Lim and Son J Transl Med (2020) 18:289 https://doi.org/10.1186/s12967-020-02455-0 Journal of Translational Medicine REVIEW Open Access Review of case defnitions for myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) Eun‑Jin Lim and Chang‑Gue Son* Abstract Background: Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating disease with unknown causes. From the perspectives on the etiology and pathophysiology, ME/CFS has been labeled diferently, which infu‑ enced changes in case defnitions and terminologies. This review sought to feature aspects of the history, develop‑ ments, and diferential symptoms in the case defnitions. Methods: A search was conducted through PubMed published to February 2020 using the following search key‑ words: case defnition AND chronic fatigue syndrome [MeSH Terms]. All reference lists of the included studies were checked. Of the included studies, the number of citations and the visibility in the literatures of the defnitions were considered for comparisons of the criteria. Results: Since the frst ’ME’ case defnition was developed in 1986, 25 case defnitions/diagnostic criteria were cre‑ ated based on three conceptual factors (etiology, pathophysiology, and exclusionary disorders). These factors can be categorized into four categories (ME, ME/CFS, CFS, and SEID) and broadly characterized according to primary disorder (ME‑viral, CFS‑unknown, ME/CFS‑infammatory, SEID‑multisystemic), compulsory symptoms (ME and ME/ CFS‑neuroinfammatory, CFS and SEID‑fatigue and/or malaise), and required conditions (ME‑infective agent, ME/ CFS, CFS, SEID‑symptoms associated with fatigue, e.g., duration of illness). ME and ME/CFS widely cover all symptom categories, while CFS mainly covers neurologic and neurocognitive symptoms. Fatigue, cognitive impairment, PEM, sleep disorder, and orthostatic intolerance were the overlapping symptoms of the 4 categories, which were included as SEID criteria. Conclusions: This study comprehensively described the journey of the development of case defnitions and com‑ pared the symptom criteria. This review provides broader insights and explanations to understand the complexity of ME/CFS for clinicians and researchers. Keywords: Myalgic encephalomyelitis, Chronic fatigue syndrome, Systemic exertion intolerance disease, Case defnition, Diagnostic criteria Background Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating disease with core symptoms of fatigue, unrefreshing sleep, postexertional malaise (PEM), *Correspondence: [email protected] and cognitive dysfunction for more than 6 months [1]. Institute of Bioscience and Integrative Medicine, Department of Korean Tis disorder afects individuals of all ages across all Medicine, Daejeon University, 62 Daehak‑ro, Dong‑gu, Daejeon, Republic socioeconomic, racial, and ethnic groups, ‘approximately of Korea estimated 1% of the population, 17 to 24 million people © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Lim and Son J Transl Med (2020) 18:289 Page 2 of 10 worldwide [2, 3]. Te clinical impact of ME/CFS left 27% Terefore, this review aims to overview the develop- of the ME/CFS patients bedridden and 29% housebound, ment of ME/CFS case defnitions, which will provide leading to 50% unable to work full time and 21% una- physicians and researchers with a comprehensive picture ble to work at all [4]. In 2015, the Institute of Medicine of the current and prominent features of ME/CFS. (IOM) in the U.S. announced that ME/CFS is a serious health problem in the form of complex multisystem neu- Methods rological disorder, which should be the focus of national Literature search strategies and data collection medical and scientifc efort using the recommended To comprehensively collect the case defnitions of ME/ name ‘systemic exertion intolerance disorder (SEID)’ [5]. CFS, we performed a search through PubMed published Outbreaks of neurological paralysis-related symptoms to February 2020 and checked all reference lists of the with systemic malaise have occurred worldwide (e.g., included studies. Te following search keywords were Los Angeles in 1934 and Iceland 1947, followed by New used: case defnition AND chronic fatigue syndrome Zealand and Nevada), and ME/CFS was frst acknowl- [MeSH Terms]. We included studies only for adult popu- edged as a form of ’poliomyelitis’ and ’benign ME’ in the lations (> 18 years), and language was limited to English 1930s and 1950s [6–8]. Later, it was known to be sporadic (Additional fle 1. Figure S1). and not rare in the general population [9]. Te etiology Two authors independently read all