Low-voltage activity in EEG during acute phase of encephalitis predicts unfavorable neurological outcome

The purpose of this study was to evaluate the risk factors responsible for the onset of postencephalitic epilepsy during the acute phase of encephalitis. We retrospectively studied 19 patients with acute encephalitis admitted to the Division of Pediatrics at our hospital from January 1989 to December 1998. Nine cases were complicated by postencephalitic epilepsy and 10 were non-complicated cases. Significant risk factors for postencephalitic epilepsy were seizures, more severe impairment of consciousness, abnormalities in cranial computed tomography/magnetic resonance imaging/single photon emission computed tomography examination and low-voltage EEG activity, which was defined as EEG findings showing less than 20 μV of background activity without the appearance of a hump or spindle, during the acute phase of encephalitis. In particular, cases with low-voltage EEG activity were all complicated by intractable epilepsy after less than 1 year. Low-voltage EEG activity during the acute phase predicts an unfavorable neurological outcome, reflecting severe widespread impairment in the whole of the cortex.