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Encephalitis is inflammation of the brain parenchyma that is usually caused by an infection. The infection can be viral or bacterial in origin, but is most commonly viral.

HSV (herpes simplex virus) is often associated with more severe disease, and if suspected, should be treated urgently with IV aciclovir.


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
  • Specific viral PCR tests (usually of blood sample, may also be of CSF)
  • 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
A large necrotic brain lesion from amoebic encephalitis.
A large necrotic brain lesion from amoebic encephalitis. This is not typical of the appearance of other types of encephalitis. This file is taken from wikimedia commons and is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.


  • Admission to a high dependency or intensive care unit may be required, especially in cases involving seizure or altered level of consciousness
  • Immediate IV aciclovir, ongoing 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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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University, and is studying for a Masters of Sports Medicine at the University of Queensland. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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