von Willebrand’s disease

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von Willebrand’s disease (vWD) is caused by a deficiency of, or abnormality in, a large plasma glycoprotein called von Willebrand factor (vWf). It is the most common inherited disorder of haemostasis in dogs, but has been diagnosed only rarely in cats. This chapter considers structure and function of von Willebrand factor; disease mechanism and classification; clinical signs; diagnostic tests; treatment; disease control.

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27.1 The role of von Willebrand factor (vWf) in primary haemostasis. Platelet adhesion is the initiating event in primary haemostasis and is mediated by vWf in blood vessels with high shear rates. Vessel injury exposes vWf that is prebound to subendothelial matrix proteins (collagen, fibronectin and vitronectin) and induces secretion of vWf from stores within Weibel–Palade bodies of endothelial cells. Stored vWf consists mostly of the more active high molecular weight multimers and is secreted preferentially into the subendothelial matrix. Plasma vWf also binds to subendothelial matrix proteins upon vessel injury. Platelets adhere to the vessel wall through vWf, which acts as a bridge between the platelet glycoprotein receptor complex, GP1b-V-IX (not shown), and exposed subendothelial matrix protein components. GPIb-V-IX does not require prior activation before engaging vWf. Once bound, platelets become activated, change shape and release platelet agonists (e.g. adenosine diphosphate (ADP), thromboxane A), which serve to recruit and activate new platelets to the injured site. Recruited platelets form aggregates via binding of fibrinogen to the αβ integrin receptor (CD41/61) on adjacent platelets, which then forms a primary platelet plug (not shown) (see Chapter 21).
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27.2 Multimeric structure of von Willebrand factor (vWf). The classification of von Willebrand’s disease (vWD) into three types is based on the multimeric composition of the protein as determined by sodium dodecyl-sulphate agarose gel electrophoresis. This technique separates out the multimers on the basis of size (molecular weight MW). Dogs with type I vWD have a quantitative defect in vWf but multimeric composition is normal. Dogs with type II vWD have a quantitative defect in vWf and are lacking the high molecular weight multimers. Dogs with type III vWD have an absolute deficiency of vWf. N indicates that the sample comes from a normal dog.
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27.4 Clinical signs associated with von Willebrand’s disease (vWD). Spontaneous epistaxis in a Scottish Terrier with severe type III vWD (< 1% vWf:Ag). Post-surgical bruising around the suture line in a Shetland Sheepdog with type III vWD. (Courtesy of Dr Marjory Brooks, Comparative Coagulation Laboratory, Cornell University)
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27.6 Flowchart outlining the use of the buccal mucosal bleeding time (BMBT) as a screening test and desmopressin (DDAVP) as prophylactic therapy before elective surgery in dogs of unknown von Willebrand’s disease status or those with vWf:Ag concentrations below a critical threshold (< 25–40%, i.e. ‘at risk’ of haemorrhage). This chart is not recommended for use in dogs with suspect or known type II or III vWD.
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