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Definition

Fever is defined as an abnormally high body temperature or a regulated rise to a new set point of body temperature. While a body temperature above 100°F(37.8°C) is considered to be a fever by some clinicians, a significant fever is usually defined as an oral temperature of 102°F (39°C) or a rectal temperature of 103°F(39.5°C). Fever is a sign of inflammation and represents the body's response to microbial invasion or to a disease process.

Hyperthermia is defined as abnormally high body temperature caused by disruption of the body's thermoregulatory mechanisms. Hyperthermia occurs when the body's metabolic heat production or environmental heat load exceeds the normal heat loss capacity (or when normal heat loss is impaired).

Description

The normal temperature of the human body fluctuates between 97°F (36.1°C) and 100°F (37.8°C), with the average being 98.6°F (37°C). There is a diurnal pattern of temperature variation in humans, with body temperature being lowest in the morning and highest in late afternoon. In women of childbearing age, there is a small rise in temperature following ovulation during the menstrual cycle, and during the first trimester of pregnancy.

The body's thermoregulatory mechanisms include changes in muscle tone, vascular tone, and sweat production, which serve to balance body heat produced by metabolism with heat lost to the environment. This balance is regulated by the hypothalamus, a small structure in the brain below the thalamus.The body's temperature regulation can be upset by environmental factors (external heat and humidity); by disorders or conditions that inhibit sweat production or evaporation; and by infectious diseases.

Fever occurs when the hypothalamus raises the body's temperature to a new set point. White blood cells called monocyte-macrophages release proteins called pyrogens when the cells encounter pathogenic microorganisms. The pyrogens act on the hypothalamus, causing it to reset the body's "thermostat" upward. The vessels that supply the skin with blood often narrow as the body's temperature is rising. This process, which is called vasoconstriction, reduces sweating and causes the body to retain more heat than it loses. Vasoconstriction in the skin and extremities allows the body to move blood toward its core, which increases the rate of metabolism and induces shivering. The chills that often accompany a fever are caused by this movement of blood to the body's core, which leaves the skin surface and extremities relatively cool. When the infection or disease process resolves, the hypothalamus attempts to reset the body's thermoregulation at its normal set point. The body's cooling mechanisms switch on as the blood returns to the extremities and skin surface, and sweating occurs.

Fever is an important component of the immune system's inflammatory response, though its role is not completely understood. A rise in body temperature has several effects. The pyrogens that trigger the resetting of the body's thermostat also increase the production of macrophages, which fight off invading bacteria or viruses. Higher temperatures also inhibit the growth of some bacteria, while at the same time activating the immune system. In addition, the increased heart rate that accompanies vasoconstriction also speeds the arrival of white blood cells to the sites of inflammation.

Causes and symptoms

Causes

Fever can be caused by a wide variety of diseases and disorders, including:

• infectious diseases, which may be caused by viruses, bacteria, fungi, rickettsiae, or parasites
• autoimmune disorders, including systemic lupus erythematosus, rheumatic fever, adult rheumatoid arthritis, Wegener's granulomatosis, polyarteritis nodosa, and Still's disease
• disorders of the central nervous system (CNS), including head injuries, multiple sclerosis, tumors of the brain or spinal cord, and cerebral hemorrhage
• cancers, including primary cancers of the colon, rectum, kidney, and liver, and metastatic cancers of the liver
• cardiovascular disorders, including myocardial infarction and pulmonary embolism
• reactions to such medications as methyldopa, quinidine, hydralazine, phenytoin, chlorpromazine, carbamazepine, anti-cancer drugs, and antibiotics
• endocrine disorders, including hyperthyroidism (thyroid storm) and pheochromocytoma
• gastrointestinal disorders, including alcoholic hepatitis, inflammatory bowel disease (IBD), and liver abscess
• inflammation associated with indwelling catheters, intravenous lines, and nasogastric tubes
• neuroleptic malignant syndrome, which is a rare and potentially fatal reaction to certain antipsychotic medications

Fever patterns

Fevers may be classified as intermittent (the patient's temperature falls to normal at some point during the day); sustained (the temperature remains high); or relapsing (periods of fever are followed by several days of normal temperatures). The fever pattern may be helpful in the differential diagnosis; for example, fevers related to medication reactions are more often sustained than intermittent. A relapsing pattern suggests malaria.

The degree of temperature elevation in a fever does not necessarily correspond to the seriousness of the illness. In general, children with fevers run higher temperatures than adults, while elderly persons and newborns may have less marked fevers.

Diagnosis

The differential diagnosis of fever is complicated by the variety of possible causes. In most cases, the patient's history—including a medication history and history of exposure to infectious diseases—vital signs, a complete blood count, and blood cultures will help to narrow the number of possible causes. In fevers of unknown origin (FUOs), however, the physician may need to schedule a chest x-ray or ultrasound and CT imaging.

