Rare forms of von Willebrand disease
von Willebrand disease (VWD) arises from deficiency and/or defect(s) of plasma von Willebrand factor (VWF). In turn, plasma VWF is an adhesive protein which primarily functions by anchoring platelets to regions of vascular injury, thereby assisting prevention of bleeding. There is a proportional reduction also in Factor VIII, due to the absence of the stabilizing and anti-proteolytic effect that VWF normally exerts. VWD is reportedly the most common inherited bleeding disorder and can be classified into quantitative and qualitative defects, with type 1 and 3 VWD respectively identifying partial and total quantitative deficiency of VWF, and type 2 VWD identifying qualitative defects of VWF. The relative incidence of each subtype of VWD differs according to the locality and the ability of clinicians and laboratories to correctly diagnose and classify cases. In general, type 1 VWD is considered the most common type of VWD, whereas types 2 and 3 represent rarer forms. However, in developing countries, and partly because of consanguinity, type 3 VWD is over-represented. This review primarily focuses on the rarer forms of VWD, which typically comprise types 2 (A, B, M and N) and 3 VWD. The review also mentions type 1 VWD, largely for completeness and comparability, and since purportedly “severe” type 1 VWD, albeit not a formally recognized subtype of type 1 VWD, would represent a relatively “rare” form of VWD.