Herpes simplex encephalitis

Herpes simplex (HSV) encephalitis is a common topic in the exam. The virus characteristically affects the temporal lobes – questions may give the result of imaging or describe temporal lobe signs e.g. aphasia.


  • fever, headache, psychiatric symptoms, seizures, vomiting
  • focal features e.g. aphasia
  • peripheral lesions (e.g. cold sores) have no relation to the presence of HSV encephalitis


  • HSV-1 is responsible for 95% of cases in adults
  • typically affects temporal and inferior frontal lobes


  • CSF: lymphocytosis, elevated protein
  • PCR for HSV
  • CT: medial temporal and inferior frontal changes (e.g. petechial haemorrhages) – normal in one-third of patients
  • MRI is better
  • EEG pattern: lateralised periodic discharges at 2 Hz


  • intravenous aciclovir

The prognosis is dependent on whether aciclovir is commenced early. If treatment is started promptly the mortality is 10-20%. Left untreated the mortality approaches 80%

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