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Encephalitis is inflammation of the parenchyma of the brain, resulting from direct viral invasion. Acute disseminated encephalomyelitis is brain and spinal cord inflammation caused by a hypersensitivity reaction to a virus or another foreign protein. Both disorders can be caused by many viruses. Symptoms include fever, headache, and altered mental status, often accompanied by seizures or focal neurologic deficits. Diagnosis requires CSF analysis and neuroimaging. Treatment is supportive and, for certain causes, includes antiviral drugs.
Etiology
Encephalitis may be a primary manifestation or a secondary (postinfectious) immunologic complication of viral infection.
Primary viral infection:
Viruses causing primary encephalitis directly invade the brain. These infections may be
Mosquito-borne arboviral encephalitides infect people during the spring, summer, and early fall when the weather is warm. Incidence in the US varies from 150 to > 4000 cases yearly, mostly in children. Most cases occur during epidemics. Among arboviruses, La Crosse virus (California virus) is identified as a cause primarily in the north central US. However, the virus is geographically widespread, and La Crosse encephalitis is probably underrecognized and accounts for most cases of arbovirus encephalitis in children. Mortality rate is probably < 1%. Until 1975, St. Louis encephalitis occurred every 10 yr, mostly in the central and eastern US; it is now rare. As of 2009, West Nile encephalitis has spread from the East Coast, where it first appeared in 1999, to all of the western states. Mortality rate is about 9%. Small epidemics of eastern equine encephalitis occur every 10 to 20 yr in the eastern US, mainly among young children and people > 55. Mortality rate is about 50 to 70%. For unknown reasons, western equine encephalitis has largely disappeared from the US since 1988.
In the US, the most common sporadic encephalitis is caused by herpes simplex virus (HSV); hundreds to several thousand cases occur yearly. Most are due to HSV-1, but HSV-2 may be more common among immunocompromised patients. HSV encephalitis occurs at any time of the year, tends to affect patients < 20 or > 40 yr, and is often fatal if untreated.
Primary encephalitis can occur as a late consequence of a viral infection. The best known types are HIV encephalopathy (which causes dementia—see Delirium and Dementia: HIV-Associated Dementia), subacute sclerosing panencephalitis (which occurs years after a measles infection and is thought to represent reactivation of the original infection—see Other Viruses: Subacute Sclerosing Panencephalitis (SSPE)), and progressive multifocal leukoencephalopathy (which is caused by reactivation of JC virus—see Brain Infections: Progressive Multifocal Leukoencephalopathy (PML)).
Immunologic reaction:
Encephalitis can occur as a secondary immunologic complication of certain viral infections or vaccinations. Inflammatory demyelination of the brain and spinal cord can occur 1 to 3 wk later (as acute disseminated encephalomyelitis); the immune system attacks one or more CNS antigens that resemble proteins of the infectious agent. The most common causes used to be measles, rubella, chickenpox, and mumps (all now uncommon because childhood vaccination is widespread); smallpox vaccine; and live-virus vaccines (eg, the older rabies vaccines prepared from sheep or goat brain). In the US, most cases now result from influenza A or B virus, enteroviruses, Epstein-Barr virus, hepatitis A or B virus, or HIV.
Pathophysiology
In acute encephalitis, cerebral edema and petechial hemorrhages occur throughout the hemispheres, brain stem, cerebellum, and, occasionally, spinal cord. Direct viral invasion of the brain usually damages neurons, sometimes producing visible inclusion bodies. Severe infection, particularly untreated HSV encephalitis, can cause brain hemorrhagic necrosis.
Acute disseminated encephalomyelitis is characterized by perivenous demyelination and absence of virus in the brain.
Symptoms and Signs
Symptoms include fever, headache, and altered mental status, often accompanied by seizures and focal neurologic deficits. A GI or respiratory prodrome may precede these symptoms. Meningeal signs are typically mild and less prominent than other manifestations. Status epilepticus, particularly convulsive status epilepticus, or coma suggests severe brain inflammation and a poor prognosis. Olfactory seizures, manifested as an aura of foul smells (rotten eggs, burnt meat), indicate temporal lobe involvement and suggest HSV encephalitis.
Diagnosis
Encephalitis is suspected in patients with unexplained alterations in mental status. Clinical presentation and differential diagnoses may suggest certain diagnostic tests, but MRI and CSF analysis (including PCR for HSV) are usually done, sometimes with other tests to identify the causative virus. Despite extensive testing, the cause of most cases of encephalitis remains unknown.
MRI:
MRI is sensitive for early HSV encephalitis, showing edema in the orbitofrontal and temporal areas, which HSV typically infects. MRI shows demyelination in progressive multifocal leukoencephalopathy and may show basal ganglia and thalamic abnormalities in West Nile and eastern equine encephalitis. MRI can also exclude lesions that mimic viral encephalitis (eg, brain abscess, sagittal sinus thrombosis). CT is much less sensitive than MRI for HSV encephalitis but can help because it is rapidly available and can exclude disorders that make lumbar puncture risky (eg, mass lesions, hydrocephalus, cerebral edema).
CSF testing:
If encephalitis is present, CSF is characterized by lymphocytic pleocytosis, normal glucose, mildly elevated protein, and an absence of pathogens using Gram stain and culture (similar to CSF in aseptic meningitis). CSF abnormalities may not develop until 8 to 24 h after onset of symptoms. Hemorrhagic necrosis can introduce many RBCs and some neutrophils into CSF, elevate protein, and modestly lower glucose.
PCR testing of CSF for many viruses (eg, HSV-1, HSV-2, varicella-zoster virus, cytomegalovirus, West Nile virus, enteroviruses, JC virus) is becoming increasingly available. PCR for HSV in CSF is sensitive and specific. However, results may not be available rapidly and, despite advances in technology, false-negative and false-positive results may still occur due to a variety of causes, not all being technical failures (eg, the blood in a mildly traumatic CSF tap may inhibit the PCR amplification step).
CSF viral cultures grow enteroviruses but not most other viruses.
CSF viral IgM titers are often useful for diagnosing acute infection, especially West Nile encephalitis, for which they are more reliable than PCR. Paired acute and convalescent serologic tests of CSF and blood must be drawn several weeks apart; they can detect an increase in viral titers specific for certain viral infections.
Brain biopsy:
Brain biopsy may be indicated for patients who are worsening, who are responding poorly to treatment with acyclovir or another antimicrobial, or who have a lesion that is still undiagnosed.
Prognosis
Mortality rate varies with cause, but severity of epidemics due to the same virus varies during different years. Permanent neurologic deficits are more likely to occur in infants.
Treatment
Supportive therapy includes treatment of fever, dehydration, electrolyte disorders, and seizures. Euvolemia should be maintained.
If HSV encephalitis is suspected, acyclovir 10 mg/kg IV q 8 h is started promptly and continued usually for 14 days. Acyclovir is relatively nontoxic but can cause liver function abnormalities, bone marrow suppression, and transient renal failure. Giving acyclovir IV slowly over 1 h helps prevent nephrotoxicity.
Last full review/revision December 2009 by Michael Jacewicz, MD
Content last modified December 2009
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