Clinical meaning
Hemophilia is an X-linked recessive bleeding disorder caused by deficiency or dysfunction of specific coagulation factors in the intrinsic pathway of the coagulation cascade. Hemophilia A (classic hemophilia, ~80% of cases) results from deficiency of factor VIII, while hemophilia B (Christmas disease, ~20% of cases) results from deficiency of factor IX. Because the genes for both factors VIII and IX are located on the X chromosome, hemophilia predominantly affects males (who have only one X chromosome), while females are typically carriers who may have mildly reduced factor levels but are usually asymptomatic. Severity is classified by factor activity level: severe (<1% factor activity) presents with spontaneous bleeding into joints (hemarthrosis) and muscles, often beginning in infancy when the child starts crawling or walking; moderate (1-5%) causes bleeding with minor trauma or surgery; mild (5-40%) may only bleed with significant trauma or surgery and may not be diagnosed until adulthood. The coagulation cascade involves two converging pathways (intrinsic and extrinsic) that culminate in the common pathway activation of factor X, leading to thrombin generation and fibrin clot formation. Factors VIII and IX function together in the intrinsic pathway as the tenase complex: activated factor IX (IXa) is the enzyme, and activated factor VIII (VIIIa) is its essential cofactor. Together, they activate factor X approximately 200,000 times more efficiently than factor IXa alone. When either factor is deficient, the intrinsic pathway is impaired, resulting in delayed and inadequate thrombin generation and unstable clot formation. Laboratory findings show prolonged activated partial thromboplastin time (aPTT, which measures the intrinsic pathway) with normal prothrombin time (PT, which measures the extrinsic pathway) and normal bleeding time/platelet function (primary hemostasis is intact). The practical nurse must recognize that hemophilia bleeding is characteristically deep (joints, muscles, internal organs) rather than mucocutaneous (which suggests platelet disorders), and that joint bleeds (hemarthrosis) are the most common and disabling complication requiring prompt factor replacement.