A case of Baker’s cyst with moderate hemophilia B confused with intraarticular bleeding during diagnosis

Abstract

Baker’s cyst is rare in children with hemophilia. A five-year-old boy with moderate hemophilia B received prophylactic Eftrenonacog alfa therapy. His left knee swelled after hitting a chair. Despite treatment with Eftrenonacog alfa, the swelling did not subside. Computed tomography showed the possibility of Baker’s cyst. However, it was diagnosed as obsolete hematoma by ultrasonography and he was treated with Nonacog alfa. Although the size of the hematoma was not reduced, the swelling was improved and he was discharged. However, the size gradually increased. Thus, we hypothesized new bleeding and he was treated with Albutrepenonacog alfa. The mass was suspected as Baker’s cyst on the basis of magnetic resonance imaging (MRI) findings. We punctured it as therapeutic diagnosis and saw hemorrhagic mucous fluid. We diagnosed bleeding in Baker’s cyst. It is likely that ultrasonography has higher sensitivity in detecting bleeding. In atypical cases of knee swelling, we should assess it by ultrasonography and MRI.