Herpes simplex encephalitis presenting as stroke-like symptoms with atypical MRI findings and lacking cerebrospinal fluid pleocytosis.

A 73-year-old woman presented with sudden onset of right hemiparesis and was diagnosed as having cerebral infarction on the basis of diffusion-weighted brain MRI, which demonstrated lesions in the left parietal cortex. On the 3rd day, the patient developed right upper limb myoclonus, aphasia, and disturbance of consciousness with high fever. On the 6th day, she was transferred to our hospital with suspected viral encephalitis, and treatment with acyclovir was started. By the 6th day, the lesions detected by MRI had expanded to the gyrus cinguli, insula and thalamus, but not to the temporal lobe. At that time, the CSF cell count was 8/μl, and this later increased to 17/μl by the 13th day. Although herpes simplex virus DNA was detected in the CSF on the 6th day, there was no evidence of CSF pleocytosis or temporal lobe abnormalities demonstrable by brain MRI throughout the whole follow-up period. This was very atypical case of herpes simplex encephalitis characterized by a stroke-like episode, atypical MRI findings, and absence of cerebrospinal fluid pleocytosis. It is important to be mindful that herpes simplex encephalitis (HSE) can have an atypical presentation, and that sufficient acyclovir treatment should be initiated until HSE can be ruled out.