Hemophilia B is an X chromosomal recessive coagulation disorder, caused by a deficiency of the coagulation factor IX. It is the rarer form of classic hemophilia. The deficiency in factor IX results in a disorder in the intrinsic pathway of the coagulation cascade and leads to signs and symptoms of a hemorrhagic diathesis with prolonged aPTT time.
Hemophilia B cannot be distinguished from hemophilia A (factor VIII deficiency) regarding its phenotype. The two disorders are distinguished by biochemical measurement of the respective enzyme activity.
More than 1,000 mutations causing hemophilia B have been identified in the F9 gene. Depending on the mutation, the enzyme activity is impaired to varying degrees; therefore the disease is categorized into different degrees of severity.
- Severe hemophilia B: factor IX activity <1%
- Moderate hemophilia B: factor IX activity 1-5%
- Mild hemophilia B: factor IX activity 5-15%
- Sub-hemophilia B: factor IX activity 15-50%
Due to the X chromosomal inheritance pattern, mainly men are affected. The prevalence is estimated to be approx. 1 in 30,000. In rare cases, female heterozygous carriers with a skewed X inactivation may develop signs and symptoms as well. Treatment consists of intravenous administration of recombinant factor IX.