Hemophilia B is an X-linked recessive coagulation disorder caused by the deficiency of coagulation factor IX (FIX). It is the rarer form of hemophilia. The deficiency of factor IX leads to a disturbance in the intrinsic pathway of the coagulation cascade and hemorrhagic diathesis with prolonged activated partial thromboplastin time (aPTT).
Phenotypically, hemophilia B is indistinguishable from hemophilia A (factor VIII deficiency). The two syndromes can be distinguished by biochemical measurement of the respective enzyme activity. FIX activity should be measured before genetic testing.
More than 1,000 pathogenic variants are known in the F9 gene that leads to hemophilia B. Depending on the type of variant, there is a varying degree of impairment of enzyme activity; as a result the disease is classified into different levels 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 X-linked inheritance, males are predominantly affected. The estimated prevalence is about 1:30,000. In rare cases, heterozygous carriers may also develop symptoms in the presence of unequal X inactivation. Therapy involves the intravenous substitution of factor IX.
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