A 16-day-old female infant with predominant brainstem and cerebellar involvement secondary to herpes simplex virus type 2 infection is reported from Children's and Women's Health Centre of British Columbia, Canada. The mother had no history of genital HSV infection and no active lesions at delivery. The infant admitted to the NICU was well for the first 2 weeks of age. At 16 days of age, she developed lethargy, hypotonia, feeding intolerance, and paroxysmal movements, treated with phenobarbital. Within 10 hours of onset of symptoms, the infant was encephalopathic and required assisted ventilation. A PCR of the CSF detected HSV type 2 DNA. Conventional MRI was normal, but diffusion-weighted MRI showed restricted diffusion in the pons, cerebellar peduncles, right cerebellar hemisphere, and vermis. Improvement followed treatment with IV acyclovir for 21 days, and the infant was discharged on oral acyclovir at 38 days. At 9 months follow-up, she had gross and fine motor delay, left esotropia, hypertonia and hyperreflexia, with preservation of alertness, interaction and cognitive function. 
COMMENT. The authors comment on the rare occurrence of brainstem HSV-2 encephalitis in a neonate, and the importance of diffusion-weighted MRI in diagnosis. HSV-2 is the most common cause of neonatal herpes infection, usually affecting premature infants, and transmitted from the mother during birth. The disease in the neonate is disseminated, localized to the CNS, or affects the skin, eyes, and mouth. CNS disease usually presents by the 2nd or 3rd week of life, and typically affects the cerebral cortex diffusely. (AAP Red Book 27th ed. 2006).