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.

Early misunderstanding and ‘Neurasthenia’

In the 19th century the concept of ‘neurasthenia’ was introduced by the psychiatrist George Beard, which defined an illness largely in women that had many of the hallmarks of ME/CFS.   Its onset 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 popularity of ‘neurasthenia’ declined in the early stages of the 20th century as its usefulness and validity were called into question.

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

Around 70 possible outbreaks of ME/CFS were documented during the course of the 20th century. At this point, the condition went by many names but all linked by their unknown cause and similar symptom set. One of the most important for the history of ME/CFS was the outbreak at the Royal Free Hospital in the UK. Over 200 staff were hospitalized and the hospital closed for several months due to an unknown illness, it was during this time that Dr. Melvin Ramsay (from the Royal Free Infectious Diseases department) would call the illness Myalgic Encephalomyelitis (ME). Dr. Ramsay definition remains one of the best 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 the many outbreaks had to be known by many other names, including ME. They put together the first description of CFS in 1988 in order to classify the disease population to improve comparability and reproducibility for research purposes. The 1988 definition was quite specific and in the later revision of 1994, the definition was loosened to form the most notable definition of CFS, the Fukuda definition.  This criteria has since been criticized for its loose definition that both significantly increases the chances of misdiagnosis and reduces the specificity of cohorts for effective research.  Furthermore, the name ‘Chronic Fatigue Syndrome’ misrepresents the severity of the illness.

The re-introduction of ME as ME/CFS

Further refinements of the case definition occurred in 2003 with the development of the Canadian Consensus Criteria (CCC) to define ME/CFS. The key change was the required symptom presentation of post-exertional malaise, a symptom named for ME and distinct from symptoms experienced in most other chronic disorders. It is also worth noting the requirement of disturbances in cognition, immune system, autonomic nervous system, and endocrine system. This was further refined in 2011 by the International Consensus Criteria (ICC). This was the first definition to not include fatigue as a required symptom and most similarly represents the definitions of ME from Dr. Ramsay several decades earlier. By removing fatigue, the ICC put more emphasis on the other symptoms of the disorder in an effort to refine the diagnosis to a set of symptoms distinct from other chronic disorders.

The improvement of research and SEID

Following the evolution of the diagnostic criteria over 2 decades, the quality and consistency of research began to improve. The increased specificity of the diagnostic criteria has resulted in tighter cohorts for comparison to healthy in research studies, the downside is the reduced prevalence of the disorder and segregation of previously diagnosed CFS patients. The recent definition decided on the name Systemic Exertion Intolerance Disease (SEID) and as indicated in the table Criteria for Diagnosis of ME/CFS, they reduced the complexity of the ME/CFS definitions of 2003 and 2011 while focusing on a small set of required symptoms most unique to ME/CFS.

Various definitions and the implications for research

From a research perspective, the more specific the definition used, the better. Furthermore, collecting samples for research from patients that have been diagnosed under the same criteria is essential for research.  The understanding of different case definitions is crucial to reading the literature of ME/CFS research and determining which studies may be relevant to each other.