What Is Fever?

Fever is usually defined as a temperature above 38.0° C to 38.4° C (100.4° F to 101° F). Parents or other caregivers usually become aware of fever because a child looks “flushed” or feels “hot,” which then leads to use of a thermometer that detects the elevated temperature. Most fever is associated with acute viral or bacterial infection and is self-limited, both in degree and duration. Fever less commonly results from malignancy, rheumatologic, or other diseases. “Teething” does not cause fever. Parents and medical professionals sometimes demonstrate “fever phobia,” the worry that any level of fever always represents a serious infection or that it may rise without limit and cause “brain damage.” This fear often prompts excessive concern, overuse of medical care, and overtreatment. Understanding the mechanism of temperature regulation, the nature of fever, and the range of “normal” body temperatures at different ages may reduce “fever phobia.”

What Mechanism Causes Fever?

Fever is a regulated elevation of body temperature that results from alteration of the set point in the hypothalamic temperature regulatory center. In health, this center maintains temperature in a narrow range around 37° C (98.6° F), although between 12 and 36 months, healthy children have temperatures that may exceed 37.5° C (99.5° F). Temperature has a diurnal pattern that is lowest in the early morning and highest in the evening. Fever develops when a stimulus, such as infection, triggers release of endogenous pyrogens (cytokines, interleukins 1α and 1β) that reset the hypothalamic set point upward. This promotes vasoconstriction that results in heat retention, plus shivering and increased metabolic rate that increase heat production. Even when elevated during fever, body temperature remains regulated by the hypothalamus and rarely exceeds 41.5° C (106° F). Disappearance of pyrogens or treatment with an antipyretic resets the “thermostat” set point downward and temperature declines.

How Does Hyperthermia Differ from Fever ?

Hyperthermia is unregulated and, unlike fever, can cause temperature to rise well above 42° C (107° F) with risk of central nervous system (CNS) damage. Hyperthermia results from prolonged exposure to excessive environmental heat, physical conditions that prevent the body from removing heat, metabolic disorders such as malignant hyperthermia that cause excessive heat production, and CNS disorders that disrupt the hypothalamic center. Hyperthermia must be identified and managed promptly to minimize morbidity and prevent mortality.

What Is “Fever without Source?”

Fever without source refers to the situation when history and physical examination can find no obvious cause for fever. Most such patients have a viral illness, but when fever is above 40° C (104° F), immunizations are incomplete, and the child appears “toxic,” the risk of a serious bacterial illness is high. The differential diagnosis includes urinary tract infection, bacteremia, pneumonia, osteomyelitis, and meningitis, all of which may be difficult to diagnose in the infant and young child. The risk that the fever represents bacteremia and meningitis has fallen dramatically since introduction of the conjugate vaccines for Haemophilus influenzae type B (Hib) and Streptococcus pneumoniae (PCV-7) (see Chapters 12 and 64). Urinary tract infection (Chapter 65) is the most common bacterial cause of fever without source in young children, especially girls. Osteomyelitis may be challenging to diagnose early in its course. Fever may be the first manifestation of Kawasaki disease, a malignancy, or a rheumatologic disorder.

What Is Fever of Unknown Origin?

Fever of unknown origin (FUO) refers to fever that persists longer than 2 weeks, most often without associated signs of acute “toxicity.” FUO is much less common than fever without source. The differential diagnosis, evaluation, and management are discussed in other resources.

How Should I Evaluate a Febrile Child?

First, use history to elicit symptoms and to characterize the illness pattern, duration, and perceived severity. Ask parents about their observations and concerns. Review the child’s risk factors, especially immunization status and exposures such as daycare. Consider any past history of fever, infection, antibiotic treatment, and recurrent or chronic disease. Observe the ill child before beginning the hands-on physical examination to identify signs of toxicity (Table 33-1). Perform a careful physical examination with the goal of identifying a source of infection or a disease that can explain the findings. Use laboratory tests and imaging judiciously.

Table 33-1 Yale Observation Scale

What Increases the Risk of Serious Bacterial Infection?