Fever is defined as an elevation of basal body temperature above 38°C (100.4°F), when measured rectally, and has a multitude of causes. Infection is recognized as the major cause of fever in the pediatric patient. In addition, drug reactions, collagen vascular diseases, neoplastic processes, recent immunization, allergic reactions, heat illness, and tissue infarction all produce fever and must be considered in the differential diagnosis.

The evaluation of the febrile child is dependent upon the child’s age. Newborns and infants are less likely to have focal findings and are unable to relate associated symptoms. Older children will often have focal finding to guide therapy, and clinical impression is more reliable.

Infection

Collagen vascular diseases

Drug fever

Tissue injury or infarction

Immunization reactions

Inflammatory disorders

Malignancy

Heat illness

Allergic reactions

Thyrotoxicosis

Hypothalamic injury

Hyperactivity

Factitious fever

Malignant hyperthermia

Aside from the common causes of fever routinely handled in emergency practice and discussed at length in other parts of this text, several causes of fever produce no striking physical findings or laboratory abnormalities. Sepsis, bacteremia, and UTI are common bacteria infections that may cause fever without localizing findings.

A young infant (<8 weeks) may have serious bacterial illness without any specific findings. Decreased feeding, irritability, excessive crying, or lethargy may be the only signs of sepsis. The infant may be pale and lethargic and have mottled skin or grunting respirations. Physical exam may not reveal any focal abnormalities.

As the child ages, history and exam become more reliable. Changes in behavior, nuchal rigidity, and localizing signs become more apparent.

In the non-toxic-appearing child 3 months to 3 years old, occult bacteremia has historically been a concern. The pneumococcal vaccine has lowered the risk of serious bacterial infection in this age group to less than 1%.