Encephalitis refers to direct inflammation of brain tissue. Patients present with abnormalities in brain function, altered mental status, personality changes, behavioral changes, sensory deficits, motor deficits, and speech disorders. Other manifestations include hemiparesis, flaccid paralysis, seizures, and paresthesias.

To distinguish encephalitis from other causes of encephalopathy, core components of the presentation of encephalitis include the presence of fever (within 72 hours before or after presentation), cerebrospinal fluid (CSF) pleocytosis (≥ 5 /mm³), or MRI or electroencephalogram (EEG) changes consistent with encephalitis.

Encephalitis is often secondary to a virus but may be caused by bacteria, fungi, or toxins. Three of the most common etiologic agents for acute encephalitis are viral including herpes simplex virus (HSV), varicella-zoster virus (VZV), and enterovirus. Alternatively, encephalitis can be caused by autoimmune conditions (anti-NMDAR encephalitis, lupus cerebritis) or inflammatory or paraneoplastic conditions, or be a consequence of stem cell transplant. Patients with encephalitis may also have concomitant meningitis (meningoencephalitis).

Mimics of encephalitis include systemic infections without direct central nervous system (CNS) infection, metabolic derangements, toxic exposures, vascular disease, and some inflammatory states.

Related topics: Behçet syndrome, antibody-mediated encephalitis, viral encephalitides (see also California encephalitis group, Oropouche fever, Venezuelan equine encephalitis, Western equine encephalitis, Eastern equine encephalitis, St. Louis encephalitis, Japanese encephalitis, West Nile virus), varicella, herpes zoster