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Fever

Part 2: Cardinal Manifestations and Presentation of Diseases · Part 2 – Cardinal Manifestations & Presentation · Chapter 20

Key Clinical Points

  1. Fever is defined as a temperature >37.7°C (99th percentile), involving an increase in the hypothalamic set point.
  2. Hyperthermia is an uncontrolled rise in body temperature exceeding the ability to lose heat; it does not respond to antipyretics.
  3. Pyrogenic cytokines (IL-1, IL-6, TNF) induce fever via PGE2 synthesis in the hypothalamus.
  4. Antipyretic drugs like acetaminophen and NSAIDs work by inhibiting cyclooxygenase (COX) to reduce PGE levels.
  5. Hyperpyrexia (>41.5°C) is often associated with severe infection or CNS hemorrhage.
  6. A 0.15°C increase in baseline temperature correlates to a 0.52% absolute increase in 1-year mortality.
  7. Fever increases oxygen consumption by 13% for every 1°C increase over 37°C.
  8. Anticytokine therapies (e.g., anti-TNF, anti-IL-1) can blunt the febrile response, potentially masking serious infections.
  9. Periodic fevers have specific patterns: Malaria (3rd/4th day), Hodgkin disease (Pel-Ebstein), and Cyclic Neutropenia (21 days).
  10. Temperature-pulse dissociation (relative bradycardia) occurs in typhoid fever, brucellosis, and leptospirosis.
  11. Acetaminophen is the preferred antipyretic in children to avoid Reye syndrome risk associated with aspirin.
  12. Blunted febrile responses are common in newborns, elderly patients, and those with renal/hepatic failure or on glucocorticoids.

DEFINITION & OVERVIEW

• Fever:

Definition: An elevation of body temperature that exceeds the normal daily variation and occurs in conjunction with an increase in the hypothalamic set point.

• Hyperthermia:

Definition: A distinct condition characterized by an uncontrolled increase in body temperature that exceeds the body's ability to lose heat; it does not respond to antipyretics.

• Hyperpyrexia:

Definition: A fever of >41.5°C (>106.7°F). ◦ Typically associated with severe infections or CNS hemorrhages.

• Normal Body Temperature Variations (Table 20-1): ◦ Mean Oral: 36.6°C (95% CI 35.7–37.3°C). ◦ Fever Threshold: >37.7°C (>99.9°F). ◦ Rectal vs. Oral: Rectal is generally 0.4°C (0.7°F) higher. ◦ Age Effect: -0.02°C for every 10-year increase. ◦ Race Effect: African-American women are 0.052°C higher than white men. ◦ Comorbidity Effect: Cancer (+0.02°C); Hypothyroidism (-0.01°C).

EPIDEMIOLOGY

• Mortality Risk: ◦ An increase in baseline temperature of 0.15°C (or 1 standard deviation) → 0.52% absolute increase in 1-year mortality.

• Measurement Methods: ◦ Use consistent sites (oral, tympanic membrane, or rectal) for monitoring; electronic devices are reliable.

• Menstrual Cycle Impact: ◦ Pre-ovulation: Lower a.m. temperature (2 weeks prior). ◦ Ovulation/Luteal Phase: Temperature rises by ~0.6°C (1°F) and remains elevated until menses.

ETIOLOGY & PATHOPHYSIOLOGY

• Pyrogens: ◦ Definition: Any substance that causes fever. ◦ Exogenous Pyrogens: Microbial products, microbial toxins, or whole microorganisms (including viruses).

• Pyrogenic Cytokines: ◦ Key molecules: IL-1, IL-6, tumor necrosis factor (TNF), and ciliary neurotropic factor (IL-6 family). ◦ Induction thresholds: ◦ IL-1 & TNF: 10–100 ng/kg required for fever. ◦ IL-6: 1–10 μg/kg required for fever.

• Hypothalamic Set Point Elevation: ◦ Mechanism: Pyrogenic cytokines trigger PGE synthesis in hypothalamic tissue and the third cerebral ventricle. ◦ Location: Highest concentration near circumventricular vascular organs (organum vasculosum of lamina terminalis). ◦ Interaction: Exogenous pyrogens/cytokines interact with the endothelium of these capillaries to initiate fever.

• Pathophysiology of Fever Induction (Figure 20-1): ◦ Initial Triggers: Infection, inflammation, or microbial toxins. ◦ Intermediate Mediators: Pyrogenic cytokines (IL-1, IL-6, TNF) and interaction with endothelial cells. ◦ Brain Signaling: Activation of hypothalamic neurons → Cyclic AMP → PGE2 synthesis. ◦ Physiological Response: Increased thermoregulatory set point → Heat conservation/production → Fever.

CLINICAL FEATURES

• History and Physical Examination: ◦ Systemic PGE in peripheral tissues causes non-specific myalgias and arthralgias. ◦ History should include exposure to others or vectors of disease.

• Laboratory Tests: ◦ CBC: Manual or automated count for neutrophil juvenile/band forms, toxic granulations, and Döhle bodies (suggestive of bacterial infection). ◦ Inflammatory Markers: CRP and ESR are most valuable for detecting occult disease in low-grade fevers. ◦ IL-6 Measurement: Useful as it induces CRP; however, IL-6 levels may fluctuate while CRP remains elevated.

