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, differential diagnosis, treatment, and prevention of heatstroke; and the diagnosis and treatment of neuroleptic malignant syndrome, serotonin syndrome, and malignant hyperthermia of anesthesia. The section on fever discusses the pathogenesis, diagnosis, and treatment of fever, as well as the possible benefits and complications of elevated body temperature. The section on FUO presents the defining criteria of FUO; the etiologic classification of FUO, including the infections and neoplasms possibly responsible; and the diagnosis of FUO, including laboratory and radiologic studies, biopsies, and the role of exploratory laparotomy. Tables describe the causes of hyperthermia; the causes of FUO as reported in various studies over five decades; and a comprehensive list of causes of FUO. An algorithm depicts the progressive steps in the pathogenesis of fever and hyperthermia. This review contains 1 highly rendered figure, 26 tables, and 107 references Key Words: collagen, vascular disease, febrile seizures, fever, heatstroke, hyperthermia, pyrogen