Viral Encephalitis


  • Rare- incidence ~0.5/100,000/year
  • More common in children and elderly (1/3 are <20 (HSV2) and 1/2 are >50 (HSV1))


  • Herpes simplex; Varicella zoster;
  • cytomegalovirus; HIV; measles; West Nile; Japanese B
  • Tick-borne
  • Rabies

Signs and symptoms

  • Usually acute onset headache, fever, focal neurological signs e.g. aphasia, hemiplegia, visual field defects etc and seizures
    • Meningism (neck stiffness, photophobia, headache) is not uncommon)
    • Other signs e.g. stupor, coma, seizures, partial paralysis, confusion, psychosis, speech/memory problems may also be present.
  • NB Herpes simplex infection tends to affect the temporal lobe, so symptoms such as hallucinations, seizures, personality changes and aphasia (particularly receptive) are not uncommon.


  • CT/MRI
    • CT may show low-density lesions in the temporal lobes
  • Lumbar puncture (once imaging excludes mass lesion) may show RBCs or xanthochromia but more commonly have a mononuclear pleocytosis and raised protein.  Glucose is normal and viral PCR may be negative or may indicate causal organism.


  • High dose IV Aciclovir (e.g. 10mg/kg TDS for 2-3 weeks)
  • Mortality of around 10%; some patients are left with residual epilepsy or cognitive impairment
  • NB Herpes simplex encephalitis is notifiable in the UK

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