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Encephalitis is inflammation of the brain parenchyma associated with clinical evidence of neurologic dysfunction1 (e.g., encephalopathy, focal neurologic deficits, speech or movement disorders, seizures). Whereas inflammation is technically a neuropathologic diagnosis, neuroradiographic and laboratory [cerebrospinal fluid (CSF) profile] evidence of inflammation are used more commonly in clinical practice, given limited tissue availability.

Meningoencephalitis refers to an inflammatory process in both the brain and the meninges. Encephalitis and meningitis (Chapter 19) have overlapping clinical features because inflammation of the brain parenchyma often extends to the adjacent meninges, and inflammation of the meninges often extends to the adjacent parenchyma.2 Encephalomyelitis refers to an inflammatory process in both the brain and the spinal cord. Rhombencephalitis refers to encephalitis predominantly localized to the brainstem.

Encephalitis may be infectious (most commonly viral, but also bacterial, fungal, or protozoan)3 or autoimmune (including parainfectious or postinfectious, paraneoplastic, and other antibody-mediated).4 This chapter focuses on infectious encephalitis; however, we will also briefly discuss autoimmune causes of encephalitis as they overlap with infections and to provide further context. Common infectious and autoimmune etiologies of encephalitis are listed in Table 20-1.

TABLE 20-1Common Infectious and Autoimmune Causes of Encephalitis

Estimating the true incidence of infectious encephalitis is difficult because there are few population-based studies and many possible pathogens. Additionally, few cases are reported to health authorities. The best available data are from large ...

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