Neurology >
Encephalitis

“Inflammation of brain parenchyma”

Risk Factors
  • Gallstones, recent biliary tract procedure or previous cholangitis.
  • Medication e.g. COCP, fibrates.
  • Lipid-rich diet (increased incidence of gallstones).
Aetiology
  • Primary viral infection (primary encephalitis)
    • Epidemic e.g. coxsackievirus, poliovirus, arbovirus, echovirus
    • Sporadic e.g. cytomegalovirus, herpes simplex (HSV-1 or HSV-2 in immunocompromised), varicella-zoster, rabies, mumps, lymphocytic choriomeningitis
    • Reactivation of latent viral infection e.g. HIV-associated encephalopathy and dementia, subacute sclerosing panencephalitis (measles reactivation), HSV-1 and herpes zoster encephalitis, progressive multifocal leukoencephalopathy
    • Rarely, SARS-CoV2 has been reported to cause encephalitis
  • Secondary immunological complication
    • With possible progression to acute disseminated encephalomyelitis -> inflammatory demyelination of the brain
    • Immune system targets CNS antigens resembling viral proteins
    • A complication of measles, rubella, chickenpox, mumps, smallpox vaccine, live-virus vaccines, influenza A/B, enterovirus, EBV, HepA, HepB or HIV
Pathophysiology
  • Acute encephalitis:
    1. Inflammation and oedema occurs in CNS structures.
    2. Direct viral invasion of brain parenchyma damages neurons.
    3. Severe infection can cause brain haemorrhagic necrosis.
  • Acute disseminated encephalomyelitis:
    • Multifocal areas of perivenous demyelination, absence of virus in the brain
Clinical Presentation
  • Fever
  • Headache
  • Altered mental status
  • Seizures and focal neurological deficits
  • GI or respiratory prodrome
  • Status epilepticus or coma = poor prognosis due to severe brain inflammation
  • Phantosmia (olfactory hallucination) indicates temporal lobe involvement -> HSV encephalitis
Investigations
  • MRI
    • Contrast-enhanced MRI -> can show orbitofrontal and temporal oedema, demyelination, basal ganglia, thalamic abnormalities)
    • Excludes lesions which may mimic viral encephalitis
  • CSF analysis (PCR for HSV/other viruses with serologic tests) -> lymphocytic leucocytosis, normal glucose, elevated protein, absence of pathogens
  • Brain biopsy if patient is worsening, has responded poorly to treatment or has an undiagnosed lesion
Management
  • Supportive care
    • Treatment of fever, dehydration, electrolyte imbalance, seizures
  • Antiviral drugs
    • Acyclovir for HSV or varicella-zoster virus encephalitis
  • Antibiotics until bacterial meningitis excluded
  • If immunologic: corticosteroids e.g. prednisolone and plasma exchange/IV immunoglobulins
Complications
  • Permanent neurological deficits in survivors of severe infection

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