Myalgic Encephalomyelitis, Chronic Fatigue Syndrome, and Systemic Exertion Intolerance Disease: Three Distinct Clinical Entities

Many researchers consider chronic fatigue syndrome (CFS) to be a synonym of Myalgic Encephalomyelitis (ME). However, the case criteria of ME and CFS define two distinct clinical entities. Although some patients will meet both case criteria, other patients can meet the diagnosis of ME and not fulfil the case criteria for CFS, while the diagnosis of CFS is largely insufficient to be qualified as a ME patient. ME is a neuromuscular disease with distinctive muscular symptoms, including prolonged muscle weakness after exertion, and neurological signs implicating cerebral dysfunction, including cognitive impairment and sensory symptoms. The only mandatory symptom of CFS is chronic fatigue. Chronic fatigue must be accompanied by at least four out of eight nonspecific symptoms: substantial impairment in short-term memory or concentration, a sore throat, tender lymph nodes, muscle pain, multijoint pain, a new type of headaches, unrefreshing sleep, and postexertional “malaise” lasting more than 24 h. So, regardless whether the name ME is appropriate or not, ME is not synonymous to CFS. That is not a matter of opinion, but a matter of definition. Due to the definitions of ME and CFS, “ME/CFS” does not exist and cannot be replaced by a new clinical entity (SEID: Systemic Exertion Intolerance Disease), as recently suggested.