Differential diagnosis between “chronic fatigue” and “chronic fatigue syndrome”

Fatigue is a common complaint experienced by most of subjects during lifetime, which affects approximately 30-50% of general population as point prevalence. 1 According to the fatigue-lasting duration, it is classified as acute (<1 month), prolonged (>1 month, <6 months), and chronic fatigue (≥6 months), respectively.Acute fatigue is generally disappears after taking a rest or treating the causative diseases, while uncontrolled prolonged and chronic fatigue limit the physical and social activities. 2 Especially, medically unexplained chronic fatigue is a debilitating status, such as idiopathic chronic fatigue (ICF) and chronic fatigue syndrome (CFS).On the other hand, to distinguish CFS from chronic fatigue or ICF is very important in clinical practice.The reason is that although patients present fatigue symptom as their main complaint in subjects suffering from chronic fatigue or CFS, CFS is considered as to being in totally different pathologic illness.3 In 2015, US Institute of Medicine (IOM) reported diagnostic criteria for CFS as follows; three mandatory symptoms, a substantial impairment in activities accompanied by fatigue persisting for more than 6 months, postexertional malaise (PEM) and unrefreshing sleep, and one optional symptom among cognitive impairment or orthostatic intolerance.4 Unlike chronic fatigue, CFS has characteristics of brain and CNS symptom and is counted as a complex, multisystem neuroimmune disease.As commonly referred to myalgic encephalomyelitis (ME)/CFS together, brain inflammation is frequently implied in pathology of CFS. 5 Above facts brought a necessity of new name which distinguishes CFS from chronic fatigue, without the word "fatigue".IOM therefore recommended "systemic exertion intolerance