Fever¶

Chapter 20 | Section 2: Alterations in Body Temperature

KEY CLINICAL POINTS¶

Fever is an elevation of body temperature due to an increase in the hypothalamic set point, distinct from hyperthermia where the set point is unchanged
Normal oral temperature mean is 36.6°C; fever is defined as >37.7°C (>99.9°F), representing the 99th percentile
Pyrogenic cytokines (IL-1, IL-6, TNF) act via PGE2 synthesis in the hypothalamus to raise the thermoregulatory set point
Hyperpyrexia (>41.5°C/106.7°F) is often associated with CNS hemorrhage or severe infections and requires urgent evaluation
Antipyretics work by inhibiting cyclooxygenase and reducing PGE2 synthesis; they do not reduce temperature in hyperthermia

1. DEFINITION & OVERVIEW¶

Fever is an elevation of body temperature that exceeds the normal daily variation and occurs in conjunction with an increase in the hypothalamic set point. This is fundamentally different from hyperthermia, where body temperature rises due to inability to dissipate heat while the hypothalamic set point remains unchanged. Body temperature is controlled by the hypothalamus through integration of signals from peripheral warmth/cold receptors in skin and the temperature of blood bathing the hypothalamic region.

1.1 Normal Body Temperature¶

According to studies of >35,000 individuals ≥ 18 years of age, the mean oral temperature is 36.6°C (95% confidence interval: 35.7-37.3°C). A temperature of >37.7°C (>99.9°F) represents the 99th percentile for healthy individuals and defines fever. Normal core body temperature range is 36.5-37.5°C (97.7-99.5°F). Factors affecting baseline temperature include: - Diurnal variation: Lower at 8 a.m., higher at 4 p.m. - Seasonal variation: Lower in summer, higher in winter - Age: Lower by 0.02°C for every 10-year increase in age - Demographics: African-American women have temperatures 0.052°C higher than white men - Comorbidities: Cancer associated with 0.02°C higher; hypothyroidism with 0.01°C lower - Ambient temperature: Higher ambient temperatures linked to higher baseline body temperatures - Menstrual cycle: In menstruating women, temperature rises ~0.6°C (1°F) with ovulation

1.2 Temperature Measurement Sites¶

Rectal temperatures: Generally 0.4°C (0.7°F) higher than oral readings
Oral temperatures: May be lower due to mouth breathing, especially in respiratory infections
Lower-esophageal temperatures: Closely reflect core temperature
Tympanic membrane: Measure radiant heat; unadjusted-mode values are 0.8°C (1.6°F) lower than rectal temperatures; more variable than oral or rectal measurements

1.3 Clinical Significance of Temperature Elevation¶

An increase in baseline temperature of 0.15°C (1 standard deviation), after controlling for age, sex, race, vital signs, and comorbidities, translates into a 0.52% absolute increase in 1-year mortality, highlighting the prognostic importance of even minor temperature elevations.

2. EPIDEMIOLOGY¶

Fever is one of the most common presenting symptoms in clinical medicine, occurring across all age groups and in multiple disease categories. Understanding the demographic and clinical factors that affect the febrile response is essential for proper interpretation.

Table 20-1: Disease Categories That Present with Fever as a Cardinal Sign¶

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

2.1 Populations with Blunted Febrile Response¶

Certain populations may have active disease without fever due to an impaired febrile response: - Newborns - Elderly patients - Patients with chronic hepatic failure - Patients with chronic renal failure - Patients receiving glucocorticoid therapy - Patients receiving anticytokine therapy - Patients receiving anti-inflammatory agents (NSAIDs)

2.2 Disease Categories Presenting with Fever¶

Fever as a cardinal manifestation spans multiple disease processes, requiring systematic evaluation to distinguish between categories.

