Long-term Neurological Complication of HHV-6 Encephalitis after Allogeneic Hematopoietic Stem Cell Transplantation

Background: Human herpesvirus 6 (HHV-6) encephalitis is a rare but serious complication of allogeneic hematopoietic stem cell transplantation (HSCT).This study investigated the incidence, clinical features, and long-term neurological sequelae of HHV-6 encephalitis in pediatric and adolescent HSCT recipients.Methods: We retrospectively reviewed 92 patients who were younger than 20 years of age at the time of undergoing allogeneic HSCT between January 2015 and December 2024.HHV-6 encephalitis was diagnosed based on neurological symptoms and the detection of HHV-6 DNA in cerebrospinal fluid using multiplex polymerase chain reaction.Patients with HHV-6 encephalitis were followed for a median of six years (range, 4.5-8.5 years) to assess long-term neurological outcomes.Results: Three patients (3.2%) developed HHV-6 encephalitis between 15 and 26 days post-transplantation, coinciding with neutrophil engraftment.Clinical presentation included fever, seizures, altered consciousness, and short-term memory loss.Neuroimaging revealed high signal intensity lesions in the limbic system.Despite prompt antiviral therapy with foscarnet and initial viral clearance, all patients developed significant long-term neurological sequelae, including persistent cognitive impairment, epilepsy (two with refractory seizures), and memory disturbances.One patient continues to require assistance with activities of daily living, while two others face challenges reintegrating into school and society.Conclusion: HHV-6 encephalitis following allogeneic HSCT in pediatric and adolescent patients can lead to severe and lasting neurological impairment, despite timely antiviral therapy.These long-term sequelae substantially affect quality of life and impose ongoing healthcare and societal burdens.Multidisciplinary long-term care is essential, and further research is warranted to improve prevention and treatment strategies.