Fatigue is one of the most common symptoms in clinical medicine. It is a prominent manifestation of a number of systemic, neurologic, and psychiatric syndromes, although a precise cause will not be identified in a substantial minority of patients. Fatigue refers to the subjective human experience of physical and mental weariness, sluggishness, low energy, and exhaustion. In the context of clinical medicine, fatigue is most typically and practically defined as difficulty initiating or maintaining voluntary mental or physical activity. Nearly everyone who has ever been ill with a self-limited infection has experienced this near-universal symptom, and fatigue is usually brought to medical attention only when it is either of unclear cause, fails to remit, or the severity is out of proportion with what would be expected for the associated trigger.
Fatigue should be distinguished from muscle weakness, a reduction of neuromuscular power (Chap. 21); most patients complaining of fatigue are not truly weak when direct muscle power is tested. Fatigue is also distinct from somnolence, which refers to sleepiness in the context of disturbed sleep-wake physiology (Chap. 27), and from dyspnea on exertion, although patients may use the word fatigue to describe those symptoms. The task facing clinicians when a patient presents with fatigue is to identify the underlying cause and to develop a therapeutic alliance, the goal of which is to spare patients expensive and fruitless diagnostic workups and steer them toward effective therapy.
EPIDEMIOLOGY AND GLOBAL CONSIDERATIONS
Variability in the definitions of fatigue and the survey instruments used in different studies makes it difficult to arrive at precise figures about the global burden of fatigue. The point prevalence of fatigue was 6.7% and the lifetime prevalence was 25% in a large National Institute of Mental Health survey of the U.S. general population. In primary care clinics in Europe and the United States, between 10 and 25% of patients surveyed endorsed symptoms of prolonged (present for >1 month) or chronic (present for >6 months) fatigue, but in only a minority was fatigue the primary reason for seeking medical attention. In a community survey of women in India, 12% reported chronic fatigue. By contrast, the prevalence of chronic fatigue syndrome, as defined by the U.S. Centers for Disease Control and Prevention, is low (Chap. 442).
Fatigue is a common somatic manifestation of many major psychiatric syndromes, including depression, anxiety, and somatoform disorders. Psychiatric symptoms are reported in more than three-quarters of patients with unexplained chronic fatigue. Even in patients with systemic or neurologic syndromes in which fatigue is independently recognized as a manifestation of disease, comorbid psychiatric symptoms or disease may still be an important source of interaction.
Patients complaining of fatigue often say they feel weak, but upon careful examination objective ...