3. ETIOLOGY & PATHOPHYSIOLOGY¶

The pathogenesis of fever involves a complex cascade beginning with exogenous or endogenous pyrogens, leading to cytokine release, prostaglandin synthesis, and ultimately elevation of the hypothalamic thermoregulatory set point.

3.1 Pyrogens¶

Pyrogens are substances that cause fever. They are classified as: Exogenous Pyrogens: - Microbial products, toxins, or whole microorganisms (including viruses) - Classic example: Lipopolysaccharide (endotoxin) from gram-negative bacteria - Endotoxin is highly pyrogenic: 2-3 ng/kg IV produces fever, leukocytosis, acute-phase proteins, and malaise - Gram-positive cell wall components are less pyrogenic than endotoxin - Superantigens from gram-positive organisms cause fever at 1-10 µ g/kg: - Staphylococcus aureus toxic shock syndrome toxin-1 (TSST-1) - Staphylococcal enterotoxins - Streptococcal pyrogenic exotoxins

3.2 Pyrogenic Cytokines¶

Pyrogenic cytokines (formerly called endogenous pyrogens) are small proteins (10,000-20,000 Da molecular mass) that regulate immune, inflammatory, and hematopoietic processes. Major Pyrogenic Cytokines: - Interleukin-1 (IL-1): Produces fever at 10-100 ng/kg - Interleukin-6 (IL-6): Requires higher doses (1-10 µ g/kg) for fever production - Tumor Necrosis Factor (TNF): Produces fever at 10-100 ng/kg - Ciliary neurotropic factor (member of IL-6 family) - Interferon-alpha (fever is a prominent side effect of therapy) Each pyrogenic cytokine is encoded by a separate gene. Primary cell types producing pyrogenic cytokines are myeloid cells and endothelial cells. Fever can occur without microbial infection in conditions such as pericarditis, trauma, stroke, and routine immunizations that induce IL-1, TNF, and/or IL-6 production.

3.3 Mechanism of Hypothalamic Set Point Elevation¶

The key molecular mediator is prostaglandin E2 (PGE2): 1. Pyrogenic cytokines (IL-1, IL-6, TNF) are released into systemic circulation 2. Circulating cytokines interact with endothelium of circumventricular vascular organs (organum vasculosum of lamina terminalis) 3. PGE2 is synthesized and elevated in hypothalamic tissue and third cerebral ventricle 4. PGE2 binds to EP-3 receptor (essential for fever; EP-3 gene deletion abolishes fever response) 5. PGE2 receptor stimulation on glial cells causes rapid release of cyclic AMP (cAMP) 6. cAMP activates neuronal endings from the thermoregulatory center 7. Elevated cAMP changes the hypothalamic set point Note: Pyrogenic cytokines do not pass from circulation into brain tissue directly; they act via circumventricular capillary endothelium.

3.4 Physiologic Response to Elevated Set Point¶

Once the hypothalamic set point is raised: 1. Heat Conservation Phase: - Vasomotor center neurons activated - Vasoconstriction begins (first noticed in hands and feet) - Blood shunted from periphery to internal organs - Decreased heat loss from skin - Patient feels cold 2. Heat Production Phase: - Shivering increases heat production from muscles - Non-shivering heat production from liver - Behavioral adjustments (adding clothing/bedding) 3. Temperature Maintenance: - Once blood temperature matches new set point, hypothalamus maintains febrile temperature 4. Defervescence (when set point returns to normal): - Vasodilation - Sweating - Behavioral changes (removing clothing)

3.5 CNS Production of Cytokines¶

Cytokines produced within the brain may account for hyperpyrexia in CNS hemorrhage, trauma, or infection: - Viral CNS infections induce microglial and possibly neuronal production of IL-1, TNF, and IL-6 - Direct injection of cytokines into brain substance or ventricles requires concentrations several orders of magnitude lower than systemic injection to cause fever - CNS cytokines can raise the hypothalamic set point bypassing circumventricular organs

4. CLINICAL FEATURES¶

The clinical presentation of fever involves characteristic symptoms and signs related to the thermoregulatory response, as well as manifestations of the underlying disease process.

