Evaluate for other (especially easily treatable) causes of symptoms
MRI brain: non-enhancing,<unk>  areas of white matter demyelination
PCR for JC virus (CSF, blood, urine)
Progressive multifocal leukoencephalopathy
Hypoxic encephalopathy
Primary CNS lymphoma
Acute toxic-metabolic encephalopathy 
Drug induced aseptic meningitis
Hypoglycemia
Japanese encephalitis
Hyperosmolar state 