Herpes simplex encephalitis is the most common cause of fatal sporadic viral encephalitis and has characteristic imaging findings. Although these abnormalities seen in HSV encephalitis are not unique, more than 90% of patients with proven HSV encephalitis will have magnetic resonance imaging abnormalities involving the temporal lobes. 

The patient was transferred from another hospital already being treated with acyclovir. Lumbar puncture shows CSF pleocytosis, but HSV PCR was negative (false negative?). 

Negative CSF HSV PCR results should always be interpreted in the context of the timing of specimen collection and the likelihood of disease. 

Clinical improvement occurred with treatment. 

The diagnosis is presumptive.