Central nervous system (CNS) complications of exanthem subitum were analyzed in 21 infants examined at the Departments of Pediatrics and Neurology, Fujita Health University School of Medicine, Toyoake, Japan. The primary infection with human herpesvirus 6 was confirmed by isolation of the virus from the blood and/or a rise in antibody titers, and the virus DNA was detected in the cerebrospinal fluid of 6 infants, including 3 with encephalitis/encephalopathy (E/E). Convulsive seizures (15 generalized and 6 focal) occurred during the pre-eruptive stage of the illness, and persisted for more than 45 minutes in 7 infants. Four patients with E/E had prolonged focal seizures and loss of consciousness, abnormal EEGs and CT scans; 3 had a pleocytosis and elevated protein in the CSF. One developed epilepsy and one died. Infants without E/E recovered with no sequelae. 
COMMENT. Exanthem subitum is a common, usually benign, infectious disease of infants, characterized by a four day fever and a rash that appears after the fever subsides. Seizures may complicate the illness during the febrile stage and before the rash erupts. Human herpesvirus 6, recently identified as the causative agent, may rarely invade the CNS and cause encephalitis.
A febrile seizure has long been recognized as the most common complication of exanthem subitum, occurring in 22 percent of 581 patients reported in 11 publications . Evidence for an encephalitic process and a direct involvement of the brain was lacking, and the degree of fever was considered sufficient to explain the frequent complication of convulsions. The present report demonstrates that seizures associated with exanthem subitum and fever are not always simple in type. They are occasionally prolonged and complex and a manifestation of encephalitis or encephalopathy.