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| ICD9 = | ICDO = | Image = | Caption = | OMIM = 306900 | MedlinePlus = 000539 | eMedicineSubj = emerg | eMedicineTopic = 240 | DiseasesDB = 5561 | MeshID = D002836 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 second most 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.Biggs RA, Douglas AS, MacFarlane RG, Dacie JV, Pittney WR, Merskey C, O'Brien JR. Christmas disease: a condition previously mistaken for haemophilia. Br Med J 1952;2:1378-1382. PMID 12997790.
Treatment Treatment (bleeding prophylaxis) is by intravenous infusion of factor IX.
Genetics 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.
Pathophysiology 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.
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