Harrison defination of fever
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Harrison's Definition of Fever
(Harrison's Principles of Internal Medicine, 22nd Edition, 2025 - Chapter on Fever)
Definition
"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 (e.g., from 37°C to 39°C)."
Based on a large study of 35,000 individuals ≥18 years of age, the mean oral temperature is 36.6°C (95% CI: 35.7-37.3°C). A temperature of >37.7°C (>99.9°F), representing the 99th percentile for healthy individuals, is used to define fever.
For clinical purposes (especially fever of unknown origin), the widely used threshold is ≥38.3°C (≥101°F) on multiple occasions.
Normal Temperature Ranges
| Site | Normal Value | Notes |
|---|---|---|
| Oral (mean) | 36.6°C (97.9°F) | Most common measurement |
| Rectal | ~0.4°C higher than oral | Best reflects core temperature |
| Tympanic (adjusted mode) | ~0.8°C higher than oral | Variable readings |
Fever vs. Hyperthermia - Key Distinction
Harrison's makes a critical distinction:
- Fever: Temperature elevation WITH a raised hypothalamic set point (driven by pyrogens). The thermoregulatory mechanism is intact but reset to a higher level.
- Hyperthermia: Temperature elevation WITHOUT a raised set point - the thermoregulatory mechanism is overwhelmed or defective (e.g., heat stroke, malignant hyperthermia, neuroleptic malignant syndrome). Antipyretics do NOT work in hyperthermia.
Mechanism of Fever (Pathogenesis)
Once the hypothalamic set point is raised:
- Vasoconstriction begins - blood is shunted from the periphery to internal organs, reducing heat loss. The patient feels cold.
- Shivering increases muscle heat production.
- Nonshivering thermogenesis from the liver also raises core temperature.
- Behavioral adjustments (adding clothing/blankets) help conserve heat.
These processes continue until blood temperature matches the new "thermostat setting." When the set point resets back to normal (fever breaks), vasodilation and sweating begin to dissipate the excess heat - the patient feels hot and sweats profusely.
Factors Affecting Baseline Temperature
- Diurnal variation: Lower at 8 AM, higher at 4 PM
- Seasonal variation: Lower in summer, higher in winter
- Age: Decreases by 0.02°C per 10-year increase in age
- Demographics: African-American women have slightly higher baseline temperatures
- Comorbidities: Cancer raises it slightly; hypothyroidism lowers it
- Menstrual cycle: Temperature rises ~0.6°C (1°F) at ovulation and stays elevated until menses
Clinical Significance
An increase in baseline temperature of 0.15°C (1 standard deviation) is associated with a 0.52% absolute increase in 1-year mortality after controlling for age, sex, race, vital signs, and comorbidities.
Source: Harrison's Principles of Internal Medicine, 22nd Edition (2025), Chapter on Fever - p. 177
Intermittent fever
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Intermittent Fever
Definition
Intermittent fever is a type of fever pattern in which the temperature rises significantly above normal and then returns completely to normal (afebrile level) at least once every 24 hours. The fever is present only for a few hours during the day.
This contrasts with:
- Continued (sustained) fever: Does not fluctuate more than 1°C during 24 hours, and never touches normal (e.g., typhoid, lobar pneumonia)
- Remittent fever: Daily fluctuations exceed 2°C but temperature never returns to normal (e.g., infective endocarditis, viral infections)
(S Das Manual on Clinical Surgery; Harrison's Principles of Internal Medicine 22E)
Subtypes of Intermittent Fever
| Subtype | Pattern | Interval | Classic Cause |
|---|---|---|---|
| Quotidian | Paroxysm every day | 24 hours | P. falciparum (early), P. knowlesi, Still's disease |
| Tertian | Paroxysm every other day | 48 hours | P. vivax, P. ovale (benign tertian); P. falciparum (malignant tertian) |
| Quartan | Two days between attacks | 72 hours | P. malariae |
"When a paroxysm of intermittent fever occurs daily, it is called quotidian; when on alternate days it is called tertian; and when two days intervene between consecutive attacks, it is called quartan."
- S Das, Manual on Clinical Surgery
Mechanism - Why is Malaria Fever Intermittent?
Harrison's explains the mechanism precisely:
"Temperatures of ≥40°C (≥104°F) damage mature parasites; in untreated infections, the effect of such temperatures is to further synchronize the parasitic cycle, with eventual production of the regular fever spikes and rigors that originally characterized the different malarias. These regular fever patterns (quotidian, daily; tertian, every 2 days; quartan, every 3 days) are seldom seen today as patients receive prompt and effective antimalarial treatment."
- Harrison's Principles of Internal Medicine 22E, Chapter on Malaria
The fever coincides with rupture of RBCs as schizonts mature and burst, releasing merozoites, hemozoin, and parasite debris, which activate macrophages and trigger cytokine release (TNF, IL-1, IL-6), causing the febrile paroxysm.
Classic Features of a Malarial (Intermittent) Paroxysm
The fever spike in intermittent fever classically has three stages:
- Cold stage (Rigor): Intense chills, shivering, blanching - lasts 15-60 minutes
- Hot stage: High fever (40-41°C), flushed skin, headache, vomiting - lasts 2-6 hours
- Sweating stage: Profuse sweating, rapid defervescence back to normal - patient feels exhausted
Other Causes of Intermittent Fever
Beyond malaria, intermittent fever patterns are seen in:
- Pyogenic abscesses (liver, subphrenic)
- Still's disease (Adult-onset): Double-quotidian pattern (two spikes per day), with evanescent salmon-pink rash
- Familial Mediterranean Fever: Discrete self-limited episodic attacks (not truly rhythmic like malaria)
- Lymphoma (Pel-Ebstein fever - an extreme variant with weeks of fever alternating with afebrile weeks)
- Brucellosis: Undulating or intermittent pattern
- Cholangitis (Charcot's triad)
Hectic (Septic/Swinging) Fever
A special extreme form of intermittent fever with very wide swings (often >2°C), temperature rising to 40-41°C with rigors then crashing to subnormal levels with drenching sweats. Seen in:
- Pyogenic abscesses
- Septicemia / bacteremia
- Miliary tuberculosis
- Infective endocarditis
Sources: Harrison's Principles of Internal Medicine 22E (2025); S Das Manual on Clinical Surgery 13th Ed; Medical Microbiology 9e; Goldman-Cecil Medicine
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