Fevers of unknown origin (FUOs)

The origin of some fevers is difficult to determine. The term "fever of unknown origin" (FUO) has been applied to cases of fever in which the patient's temperature remains over 101°F (38.3°C) for three weeks and in which the diagnosis is not apparent after seven or more days of studies. The most common cause of FUOs is an infectious disease of some kind, although many cancers present initially as FUOs. About 15% of FUOs never receive a definite diagnosis.

Temperature measurement

A variety of different devices are available to register body temperature, including the traditional glass thermometers used for oral or rectal temperature readings. More recently, sophisticated electronic devices have been introduced that register body temperature as a digital readout. For younger children and infants, there are fore-head meter strips and pacifiers with temperature-sensitive readouts.

Treatment

Routine measures

Most fevers are caused by common infections, are short-lived, and do not require symptomatic treatment. The patient's comfort, however, may be improved by measures to remove body heat, antipyretic medications, and fluid replacement. Measures to remove body heat include alcohol or cold sponges; ice bags; and ice baths. The most common antipyretic medications are aspirin or acetaminophen, 350–650 mg every four hours. Children or adolescents with influenza-like symptoms should not be given aspirin without consulting a physician because of the possibility of developing Reye's syndrome, a rare disorder characterized by vomiting and liver dysfunction.

Fluid replacement is an important part of fever treatment. Extra fluid is required to prevent electrolyte imbalance as well as to replace fluid lost through perspiration.

Emergency treatment

Body temperature over 105.8°F (41°C) in an adult is a medical emergency and requires immediate treatment.

The following indications in specific patient groups may require emergency treatment:

• newborns (three months or younger) with fever of 100.5°F (38°C) or higher
• infants or children with fever of 103°F (39.4°C) or higher
• adults with fever of 101°F or higher lasting longer than three days; or a temperature of 100.5°F lasting longer than three weeks without other symptoms

A very high fever in a small child can trigger febrile seizures, and therefore requires immediate treatment. A fever accompanied by a stiff neck, severe headache, throat swelling, and changes in mental status may indicate the presence of meningitis or another serious infection, and should be brought to the immediate attention of a physician. High fever does not always produce sweating or diaphoretic symptoms, which indicates weakness in the body's defenses against severe disease.

Prognosis

Most fevers caused by infection end as soon as the immune system rids the body of the pathogen and do not produce any lasting effects. Exceptions include fevers associated with meningitis, typhoid, or scarlet fever. High fevers can produce major disturbances of the body's metabolism and alter the body's responses to medications. In addition, a high fever during the first trimester of pregnancy increases the risk of such birth defects as anencephaly (absence of a portion of the skull and brain).

The prognosis for fevers associated with such chronic conditions as autoimmune disorders depends on the stage of the disease and its management.

Health care team roles

Nurses assess and record the fever patient's signs and symptoms. Nurses as well as physicians can obtain relevant information from the patient's medical history. Home health care and visiting nurses may also help to educate patients about at-home treatment of fevers. In some cases, home care nurses may counsel patients and caregivers over the phone, advising when the condition warrants in-person medical attention.

Prevention

Since most fevers are caused by infectious agents, careful attention to proper methods of food handling, hand washing, and similar public health measures is an important form of prevention. Fevers related to medication reactions can sometimes be prevented by substitution of drugs that are less likely to produce such reactions. Fevers related to indwelling catheters and similar devices may be avoided by removing them as soon as they are no longer required.

Hyperthermia related to environmental conditions may be prevented by proper attention to climatic conditions, adequate fluid and electrolyte intake, and acclimatization to hot environments. Educating the public about the early signs of heat disorders is another important preventive measure.

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KEY TERMS

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Antipyretic—A drug that lowers body temperature, such as aspirin or acetaminophen.

Autoimmune disease—A disorder in which the immune system attacks the body's own cells.

Febrile seizure—Convulsions brought on by high body temperature and inflammation of the brain or brain covering.

Hyperthermia—Abnormally high body temperature that occurs when the body's metabolic heat production or environmental heat load exceeds the normal heat loss capacity (or when heat loss is impaired). Heat stroke is an example of hyperthermia.

Hypothalamus—A region in the posterior section of the forebrain that regulates body temperature, sleep cycles, and the activity of the pituitary gland.

Intermittent—A fever pattern marked by the patient's temperature returning to normal at some point during the day.

Macrophage—A type of large white blood cell that helps the body fight off pathogenic microorganisms by ingesting them.

Malignant hyperthermia—A rare inherited condition in which a person develops a very high fever when given general anesthetics or muscle relaxants in preparation for surgery.

Pyrogen—A substance that produces a rise in body temperature. Bacterial toxins are one type of pyrogen.

Relapsing—A fever pattern in which periods of fever alternate with several days of normal temperatures.

Reye's syndrome—A rare disorder that occurs primarily in children recovering from a viral illness and associated with aspirin usage. It is characterized by vomiting, swelling of the brain, and liver dysfunction.

Vasoconstriction—Narrowing of the blood vessels, which serves to conserve body heat and maintain or raise blood pressure.

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Lisette Hilton