Hyperthermia, Fever, and Fever of Undetermined Origin

Abnormal elevation of body temperature, or pyrexia, can occur in one of two ways: hyperthermia or fever. In hyperthermia, thermal control mechanisms fail, so that heat production exceeds heat dissipation. In contrast, in fever, the hypothalamic thermal set point rises, and intact thermal control mechanisms are brought into play to bring body temperature up to the new set point. The distinction between fever and hyperthermia is more than academic: hyperthermia is best treated with drugs that lower the thermal set point, such as aspirin, other cyclooxygenase inhibitors, or acetaminophen. Fever of undetermined origin (FUO) presents one of the most challenging and perplexing problems in clinical medicine. Such fevers may persist for weeks or months in the absence of characteristic clinical findings or clues. Ultimately, most such obscure fevers prove to be caused by common diseases presenting in an atypical fashion rather than by rare and exotic illnesses. This chapter discusses fever and hyperthermia and further clarifies the distinction between fever and hyperthermia. The section on hyperthermia discusses the etiology of hyperthermia; the epidemiology, pathophysiology, diagnosis, dif