Encephalitis in the pediatric population.

Introduction The broad definition of the term “encephalitis,” that is, inflammation of the brain, necessitates acknowledgment of the enormous inclusivity of the topic. The most common interpretation of the term implies a direct invasion of the brain by an infectious pathogen, most commonly, viral, fungal, or parasitic. The topic also includes examples of meningitis mediated by bacteria or other agents, which can produce extrameningeal symptoms such as lethargy or seizures, in which case, the combined term “meningoencephalitis” is used. There are also many examples of encephalitis not due to direct central nervous system (CNS) infections. Inflammatory processes due to an acute or chronic illness can result in an acute immune-mediated encephalitis, such as acute disseminated encephalomyelitis (ADEM), lupus cerebritis, and paraneoplastic syndromes. Agents or conditions that produce slowly progressive CNS symptoms, such as tertiary syphilis or “slow viruses” (the prion protein encephalopathies), also are considered examples of encephalitis. Table 1 lists only a limited number of the many pathogens and pathologic conditions that can cause either acute or subacute encephalitis. In this discussion, we will mainly address examples of acute encephalitis related to direct CNS infection and para-infectious processes involving the CNS. These examples embrace the major portion of the spectrum of disease presentation, course, and recovery, as well as mechanisms of cerebral injury. In addition to the taxonomic classification in Table 1, causes of infectious encephalitis often are grouped according to the most common methods of transmission. “Arboviruses” are those spread by insect vectors, such as West Nile virus (WNV) and the equine encephalitis group (both by mosquitoes). Zoonotic causes of encephalitis not spread by intermediary insect vectors include many of the parasitic infections (larva migrans) and rabies. Community-acquired encephalitides, such as enterovirus, adenovirus, and latechildhood herpesvirus infections, generally are spread by person-to-person contact. Vertically transmitted pathogens include neonatal herpes simplex (HSV), rubella virus, Abbreviations