4.1 Typical Fever Presentation¶

Most fevers: Body temperature increases by 1-2°C
Vasoconstriction: Cold extremities (hands and feet)
Chills and rigors: Shivering to increase heat production
Myalgias and arthralgias: Due to peripheral PGE2 production
Malaise and systemic symptoms

4.2 Hyperpyrexia¶

Definition: Fever >41.5°C (>106.7°F) Causes: - Most commonly: CNS hemorrhages - Severe infections - CNS trauma or tumor Note: In the preantibiotic era, infectious fever rarely exceeded 106°F, suggesting a natural 'thermal ceiling' mediated by neuropeptides functioning as central antipyretics.

4.3 Hypothalamic Fever¶

Elevated temperature caused by abnormal hypothalamic function due to: - Local trauma - Hemorrhage - Tumor - Intrinsic hypothalamic malfunction Important: Most patients with hypothalamic damage have subnormal, not supranormal, body temperatures.

4.4 Characteristic Fever Patterns¶

Certain infections and conditions have distinctive fever patterns: - Plasmodium vivax malaria: Fever every third day (tertian) - Plasmodium malariae: Fever every fourth day (quartan) - Borrelia infection (relapsing fever): Days of fever followed by afebrile period, then relapse - Pel-Ebstein pattern (Hodgkin disease, lymphomas): Fever lasting 3-10 days followed by afebrile periods of 3-10 days - Cyclic neutropenia: Fevers every 21 days accompanying neutropenia - Periodic fever syndromes: Familial Mediterranean fever, TRAPS (varying periodicity and clinical features)

4.5 Temperature-Pulse Dissociation (Relative Bradycardia)¶

Occurs in: - Typhoid fever - Brucellosis - Leptospirosis - Some drug-induced fevers - Factitious fever

5. DIFFERENTIAL DIAGNOSIS¶

The critical differential is between fever (elevated hypothalamic set point) and hyperthermia (uncontrolled heat gain without set point change).

5.1 Fever vs Hyperthermia¶

FEVER: - Hypothalamic set point is elevated - Involves pyrogenic molecules (cytokines, PGE2) - Skin may be cold due to vasoconstriction - Responds to antipyretics - Rarely exceeds 41.5°C except in CNS disease HYPERTHERMIA: - Hypothalamic set point unchanged - Does NOT involve pyrogenic molecules - Results from: - Exogenous heat exposure - Endogenous heat production exceeding dissipation capacity - Drugs interfering with thermoregulation - Skin is hot but DRY (in heat stroke, drugs blocking sweating) - Does NOT respond to antipyretics - Can be rapidly fatal Important: Distinguishing fever from hyperthermia can be difficult in emergencies. Consider events immediately preceding temperature elevation (heat exposure, drug treatment).

5.2 Causes of Fever by Category¶

Infectious Diseases: - Bacterial, viral, fungal, parasitic infections - Most common cause of fever Autoimmune and Noninfectious Inflammatory Disorders: - Autoimmune diseases (fever at some point in most) - Autoinflammatory diseases (recurrent fevers characteristic) - Adult and juvenile Still disease - Familial Mediterranean fever - Hyper-IgD syndrome - Idiopathic pericarditis - Gout - Inflammasome disorders (NLRP3, pyrin) Cancer: - Particularly lymphomas, leukemias - Associated with 0.02°C higher baseline temperature Medication Related: - Vaccines - Drug fever - Interferon-alpha therapy Endocrine Disorders: - Hyperthyroidism Intrinsic Hypothalamic Malfunction: - Rare; usually causes subnormal temperatures

6. INVESTIGATIONS & DIAGNOSIS¶

A systematic approach to diagnosis includes thorough history, physical examination, and targeted laboratory evaluation.

