Human herpesvirus 6 encephalitis.

Human herpesvirus (HHV) 6, the etiologic agent of roseola, is nearly universally acquired during childhood. The virus establishes lifelong infection, including within the central nervous system (CNS), and replicates within several CNS cell types. HHV-6 has been linked to CNS disease during primary infection, including febrile seizures and possibly hippocampal injury. HHV-6 may also be associated with neurologic disease later in life, particularly in transplant patients. Recent reports offer evidence that HHV-6 reactivation may underlie a characteristic limbic encephalitis syndrome following hematopoietic cell transplant; the cardinal features of this syndrome include memory loss, insomnia, electroencephalographic evidence of temporal lobe seizure activity, MRI signal intensity abnormalities of the mesial temporal lobe, and the syndrome of inappropriate release of antidiuretic hormone. HHV-6 DNA is frequently detectable by nucleic acid amplification tests in the cerebrospinal fluid and peripheral blood upon symptom onset, which may provide a screening strategy in high-risk patients. Possible associations of HHV-6 with meningoencephalitis, mesial temporal lobe epilepsy, and multiple sclerosis in apparently immunocompetent hosts are under investigation.