ME: Chronic fatigue syndrome or a distinct clinical entity?

Myalgic encephalomyelitis (ME) is a disabling condition characterised by profound fatigue following minimal exertion and a delay in recovery after exertion ends. In 1988, when specialists introduced the concept of chronic fatigue syndrome (CFS), it was assumed that the illness in question was identical to ME but the lack of research criteria for the latter has prevented the testing of this assumption. In this article, we propose criteria and guidelines which can be used to study ME and determine whether it is a synonym for CFS or one of several subgroups contained within the CFS construct. If clinically meaningful differences are identified, the criteria may help to increase diagnostic precision and facilitate further research into the cause, course and treatment of ME.