Encephalitis is inflammation of the brain parenchyma that is usually caused by a viral infection.


Encephalitis is usually viral in origin. Causative organisms include:

  • Echoviruses
  • Coxsackie
  • Mumps
  • Herpes simplex – This often causes the severest forms of encephalitis (particularly HSV-1) and is thought to reach the brain via the olfactory nerves
  • Adenoviruses
  • Varicella zoster
  • Measles
  • Influenza
  • Arboviruses  – Especially Japanese B arbovirus which is the leading cause of encephalitis in Asia. The West Nile virus, first reported in New York, has spread to Australia, Africa, Asia and parts of Europe. Arboviruses are commonly transmitted via mosquito bites.
  • Cytomegalovirus (CMV)
  • Epstein-Barr virus (EBV)
  • Rabies
  • HIV (during seroconversion)

However, encephalitis can occasionally be due to non-viral causes. Causative organisms include:



An intracranial infection provokes an inflammatory response causing inflammation of the cortex, white matter, basal ganglia and brain stem depending on the causative organism.

Clinical Features

In most cases, the patient will present with a mild self-limiting illness with headache, drowsiness, pyrexia and malaise. More severe signs and symptoms typically occur when there is meningeal or significant parenchymal involvement. This is particularly true with herpes simplex type 1 viral infection:
Meningeal signs:

  • Headache
  • Photophobia
  • Neck stiffness
  • Vomiting
  • Positive Kernig’s sign

Parenchymal signs:

There may also be signs indicative of the underlying cause:

  • Herpes simplex – Cold sores
  • Mumps – Parotid gland swelling
  • Japanese B arbovirus and West Nile virus – Mosquito bites
  • Rabies – Hydrophobia, delusions, hallucinations, anxiety


Differential Diagnosis

  • Diabetic ketoacidosis
  • Hepatic encephalopathy
  • Hypoglycaemia
  • Beri-beri (give thiamine immediately if this is suspected)
  • Drug overdose
  • SLE
  • Hypoxia


  • Viral serology – However, you shouldn’t wait for cause to be known before starting treatment
  • PCR of blood
  • Lumbar puncture – Only if imaging has excluded an intracranial mass. CSF analysis will usually show excess lymphocytes. Glucose level will usually be normal but is sometimes reduced. Protein is occasionally elevated. A normal CSF doesn’t exclude encephalitis.
  • CT scan – Will often show cerebral oedema
  • MRI scan – Will show subtle inflammation that is undetectable on the CT scan and is particularly useful in showing characteristic changes in the temporal lobes which occur in HSV-1 infection
  • EEG – May show slow wave changes and periodic complexes. If this is confined to the temporal lobes, a diagnosis of HSV encephalitis is suggested
  • If indicated: IgM toxoplasma titre, malaria film, HIV test, TB skin prick test etc



  • Admission to a high dependency or intensive care unit may be required
  • Immediate IV aciclovir for 2-3 weeks if herpes simplex encephalitis is suspected
  • Anticonvulsants if the patient experiences seizures
  • Dexamethasone to treat raised intracranial pressure



Even with optimum treatment, mortality is 10-30%. A significant number of those who survive have long term neurological complications such as cognitive impairment and epilepsy.

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