Causes
Encephalitis is most commonly viral. Herpes simplex virus type 1 is the most important sporadic cause because it is treatable and, untreated, is often fatal; it characteristically affects the temporal lobes. Other causes include arboviruses (such as West Nile), enteroviruses, and, in immunocompromised patients, CMV and others. Autoimmune encephalitis (such as anti-NMDA receptor encephalitis) is an increasingly recognized non-infectious cause.
Encephalitis vs Meningitis
Meningitis is inflammation of the meninges and presents with fever, headache, and neck stiffness, with preserved brain function. Encephalitis involves the brain parenchyma itself, so its hallmark is altered mental status — confusion, behavioral change, or decreased consciousness — along with focal neurologic deficits and seizures. The two can overlap (meningoencephalitis), and the presence of brain dysfunction points to encephalitis.
Diagnosis
Evaluation includes neuroimaging and lumbar puncture. MRI is more sensitive than CT and may show temporal lobe involvement in HSV encephalitis. Cerebrospinal fluid typically shows a lymphocytic pleocytosis with elevated protein and normal glucose, and HSV is confirmed by polymerase chain reaction (PCR) of the CSF, the test of choice. An EEG may show characteristic temporal changes.
Treatment
Because HSV encephalitis is treatable and rapidly dangerous, empiric intravenous acyclovir is started immediately whenever it is suspected, without waiting for PCR results, and continued if HSV is confirmed. Supportive care manages seizures and raised intracranial pressure. Other specific causes are treated accordingly, and autoimmune encephalitis is treated with immunotherapy. Prompt acyclovir markedly improves outcomes in HSV disease.