INTRODUCTION

Viral infections of the central nervous system (CNS) are uncommon but potentially devastating clinical events. As a group, these infections range from benign, self-limited forms of meningitis to full-blown and often fatal cases of acute encephalitis to chronic, persistent diseases. Encephalitis literally refers to inflammation of the brain parenchyma, and such a host response is common with many viral infections that spread to this site (Figure 75.1). In contrast, meningitis results when infection and the associated inflammatory response are limited to the leptomeninges and the subarachnoid space. In reality, the two syndromes often occur together—hence the term meningoencephalitis. Acute viral encephalitis or acute viral meningoencephalitis is suggested by various signs and symptoms indicative of brain parenchymal invasion (mental status and behavioral changes, seizures, and focal neurological deficits) accompanied by fever. Such patients require emergent evaluation for what can be a life-threatening infectious illness that sometimes has a treatable cause.

ETIOLOGY

More than 100 different viruses can infect the human CNS, but a much smaller number cause the vast majority of viral encephalitis cases. The most relevant pathogens come from the Herpesviridae, Picornaviridae, Retroviridae, Paramyxoviridae, and a group of RNA viruses formerly called the arboviruses, now designated as the Togaviridae, Flaviviridae, Bunyaviridae, and Reoviridae families (Table 75.1). The acquired immunodeficiency syndrome (AIDS) epidemic and the therapeutic use of immunosuppression in transplant recipients and oncology patients have resulted in the identification of new infectious disease processes that can cause signs and symptoms consistent with acute encephalitis.