Encephalitis is an inflammation of the brain parenchyma. It can be caused by infectious (usually viral) agents or due to immunologic mechanisms, as in post-infectious encephalitis. Approximately 20,000 cases occur per year in the United States, most of them mild. Viruses enter the host through the skin or through the respiratory, gastrointestinal or urogenital tracts. The virus then enters the CNS either by hematogenous spread, by retrograde transmission along neuronal axons, or by direct invasion following infection of the nasal mucosa. Viral replication in neural cells leads to cell dysfunction and death, with resulting cerebral edema and increased ICP.
Herpes simplex virus (HSV) is the most common cause of viral encephalitis in the United States (about 10% of cases). Disease in neonates is often caused by HSV type 2 acquired from perinatal transmission, but HSV type 1 is the most common cause in all other patients. The mortality rate is greater than 70% if left untreated, but is approximately 30% with treatment. Survivors commonly have significant neurologic sequelae.
Other viruses include arboviruses (such as St. Louis encephalitis, eastern or western equine encephalitis, West Nile encephalitis, LaCrosse encephalitis), rabies, mumps, measles, and varicella. The arthropod-borne encephalitides are transmitted by mosquitoes from a host, such as wild birds, to horses or humans. Outbreaks occur periodically in the United States in the summer and fall.
Post-infectious encephalomyelitis may follow infection with various viruses, most commonly varicella and influenza in the United States, but measles is the most common cause worldwide. It often follows a vague viral syndrome, usually of the respiratory tract, and occurs days to weeks after the preceding illness. It is thought to be an autoimmune phenomenon that is initiated by the viral pathogen. Demyelination is a prominent pathological finding of this disease.
Clinical manifestations include headache, fever, altered level of consciousness, disturbances in behavior and speech, and generalized seizures. Focal neurologic findings generally indicate HSV infection and include ataxia, cranial nerve defects, hemi-paresis, or focal seizures. Skin lesions such as the characteristic exanthem of varicella or vesicles that suggest HSV type 1 or 2 may be present. History may reveal the bite of a potentially rabid animal, and knowledge of disease outbreaks in the community (such as mosquito-borne viruses) may help to guide diagnosis. Differential diagnosis includes metabolic diseases, toxic disorders, mass lesions, and acute demyelinating disorders.
Cerebrospinal fluid must be obtained to exclude bacterial meningitis. CSF should be sent for cell count, protein, glucose, Gram stain, bacterial and viral cultures, and HSV PCR. In viral encephalitis, CSF cell count usually shows fewer than 500 cells/mm3, with a mononuclear prominence, though the cells may be predominantly polymorphonuclear early in the course of illness. CSF protein is normal to moderately elevated and glucose is usually normal. Detection of herpes virus by polymerase chain reaction (PCR) of CSF leads to a rapid diagnosis of HSV infection. Viral isolation from other body sites, such as the nasopharynx, skin lesions, urine, and feces may aid in diagnosis. Blood samples for CBC with differential, platelet count, electrolytes, BUN, creatinine, glucose, blood culture, and viral titers should be obtained. Detection of viral antibodies does not aid in acute diagnosis or management, but may be useful later to clarify the cause of the infection.
CT or MRI may show focal parenchymal involvement or temporal lobe edema in HSV encephalitis. EEG shows characteristic periodic spike wave temporal lobe activity and slow wave complexes in HSV encephalitis. It is usually normal in other encephalitides.
Management of non-herpes viral encephalitis is supportive, though herpes simplex virus and varicella-zoster encephalitis are treated with acyclovir, 30-60 mg/kg/day divided q 8 h, in addition to supportive measures. Acyclovir should be administered until the virus is identified, especially if there is evidence of focality on physical or neurodiagnostic evaluation. Antibiotics should also be given until a bacterial etiology is excluded. Medical management of rabies is prevention of infection by the administration of rabies vaccine and rabies immune globulin after an exposure to a potentially rabid animal. Once infection occurs, it is uniformly fatal. Arboviral infections can be prevented by using insect repellents and protective clothing to avoid mosquito bites. Treatment of infection is supportive.
Epidemics of arthropod-borne encephalitis can occur during natural disasters if heavy rainfall and flooding lead to increased numbers of mosquitoes, or if insect control measures are disrupted. There have been recent epidemics of West Nile virus in the United States. In addition, the CDC lists Venezuelan, eastern, and western encephalitis as potential biological weapons. An unexplained large number of cases should raise suspicion for a potential act of bioterrorism.
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