Haemophilia C (also known as plasma thromboplastin antecedent (PTA) deficiency or Rosenthal syndrome) is a mild form of haemophilia affecting both sexes. However, it predominantly occurs in Jews of Ashkenazi descent. It is the fourth most common coagulation disorder after von Willebrand's disease and haemophiliaA and B. In the USA it is thought to affect 1 in 100,000 of the adult population, making it 10% as common as haemophilia A.
It is caused by a deficiency of coagulationfactor XI and is distinguished from haemophilia A and B by the fact it does not lead to bleeding into the joints. Furthermore, it has autosomal inheritance, since the gene for factor XI is located on chromosome 4 (close to the prekallikrein gene); and it is not completely recessive, individuals who are heterozygous also show increased bleeding. Many mutations exist, and the bleeding risk is not always influenced by the severity of the deficiency. Treatment is usually not necessary, except in relation to operations, leading to many of those having the condition not being aware of it. In these cases, fresh frozen plasma or recombinant factor XI may be used, but only if necessary. The afflicted may often suffer nosebleeds, and females can experience heavy menstrual bleeding. Haemophilia C was first discovered in a young Ashkenazic Jewish American in the 1950s.