The mechanism of the development of fever – pathogenesis
An increase in body temperature is a common clinical symptom, it is noted in a wide variety of diseases and occurs in different ways. Understanding the basics of pathophysiology is important for the rational treatment of children with high body temperature. There are two types of increase in body temperature – fever and heat diseases. Fever is a condition in which the center of thermoregulation tends to increase body temperature. Thermal diseases are those conditions in which the body temperature rises as a result of the action of internal or external factors, contrary to attempts by the thermoregulation center to maintain a normal temperature.
Fever-like thermoregulatory reaction of the body
In clinical practice, the most common mechanism for increasing body temperature is an increase in the reference temperature of the “set point” located in the preoptic region of the anterior part of the hypothalamus (PHP). Under these conditions, the “set point” is adjusted to a higher than normal temperature and perceives the existing temperature of the “core” as very low, although in fact it is normal. “Set point” gives commands to increase body temperature by increasing the activity of the heat production center and reducing the activity of the heat transfer center.
Strictly speaking, only such a state in which the center of thermoregulation itself actively tends to a higher than normal body temperature, has the right to be called fever. All other conditions that occur with fever should be correctly referred to as fever (it is also legitimate to use the terms: pyrexia, hyperpyretic, hyperthermia, or thermal illness). Although at present all types of fever, often quite inaccurately, are termed fever, in this book we will distinguish between fever and other types of hyperthermia.
In fever, an incorrectly programmed “set point” causes the hypothalamic prepotic region to function as if the febrile temperature was normal. At the same time, a new equilibrium is achieved between heat production and heat transfer already against a background of increased body temperature. Thermoregulatory control remains effective, but at a higher than normal level. In dogs with fever, there is an excessive thermogenic response to cold and a reduced ther’molitic response to heat.
Animals react to changes in the ambient temperature in such a way as to keep the body in balance with the new temperature of the “set point”.
Cooper described several interesting patients with baseline hypothermia caused by various CNS diseases or unknown causes. It is possible that in these patients the mechanism of thermoregulation was not damaged, but the “set point” temperature was lower than normal. With the introduction of pyrogen, they noted an increase in body temperature. The degree of this increase was the same as in patients with a normal baseline, but the temperature, of course, did not reach the same absolute values.
Lipton reported a patient with CNS sarcoidosis and profound hypothermia. This patient retained the ability to develop fever, which indicates the independence of the body temperature control mechanism from the mechanism of the onset of fever. Stitt stressed that the increase in body temperature during fever is a centrally adjustable rise, which is completely guided, controlled and protected by functional thermoregulatory mechanisms.