Description

• Inflammatory process of the brain associated with clinical evidence of neurologic dysfunction
• System(s) affected: Nervous
• Synonym(s): Meningoencephalitis

Epidemiology

Incidence
3.5–7.4/100,000 persons/year

• Seasonal variation (e.g., arboviruses, enteroviruses)
• Nonseasonal: Most others (e.g., herpes simplex virus [HSV])
• Vaccines have altered prevalence and seasonality (e.g., mumps is no longer common and not seasonal)

Risk Factors

• Age: Increased incidence among infants and elderly
• Contact with animals or insect vectors
• Impaired immune status
• Occupation (e.g., lab or animal-care workers)
• Recreational activities (e.g., camping, hunting)
• Transfusion and transplantation
• Travel to endemic areas
• Recent vaccinations/unvaccinated status

General Prevention

• Appropriate clothing to protect against mosquitos
• Use of mosquito repellants (DEET, Picaridin)
• Avoidance and removal of ticks
• Elimination of mosquito breeding sources
• Vaccines, when available (e.g., mumps, measles, polio, rabies, Japanese encephalitis)

Pathophysiology

• Most common entry site is through the respiratory or GI tract, followed by hematogenous spread.
• Rarer modes of entry include neurotropic spread (e.g., rabies)
• Specific cell lines may be infected and are associated with specific symptom complexes:
  • Neurons: Associated with seizures
  • Oligodendroglia: May cause demyelination alone, cortical infection, or reactive parenchymal swelling; changes in state of consciousness
  • Brain stem neurons: Coma, respiratory failure
  • Microglia, macrophages: Neurologic dysfunction
• Pathologic changes seen with postinfectious and postvaccinal encephalomyelitis include perivascular infiltration of mononuclear inflammatory cells.

Etiology

• Most commonly identified etiologies in the US: HSV, West Nile virus, enteroviruses
• Despite extensive evaluations, the etiologic agent frequently is not identified.

Commonly Associated Conditions

• Seizures
• Hyperthermia
• Increased intracranial pressure (ICP)
• Inappropriate antidiuretic hormone (ADH) secretion