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Fever in Adults

Fever in Adults Overview

A fever (also termed pyrexia) is a higher-than-normal body temperature. It is a symptom caused by a variety of illnesses.

Every one of us has experienced the wave of chills and exhaustion that a fever causes. Fever usually occurs in response to an infection as with the flu virus or inflammation that occurs with tissue injury or disease. However, many other causes of fever are possible, including drugs, poisons, cancer, heat exposure, injuries or abnormalities to the brain, or disease of the endocrine (hormonal or glandular) system.

A fever rarely comes without other symptoms. It is often accompanied by specific complaints, which may help to identify the illness causing the fever. This can help the doctor determine which treatment is necessary.

  • Normal body temperature can vary depending on the individual, the time of day, and even the weather. For most people, a temperature of 98.6 F (37 C) is baseline.
  • Temperature is usually controlled by the part of the brain called the hypothalamus. The hypothalamus is like a thermostat for the body. It maintains normal temperature through heating mechanisms, such as shivering and increased metabolism, and cooling mechanisms, such as sweating and dilating (opening) blood vessels close to the skin.
  • Fever occurs when the body's immune response is triggered by pyrogens (fever-producing substances). Pyrogens usually come from a source outside the body and, in turn, stimulate the production of additional pyrogens inside the body. Pyrogens tell the hypothalamus to increase the temperature set point. In response, our body begins to shiver; our blood vessels constrict (close); we get under the covers in an attempt to reach the new temperature that is higher than our baseline. However, other pyrogens can be produced by the body, usually in response to inflammation; these are referred to as cytokines (also termed endogenous pyrogens).
    • Pyrogens (fever-producing substances) that occur outside the body include the following:
      • Viruses
      • Bacteria
      • Fungi
      • Drugs
      • Toxins

Body temperature measurements are usually measured by temperature devices inserted on or into the rectum, mouth, axilla (under the armpit), skin, or ear. Some devices (laryngoscopes, bronchoscopes, rectal probes) may have temperature-sensing probes that can record temperature continually. The most common way to measure body temperature was (and still is in many countries) with a mercury thermometer; because of glass breakage and the possibility of subsequent mercury contamination, many developed countries use digital thermometers with disposable probe covers to measure temperature from all of the body sites listed above. Disposable temperature-sensitive strips that measure skin temperature are also used. Oral temperatures are most commonly measured in adults, but rectal temperatures are the most accurate because environmental factors that increase or decrease temperature measurements have the least effect on the rectal area. Rectal temperatures, when compared to oral temperatures taken at the same time, are about 1.8 F (0.6 C) higher. Consequently, an accurate measurement of body temperature (best is rectal core temperature) of 100.4 F (38 C) or above is considered to be a "fever."

Low-grade fevers range from about 100 F-101 F while high-grade fevers range from about 103 F-104 F. Dangerous temperatures are high-grade fevers that range from over 104 F-107 F or higher (extremely high fevers are also termed hyperpyrexia). The preceding fever values may vary somewhat according to different clinicians and the condition and age of the patient, but they offer a reader a way to judge the terms "low," "high," and "dangerous" when they are used in reference to fever in the medical literature.

Other terms are used to describe fever or fever types:

  • Prolonged fever is fever lasting longer than about 10-14 days.
  • Constant fever is also termed continuous fever; it is usually low-grade fever and does not change by much (by about 1 degree F over 24 hours).
  • Chronic: Fever lasts longer than three to four days; some researchers consider intermittent fevers that recur over months to years as "chronic" fevers.
  • Intermittent: Temperature either varies from normal to fever levels during a single day or fever may occur one day and recur in about one to three days.
  • Remittent: Fevers come and go at regular intervals
  • Hyperpyrexia: Fevers that are equal to or above 106.7 F that constitute a medical emergency.

In addition, there are well over 40 diseases that have "fever" as part of the disease name (for example, rheumatic fever, scarlet fever, cat scratch fever, Lassa fever, and many more). Each disease has fever as one of its symptoms; countless other conditions may have fever as a symptom.

Cytokines or endogenous (body-generated) pyrogens can cause many of the same features mentioned above. Cytokine release is triggered by inflammation and many immune-mediated diseases. People may have both infectious (also termed exogenous) pyrogens and cytokines generating fevers at the same time, depending on their disease processes. The major cytokines involved in fever generation are interleukins 1 and 6 along with tumor necrosis factor (TNF)-alpha.

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Read What Your Physician is Reading on Medscape

Fever of Unknown Origin »

Fever of unknown origin (FUO) was defined in 1961 by Petersdorf and Beeson as the following: (1) a temperature greater than 38.3°C (101°F) on several occasions, (2) more than 3 weeks' duration of illness, and (3) failure to reach a diagnosis despite one week of inpatient investigation.

Read More on Medscape Reference »


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