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INTRODUCTION

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The condition known as von Willebrand disease (VWD) is a result of quantitative and qualitative abnormalities in von Willebrand factor (VWF), a plasma protein serving as a carrier for factor VIII and as an adhesive link between platelets and damaged blood vessel walls. Table 79–1 presents the nomenclature used in discussing the functions of VWF.

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TABLE 79–1VON WILLEBRAND FACTOR AND FACTOR VIII TERMINOLOGY
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ETIOLOGY AND PATHOGENESIS

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  • VWF is synthesized in endothelial cells and megakaryocytes.

  • Post-translational modification of the molecule involves glycosylation, sulfation, and multimer formation through extensive disulfide bond formation.

  • VWF is stored in platelets and in Weibel-Palade bodies in endothelial cells.

  • Secretion of VWF from Weibel-Palade bodies is both constitutive and regulated. High-molecular-weight multimers with the greatest activity are released in response to agents such as thrombin in vitro or desmopressin (DDAVP) in vivo.

  • A specific VWF-processing protease can reduce the size of high-molecular-weight multimers in plasma.

  • VWF plays an important role in platelet aggregation at sites of vessel injury.

  • VWF stabilizes factor VIII through formation of a noncovalent complex between the two proteins.

  • A large number of mutations of the VWF gene have been discovered, and more than 20 distinct subtypes of VWD have been described. Table 79–2 presents a simplified classification of VWD.

  • Types 1 and 3 are deficiencies of normal VWF, either partial (type 1) or complete (type 3).

  • Type 2 includes the qualitative abnormalities of VWF structure and/or function. The quantity of VWF (VWF antigen) in type 2 disease may be normal but is usually reduced.

  • Platelet-type VWD is an inherited platelet abnormality due to a mutation in glycoprotein Ib (CD42b, c). It is discussed in Chap. 75.

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