Von Willebrand factor and von Willebrand disease
Von Willebrand factor (vWF) is a large protein in the blood that helps platelets attach to a damaged vessel wall and protects factor VIII, a clotting protein.
Remember
Von Willebrand factor is both glue for platelets and bodyguard for factor VIII, so when it is deficient or abnormal, bleeding typically comes from the skin and mucous membranes.
Memory aid
glue for platelets, bodyguard for factor VIII
Think of vWF as glue that holds platelets to the damaged vessel wall and as a bodyguard that protects factor VIII.
Von Willebrand factor (vWF) is a large protein made by the cells lining blood vessels, and it has two jobs. First, it is the glue for platelets (the small cell fragments that form the first plug). At a damaged vessel wall it binds collagen exposed in the subendothelium (the layer beneath the vessel lining), and platelets attach to it. Second, it is the bodyguard for factor VIII: it carries factor VIII in the blood and protects it from early breakdown.
In von Willebrand disease, the commonest inherited bleeding disorder, vWF is low or abnormal. The platelet plug forms poorly, and factor VIII levels can fall as well. That gives bleeding from the skin and mucous membranes (mucocutaneous bleeding): easy bruising, nosebleeds (epistaxis) and heavy periods (menorrhagia), and bleeding after minor trauma or surgery.
The bleeding time (how long a standard skin cut keeps bleeding) can be affected, but it is now rarely used. Because factor VIII can fall, the APTT (activated partial thromboplastin time, a test of the intrinsic clotting pathway) may be mildly prolonged. The diagnosis rests on vWF levels and activity plus factor VIII.
Bleeding into muscles and joints points to haemophilia (for example haemophilia A, isolated factor VIII deficiency) instead, because it is rare in von Willebrand disease. Many patients respond to desmopressin, a drug that raises vWF levels.
Key facts
- Two jobs
- vWF helps platelets stick to a damaged vessel wall and protects factor VIII from early breakdown.
- Typical bleeding
- nosebleeds, heavy periods and bleeding after minor trauma or surgery, while bleeding into muscles and joints is rare.
- Tests
- the APTT can be slightly prolonged, and the diagnosis rests on vWF levels and activity plus factor VIII.
- Test pattern
- in von Willebrand disease both the bleeding time and the APTT can be affected, whereas haemophilia prolongs the APTT with a normal bleeding time.
- Frequency
- von Willebrand disease is the most common inherited bleeding disorder.
- Treatment
- many patients respond to desmopressin, a drug that raises vWF levels.
Exam traps
- Immune destruction of platelets can cause bleeding from mucous membranes, but in isolated thrombocytopenia (a low platelet count on its own) the APTT may stay normal, because that test checks the plasma clotting factors rather than the platelet count.
- Isolated factor VIII deficiency is haemophilia A, which prolongs the APTT, but it is a clotting factor defect rather than a platelet adhesion defect. It classically causes bleeding into joints and deep tissues, not recurrent nosebleeds and menorrhagia.
- Thrombin is the enzyme that converts fibrinogen to fibrin. Von Willebrand factor is not that enzyme: it supports platelet sticking and carries factor VIII instead.
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