Temporal lobe involvement in Japanese encephalitis: problems in differential diagnosis.

BACKGROUND AND PURPOSE On MR imaging and CT, Japanese encephalitis (JE) shows lesions in the thalami, substantia nigra, basal ganglia, cerebral cortex, cerebellum, brain stem, and white matter, whereas temporal lobe involvement is characteristically seen in Herpes simplex encephalitis (HSE). Temporal lobe involvement in JE may cause problems in differentiating it from HSE. We undertook this study to show the temporal lobe involvement pattern in JE and highlight differentiating features from temporal lobe involvement in HSE. METHODS Sixty-two patients with JE underwent CT or MR imaging or both. MR imaging was done in 53 and CT in 53. The diagnosis of JE was confirmed by cerebrospinal fluid (CSF) IgM enzyme-linked immunosorbent assay. RESULTS Eleven (17.7%) patients showed temporal lobe involvement with abnormal MR imaging in all. All the patients showed hippocampal involvement. Two patients showed extension of lesions into the amygdala and uncus with insular involvement in 1. The rest of the temporal lobe was spared. All patients had thalamic and substantia nigra involvement with basal ganglia involvement in 7. Six of 9 CT scans were abnormal and the temporal lesions were seen in 2. CONCLUSIONS The temporal lobe involvement pattern is fairly characteristic and mostly involves the hippocampus, usually sparing the rest of the temporal lobe. This and the concurrent involvement of the thalami, substantia nigra (SN), and basal ganglia allow differentiation from HSE. However, if the temporal lobe involvement is more severe, laboratory tests may be the only way to differentiate it from HSE, and it may be prudent to start antiviral therapy in the interim period.