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Fatigue is synonymous with lethargy, tiredness, lassitude, being run down, or lack of energy. It has no standard definition shared by the lay public and the medical establishment; so many patients use it to denote related conditions such as simply a need for sleep.(1) Fatigue may be related to an underlying medical condition, poor nutrition, stress, or it may exist independently. Patients who complain of fatigue do not exercise as much as patients who do not experience fatigue. Smokers tend to experience fatigue more than nonsmokers, and patients with a high body mass index also experience more fatigue.(1)
Differentiation of sleepiness from fatigue can be difficult, particularly given the imprecise use of these terms. The distinction can be useful in the differentiation of patients with complaints of fatigue or tiredness in the setting of disorders such as fibromyalgia, chronic fatigue syndrome, or endocrine deficiencies such as Addison's disease or hypothyroidism. While patients with these disorders can typically distinguish their daytime symptoms from the sleepiness that occurs with sleep deprivation, substantial overlap can occur.(2) This may be particularly true when the primary disorder results in chronic sleep disruption or in abnormal sleep.
Patients who describe intermittent "weakness," but actually have fatigue, suffer from asthenia, which can be separated from true weakness by the fact that patients do not lack the ability to do a task, but rather the ability to perform it repetitively.(3) Such fatigue is characteristic of serious renal, hepatic, cardiac, or pulmonary disease.
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