Normal body temperature differs between the person and the time of the day. it is even highest in preschool children. However, fever is defined as a core body (rectal) temperature ≥ 38.0° C (100.4° F).

Pathophysiology of Fever in Infants and Children

Fever is the sign that shows up in response to the release of endogenous pyogenic mediators called cytokines (in particular interleukin-1 [IL-1]), which stimulate the production of prostaglandins by the hypothalamus; prostaglandins readjust and elevate the temperature set point.

Fever helps in fighting infection and, although being uncomfortable, does not necessitate treatment in an otherwise healthy child. However, it is known that fever increases the metabolic rate and the demands on the cardiopulmonary system. So it can prove detrimental to children with pulmonary or cardiac compromise or neurologic impairment. Further, it can act as a catalyst for febrile seizures, a typically benign childhood condition.

Duration of fever-

Acute fever

Most acute fevers in infants and young children are due to infection. E.g. Viral respiratory or gastrointestinal infections (most common causes overall), Certain bacterial infections (otitis media, pneumonia, urinary tract infections) etc. However, potential infectious causes of acute fever differ according to the child’s age. 

Neonates (infants < 28 days) often fail to contain infection locally and, thus are at higher risk of serious invasive bacterial infections. Common perinatal illnesses may include bacteremia (viremia with herpes simplex), pneumonia, pyelonephritis, meningitis, and/or sepsis.

Most febrile children (1 month to 2 years of age) without an obvious focus of infection on examination (fever without source [FWS]) suffer from a self-limited viral disease, but some (< 1% in the post conjugate vaccine era) of such patients are early in the course of a serious infection (eg, bacterial meningitis). Thus, the patient with FWS is frequently doubted for occult bacteremia. 

Noninfectious causes of acute fevers are Kawasaki disease, heatstroke, toxic ingestions (eg, drugs with anticholinergic effects), some vaccinations like pertussis vaccination or measles vaccination. These fevers typically last from a few hours to a day and do not demand evaluation if the child is otherwise healthy. Teething is not associated with significant or prolonged fevers.

Acute recurrent or periodic fever

In this, episodes of fever alternate with periods of normal temperature.

Chronic fever

Daily fever for ≥ 2 weeks, without concludable initial cultures and other investigations report is considered fever of unknown origin (FUO).

Potential causes include localized or generalized infection, connective tissue disease, and cancer. Miscellaneous specific causes include inflammatory bowel disease, diabetes insipidus with dehydration, and disordered thermoregulation. Respiratory infections account for almost 50% of cases of infection-associated FUO.

Evaluation of Fever in Infants and Children

History

History of present illness, Important associated symptoms, Drug history, Factors predisposing to the infection should be reviewed.

A review of systems should be done to note symptoms suggesting possible causes. A history of repeated infections or symptoms suggesting a chronic illness should be identified. 

Past medical history should note previous fevers or infections and known conditions predisposing to infection. 

Physical examination

Vital signs are reviewed, along with abnormalities in temperature and respiratory rate. Blood pressure should also be measured in ill-appearing children. A child with cough, tachypnea, or labored breathing requires pulse oximetry. The child’s overall appearance and response to the examination are important to look at.

Red flag Signs-

• Age < 1 month
• Lethargy, listlessness, or toxic appearance
• Respiratory distress
• Petechiae or purpura
• Inconsolability

Interpretation of findings

A temperature of ≥ 39° C in children < 36 months indicates a higher risk of serious bacterial infection. Other vital signs also play a significant role in diagnosis. 

Hypotension raises concerns for hypovolemia, sepsis, or myocardial dysfunction. Tachycardia in the absence of hypotension may be due to fever (10 to 20 beats/minute increase for each degree above normal) or hypovolemia. An increased respiratory rate may be interpreted as a response to fever, the pulmonary source of the illness, or respiratory compensation for metabolic acidosis.

Regardless of clinical findings, a neonate with fever requires immediate hospitalization and testing to rule out a dangerous infection. Young infants requirement for hospitalization depends on screening laboratory results and the likelihood for a follow-up.

Source: https://www.msdmanuals.com/en-in/professional/pediatrics/symptoms-in-infants-and-children/fever-in-infants-and-children