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Fever also is known as pyrexia and febrile response is defined as having a temperature above the normal range due to an increase in the body’s temperature set-point. There is not a single agreed-upon upper limit for normal temperature with sources using values between 37.5 and 38.3 °C (99.5 and 100.9 °F). The increase in set-point triggers increased muscle contractions and causes a feeling of cold. This results in greater heat production and efforts to conserve heat. When the set-point temperature returns to normal, a person feels hot, becomes flushed, and may begin to sweat. Rarely a fever may trigger a febrile seizure. This is more common in young children. Fevers do not typically go higher than 41 to 42 °C (105.8 to 107.6 °F).
Types of Fever
Fevers can be classified according to how long they last, whether or not they come and go, and how high they are.
A fever can be:
- low grade, from 100.5–102.1°F or 38.1–39°C
- moderate, from 102.2–104.0°F or 39.1–40°C
- high, from 104.1–106.0°F to or 40.1-41.1°C
- hyperpyrexia, above 106.0°F or 41.1°C
The height of the temperature may help indicate what type of problem is causing it.
Length of time
A fever can be
- acute if it lasts less than 7 days
- sub-acute, if it lasts up to 14 days
- chronic or persistent, if it persists for over 14 days
Depending on the performance of the various types of fever
The pattern of temperature changes may occasionally hint at the diagnosis
- Continuous fever – Temperature remains above normal throughout the day and does not fluctuate more than 1 °C in 24 hours, e.g. lobar pneumonia, typhoid, meningitis, urinary tract infection, or typhus. Typhoid fever may show a specific fever pattern (Wunderlich curve of typhoid fever), with a slow stepwise increase and a high plateau. (Drops due to fever-reducing drugs are excluded.)
- Intermittent fever – The temperature elevation is present only for a certain period, later cycling back to normal, e.g. malaria, kala-azar, pyemia, or septicemia. Following are its types
- Quotidian fever, with a periodicity of 24 hours, typical of Plasmodium falciparum or Plasmodium knowlesi malaria
- Tertian fever (48-hour periodicity), typical of Plasmodium vivax or Plasmodium ovale malaria
- Quartan fever (72-hour periodicity), typical of Plasmodium malaria.
- Remittent fever – Temperature remains above normal throughout the day and fluctuates more than 1 °C in 24 hours, e.g., infective endocarditis, brucellosis.
- Pel-Ebstein fever – A specific kind of fever associated with Hodgkin’s lymphoma, is high for one week and low for the next week and so on. However, there is some debate as to whether this pattern truly exists.
A neutropenic fever, also called febrile neutropenia, is a fever in the absence of normal immune system function. Because of the lack of infection-fighting neutrophils, a bacterial infection can spread rapidly; this fever is, therefore, usually considered to require urgent medical attention. This kind of fever is more commonly seen in people receiving immune-suppressing chemotherapy than in apparently healthy people.
Febricula is an old term for a low-grade fever, especially if the cause is unknown, no other symptoms are present, and the patient recovers fully in less than a week
Symptoms of Fever
- Temperature greater than 100.4 F (38 C) in adults and children
- Shivering, shaking, and chills
- Aching muscles and joints or other body aches
- Intermittent sweats or excessive sweating
- Rapid heart rate and/or palpitations
- Skin flushing or hot skin
- Feeling faint, dizzy, or lightheaded
- Eye pain or sore eyes
- Loss of appetite
- Fussiness (in children and toddlers)
- Also important to note in children are symptoms that can accompany an infection, including sore throat, cough, earache, vomiting, and diarrhea
- With very high temperatures (>104 F/40 C), convulsions, hallucinations, or confusion is possible. Always seek medical attention for a high fever or if these symptoms occur.
Diagnosis of Fever
Fever is a common symptom of many medical conditions:
- Infectious disease, e.g., influenza, primary HIV infection, malaria, Ebola, infectious mononucleosis, gastroenteritis, Lyme disease
- Various skin inflammations, e.g., boils, abscess
- Immunological diseases, e.g., lupus erythematosus, sarcoidosis, inflammatory bowel diseases, Kawasaki disease, Still disease, Horton disease, granulomatosis with polyangiitis, autoimmune hepatitis, relapsing polychondritis
- Tissue destruction – which can occur in hemolysis, surgery, infarction, crush syndrome, rhabdomyolysis, cerebral bleeding, etc.
- Reaction – to incompatible blood products
- Cancers – most commonly kidney cancer and leukemia and lymphomas
- Metabolic disorders – gout, porphyria
- Inherited metabolic disorders,
Treatment of Fever/Pyrexia
The following fever-reducing medications may be used at home:
- Acetaminophen (Tylenol and others) can be used to lower a fever. The recommended pediatric dose can be suggested by the child’s pediatrician. Adults without liver disease or other health problems can take 1,000 mg (two “extra-strength” tablets) every six hours or as directed by a physician. The makers of Tylenol state the maximum recommended dose of acetaminophen per day is 3,000 mg, or six extra-strength tablets per 24 hours unless directed by a doctor. Regular-strength Tylenol tablets are 325 mg; the recommended dosage for these is two tablets every four to six hours, not to exceed 10 tablets per 24 hours. If your fever is accompanied by vomiting and you are unable to keep oral medications down, ask a pharmacist for acetaminophen suppositories, which are available without a prescription.
- Ibuprofen can also be used to break a fever in patients over 6 months of age. Discuss the best dose with a doctor. For adults, generally, 400 mg to 600 mg (two to three 200 mg tablets) can be used every six hours as fever reducers.
- Naproxen is another nonsteroidal anti-inflammatory drug (NSAID) that can temporarily reduce fever. The adult dose is two tablets every 12 hours.
- Aspirin should not be used for fever in children or adolescents. Aspirin use in children and adolescents during a viral illness (especially chickenpox and influenza, or flu) has been associated with Reye’s syndrome. Reye’s syndrome is a dangerous illness that causes prolonged vomiting, confusion, and even coma and liver failure.