Twenty‐one infants who had virologically confirmed exanthem subitum and central nervous system (CNS) complications were studied to elucidate the clinical fearures, laboratory and virological findings, and outcome. The primary infection with human herpesvirus 6 was confirmed by isolation of the virus from blood, a significant rise in the antibody titeres to the virus, or both. All convulsive seizures (15 generalized and 6 focal) occurred during the pre‐eruptive stage of exanthem subitum. Four infants with encephalities/encephalopathy had more severe clinical features with abnormalities demonstrated on electroencephalograms and cerebral computed tomograms. All infants except those with encephalities/encephalopathy recovered without any sequelae. One infant with encephalities/encephalopathy developed epilepsy and another one died. Human herpesvirus 6 DNA amplified by the nested polymerase chain reaction method was detected in the cerebrospinal fluid of 6 infants, including 3 with encephalities/encephalopathy, of 11 parients examined by the fifthy day of the illness. These findings suggest thar CNS complications including encephalitis/encephalopathy occur at the pre‐eruptive stae of exanthem subitum, that human herpesvirus 6 invades the CNS in some patients, and that the outcome is not always benign.
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