Description

• Acute inflammation of the brain
• 20,000 cases in the U.S. annually
• Mortality: 10%
• Inflammatory reaction occurs within brain parenchyma with destruction of neurons, parenchymal edema, and petechial hemorrhages
• Route of CNS infection usually hematogenous:
  • Respiratory or GI tract
  • Blood transfusion
  • Organ transplant
• Neural migration occurs with rabies, herpes simplex virus (HSV), and varicella zoster virus (VZV) encephalitis

Etiology

• Viral is most common
• Noninfectious
  • Autoimmune, paraneoplastic, collagen vascular disease
• 50% of cases have no identifiable cause

• HSV:
  • 10–20% of all encephalitides
  • Primary or reactivation
  • Early treatment improves prognosis
• Arbovirus:
  • 10–15% of all encephalitides
  • Zoonotic transmission (mosquitoes, ticks) in warm months
  • Eastern equine causes fulminant encephalitis:
    • Tropism for the hippocampus
    • Abrupt onset of headache, fever, vomiting progressing to coma
  • Western equine occurs mostly in the western 2/3 of the U.S.:
    • Often preceded by nonspecific upper respiratory/GI tract symptoms
  • Japanese – most prevalent arboviral encephalitis worldwide:
    • Indolent course of fever, headache, myalgias, and fatigue followed by confusion, delirium, masklike facies, and parkinsonism, seizures, brainstem dysfunction, coma, and death
• Flavivirus:
  • West Nile virus – increased incidence in North America:
    • Found in mosquitoes and birds
    • Febrile illness, often with rash
    • Headache
    • Lymphadenopathy
    • Polyarthropathy
    • Increased morbidity/mortality in elderly patients
  • Flaccid paralysis can lead to respiratory failure with 50% mortality
• Enteroviral:
  • Occurs mainly in children <10 yr old
  • Relatively benign course with little or no long-term sequelae
• Measles encephalitis:
  • Occurs several days to 2–3 wk after primary infection and rash, or after years of latent infection
  • Abrupt onset and rapid progression to coma
  • Seizures common (50–60%)
  • Postimmunization incidence of 1 per 1 million vaccinated
• HIV encephalitis:
  • Lower CD4 counts predispose to encephalitis
  • Typical features include motor spasticity and dementia
  • Involvement of white matter with extensive neural degeneration
• Rhabdovirus: Rabies
  • CNS infection in the absence of systemic infection

• Mycoplasma pneumoniae
• Toxoplasma gondii
• Rickettsia rickettsii
• Mycobacterium tuberculosis
• Borrelia burgdorferi
• Bartonella henselae

• Histoplasma
• Cryptococcus neoformans
• VZV
• Listeria monocytogenes
• Cytomegalovirus (CMV)
• T. gondii
• Human herpesvirus type 6 (HHV-6)

• Anti-LGI1 encephalitis
• Anti-N-methyl-D-aspartate receptor (anti-NMDAR) encephalitis