Haemophilia B (also spelled Hemophilia B or Hæmophilia B) is a blood clotting disorder caused by a mutation of the Factor IX gene, leading to a deficiency of Factor IX. It is the least common form of haemophilia, rarer than haemophilia A. It is sometimes called Christmas disease after Stephen Christmas, the first patient described with this disease. In addition, the first report of its identification was published in the Christmas edition of the British Medical Journal.
Treatment (bleeding prophylaxis) is by intravenous infusion of factor IX. Factor IX has a longer half life than factor VIII (Deficient in Haemophilia A) and as such factor IX can be transfused less frequently.
The factor IX gene is located on the X chromosome
(Xq27.1-q27.2). It is an X-linked
recessive trait, which explains why, as in haemophilia A, only males are usually affected.
Factor IX deficiency leads to an increased propensity for haemorrhage
. This is in response to mild trauma or even spontaneously, such as in joints (haemarthrosis
) or muscles.