Pathophysiology
Summary
Von Willebrand disease is the most common inherited coagulopathy and is characterized by a deficiency in von Willebrand factor (vWf). Von Willebrand factor is found in the sub-endothelium of arteries and veins and is crucial for blood clotting. When endothelial injury occurs, vWf aids in platelet aggregation by linking glycoprotein 1b (GP1b) on platelets to exposed collagen on the endothelium. A deficiency in vWf results in prolonged bleeding time and symptoms like easy bruising, epistaxis, prolonged skin bleeding, & heavy menstrual bleeding in females.
In von Willebrand disease, both platelet function and the intrinsic coagulation pathway are affected. Specifically, reduced levels of von Willebrand factor (vWf) increase the susceptibility of factor VIII to degradation, thereby shortening its half-life. As a result, the activity of the intrinsic coagulation pathway is diminished, manifesting as a prolonged activated partial thromboplastin time (aPTT).
The ristocetin test assesses vWf's ability to mediate platelet aggregation by adding ristocetin to a blood sample, activating GP1b receptors on platelets. In von Willebrand disease, this test is negative due to low vWf levels, resulting in decreased GP1b receptor binding and reduced platelet aggregation.
The synthetic vasopressin analog desmopressin (DDAVP) stimulates endothelial cells to release stored vWf and can be used for treatment. For severe hemorrhages, cryoprecipitate is employed.
Acquired von Willebrand disease can be due to aortic stenosis, which degrades vWf multimers, or coexist with angiodysplasia, causing persistent gastrointestinal bleeding.
Hemophilia represents a group of inherited coagulopathies predominantly affecting the intrinsic coagulation pathway. Two main types of this disorder are hemophilia A and hemophilia B, each distinguished by the deficiency of specific clotting factors.
Hemophilia A is an X-linked recessive disorder caused by a factor VIII deficiency, affecting both the intrinsic and common coagulation pathways by reducing factor X activation. Clinically, the disorder leads to ‘coagulopathic' bleeding, commonly manifesting as hemarthroses, as well as intramuscular and intracranial hemorrhages. Even dental procedures can trigger bleeding episodes. Diagnosis is confirmed by a prolonged aPTT, indicative of intrinsic pathway disruption. Desmopressin can also be used to treat hemophilia A, as it induces platelets and endothelial cells to release factor VII.
Hemophilia B is similar to hemophilia A but is caused by a deficiency in factor IX. Hemophilia B is also an X-linked recessive disorder that affects the intrinsic pathway, leading to a prolonged aPTT and ‘coagulopathic’ bleeding similar to hemophilia A.
Acquired hemophilia is a phenomenon that can occur in conditions including SLE, pregnancy, postpartum, rheumatoid arthritis, and drugs like penicillin. These conditions promote the formation of anti-factor VIII antibodies that accelerate the clearance of factor VIII, resulting in the clinical manifestations of hemophilia. Diagnosis involves a mixing study, where normal serum is combined with the patient's serum. If aPPT remains prolonged, this suggests factor VII inactivation due to antibodies. Treatment includes corticosteroids to counteract the antibody-mediated degradation of factor VIII.
Lesson Outline
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FAQs
Antiphospholipid syndrome (APS) is an acquired form of thrombosis often linked to autoimmune conditions like rheumatoid arthritis, HIV, and systemic lupus erythematosus. It is marked by the presence of specific autoantibodies, including anticardiolipin, lupus anticoagulant, and anti-beta2-glycoprotein. The syndrome can cause both venous and arterial thrombosis, typically in younger patients. It is also associated with complications such as non-bacterial thrombotic endocarditis and pregnancy-related issues like recurrent miscarriages.
Hyperviscosity syndrome is a condition characterized by increased blood viscosity, which can elevate the risk of thrombosis. Conditions like polycythemia, which increases red blood cell count, and elevated levels of immunoglobulins in diseases like multiple myeloma, can contribute to hyperviscosity. This syndrome can manifest with neurological symptoms such as confusion, as well as visual disturbances like blurred vision, due to stasis in cerebral and ocular vessels.
Factor V Leiden is the most prevalent inherited thrombosis syndrome and is more common among younger Caucasians. A mutation in this syndrome prevents protein C from inactivating factor V, leading to increased thrombin activity and a higher risk of venous thrombosis. Certain lifestyle factors like smoking, use of oral contraceptives, pregnancy, and prolonged immobilization can exacerbate the risk of thrombosis in individuals with this condition.
The prothrombin G20210A mutation is an inherited form of thrombosis that leads to excessive production of prothrombin, increasing the risk of venous thrombosis. This condition commonly manifests as deep vein thrombosis (DVT) and pulmonary embolism (PE), similar to other inherited thrombosis syndromes.
Deficiencies in protein C, protein S, and antithrombin III are types of inherited thrombosis syndromes that elevate the risk of venous thrombosis. These deficiencies increase the activity of coagulation factors Va and VIIIa, as well as thrombin. Individuals with these deficiencies should avoid warfarin treatment, as it further reduces protein C and S activity, increasing the thrombotic effect. In cases of antithrombin III deficiency, heparin is ineffective, and direct thrombin inhibitors are the preferred treatment.
