ME, a neurological disease[20,21], has been described in the medical literature since 1934 under various names[22], e.g., epidemic neuromyasthenia and atypical poliomyelitis, often on account of outbreaks[23-25]. Characteristic symptoms of ME, classified as a disease of the nervous system by the WHO since 1969[26], are: muscle weakness, neurological dysfunction, especially of cognitive, autonomic and neurosensory functions; variable involvement of the cardiac and other systems; a prolonged relapsing course; but above all general or local muscular fatigue after minimal exertion with prolonged recovery times (post-exertional “malaise”)[20].

The diagnostic criteria for myalgic encephalomyelitis (ME) and chronic fatigue syndrome (CFS) define two distinct clinical entities. Cognitive impairment and post-exertional “malaise” (a long-lasting aggravation of typical symptoms, e.g., muscle weakness and cognitive “brain fog”, after minor exertion) are obligatory for the diagnosis ME, while chronic fatigue is the only mandatory symptom for the diagnosis CFS.

individuals with ME/CFS experience different types of fatigue than what are reported in the general populations

the disabling fatigue experienced by individuals with ME/CFS differs from that associated with other illnesses or everyday activity

Finally, individuals with severe pathological fatigue might experience states that are very different from what a healthy individual experiences when fatigued.

Many people confuse “chronic fatigue” (which is a symptom of many chronic conditions, including rheumatoid arthritis, fibromyalgia, or lupus) or “general tiredness” (which is lifestyle-related), with the specific illness “chronic fatigue syndrome” (ME/CFS). To clarify the differences, here are some of the major symptoms of ME/CFS:

- unexplained physical and mental fatigue for an extended period of time
- post-exertional malaise (meaning an inappropriate loss of physical and mental stamina and a worsening of symptoms after any effort)
- sleep dysfunction
- pain
- neurological/cognitive manifestations
- autonomic manifestations, such as orthostatic intolerance
- neuroendocrine symptoms, such as subnormal body temperature
- immune system changes, such as recurrent flu-like symptoms.