Bilateral, symmetric signal abnormality in the hippocampal formations in an immunocompromised patient with worsening encephalopathy was concerning for an infectious process such as HHV-6. 

Differential considerations included other infectious sources such as HSV encephalitis and seizure. The patient was not found to be seizing on EEG monitoring. The lack of additional sites of parenchymal involvement (e.g. cingulate gyrus) was unusual for HSV. Additionally, CSF analysis was positive for HHV-6 DNA on PCR and negative for HSV, suggesting HHV-6 as the causative agent.