The Canadian ME/CFS Clinical Case Definition and Guidelines for Medical Practitioners (Overview) provides a comprehensive breakdown of the symptoms, diagnosis and management of ME/CFS. While aimed at medical practitioners, it is also of great value to people who have ME/CFS, assisting with a greater understanding of the condition.

Symptoms required for a diagnosis of ME/CFS

1. Fatigue: New onset unexplained, physical and mental fatigue. Inappropriate loss of physical/mental stamina.

The "fatigue" of ME/CFS is an overwhelming physical and mental exhaustion, unlike the tiredness that well-people experience after strenuous exercise or a day's work or the "chronic fatigue" associated with many chronic illnesses. The pathological "fatigue" experienced in ME/CFS may combine exhaustion, weakness, heaviness, general malaise, lightheadedness, and sleepiness that can be overwhelmingly debilitating' and energy is not promptly restored by rest, as is the case with "normal fatigue".

2. Post-exertional malaise: A pathological slow recovery after both mental and physical function, 24 hours or longer.

This is the most distinctive symptom. Post-exertional malaise is a worsening of symptoms following physical or mental activity with a prolonged recovery time. It includes a disproportionate loss of mental and physical stamina, rapid muscle and cognitive fatiguing and a worsening of other symptoms. The worsening may be delayed 24 hours or more and recuperation to pre-activity levels is slow, generally taking at least a day and sometimes weeks or months. The amount of activity required to worsen symptoms can be minimal and varies from person to person and from hour to hour and day to day.

3. Sleep dysfunction: Unrefreshing sleep, changed sleep patterns or quantity.

4. Pain: Joint or muscle aches or headaches

5. Neurological-cognitive manifestations, two or more should be present: Short-term memory problems, poor concentration, word-finding difficulties, inability to calculate numbers, disorientation, difficulty with processing information, categorising words and word retrieval, perceptual and sensory changes, inability to comprehend/retain what is read, slurring of speech, vision disturbances (blurring, eye pain), numbness, tingling or burning sensations in the extremities, ringing in the ears, balance problems, muscle weakness, overload phenomena either cognitive, sensory or emotional e.g. photophobia or hypersensitivity to noise, touch or sound, which can lead to crash and/or anxiety.

6. At least one symptom from two of the following categories:

6.1 Autonomic: Orthostatic intolerance or difficulty standing, light-headedness, chest palpitations cardiac arrythmias, shortness of breath, dizziness, pale, nausea, irritable bowel symptoms, bladder dysfunction

6.2 Neuro-endocrine: Changes in body temperature, sweating episodes, cold extremities, marked changes in weight or appetite; symptoms worsen with stress.

6.3 Immune: Tender lymph nodes, sore throat, flu-like symptoms. Allergies and sensitivities to odours, chemicals, medications, low grade fever, rashes.

The particular cluster of symptoms experienced, the severity of individual symptoms and overall illness severity vary from person to person and during the course of the illness. For example, for some people headaches may be a more severe and incapacitating symptom than, say, cognitive impairment. Flu-like symptoms are often more common in the early stages.

Symptoms can fluctuate from hour to hour and day to day and may also be made worse by, for example, infection, anaesthetics, immunisation, physical trauma, chemical exposure, increased stress and air travel as well as physical and/or mental activity beyond personal limits.

Adapted from the Canadian Guidelines Overview document produced in 2005; the full version of the 2003 article 'Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Clinical Working Case Definition, Diagnostic and Treatment Protocols', was published in the Journal of Chronic Fatigue Syndrome, vol. 11, no. 1.