is a pathologically slow recovery period – usually 24 hours or longer.

3. Sleep Dysfunction (*) (Required)

There is unrefreshing sleep or sleep quantity or rhythm disturbances such as reversed or chaotic diurnal sleep rhythms. Note that patients without sleep dysfunction can still meet the diagnostic criteria if their illness began with an infection — see (*) below.

4. Pain (*) (Required)

There is a significant degree of myalgia. Pain can be experienced in the muscles, and/or joints, and is often widespread and migratory in nature. Often there are significant headaches of new type, pattern or severity. Note that patients without pain can still meet the diagnostic criteria if their illness began with an infection — see (*) below.

5. Neurological / Cognitive Manifestations (Two or more required)

Two or more of the following difficulties should be present: 

• Confusion

• Impairment of concentration and short-term memory consolidation

• Disorientation

• Difficulty with information processing, categorizing and word retrieval (Word-finding problems)

• Perceptual and sensory disturbances (for example spatial instability and disorientation and inability to focus vision)

Ataxia, muscle weakness and fasciculations are common. There may be overload1 phenomena: cognitive overload, sensory overload (for example photophobia and hypersensitivity to noise) and/or emotional overload, which may lead to crash2 periods and/or anxiety.

6.  At Least One Symptom From Two of the Following Three Categories: autonomic, neuroendocrine, immune

(a) Autonomic Manifestations

• Orthostatic intolerance (either neurally mediated hypotension, postural orthostatic tachycardia syndrome or delayed orthostatic hypotension)

• Light-headedness

• Extreme pallor

• Nausea and irritable bowel syndrome

• Urinary frequency and bladder dysfunction

• Heart palpitations with or without cardiac arrhythmias

• Exertional dyspnea

(b) Neuroendocrine Manifestations

• Loss of thermostatic stability (subnormal body temperature and marked diurnal fluctuation)

• Sweating episodes, recurrent feelings of feverishness

• cold extremities (cold hand and feet)

• Intolerance of extremes of heat and cold

• Marked weight change (anorexia or abnormal appetite)

• Loss of adaptability and worsening of symptoms with stress

(c) Immune Manifestations

• Tender lymph nodes

• Recurrent sore throat

• Recurrent flu-like symptoms

• General malaise

• New food sensitivities, medication sensitivities and/or chemical sensitivities

7. The Illness Persists for at Least Six Months (Required - 3 months for Pediatric)

It usually has a distinct onset, (**) although it may be gradual. Preliminary diagnosis may be possible earlier. Three months is appropriate for children.

(from MEAction https://me-pedia.org/wiki/Canadian_Consensus_Criteria)