Fever

 

• Background

  Pathophysiology

  • Various definitions, usually varying between 37.5-38.3°C. In the UK National Early Warning Score (NEWS), it is 38°C.
  • We feel cold during the temperature rise, then warm once it's reached.
  • Due to endogenous pyrogens (IL1, IL6, TNFα) or exogenous pyrogens (LPS, exotoxins) activating prostaglandin E2 to act on the thermoregulatory centre in the hypothalamus.
  • Aka controlled hyperthermia, whereas uncontrolled hyperthermia (or just 'hyperthermia') is thermoregulatory dysfunction due to heat stroke or drugs such as antipsychotics or SSRIs.
  • Causes are infectious, autoimmune, or malignant.

  Measurement

  • In routine clinical practice, temperature is measured peripherally e.g. mouth, axilla, tympanic membrane. This only has 65% sensitivity (but 95% specificity) for detecting true fever (as measured by central temperature measurement e.g. rectal, oesophageal).
  • So the 'absence' of fever on routine observations should not be used to rule out serious infection if there are other signs of it.

  Patterns

  The association of particular fever patterns with particular diseases has been described, though their clinical utility (e.g. sensitivity and specificity), is variable. Classic patterns include:

  • Continuous e.g. lobar pneumonia, infective endocarditis.
  • Remittent: continuously elevated temp with higher spikes e.g. infective endocarditis.
  • Intermittent: normal temp with febrile spikes e.g. abscesses, sepsis.
  • Relapsing: sub-type of intermittent fever, with spikes separated by days e.g. malaria, Pel-Ebstein fever (week to week) of Hodgkin's lymphoma.
  • Step-ladder rise to plateau: typhoid.

• DDx: Night sweats

  • Infection: TB, abscess, endocarditis, osteomyelitis, HIV, any other infection.
  • Lymphoproliferative disease.
  • Drugs: antidepressants, antipsychotics.
  • Endocrine: thyrotoxicosis, diabetes.
  • Others: menopause, anxiety, idiopathic hyperhidrosis.

• Rigors

  • Uncontrollable limb shaking ± teeth chattering. May look like a seizure but there is no loss of consciousness.
  • A signs of severe infection, usually requiring hospital admission.
  • Causes: sepsis, pneumococcal disease, purulent collection (abscess/empyema), malaria.

• Fever of unknown origin

  Definition

  • Temperature >38°C for >3 weeks, which is still undiagnosed after 1 week of hospital investigation.
  • Sub-types of fever of unknown origin: neutropenic, HIV, nosocomial.

  Causes

  • Infection: abscess (chest/abdo/pelvic), infective endocarditis, TB, osteomyelitis, UTI, biliary infection, non-bacterial (viral, fungal, parasite).
  • Connective tissue disease: polymyalgia rheumatica, temporal arteritis, adult-onset Still's disease, SLE, RA, PAN.
  • Cancer: lymphoma, leukaemia, renal cell cancer.
  • Others: drugs, IBD, PE, sarcoidosis, amyloidosis, thyrotoxicosis, Addison's.

  The older the patient and the longer the fever, the more likely a non-infectious cause.

• Septic screen

  • Bloods: FBC, blood culture, CRP.
  • Others: urinalysis, lumbar puncture, chest XR.
  • Remember that CRP/ESR can also be raised in autoimmune disease, cancer, MI, and burns.