Asymptomatic self-limiting white matter lesions in the chronic phase of herpes simplex encephalitis

We report a 9-month-old girl with herpes simplex encephalitis. T2-weighted magnetic resonance imaging (MRI) on the 8th day of illness showed increased signal intensity in the gray and white matter of the bilateral lobes, especially the right temporo-parietal lobe. High voltage slow waves were recorded in the right parieto-occipital region on electroencephalography. She was discharged on the 34th day of illness without sequelae. On discharge, the high signal intensity on MRI had improved and the abnormal slow waves on electroencephalography had disappeared.At 4 months after the onset of illness, MRI revealed decreased signal intensities in T1-weighted images and increased signal intensities in T2-weighted images in the periventricular white matter adjacent to the posterior horns of the lateral ventricles. The white matter lesions had expanded at 9 months after the onset of illness. At 14 months after the onset of illness, the white matter lesions were reduced in size, and at 2 years after the onset of illness, they had completely disappeared. During 2 years follow-up after discharge, however, there were no neurological abnormalities corresponding to the lesions and her psychomotor development was normal. Although the mechanism underlying these white matter lesions had not been elucidated, an immune-mediated complication such as edema or demyelination is suspected. Repeat MRI studies over a long period are necessary to follow up a patient with herpes simplex encephalitis.