Diagnosis unlikely to be made in the ED
Evaluate for alternative diagnoses
MRI Brain- usually increased T2 signal involving one or both temporal lobes.
LP- CSF w/lymphocytosis (though usually <100), elevated protein, normal glucose, elevated IgG index, oligoclonal bands
Limbic encephalitis
Hypoxic encephalopathy
Japanese encephalitis
Acute toxic-metabolic encephalopathy 
Anti-NMDA receptor encephalitis
Hypoglycemia
Drug induced aseptic meningitis
Hyperosmolar state 