Herpes simplex encephalitis with normal brain magnetic resonance imaging and normocellular initial cerebrospinal fluid.

However, after four days, the patient's HSE diagnosis was confirmed by the detection of herpes simplex virus type 1 (HSV1) polymerase chain reaction (PCR) testing. This case highlights the importance of utilizing multiple diagnostic aids and not solely relying on initial test results, as infections may not appear in CSF analysis or MRI scans initially. Furthermore, this case also emphasizes the necessity of initiating empirical treatment based on clinical signs and symptoms, even in cases where diagnostic tests initially appear negative.