Chronic Fatigue: Definition and Overlap with Fibromyalgia

Abstract

This chapter includes a rather simplified definition and the diagnostic criteria of fibromyalgia syndrome (FMS) and a more detailed one for myalgic encephalomyelitis/chronic fatigue syndrome (CFS). Definitions will lead to a discussion on whether CFS is the same or a different illness from FMS, since a clinical overlap indeed exists between the two syndromes, and there have been notions in the literature claiming that they are variants of the same illness—called “the unitarian hypothesis.” A review of the existing literature will be presented in an attempt to delineate similarities and differences between the two syndromes.