6.1 History Taking¶

Key elements: - Chronology of events preceding fever - Exposure to symptomatic individuals - Exposure to disease vectors - Travel history - Medication history (including anticytokine therapy, glucocorticoids, NSAIDs) - Underlying medical conditions

6.2 Temperature Measurement¶

Use electronic devices for oral, tympanic membrane, or rectal temperatures
Use the same site consistently to monitor febrile disease
Be aware of populations with blunted febrile response

6.3 Laboratory Tests¶

Complete Blood Count with Differential: - Manual differential or instrument sensitive to: - Neutrophil juvenile/band forms - Toxic granulations - Döhle bodies (suggestive of bacterial infection) - Neutropenia may indicate viral infection - Leukocytosis with absolute neutrophilia often due to IL-1 and IL-6 Inflammatory Markers (most valuable for low-grade fever or suspected occult disease): - C-reactive protein (CRP): Remains elevated during febrile disease - Erythrocyte sedimentation rate (ESR) - IL-6 levels (induces CRP; levels may vary during febrile disease) Note: Measuring circulating IL-1 or TNF is NOT helpful - levels often below detection limit or do not coincide with fever.

6.4 Special Considerations in Anticytokine Therapy¶

Patients on anticytokine therapy (anti-TNF, anti-IL-1, anti-IL-6, anti-IL-12/23, anti-IL-17) for Crohn disease, rheumatoid arthritis, or psoriasis: - Increased risk of infection due to lowered host defenses - Risk of latent TB dissemination (especially with anti-TNF) - Risk of systemic candidiasis (especially with anti-IL-17) - Febrile response may be blunted - Low-grade fever warrants early and rigorous diagnostic evaluation

7. MANAGEMENT & TREATMENT¶

Treatment of fever involves both reducing the elevated hypothalamic set point and facilitating heat loss, while also treating the underlying cause.

Comparison of Antipyretic Agents¶

Agent	Mechanism	Advantages	Disadvantages	Special Considerations
Acetaminophen	Inhibits brain cyclooxygenase via P450 oxidation; may inhibit COX-3	No platelet effects; No GI toxicity; Effective oral antipyretic	Hepatotoxicity with overdose	Preferred agent; Safe in children
Aspirin	Cyclooxygenase inhibition (peripheral and central)	Effective antipyretic; Anti-inflammatory	Platelet dysfunction; GI toxicity	AVOID in children (Reye syndrome risk)
Ibuprofen/NSAIDs	Cyclooxygenase inhibition; May reduce IL-1-induced IL-6	Effective antipyretic; Anti-inflammatory	Platelet dysfunction; GI toxicity	Safe oral option for children
COX-2 Inhibitors	Selective COX-2 inhibition; May reduce IL-6 production	Excellent antipyretic; Less GI toxicity	Cardiovascular risk with chronic use	Consider in patients with GI risk
Glucocorticoids	Inhibit phospholipase A2; Block pyrogenic cytokine mRNA transcription	Dual mechanism; Potent anti-inflammatory	Multiple systemic side effects; Infection risk	Not first-line for fever alone

7.1 Decision to Treat Fever¶

Key Principle: Fever itself is not an illness but an ordinary response to a perturbation of normal host physiology. Most fevers are associated with self-limited infections (e.g., common viral diseases). Evidence regarding antipyretic use: - No significant clinical evidence that antipyretics delay resolution of viral or bacterial infections - No evidence that fever facilitates recovery or acts as immune adjuvant - Treatment of fever with routine antipyretics does no harm and does not slow resolution of common infections Situations where withholding antipyretics may be helpful: - Evaluating effectiveness of antibiotic therapy in bacterial infections (especially without positive cultures) - Facilitating diagnosis of unusual febrile diseases - Detecting temperature-pulse dissociation patterns

