Bleeding Disorders

Core Idea

Bleeding disorders result from defects in vascular integrity, platelet function, or coagulation factors. Classification by hemostatic phase reveals pathophysiology: platelet disorders cause mucocutaneous bleeding, and factor deficiencies cause deep tissue or joint bleeding.

How It's Best Learned

Correlate laboratory findings (bleeding time, PT, aPTT, platelet count) with clinical presentation. Study hemophilia A and B as paradigms of factor X deficiency.

Common Misconceptions

Low platelet count does not always cause bleeding—function matters more than quantity. Vitamin K deficiency affects multiple factors (II, VII, IX, X), not just one.

Explainer

From your study of hemostasis, you know that stopping a bleed requires two sequential processes: primary hemostasis, in which platelets adhere to damaged endothelium and form a soft platelet plug, and secondary hemostasis, in which the coagulation cascade generates thrombin and solidifies the clot with a fibrin mesh. Bleeding disorders map almost perfectly onto which of these processes is broken. Understanding the clinical pattern of bleeding — where it occurs and how severe it is — immediately localizes the defect.

Platelet disorders disrupt primary hemostasis and cause mucocutaneous bleeding: nosebleeds, gingival oozing, petechiae (pinpoint skin hemorrhages), and prolonged bleeding from small cuts. The platelet plug is the first responder at small vessels where turbulent flow and mechanical stress are constant. When that plug is defective, small-vessel wounds don't seal. Platelet disorders come in two flavors: *thrombocytopenia* (too few platelets — from bone marrow failure, immune destruction, or sequestration) and *thrombopathia* (enough platelets but they function poorly — as in aspirin use, which irreversibly inhibits thromboxane A₂, or von Willebrand disease, where the platelet adhesion ligand is missing). Crucially, a platelet count of 80,000/µL with normal function may bleed less than a count of 150,000/µL with severely impaired function.

Coagulation factor deficiencies disrupt secondary hemostasis and cause deep-tissue bleeding: hemarthrosis (bleeding into joints), muscle hematomas, and intracranial hemorrhage. The platelet plug can still form — it's intact — but without a fibrin meshwork to reinforce it, the clot is fragile and fails under the higher pressures in large vessels and joint spaces. Hemophilia A (factor VIII deficiency) and Hemophilia B (factor IX deficiency) are the paradigm cases. Both are X-linked recessive, and both affect the intrinsic pathway of coagulation, which you can trace in the cascade: factor IX activates factor X in a complex with factor VIII. A deficiency in either abolishes this amplification step, starving the cascade of thrombin.

Laboratory tests discriminate between these categories. The bleeding time (or PFA-100) measures platelet function and is prolonged in platelet disorders but normal in isolated factor deficiencies. The prothrombin time (PT) tests the extrinsic pathway (factors VII, X, V, II, I) and is elevated in vitamin K deficiency or warfarin use, since vitamin K is required to carboxylate factors II, VII, IX, and X. The activated partial thromboplastin time (aPTT) tests the intrinsic pathway and is elevated in hemophilia A or B. A patient with a normal platelet count and normal PT but elevated aPTT with joint bleeding almost certainly has hemophilia — the pattern alone tells you where the break in the hemostatic system lies.