The hemophilias are a group of related bleeding disorders that most commonly are inherited. Inherited bleeding disorders include abnormalities of coagulation factors and platelet function; the most common of these is von Willebrand disease. However, when the term "hemophilia" is used, it most often refers to the following two disorders:
- Factor VIII deficiency (hemophilia A)
- Factor IX deficiency (hemophilia B, Christmas disease)
The clinical manifestations and diagnosis of these conditions will be reviewed here. Therapy with replacement factors and the development of inhibitors are discussed separately. (See "Treatment of hemophilia" and "Factor VIII and factor IX inhibitors in patients with hemophilia".)
Hemophilia A and B are X-linked recessive diseases. They exhibit a range of clinical severity that correlates well with assayed factor levels. Severe disease is defined as <1 percent factor activity, whereas 1 to 5 percent and >5 percent of normal are defined as moderate and mild disease, respectively [1,2]. Males within a family have the same level of deficiency because they share the same genetic defect.
The combined incidence of hemophilia A and B is 1 in 5000 live male births. Approximately 80 percent have hemophilia A, two-thirds of whom have severe disease. In contrast, almost one-half of individuals with hemophilia B (which occurs in 1 in 25,000 to 1 in 30,000 births) have factor IX levels above 1 percent (ie, moderate or mild disease). Hemophilia A and B of comparable severity bleed with similar frequency .