Last updated: September 12, 2023

Summarytoggle arrow icon

Encephalitis is an inflammation of the brain parenchyma, caused by viral (most common), bacterial, parasitic, or fungal infection (infectious encephalitis) or an antibody-mediated autoimmune response (autoimmune encephalitis). Prodromal symptoms include fever, headache, and nausea, with progression to neurological and psychiatric symptoms within hours to weeks, including altered mental status, seizures, focal neurological deficits, hallucinations, and psychosis. Diagnosis is based on laboratory studies (serology, CSF analysis), MRI of the head, and EEG. Epidemiologic clues (e.g., season of the year, geographic location, travel, and exposure history) and clinical features may guide the diagnostic process. Additional testing with antibody detection of specific pathogen antibodies or autoantibodies is not necessary for all patients and should be performed as directed by clinical suspicion. Brain biopsy is limited to idiopathic cases that do not respond to empiric treatment. Paraneoplastic encephalitis, mediated by autoantibodies targeted against intracellular antigens, may be the first manifestation of malignancy, which is why malignancy screening may be part of the diagnostic workup. Treatment is based on the etiology and may include antiinflammatory or immunomodulatory treatment. Acyclovir or empirical antibiotic treatment should be started immediately if viral or bacterial encephalitis is suspected, respectively.

Definitiontoggle arrow icon

Anatomical classification

Etiological classification

Etiologytoggle arrow icon

Infectious encephalitis

Causes of infectious encephalitis include viruses, bacteria, and, less frequently, fungi and protozoa.

Viral encephalitis

Bacterial encephalitis

Autoimmune encephalitis

Classification of autoimmune encephalitis
Characteristics Associated with intracellular neuronal antigens Associated with surface neuronal antigens
Tumor association

Clinical featurestoggle arrow icon

Subacute onset of psychiatric symptoms in combination with seizures and/or focal neurological deficits should raise clinical suspicion for encephalitis.

Diagnosticstoggle arrow icon


Management of encephalitis should focus on identifying and treating the underlying causes of encephalitis (see “Etiology” above).Diagnostic and treatment steps should be initiated simultaneously and empiric treatment should not be delayed for diagnostic steps.


A normal brain MRI does not exclude the diagnosis of encephalitis.

Always consider HSE when imaging suggests potential meningoencephalitis and temporal lobe involvement; bilateral temporal lobe abnormality is pathognomonic of HSE.

Laboratory studies

Other studies

Differential diagnosestoggle arrow icon

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon


Infectious encephalitis

Immune-mediated encephalitis

Referencestoggle arrow icon

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