Fever is one of the most common chief complaints in pediatric patients presenting to the emergency department, accounting for up to 20% of pediatric ED visits.1 Fever tends to be of a higher clinical importance in neonates and infants younger than 3 months, as they are immunologically immature and incompletely vaccinated. Children younger than 3 months have unique risks for serious bacterial infection, bacteremia and occult bacteremia, making the recognition of fever and transport to an ED for evaluation extremely important. Assessing these patients can be difficult and misleading due to the absence of traditional signs and symptoms of severe disease. To help you best appreciate and understand fever in this unique patient population, this article will review maintenance of normal body temperature, purpose of fever, development of the immune system in the neonate and young infant, and assessment and management of the neonate and young infant with fever.
Normal Body Temperature
Body temperature is regulated by thermosensitive neurons located in the hypothalamus that respond to signals from two sources: peripheral nerves that transmit information from specific warmth/cold receptors in the skin, and from the temperature of the various blood-bathing tissue regions in the body. These signals are integrated by the thermoregulatory center in the hypothalamus, and thermoregulatory responses are initiated to raise or lower temperature as needed. These responses include behavioral actions, such as seeking a warmer or cooler environment, as well as physiologic responses, such as redirecting of blood to and from capillary beds, increased or decreased sweating and fluid volume regulation.
A normal body temperature is usually easily maintained by balancing the heat lost through the skin and lungs with the heat produced by metabolic activity, such as from muscle and liver function. In a neutral temperature environment, the metabolic activity of the human body produces more than enough heat to maintain a core body temperature of about 37ºC (98.6°F) in an adult.
An infant’s normal body temperature will vary based on age, activity and time of day. Infants tend to have higher average temperatures than older children and adults (37.5°C or 99.5°F). An infant’s normal core body temperature will vary by as much as 1°–2°F throughout the day. Typically, core body temperature will rise during the day and be slightly lower during the night while the infant is sleeping. This circadian temperature rhythm, or diurnal variation, will occur without pathology being present. Fever is defined as a body temperature elevated above the norm. It is generally accepted that for the appropriately dressed child at rest, fever is present with a rectal temperature of 38°C (100.4°F) or an oral temperature of 37.2°C (99.0°F) or higher.2
In the prehospital environment, temperature in the infant is most often measured via rectal, oral or tympanic membrane (TM) thermometer. Rectal temperatures are usually the most accurate and are about 0.6°C (1.0°F) higher than oral readings, most likely because breathing through the mouth results in a decreased average oral temperature. Axillary measurement is less accurate—typically 0.6°C (1.0°F) lower than oral measurement. TM thermometers, while convenient, carry a risk of error.3 They measure the radiant heat from the tympanic membrane and nearby ear canal and display either the absolute value or an adjusted value. The adjusted value is calculated by taking the absolute value and adjusting the result based on nomograms relating the radiant temperature measured to actual core temperatures obtained in clinical studies.
Fever is an elevation in body temperature that exceeds the normal daily variation and occurs secondary to an increase in the hypothalamic set point. As such, fever is regulated in the same manner that normal temperature is maintained in a normal environment, the difference being that the body's thermostat (the hypothalamus) has been reset at a higher temperature (a higher hypothalamic set-point). This shift from a normothermic to a febrile set point is analogous to resetting a thermostat to raise room temperature.