[Human Herpesvirus-6B Encephalitis]

Human herpesvirus 6B (HHV-6B) is a β-herpesvirus that establishes latency following primary infection during early childhood. In immunocompromised individuals, particularly those who have undergone allogeneic hematopoietic stem cell transplantation, HHV-6B reactivation occurs in 30-80% of cases, with notably high rates observed after cord blood or HLA-mismatched transplants. HHV-6B encephalitis poses a serious and often life-threatening neurological complication for these transplant recipients. The mortality rate associated with this condition remains approximately 30-50%, and many survivors endure long-term neurocognitive sequelae. Clinical manifestations include altered consciousness, memory impairment, and seizures, which are often preceded by hyponatremia resulting from the syndrome of inappropriate antidiuretic hormone secretion. A definitive diagnosis requires the detection of HHV-6B DNA in cerebrospinal fluid by polymerase chain reaction, the exclusion of inherited chromosomally integrated HHV-6, and the ruling out of other potential etiologies. Due to the rapid progression and poor prognosis associated with delayed treatment, empirical antiviral therapy should be initiated promptly whenever HHV-6B encephalitis is suspected. In Japan, foscarnet is recommended as the first-line treatment due to its superior clinical outcomes and antiviral potency, whereas ganciclovir serves as a second-line alternative. Recent advancements in early diagnosis and timely treatment have contributed to improved survival outcomes.