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TO THE EDITOR: Hemophilia B, also known as Christmas disease, is an X-linked disorder caused by either the absence or reduced biosynthesis of clotting factor IX. This disorder affects approximately 1 in 30,000 male individuals worldwide [1]. It is five times less common than hemophilia A. On the other hand, Down syndrome (DS), the most common human chromosomal anomaly, results from trisomy of chromosome 21 and leads to mental retardation. Besides, it represents many consistent phenotypes including characteristic facies, intellectual disability, congenital heart diseases, and gastrointestinal abnormalities. In particular, hematological abnormalities include transient abnormal myelopoiesis, acute megakaryoblastic leukemia, and transient thrombocytopenia/polycythemia/neutrophilia [2]. We present a case with a rare phenotype, i.e., the coexistence of hemophilia B and DS: one X-linked disorder and the other a chromosomal disorder. Here, we also describe the management of this rare coexistence.
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