7.2 Indications for Treating Fever¶

Treatment of fever is highly recommended in patients where: - Increased oxygen demand is detrimental (for every 1°C increase over 37°C, there is a 13% increase in oxygen consumption) - Cardiovascular or pulmonary compromise exists - Neurological conditions where fever may worsen outcomes

7.3 Mechanisms of Antipyretic Agents¶

Antipyretics work by reducing PGE2 synthesis in the thermoregulatory center: Cyclooxygenase Inhibitors: - Reduce PGE2 synthesis by inhibiting cyclooxygenase - Antipyretic potency directly correlates with brain cyclooxygenase inhibition - Rate-limiting step: Release of arachidonic acid from cell membrane Acetaminophen: - Poor cyclooxygenase inhibitor in peripheral tissue - Lacks notable anti-inflammatory activity peripherally - In brain: Oxidized by P450 cytochrome system; oxidized form inhibits cyclooxygenase - May also inhibit COX-3 (found only in CNS) Glucocorticoids (act at two levels): - Reduce PGE2 synthesis by inhibiting phospholipase A2 (needed to release arachidonic acid) - Block transcription of mRNA for pyrogenic cytokines NSAIDs: - Ibuprofen and COX-2 inhibitors may reduce IL-1-induced IL-6 production - This may contribute to their antipyretic activity Important: Chronic high-dose therapy with antipyretics does NOT reduce normal core body temperature (PGE2 plays no role in normal thermoregulation).

7.4 Antipyretic Drug Selection¶

PREFERRED AGENT: Acetaminophen - Effective oral antipyretic - Does not adversely affect platelets - No gastrointestinal adverse effects ASPIRIN AND NSAIDs: - Equally effective to acetaminophen in reducing fever - Adverse effects: Platelet dysfunction, gastrointestinal toxicity PEDIATRIC USE: - Acetaminophen OR oral ibuprofen - AVOID ASPIRIN: Increases risk of Reye syndrome with certain viral infections ALTERNATIVE ROUTES (if patient cannot take oral): - Parenteral preparations of NSAIDs - Rectal suppositories of various antipyretics

7.5 Anticytokine Therapy for Autoinflammatory Diseases¶

Autoinflammatory diseases characterized by recurrent fevers respond to IL-1 blockade: Diseases responding to IL-1 blockade: - Familial Mediterranean fever - Adult and juvenile Still disease - Hyper-IgD syndrome - Idiopathic pericarditis - Gout - NLRP3 inflammasome disorders - Pyrin disorders IL-1 blocking agents: - Anakinra (IL-1 receptor antagonist) - Canakinumab (anti-IL-1 β monoclonal antibody) Note: Although these fevers are mediated by IL-1 β , patients also respond to conventional antipyretics.

7.6 Key Distinction: Fever vs Hyperthermia Treatment¶

CRITICAL: Antipyretics do NOT reduce elevated temperature in hyperthermia. In fever (even hyperpyrexia): - Adequate doses of aspirin or acetaminophen usually result in some decrease in body temperature In hyperthermia: - Antipyretics are ineffective - Hyperthermia can be rapidly fatal - Requires physical cooling measures and treatment of underlying cause

8. PROGNOSIS & COMPLICATIONS¶

The prognosis of fever depends entirely on the underlying cause. However, the fever itself has metabolic and physiologic consequences that must be considered.

8.1 Metabolic Consequences of Fever¶

Increased oxygen demand: 13% increase in oxygen consumption for every 1°C above 37°C
Increased metabolic rate
Fluid losses from sweating
Potential cardiovascular stress

8.2 Prognostic Significance of Temperature¶

An increase in baseline temperature of 0.15°C is associated with 0.52% absolute increase in 1-year mortality
Hyperpyrexia (>41.5°C) is a medical emergency requiring immediate evaluation and treatment
Hypothermia can develop in patients with septic shock (poor prognostic sign)

8.3 Complications of Hyperthermia (Not Fever)¶

Hyperthermia can be rapidly fatal and represents a medical emergency: - Heat stroke syndromes - Rhabdomyolysis - Multi-organ dysfunction - Death Note: These complications relate to hyperthermia, not to fever with an elevated set point.

