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