Acute Viral Encephalitis

Encephalitis means inflammation of the brain. Acute encephalitis associated with viral infections includes two distinct clinical-pathological diseases. The form referred to simply as acute viral encephalitis is direct infection of neural cells with perivascular inflammation, neuronal destruction, neuronophagia, and tissue necrosis, and this pathology is centered primarily in the gray matter. The other disease, postinfectious encephalomyelitis (acute disseminated encephalomyelitis), is an illness that follows a variety of viral and some bacterial infections; there is no evidence of direct infection ofneural cells, but there is widespread perivenular inflammation and demyelination localized to the white matter of the brain. Clinically, the distinction is often difficult unless the demyelinating disease complicates an exanthem. Historically, approximately two-thirds of the fatal cases of encephalitis were acute viral encephalitis, and one-third were postinfectious encephalomyelitis. The number of cases ofpostinfectious encephalomyelitis has decreased greatly, however, with the elimination of the use of vaccinia virus for prevention of smallpox and the institution of immunization against measles, mumps, and rubella. The reported incidence of acute encephalitis is between 3.5 and 7.4 cases per 100,000 patient-years [1, 2]. It is more common in children, among whom the incidence is > 16 cases per 100,000 patient-years [3]. Nearly 100 different agents have been associated with encephalitis, but the most important lifethreatening causes of acute neuronal and glial infection are herpes simplex virus and the arthropod-borne viruses (arboviruses); the most common antecedent illness related to postinfectious encephalomyelitis now is nonspecific respiratory disease. The most important issues in the differential diagnosis of encephalitis are to rule out nonviral diseases, which may require urgent treatment, and to properly identify cases due to herpes simplex virus, where morbidity and mortality can be greatly reduced with specific antiviral therapy.