Concept of Fever and Principles of Classification
Fever is a typical pathological process characterized by the resetting of thermoregulation and an increase in body temperature in response to pyrogenic factors. In clinical practice, fevers are classified by several criteria: degree of temperature elevation, duration, nature of the temperature curve, and etiology.
The most practically significant is the classification by degree of body temperature elevation, measured in the axilla of an adult patient at rest. This specific criterion determines patient management tactics, the need for antipyretic therapy, and the prognosis.
Low-Grade Fever
Low-grade fever (subfebrile fever) is an increase in body temperature ranging from 37.1 to 38.0 °C. This type of fever often accompanies chronic inflammatory processes, dormant infection foci, endocrine disorders (e.g., thyrotoxicosis), oncological diseases, tuberculosis, and rheumatic diseases.
Prolonged low-grade fever of unknown origin requires thorough examination: taking a history, physical examination, complete blood count (CBC), biochemical tests, and chest radiography. Low-grade fever is often unaccompanied by marked malaise and may be detected incidentally during thermometry.
Febrile Fever
Febrile (moderate) fever is characterized by a temperature increase within the range of 38.1–39.0 °C. This is the most common variant of febrile reaction in acute bacterial and viral infections, pneumonia, pyelonephritis, tonsillitis, and influenza.
Clinically, febrile fever manifests as chills, headache, myalgia, tachycardia, increased respiratory rate, and a feeling of being unwell (malaise). Concomitant changes in the CBC (leukocytosis with a left shift, accelerated ESR) help determine the infectious-inflammatory nature of the condition.
Pyretic and Hyperpyretic Fevers
Pyretic (high) fever is a temperature elevation up to 39.1–41.0 °C. This temperature level is typical for severe bacterial infections, septic conditions, malaria, lobar pneumonia, and typhus. Pyretic fever is accompanied by pronounced intoxication and altered consciousness; febrile seizures are possible, especially in children.
Hyperpyretic fever is a temperature elevation above 41.0 °C. This is a life-threatening condition involving protein denaturation, cerebral edema, and possible multiple organ dysfunction syndrome. Hyperpyretic fever is most commonly observed in meningitis, severe sepsis, hypothalamic hemorrhages, and malignant hyperthermia.
- Low-grade (subfebrile): 37.1–38.0 °C
- Febrile (moderate): 38.1–39.0 °C
- Pyretic (high): 39.1–41.0 °C
- Hyperpyretic (excessive): above 41.0 °C
Hectic Fever
Hectic (exhausting) fever belongs to the classification by the nature of the temperature curve and is characterized by daily temperature fluctuations within 3–5 °C with sharp spikes and rapid drops, frequently accompanied by drenching sweats and chills.
This type of fever is most characteristic of sepsis, severe purulent processes, abscesses, pleural empyema, generalized tuberculosis, and Hodgkin's lymphoma. Hectic fever severely depletes the patient: rapid weight loss, weakness, dehydration, and cardiovascular dysfunction are observed.
Clinical Significance and Examination Tactics
Determining the type of fever begins with correct thermometry: measurements are taken in the axilla twice a day (in the morning at 7–9 AM and in the evening at 5–7 PM), and if necessary, every 2–3 hours to plot a temperature chart. Analysis of the temperature curve combined with history, auscultation, percussion, and laboratory tests allows for a presumptive nosological diagnosis.
The degree of temperature elevation dictates emergency tactics: in adults with low-grade and moderate febrile fever, antipyretics are generally not indicated if general well-being is satisfactory. Pyretic and hyperpyretic fevers require active antipyretic therapy, physical cooling methods, and hospitalization to determine the underlying cause.
Key points
- The classification of fevers by the degree of temperature elevation includes four main grades: low-grade (subfebrile), febrile, pyretic, and hyperpyretic.
- Low-grade fever (37.1–38.0 °C) frequently accompanies chronic infections, tuberculosis, thyrotoxicosis, and oncopathology.
- Febrile fever (38.1–39.0 °C) is the most typical response to acute infectious processes.
- Pyretic fever (39.1–41.0 °C) is characteristic of severe infections and sepsis, and requires active therapy.
- Hyperpyretic fever (above 41.0 °C) is life-threatening due to protein denaturation and cerebral edema.
- Hectic fever is characterized by daily fluctuations of 3–5 °C and is typical for sepsis and purulent processes.
- Patient management tactics are determined by the temperature level, general condition, and results of a comprehensive examination.