Haemophilia A and B

Overview & Definition

Picture the eighteen-month-old boy brought in with a swollen, warm, painful right knee that appeared overnight after he crawled, his mother telling you he has had two similar episodes and that her brother was a "bleeder" who needed injections. That boy has severe haemophilia, and the overnight swollen joint is a haemarthrosis. The family history on the mother's side is the X-linked recessive clue, and the joint bleed is the clinical signature of secondary haemostasis failing. [1]

Haemophilia is an inherited bleeding disorder caused by a deficiency of one of the coagulation factors of the intrinsic pathway that generates thrombin. Haemophilia A is deficiency of factor VIII, encoded by the F8 gene at Xq28, and accounts for about 80 to 85 percent of cases. Haemophilia B is deficiency of factor IX, encoded by the F9 gene at Xq27, also called Christmas disease after the first patient described, and accounts for 15 to 20 percent. Both are inherited X-linked recessive, so they almost exclusively affect males, while females are usually asymptomatic carriers. The two conditions are clinically indistinguishable, share the same severity scale, and are distinguished only by the specific factor assay. [2] [3]

The defining principle is that severity is determined not by the gene mutation but by how much residual factor activity the child has left. A child with severe haemophilia has a factor level under 1 percent of normal and bleeds spontaneously into joints and muscles. A child with mild haemophilia, a level over 5 to 40 percent, bleeds only after surgery or major trauma and may not be diagnosed until adulthood. This single relationship between residual factor activity and clinical phenotype is the backbone of every management decision that follows. [1] [4]