Fever and hyperthermia

“Humanity has but three great enemies: fever, famine and war; of these by far the greatest, by far the most terrible, is fever.” William Osler, from his address to the 47th annual meeting of the American Medical Association, 1896

Fever is defined as an increase in body temperature as a result of a pathophysiologic response that increases the body’s normal thermoregulatory set point. This set point varies with age, gender, the time of the day, and other factors. Although there is no universally agreed-upon definition of what constitutes a febrile temperature, a threshold of ≥38.3°C is perhaps the most commonly applied. Whatever threshold is used, it is important to remember that body temperature varies depending on the route by which it is measured. Because peripheral temperature measurements such as via the axilla or elsewhere on the skin can underestimate core temperature by 1°C or more, continuous monitoring of core temperature is recommended in patients at risk of life-threatening temperature elevations and when precise control of temperature is desirable.

In clinical practice, fever is not always readily distinguishable from hyperthermia. Hyperthermia occurs when the body temperature is elevated but the thermoregulatory set point is normal. It is the result of environmental exposure to heat, increased heat generation, impaired heat loss, or a combination of these, and it implies that normal homeostatic thermoregulatory mechanisms have been overwhelmed. Elevated body temperature, caused by either fever or hyperthermia, is commonly encountered in patients who are in the intensive care unit (ICU). Although infection must always be at the front of one’s mind when an elevated body temperature is encountered, there is also a broad differential diagnosis of noninfectious etiologies to consider ( Box 4.1 ). Although many patients develop a fever during the course of an ICU admission, persistent fever is uncommon, except in patients with neurologic conditions. In patients who do not have such conditions, fever typically abates within a day or two unless there is an undrained collection of pus. There are exceptions, and a few of these are notable. High fevers that persist for days or weeks are often a feature of influenza pneumonitis and are almost invariably encountered in patients with severe necrotizing pancreatitis. Among patients with a range of neurologic conditions, including subarachnoid hemorrhage, stroke, and traumatic brain injury, fever is particularly common and often persistent. In young patients with severe traumatic brain injury, persistent high fevers, tachycardia, and hypertension sometimes present a difficult management problem in the weeks after the primary brain injury. One uncommon neurologic condition where persistently elevated body temperature is routinely encountered is anti–N-methyl-D-aspartate (NMDA) receptor autoantibody encephalitis. This condition warrants specific mention because patients with it often need prolonged intensive care, and potentially life-threatening body temperature elevation is sometimes a feature of the disease.