Pathophysiology

1) infection and inflammation of brain parenchyma 2) sometimes occurs with meningeal and spinal cord involvement

Signs and Symptoms

1) high fever 2) headache (often severe) 3) stiff neck 4) mental status changes (from lethargy to confusion/delirium/hallucinations to coma) 5) seizures 6) focal neurological deficits – ataxia, aphasia, hemiparesis, ocular palsies

Characteristic Test Findings

Lumbar puncture – 1) CSF findings – identical to viral meningitis: increased lymphoctyes, normal glucose (except in mumps, which is decreased), and normal or slightly elevated protein 2) diagnosis is by CSF PCR

Histology/Gross Pathology

1) inflammatory exudate with lymphoctyic infiltration of small arteries and veins 2) hemorrhage 3) in herpes simplex 1, temporal lobes are more often affected 4) in Epstein-Barr cases, atypical lymphocytes are found in brain parenchyma

Associated Conditions

1) herpes simplex type 1 2) varicella 3) enteroviruses 4) mumps 5) arboviruses – easter equine, western equine, Venezuelan, California viruses 6) west Nile virus

Inheritance/Epidemiology

20,000 cases/year in USA

Treatment

1) acyclovir empirically until herpes simplex virus type 1 is ruled out 2) ganciclovir and foscarnet in CMV 3) ribavirin in California (LaCrosse) virus 4) prophylactic antiseizure medications 5) fluid restriction

Tips for USMLE

1) eastern equine virus is usally the most severe 2) mumps is unusual among viral encephalitis because glucose is decreased instead of normal 3) neurologic findings are usually more dramatic and acute than in bacterial causes