Differentiation of viral from postinfectious encephalitis (See Also Chap. 35). The acute encephalitis syndrome described above may take two forms: the more common direct invasion of brain and meninges (true viral encephalitis) and a postinfectious encephalomyelitis that is presumably based on an autoimmune reaction to the systemic viral infection but in which virus is not present in neural tissue. The distinction between postinfectious encephalomyelitis and infectious encephalitis may be difficult, especially in younger patients who have a proclivity to develop the postinfectious variety with fever. The latter, termed acute disseminated encephalomyelitis (ADEM), occurs after a latency of several days, as the infectious illness is subsiding. It is expressed by a low-grade fever and cerebral symptoms such as confusion, seizures, coma, or ataxia. The spinal fluid shows slight inflammation and elevation of protein—sometimes a more intense reaction, and there are usually characteristic confluent, scattered, bilateral lesions in the white matter in imaging studies, findings that differ from those of viral encephalitis. When there is no coexistent epidemic of encephalitis to suggest the diagnosis, or the preceding systemic illness is absent or obscure, a differentiation between the two may not be possible on clinical grounds alone. The fever is generally higher in the infectious type but even this difference does not always hold in young children with ADEM. The encephalitis that follows certain childhood exanthems (postexanthematous) and vaccinations at any age (postvaccinal) are essentially forms of ADEM.
Because ADEM is predominantly an inflammatory and demyelinative process, we mention it here but discuss its clinical features and imaging more fully in Chap. 35, with the other demyelinating diseases such as multiple sclerosis, with which it shares some features. We also place in special categories further on the now rare Reye syndrome of postinfectious acute encephalopathy with hepatic failure that follows influenza, and other viral infections and postinfectious cerebellitis.
Whereas numerous viral, bacterial, fungal, and parasitic agents are cited as causes of the encephalitis syndrome, only the viral ones are considered here, for they are the most common and it is to these that one usually refers when the term encephalitis is used. The nonviral forms (mycoplasmal, rickettsial, Lyme, etc.) are considered in Chap. 31, under “Encephalitis Caused by Bacterial
Infections,” and should be reviewed with this section.
According to the Centers for Disease Control and Prevention, approximately 20,000 cases of acute viral encephalitis are reported annually in the United States. Death occurs in 5 to 20 percent of these patients and residual signs, such as mental deterioration, amnesic defect, personality change, recurrent seizures, and hemiparesis, are seen in approximately another 20 percent. However, these overall figures fail to reflect the widely varying incidence of mortality and residual neurologic abnormalities that follow infections by different viruses. In herpes simplex encephalitis, for example, approximately 50 percent of patients die or are left with some impairment, and in eastern equine encephalitis, the figures are even higher. On the other hand, death and serious neurologic sequelae have been observed in only 5 to 15 percent of those with western or eastern equine and West Nile infections and in even fewer patients with Venezuelan, St. Louis, and La Crosse encephalitides.