Subendothelial collagen and von Willebrand factor
Trigger · Primary haemostasis GPIb Carries VIII
Endothelial breach exposes collagen and the von Willebrand factor multimers released from endothelial Weibel-Palade bodies and platelet alpha granules. vWF tethers platelets to the wall through glycoprotein Ib at high shear, and separately carries factor VIII in plasma and protects it from clearance and from activated protein C.
Detail
- Sources
- Endothelial Weibel-Palade bodies and platelet alpha granules; released by thrombin, hypoxia and desmopressin (DDAVP)
- Binds
- Subendothelial collagen; platelet GPIb-IX-V for adhesion and GPIIb/IIIa for aggregation; factor VIII
- Carrier function
- Bound factor VIII has a half-life of about 12 hours; unbound factor VIII survives only about 2 hours
- Processed by
- ADAMTS13, which cleaves ultra-large vWF multimers as they unfold under shear
- Measured by
- vWF antigen, vWF activity (ristocetin cofactor or GPIbM assay), factor VIII activity and multimer analysis
When it goes wrong
Von Willebrand disease — type 1 partial quantitative deficiency (about 75% of cases), type 2 qualitative, type 3 near-total absence
The commonest inherited bleeding disorder; autosomal, so it affects both sexes equally. Mucocutaneous bleeding: epistaxis, easy bruising, menorrhagia and bleeding after dental extraction. Platelet count normal; APTT prolonged only once factor VIII falls. DDAVP for type 1, vWF-containing factor VIII concentrate for type 3
ADAMTS13 deficiency, inherited or from an acquired autoantibody
Thrombotic thrombocytopenic purpura: microangiopathic haemolytic anaemia, thrombocytopenia, fever, renal impairment and fluctuating neurology, with a normal PT, APTT and fibrinogen; treat with urgent plasma exchange
Severe aortic stenosis or a left ventricular assist device shearing large vWF multimers
Acquired von Willebrand syndrome with bleeding from gastrointestinal angiodysplasia — Heyde syndrome, which resolves after valve replacement
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