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What is fatigue? It is a sensation that we all feel at times and which has a variety of synonyms such as tiredness, exhaustion, and weariness.1 In this article we are concerned with a general feeling of fatigue or exhaustion and not with fatigue specific to a part of the body; this is sometimes called central as opposed to peripheral fatigue.2 Although one may interpret a complaint of “fatigue” as meaning whatever the patient wants it to mean, it is useful to apply more precision by exploring exactly what the patient means by the term. For example, to differentiate the core concept of fatigue as a lack of ability to initiate or sustain activities because of a feeling of lack of energy, from other meanings such as the “lack of interest and motivation” that is typical of depression, and the “tendency to fall asleep” that is typical of sleep disorders.
When is fatigue abnormal? Fatigue as a symptom must be differentiated from normal tiredness. Given that the severity of fatigue is continuously distributed in the population (fig 1), we need to draw some kind of cutoff to define fatigue of clinically important severity, in the same way that we do for blood pressure and depression. One approach is to define this as fatigue that has persisted for a period of time (say a month or more) and that is of sufficient severity to interfere with the patient’s activities or to be otherwise severely troublesome. A patient with fatigue of this severity and duration may then be referred to as “a case” of clinically significant fatigue.