the titles, abstracts, of the illness has yet to be revealed, which has led to no and full text retrieved by the search. Te literatures were established objective diagnostics, pathophysiology or viewed forward the background of case defnitions and therapeutics [4]. Accordingly, many expert groups have analyzed according to time-line based changes. Of those developed case defnitions based on clinical features. To included studies, comparisons of the criteria are limited date, over 20 ME/CFS case defnitions have been estab- to the eight defnitions (Ramsay, International Consen- lished by diferent groups in various countries [10]. Tese sus Criteria (ICC), Holmes, Australian, Oxford, Fukuda, defnitions refect the historical fow of the clinical fea- Canadian Consensus Criteria (CCC), systemic exertion tures and characteristics of the illness emphasized in a intolerance disease (SEID)) that were selected based on diferent way according to the perspectives of researchers the number of Google Scholar citations and the visibility [11]. in the literatures (Additional fle 1. Table S1). Recognition of the changes in the development of the case defnitions of ME/CFS is necessary for physicians Results and researchers to better understand the illness. Dif- Past and present of ME/CFS ferential diagnosis is crucial in the therapeutic process Since the frst recognition of ME/CFS in an outbreak to enhance treatment efcacy; however, a high number in Los Angeles 1934, the illness has undergone various of misdiagnoses can lead to delays in the diagnosis and changes in terminology and case defnition [17]. Dr. G. treatment of ME/CFS [12]. Approximately 37% of ME/ Beard (1839–1883) frst disclosed the illness in his book CFS-like patients had experienced alternative diagno- and introduced the term ’neurasthenia’ in the 1860 s [18]. ses, such as psychiatric, pain, or sleep disorders, in clin- Later, the features of neurologic symptoms during the ics [13]. Furthermore, the application of particular case U.K. outbreaks led to naming the illness ‘benign ME’ [19], defnitions profoundly impacts epidemiological stud- then Ramsay created the ’ME’ case defnition in 1986 ies of ME/CFS [14]. Te prevalence of ME/CFS could [20]. Serial outbreaks of the illness led to proposing that widely vary based on the application of case defnitions; the condition was linked to viral infection, which altered for example, there were fvefold diferences in prevalence its name to ‘chronic Epstein-Barr virus syndrome (EBVS)’ using the Fukuda (0.89%) and the Holmes defnition in 1982 [21] and ’postviral fatigue syndrome (PVFS)’ in (0.17%) [15]. 1985 [22]. In 1988, insufcient evidence in connection Numerous studies have also documented skepticism with the virus and numerous sporadic cases in the gen- among physicians about ME/CFS being a distinct clini- eral population led to the Centers for Disease Control cal entity, and they do not feel confdent in making the and Prevention (CDC) to create the new term ‘CFS’ (Hol- diagnosis [12, 16]. One of the reasons is a lack of under- mes defnition), which was proposed to more inclusively standing of ME/CFS, which is likely to result from the describe the symptom complex, including psychological complicated backgrounds of this disorder, including symptoms [23]. In 2003, the ’ME/CFS’ by CCC was pub- indefnite terminologies and etiology. In fact, ME/CFS lished embracing the clinical features of both ’ME’ and has been named diferently (e.g., postviral fatigue syn- ’CFS’ [24]. Te conception of ’ME’ or ’ME/CFS’ adopted drome, neurasthenia) depending on the perspectives of the notion of neuroinfammation [24, 25]. the researchers; likewise, diagnostic criteria or case def- Te terminologies of ME, CFS, and/or ME/CFS nitions have also been changed accordingly. have been and interchangeably used until present. Te Lim and Son J Transl Med (2020) 18:289 Page 3 of 10 complexity of those indefnite terminologies is shown ’neurosis’ that was indicative of psychiatric origin by in the international code for disease (ICD) system. Te Dr. S. Freud (1856–1939), who believed the illness was WHO initially classifed the illness as a neurological caused by unresolved conficts in the unconscious mind disorder in the ICD-8 (code 796.0) in 1969 [26]. Subse- [31]. Similarly, in the 1970s, McEvedy alleged its psy- quently, the ICD-10 (2016) classifed it as PVFS (code chological origin with the term ‘myalgia nervosa’ [32]. G93.3) indexing only ‘benign ME’ [27]—‘benign’ was Te debate on the origin of illness (psychological versus dropped in the 2019 version [28].