Fever, or pyrexia, is an elevation of body temperature above the normal range due to a resetting of the hypothalamic set point. It is a common clinical sign that can occur in numerous disorders affecting almost any body system. While low-grade or brief fevers may have minimal diagnostic significance, a persistent high fever represents a medical emergency and may indicate serious underlying pathology.

Classification

• By intensity:

  • Low: 99–100.4°F (37.2–38°C)

  • Moderate: 100.5–104°F (38–40°C)

  • High: >104°F (>40°C)

  • Hyperpyrexia: >106°F (>41.1°C), may cause unconsciousness and permanent brain damage

• By pattern:

  • Remittent: Daily fluctuations above normal

  • Intermittent: Temperature returns to normal daily, may be hectic/septic with chills and sweating

  • Sustained: Persistently elevated with minimal fluctuation

  • Relapsing: Alternating febrile and afebrile periods

  • Undulant: Gradual rise and fall over days

• By duration:

  • Brief: <3 weeks

  • Prolonged: Fever of unknown origin when careful evaluation fails to identify a cause

Emergency Interventions

For fever >106°F, initiate the following immediately:

1. Assess vital signs and level of consciousness (LOC).

2. Administer an antipyretic (e.g., acetaminophen) as per protocol.

3. Apply rapid cooling measures: ice packs to axillae and groin, tepid sponging, or cooling blankets.

4. Monitor rectal temperature continuously to prevent overcooling or shivering.

5. Evaluate for underlying life-threatening causes, including sepsis, CNS lesions, heatstroke, or thyroid storm.

History and Physical Examination

History:

• Onset, duration, peak temperature, and pattern of fever

• Associated symptoms: chills, fatigue, malaise, pain, rash

• Exposure history: recent travel, animal contact, endemic regions

• Medical history: immunosuppression, recent surgery, trauma, medications, anesthesia

Physical Examination:

• May range from focused (single system) to comprehensive (all systems)

• Look for signs of infection, inflammation, organ dysfunction, or systemic illness

• Evaluate for rash, lymphadenopathy, hepatosplenomegaly, neurological deficits, or cardiovascular compromise

Pathophysiology

• Fever results from the hypothalamic thermoregulatory set point being raised by pyrogens, including bacterial toxins, viral antigens, immune complexes, or cytokines (e.g., IL-1, IL-6, TNF-α).

• The thermoregulatory center responds by initiating heat-conserving mechanisms (vasoconstriction, shivering) to reach the new set point.

• Fever serves as a host defense mechanism by enhancing immune responses and inhibiting pathogen replication.

Medical Causes

Table: Causes of Fever with Distinctive Features and Key Clinical Notes

Notes for Clinical Use:

• Patterns of fever (intermittent, sustained, relapsing) often point to specific causes.

• Pediatric and geriatric patients may present atypically; high vigilance is required.

• Always correlate fever with vital signs, organ function, lab results, and history to guide further workup.

• Fever >106°F or with CNS/cardiovascular compromise is an emergency.

Emergency Considerations

• For hyperpyrexia (>106°F), aggressive cooling, airway protection, and hemodynamic support are critical.

• Monitor for complications: seizures, dehydration, hypotension, and altered mental status.

• Laboratory evaluation should include: CBC, blood/urine/sputum cultures, electrolytes, liver/renal function tests, and imaging as indicated.

Special Considerations

• Patient Comfort: Maintain stable room temperature, frequent clothing/bedding changes

• Hydration: Encourage increased fluid intake unless contraindicated

• Medication: Follow antipyretic dosing schedules to minimize chills and diaphoresis

• Documentation: Record temperature trends to assess response to treatment

Pediatric Pointers

• Children exhibit higher, more rapid, and fluctuating fevers

• Monitor for febrile seizures and prevent complications with tepid cooling

• Avoid aspirin in children with viral infections due to risk of Reye’s syndrome

• Common causes: varicella, croup, meningitis, mumps, otitis media, roseola, rubella

Geriatric pointers

• Elderly patients may have blunted febrile responses due to impaired thermoregulation

• High-risk for heatstroke during environmental heat stress

• Fever may be an unreliable indicator of serious infection

Patient counseling

• Teach proper temperature measurement at home

• Emphasize hydration and adherence to antipyretic therapy

• Educate on recognizing signs of severe illness requiring immediate medical attention

References

1. Graham, J. G., MacDonald, L. J., Hussain, S. K., Sharma, U. M., Kurten, R. C., & Voth, D. E. (2013). Virulent Coxiella burnetii pathotypes productively infect primary human alveolar macrophages. Cellular Microbiology, 15(6), 1012–1025.

2. Van der Hoek, W., Dijkstra, F., Schimmer, B., Schneeberger, P. M., Vellema, P., Wijkmans, C., … van Duynhoven, Y. (2010). Q fever in the Netherlands: An update on the epidemiology and control measures. European Surveillance, 15, 19520.

3. Mandell, G. L., Bennett, J. E., & Dolin, R. (2010). Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases (7th ed.). Philadelphia: Elsevier.

4. Nelson, L. S., & Goldfrank, L. R. (2011). Goldfrank’s Toxicologic Emergencies (9th ed.). New York: McGraw-Hill.