9. SPECIAL CONSIDERATIONS¶

Certain patient populations and clinical situations require modified approaches to fever evaluation and management.

9.1 Populations with Blunted Febrile Response¶

These patients may have serious infections without fever: - Newborns - Elderly patients - Chronic hepatic failure - Chronic renal failure - Patients on glucocorticoid therapy - Patients on anticytokine therapy - Patients on chronic NSAID therapy

9.2 Patients on Anticytokine Therapy¶

Specific risks by agent: - Anti-TNF therapy: Risk of latent Mycobacterium tuberculosis dissemination - Anti-IL-17 (psoriasis): Increased risk of systemic candidiasis - All anticytokine agents: Increased risk of routine bacterial and opportunistic infections Management: - Low-grade fever warrants early and rigorous diagnostic evaluation - Fever is often a presenting sign in reported infection cases - Extent of febrile response blunting is unknown

9.3 Pediatric Considerations¶

Use acetaminophen or oral ibuprofen
NEVER use aspirin due to Reye syndrome risk with viral infections
Be vigilant in newborns who may not mount fever despite serious infection

9.4 Pregnancy¶

Acetaminophen is generally considered safe
Avoid NSAIDs, especially in third trimester
Fever evaluation should proceed urgently due to fetal risks

10. KEY POINTS & CLINICAL PEARLS¶

Essential concepts for clinical practice regarding fever evaluation and management.

Quick Reference: Fever vs Hyperthermia¶

Feature	Fever	Hyperthermia
Hypothalamic set point	Elevated	Normal (unchanged)
Pyrogenic molecules involved	Yes (IL-1, IL-6, TNF, PGE2)	No
Skin characteristics	May be cold (vasoconstriction)	Hot and DRY
Response to antipyretics	Temperature decreases	NO response
Typical maximum	Rarely exceeds 41.5°C	Can exceed 41.5°C rapidly
Urgency	Depends on cause	Medical emergency
Treatment approach	Antipyretics effective	Physical cooling required

10.1 Diagnostic Pearls¶

Temperature >37.7°C (>99.9°F) defines fever (99th percentile for healthy adults)
Rectal temperatures are ~0.4°C higher than oral readings
Temperature-pulse dissociation suggests typhoid, brucellosis, leptospirosis, drug fever, or factitious fever
CRP and ESR are most valuable for detecting occult disease in low-grade fever
Measuring circulating IL-1 or TNF is NOT helpful clinically

10.2 Pathophysiology Pearls¶

PGE2 acting on EP-3 receptors is essential for fever generation
Endotoxin causes fever at doses as low as 2-3 ng/kg IV
Superantigens cause fever at 1-10 µ g/kg
Cytokines do not cross into brain tissue directly; they act via circumventricular endothelium
Most patients with hypothalamic damage have LOW, not high, temperatures

10.3 Treatment Pearls¶

Fever itself is not harmful; no evidence that treating fever delays infection resolution
Acetaminophen is preferred antipyretic (no platelet or GI effects)
Antipyretics work ONLY in fever, NOT in hyperthermia
If antipyretics don't reduce temperature, consider hyperthermia
Chronic high-dose antipyretics do not reduce normal body temperature
For every 1°C above 37°C, oxygen consumption increases 13%

10.4 Red Flags¶

Hyperpyrexia (>41.5°C): Usually CNS hemorrhage or severe infection - urgent evaluation required
Hot, dry skin with high temperature: Think hyperthermia, not fever
Fever not responding to antipyretics: Consider hyperthermia
Low-grade fever in anticytokine therapy patients: Conduct rigorous diagnostic evaluation
Hypothermia in suspected infection: May indicate septic shock