Background: Herpes simplex encephalitis (HSE) occurs without regional and seasonal predilections. HSE is important
to differentiate from arboviral encephalitis in endemic areas because of therapeutic potential of HSE.
This study evaluates clinical features, MRI and laboratory findings which may help in differentiating HSE from
Japanese encephalitis (JE).
Methods: Confirmed patients with JE and HSE in last 10 years were included. The presenting clinical symptoms
including demographic information, seizure, behavioral abnormality, focal weakness and movement disorders
were noted. Cranial MRI was done and location and nature of signal alteration were noted. Electroencephalography
(EEG), cerebrospinal fluid (CSF), blood counts and serum chemistry were done. Outcome was measured by
modified Rankin Scale (mRS). Death, functional outcome and neurological sequelae were noted at 3, 6 and
12 months follow up, and compared between HSE and JE. Outcome was categorized as poor (mRS; N 2) and
good (mRS ≤ 2).
Results: 97 patients with JE and 40 HSE were included. JE patients were younger than HSE and occurred in post
monsoon period whereas HSE occurred throughout the year. Seizure (86% vs 40%) and behavioral abnormality
(48% vs 10%) were commoner in HSE; whereas movement disorders (76% vs 0%) and focal reflex loss (42% vs
10%) were commoner in JE. CSF findings and laboratory parameters were similar in both the groups. Thalamic
involvement in JE and temporal involvement in HSE were specific markers of respective encephalitis. Delta
slowing on EEG was more frequent in JE than HSE. 20% JE and 30% HSE died in the hospital, and at 1 year follow
up JE patients showed better outcome compared to HSE (48% vs 24%). Memory loss (72% vs 22%) was the predominant
sequelae in HSE.
Conclusion: Seizure and behavioral abnormality are common features in HSE whereas focal reflex loss is commoner
in JE. In a patient with acute encephalitis, thalamic lesion suggests JE and temporal lobe involvement
HSE. Long term outcome in JE is better compared to HSE.