Chronic Fatigue Syndrome (Systemic Exertion Intolerance Disease; SEID; Myalgic Encephalomyelitis; ME / CFS) By Stephen Gluckman, MD, Perelman School of Medicine at The University of Pennsylvania Reviewed / Revised Sep 2021 | Modified Sep 2022 View Patient Education Etiology Symptoms and Signs Diagnosis Prognosis Treatment Key Points More Information Topic Resources Diagnostic Criteria for Chronic... Chronic fatigue syndrome (CFS, also called myalgic encephalomyelitis / chronic fatigue syndrome [ME / CFS]) is a syndrome of life-altering fatigue lasting > 6 months that is unexplained and is accompanied by a number of associated symptoms.