Herpes Simplex Encephalitis presentation in Infants and Toddlers: what should we expect?

Herpes simplex virus encephalitis (HSE) is a treatable form of acute sporadic encephalitis with a high mortality rate of 50-70% if left untreated or inadequately treated. Diagnosis relies on identifying the classical involvement of the medial and inferior aspects of the temporal lobe.

Although uncommon in children below three years old, HSE in this age group exhibits a unique presentation involving hematogenous spread leading to cortical or white-matter lesions across all lobes, insulae, or thalami per the vascular distribution pattern.

A case report highlights an 18-month-old girl who initially presented with febrile encephalopathy. Despite a normal MRI on day 5, suspicion of viral encephalitis emerged in the second week of illness, leading to the administration of acyclovir (30 mg/kg/d for a week). A repeated MRI after ten days revealed cortical hyperintensities in the left parietal lobe. Acyclovir was discontinued due to the atypical nature and negative HSV-IgM/IgG results. In the fourth week, the child exhibited choreoathetosis, reduced sleep, and irritability. A subsequent MRI indicated T1-hypointense and corresponding T2/FLAIR-hyperintensities in the left temporo-parieto-occipital region and bilateral thalami, indicating Post-HSE autoimmune encephalitis. Positive HSV-1 IgM/IgG and CSF-NMDAR confirmed the final diagnosis as HSE with NMDAR antibody encephalitis. The child showed improvement after two months of immunotherapy and supportive care.

In HSV encephalitis, the imaging pattern differs in younger children, with up to 55% experiencing extratemporal involvement, including the frontal, parietal, occipital lobes, thalami, limbic system, and brain stem. Pediatricians must thus be mindful of its different presentation in infants and toddlers.

Kamate M, Basavanagowda T. Is Herpes Simplex Encephalitis Different in Infants and Toddlers?. Indian J Pediatr. 2023; 90:1269. https://doi.org/10.1007/s12098-023-04784-6