Encephalitis causes significant morbidity and mortality and raises difficult diagnostic and management challenges. The etiology is often illusive and, given the lack of literature on the subject, there is little information physicians can offer in the way of prognosis or etiology-specific treatment. This chapter defines encephalitis and describes its major causes with emphasis on the presentation, pathology, and management of viral encephalitis in pediatric populations.
Encephalitis is defined as inflammation of the brain tissue, infectious or otherwise, causing alterations in cerebral function. The patient with encephalitis often presents with fever, headache, altered mental status, behavioral changes, focal neurological signs, and seizures. Meningoencephalitis describes the clinical entity in which the inflammation extends to the subarachnoid spaces and meninges. When the spinal cord is involved in the inflammatory process, the term encephalomyelitis is used. Noninfectious encephalitis is an antibody-mediated inflammation of the brain parenchyma, which may be triggered by immune response to a viral illness or tumor.
Estimating the true incidence of encephalitis is difficult, as most cases are not reported to local health departments. The most accurate estimates are those concerning the subset of arthropod-borne viral, or arboviral, encephalitides attributed to tracking efforts at the Center for Disease Control and Prevention (CDC), which estimates between 250 and 3000 cases occurring annually.1 The California Encephalitis Project documented all hospitalized cases of encephalitis in California from 1991 to 1999 and found 35–50 cases per 100,000 people annually. Encephalitis occurred in the highest numbers in infants, followed by the elderly. A specific cause was reported in approximately 45% of 13,939 cases; HSV accounted for 14% of all cases, while arboviral disease was identified in less than 1% of the cases (West Nile Virus had not yet become established in California).2
Because of the protection of the blood–brain barrier (BBB), the lack of a lymphatic system, and the absence of major histocompatibility complexes (antigen presenting cells), the brain was historically considered an immunologically isolated organ without the same vulnerabilities to infectious agents or immune responses as other body systems. It is increasingly apparent that the BBB is a far more dynamic entity than previously thought.
The BBB is made of capillary endothelial cells, astrocytes and pericytes with unique properties not seen in other organ systems. These anatomic differences include narrow tight junctions, lack of fenestrations, decreased transport, and a continuous basement membrane. Electrically, the surfaces of these cells are negatively charged, and therefore, repel proteins and other negatively charged molecules. Specific areas within the central nervous system (CNS) differ in their levels of BBB permeability. For example, the choroid plexus endothelium has fenestrations, allowing free entry of immune cells to the cerebrospinal fluid (CSF). The ependymal lining of the ventricles lacks tight junctions, which permits drainage of CNS antigens into the CSF.
In a study of foreign tissue graft survival in the brain parenchyma, Medawar demonstrated decreased cytotoxic T-cell response in the CNS compared with ...