Myalgic Encephalomyelitis or Chronic Fatigue Syndrome (ME/CFS) is a complex illness with a complex history. The best way to understand ME/CFS as we know it today is to understand the history of how it was first defined and how the diagnostic criteria has evolved over the past few decades.

Summary of diagnostic criteria since 1988.

What's in a name after all?

History

Early misunderstanding
and ‘Neurasthenia’

In the 19th century the concept of ‘neurasthenia’ was introduced by psychiatrist George Beard, which defined an illness largely found in women with symptoms very similar to ME/CFS.

It was generally triggered by an infection and it was described as a "...condition of nervous exhaustion, characterized by undue fatigue on [the] slightest exertion, both physical and mental... the chief symptoms are headache, gastrointestinal disturbances, and subjective sensations of all kinds.” The diagnosis of neurasthenia declined in the early 20th century as its usefulness and validity were called into question.

Outbreaks of the 20th century
and the first definition of ME

Throughout the 20th century, there were about 70 possible documented outbreaks of ME/CFS. The condition went by many names but all were linked by their unknown cause and similar symptoms.

One of the most significant outbreaks in the history of ME/CFS was at the Royal Free Hospital in the United Kingdom. Over 200 staff were hospitalized, and the hospital closed for several months due to an unknown illness. During this time, Dr. Melvin Ramsay of the Royal Free Infectious Diseases department called the illness Myalgic Encephalomyelitis (ME).

Dr. Ramsay's definition remains one of the most accurate clinical descriptions of the illness, and several years later the condition would be recognized by the World Health Organisation in 1969.

Centers for Disease Control and Prevention (CDC) and CFS

Several significant US outbreaks led the CDC to put together a working group to reach a consensus on the definition and clinical description of these illnesses. This group recognized that the many outbreaks had to be known by many other names, including ME. They put together the first description of Chronic Fatigue Syndrome (CFS) in 1988 in order to classify the disease population to improve comparability and reproducibility for research purposes.

In 1994, the definition of CFS was loosened. This criteria has since been criticized for its broad definition that significantly increases the chances of misdiagnosis and impacts the effectiveness research. Furthermore, the name has been faulted for misrepresenting the severity of the illness.

The re-introduction of ME as ME/CFS
In 2003, the development of Canadian Consensus Criteria further refined the definition of ME/CFS. The key change was requiring the presentation of the symptom post-exertional malaise, a symptom named for ME and distinct from symptoms experienced in most other chronic disorders. The International Consensus Criteria removed fatigue as a required symptom in 2011, putting more emphasis on the other symptoms of the disorder in an effort to refine the diagnosis to symptoms which are distinct from other chronic disorders.
The improvement of research
With an improved diagnostic criteria, the quality and consistency of research into ME/CFS began to improve. The specificity of the diagnostic criteria has resulted in tighter cohorts for comparison to healthy subjects in research studies. The specific criteria, however, also reduced the prevalence of the disorder, including among previously diagnosed CFS patients who no longer fit this symptom set.
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