To the Editor: Herpes simplex virus encephalitis (HSE) is a treatable cause of acute sporadic encephalitis with a mortality rate of 50-70% with no or incomplete treatment [1, 2]. Diagnosis is based on the classical involvement of the medial and inferior aspects of the temporal lobe. However, this is not usually seen in children below 3 y, where hematogenous spread, either by diffusing through the blood-brain-barrier or by infecting the endothelial cells of blood vessels in the brain, leads to cortical or white-matter lesions that involves all lobes, insulae, or thalami corresponding to the vascular distribution pattern [1,2,3]. Unfortunately, the awareness of this fact is lacking amongst radiologists and pediatricians thereby leading to missed/delayed diagnosis.

An 18-mo-old girl presented with febrile encephalopathy. MRI on day-5 was reported normal. Suspecting viral encephalitis in 2nd wk of illness, acyclovir was given (30 mg/kg/d for a wk). MRI repeated after 10 d showed cortical hyperintensities in left parietal lobe. As this was not classical of HSE and HSV-IgM/IgG were negative, acyclovir was stopped. She presented to us in the 4th wk with choreoathetosis, reduced sleep, and irritability. On reviewing the initial MRI, we found mild T2-hyperintensities in the parieto-occipital region with diffusion restriction (Supplementary Fig. S1). MRI done in the 4th wk of illness showed T1-hypointense and corresponding T2/ FLAIR-hyperintensities in left temporo-parieto-occipital region and bilateral thalami. Post-HSE autoimmune encephalitis was thought of and worked up [4]. Repeat HSV-1 IgM/IgG, and CSF-NMDAR (N-Methyl D-Aspartate Receptor) were positive thereby providing a final diagnosis of HSE with NMDAR antibody encephalitis. She improved after two months of immunotherapy and supportive care.

In HSV encephalitis, the imaging pattern is different from that what is seen in older children. Extratemporal involvement can be seen in up to 55%, including frontal, parietal, occipital lobes, thalami, limbic system, and brain stem. Pure extratemporal involvement can occur in 15% [1].