Fatal outcome of BK virus encephalitis in an allogeneic stem cell transplantation recipient

BK virus (BKV) encephalitis is a rare complication after hematopoietic stem cell transplantation (HSCT). A 43-year-old woman with recurrent follicular lymphoma after autologous HSCT received allogeneic bone marrow transplantation from a human leukocyte antigen-matched related donor. Neutrophil engraftment was achieved on post-transplant day 13. Memory loss and noncooperative attitude toward the medical staff were observed on day 16, and her mental status worsened progressively. Magnetic resonance imaging (MRI) showed nonspecific findings on day 19; however, cerebrospinal fluid (CSF) analysis including real-time polymerase chain reaction on day 20 revealed elevated levels of BKV 4.67 × 10⁴ copy/mL. BKV encephalitis was diagnosed based on CSF findings, intravenous administration of immunoglobulin and cidofovir was started, and the immunosuppressive agent dose was reduced. Diffusion-weighted MRI on day 28 showed signal abnormalities in the bilateral periventricular white matter. Although the follow-up CSF analysis on day 35 was negative for BKV, her mental status and MRI findings did not improve, and she died on day 55 because of respiratory failure. This case emphasizes the importance of considering BKV encephalitis as a differential diagnosis of post-transplant encephalitis, considering the central nervous system-associated immune reconstitution inflammatory syndrome in patients with worsening central nervous system findings after eradication of BKV in the CSF.