• Periodic Fevers: ◦ Malaria (P. vivax): Every 3rd day. ◦ Malaria (P. malariae): Every 4th day. ◦ Relapsing Fever: Borrelia infection (days of fever followed by several days afebrile). ◦ Pel-Ebstein Pattern: 3–10 days fever → 3–10 days afebric (classic for Hodgkin disease/lymphomas). ◦ Cyclic Neutropenia: Every 21 days. ◦ Syndromes: Familial Mediterranean fever, TNF receptor–associated periodic syndrome (TRAPS).

DIFFERENTIAL DIAGNOSIS

• Fever vs. Hyperthermia: ◦ Mechanism: Fever = set point elevation; Hyperthermia = uncontrolled heat. ◦ Skin Appearance: ◦ Hyperthermia (e.g., heat stroke): Hot and dry skin. ◦ Fever: Can be cold/clammy due to vasoconstriction. ◦ Antipyretic Response: ◦ Hyperthermia: No response to antipyretics. ◦ Fever/Hyperpyrexia: Respond to adequate doses of aspirin or acetaminophen.

• Anticytokine Therapy Risks: ◦ Blunted Response: Patients on anti-TNF, anti-IL-1, anti-IL-6, anti-IL-12/23, anti-IL-17, or glucocorticoids may have blunted febrile responses. ◦ Clinical Concern: Low-grade fever in these patients requires early/rigorous evaluation as it may signal infection (e.g., M. tuberculosis or fungal infections like systemic candidiasis).

INVESTIGATIONS & DIAGNOSIS

  1. Measure Temperature: ◦ Use consistent site (oral, rectal, tympanic). ◦ Note: Rectal is 0.4°C higher than oral.
  2. Assess Clinical Context: ◦ Distinguish fever (set point elevation) from hyperthermia (uncontrolled heat).
  3. Laboratory Evaluation: ◦ CBC with differential (check for band forms, toxic granulations). ◦ CRP and ESR to detect occult disease.
  4. Evaluate for Anticytokine Use: ◦ Identify if blunted febrlesponse masks infection in patients on anti-TNF, anti-IL-1, etc.
  5. Identify Periodic Patterns: ◦ Malaria (3rd/4th day), Hodgkin (Pel-Ebstein), Cyclic Neutropenia (21 days).

MANAGEMENT & TREATMENT

• General Principles: ◦ Fever is a response to perturbation, not an illness. ◦ Antipyretics do not delay resolution of viral/bacterial infections or hinder immune function. ◦ Withholding antipyretics in bacterial infections may help evaluate antibiotic efficacy and prevent masking of poorly treated infection.

• Treatment Plan: 1. Identify Goal: → Reduce elevated hypothalamic set point AND facilitate heat loss. 2. Select Antipyretic Agent: → Acetaminophen: Preferred due to lack of GI/platelet effects; works via PGE reduction (via COX inhibition). → NSAIDs (Ibuprofen, etc.): Effective but may affect platelets/GI tract. 3. Pediatric Protocol: → Use acetaminophen or oral ibuprofen. → Avoid aspirin due to Reye syndrome risk. 4. Alternative Administration: → If oral is not possible: Parenteral NSAIDs or rectal suppositories.

PROGNOSIS & COMPLICATIONS

• Mortality Risk: ◦ 0.15°C increase in baseline → 0.52% absolute increase in 1-year mortality.

• Metabolic Demand: ◦ Every 1°C increase over 37°C → 13% increase in oxygen consumption.

• Anticytokine Complications: ◦ Blunted response may mask infections like M. tuberculosis or systemic candidiasis (specifically with anti-IL-17).

SPECIAL CONSIDERATIONS

• Blunted Febrile Response: ◦ High Risk Groups: Newborns, elderly patients, chronic hepatic/renal failure. ◦ Medication Factors: Patients on glucocorticoids or anticytokine therapies (anti-TNF, anti-IL-1, etc.).

• Temperature-pulse Dissociation: → Observed in: Typhoid fever, brucellosis, leptospirosis, drug-induced fevers, and factitious fever. → Characterized by: Relative bradycardia.

KEY PEARLS & CLINICAL TRAPS

• Fever vs. Hyperthermia: ◦ Key distinction is the response to antipyretics and the underlying mechanism (set point vs. heat dissipation).

• Antipyretic Mechanism: ◦ Inhibition of cyclooxygenase (COX) reduces PGE levels in the thermoregulatory center.

• Clinical Significance of Blunted Response: ◦ Low-grade fever in patients on anticytokines is a major red flag for infection.

• Oxygen Demand: ◦ Fever significantly increases oxygen consumption (13% per 1°C over 37°C), which is critical in critically ill patients.

Reference Tables

TABLE 20-1 Disease Categories That Present with Fever as a Cardinal Sign Infectious diseases Autoimmune and…

Harrison's 22e, p.135

  • Infectious diseases
  • Autoimmune and noninfectious inflammatory disorders
  • Cancer
  • Medication related (e.g., vaccines, drug fever)
  • Endocrine disorders (e.g., hyperthyroidism)
  • Intrinsic hypothalamic malfunction