The term pyrexia refers to a feverish condition and conversely, apyrexia is synonymous with the absence of fever. The fever is defined by an elevation of body temperature above 38 °C. Between 37.7 °C and 37.9 °C, it is a fever. The temperature can be taken at the rectal level, the most reliable measurement, but also at the armpit (add 0.5 °C) and at the oral level (add 0.4 °C). A distinction must be made between pyrexia and hyperthermia, which cause an increase in body temperature, but result from different mechanisms. Pyrexia is a pathological phenomenon triggered in certain contexts by substances called pyrogens.
What is thermoregulation?
Body temperature is kept constant around 37 °C by a thermoregulatory mechanism. There are variations in body temperature of around 0.5 °C throughout the day with a low around 3 am and a high around 6 pm
The seat of thermoregulation is located in the anterior hypothalamus, a gland located at the base of the brain. There are thermoreceptors in the body which constantly analyze the temperature in the bloodstream and inform the hypothalamus.
When the temperature exceeds the set point, the hypothalamus sends signals promoting thermolysis (heat loss). This results in vasodilation of the arterioles, a deviation of the deep venous return to the superficial venous network, and the production of sweat .
When the temperature is below the threshold, the hypothalamus stimulates thermogenesis (heat production) which results in chills and vasoconstriction of the arterioles.
Pyrexia and hyperthermia
What is hyperthermia?
Hyperthermia refers to an increase in body temperature due to disturbance of thermoregulation. It is induced by an increase in thermogenesis or by a decrease in thermolysis, following intense physical exercise, heat stroke.
There are rare but serious situations of hyperthermia, following general anesthesia (malignant hyperthermia), or even triggered by medication (neuroleptic malignant syndrome).
What is pyrexia?
Pyrexia is caused by pyrogenic substances released by cells or germs during states of inflammation. Pyrexia is said to be a defense system intended to inhibit bacterial growth.
Pyrogens act on the center of thermoregulation and cause an upward shift in the point of thermal equilibrium: the body is perceived as too cold, which activates thermogenesis (muscle chills, vasoconstriction). When the fever subsides, the set point returns to normal, the body is too hot which triggers thermolysis (vasodilation, sweating ).
Causes of pyrexia
Pyrexia: infectious causes
The fever very often results from a viral infection, generally benign ( tonsillitis, nasopharyngitis, childhood illnesses, bronchitis ). Other infectious causes include:
- Bacterial infections can affect all organs: nervous system, mouth and respiratory tract, digestive tract, urinary tract, skin, etc. A fever in a person with a prosthesis raises concerns about an infection on the prosthesis, just as in a person with a prosthetic heart valve an infection called infective endocarditis is suspected.
- Parasitic infections (malaria after travel).
- Fungal infections (fungi) in immunocompromised people.
Non-infectious causes of pyrexia
Some of the non-infectious causes include:
- vascular causes such as venous thromboembolic diseases ( phlebitis, pulmonary embolism ) and infarctions;
- certain inflammatory diseases: lupus erythematosus, rheumatoid arthritis, chronic inflammatory bowel disease;
- cancers, hyperthyroidism, certain allergic reactions as well as vaccinations.
What are the risks in case of fever?
The fever itself can cause dehydration or even convulsions in young children, especially above 40 °C, or even organ failure above 41-42 °C.
Complications in case of pyrexia are also related to the disease in question. When the origin is bacterial, the germ can, in fragile people (young children, the elderly, immunosuppression), spread to the bloodstream ( sepsis ) and, in the most serious cases, cause septic shock.
In young children, the risk of severe bacterial infection is higher if they are less than 3 months old and particularly before 6 weeks of age. A fever associated with purpura (red or purplish skin patches) in a young child raises concerns about an invasive meningococcal infection and is a life-threatening emergency.
In case of pyrexia, while waiting to consult the doctor, take care to lower the body temperature:
- physical measures: undressing, stopping the heating, ventilating the room, using wet towels or lukewarm water baths;
- drinks in abundance;
- antipyretic treatment ( Paracetamol ).
The doctor looks for the cause of the fever and prescribes an appropriate treatment. In case of viral infection, treatment is often symptomatic, while for bacterial infection antibiotic treatment is required.
When there are signs of seriousness (signs of sepsis, immunocompromised person, elderly person, infant under 3 months), hospitalization is necessary in order to set up appropriate treatment and monitoring.