Von Willebrand Disease results in a functional platelet disorder and a deficiency of Factor VIII of the coagulation cascade.
vWD is the most common inherited bleeding disorder and is inherited in an autosomal dominant fashion.
Without vWF to facilitate the interaction between platelets and exposed collagen on damaged endothelium, the platelet plug does not form.
In the Bleeding Time test, rarely performed today (but still tested on!), a small incision is made on the underside of the patient's forearm and the time until bleeding ceases is recorded. Because there is a defect in platelet plug formation with vWD, the bleeding time is classically increased.
Von Willebrand Disease has three main subtypes and depending on the subtype the PTT may be Increased or Normal. Since vWF acts to protect Factor VIII from proteolysis, vWF deficiency results in decreased levels of Factor VIII. Factor VIII plays a role in the intrinsic coagulation pathway which is monitored using PTT. Without Factor VIII to act in this pathway, PTT is increased.
vWF acts to protect Factor VIII from proteolysis. Thus if vWF is decreased Factor VIII will not be protected and will be decreased.
Although the platelets do not function properly since they cannot form the platelet plug, the platelet count is normal. This lab finding helps distinguish vWD from other diseases such as Idiopathic Thrombocytopenic Purpura (ITP) where the platelet count is decreased but the bleeding time is increased like in vWD.
This is a synthetic analog of vasopressin (antidiuretic horome) that causes vWF to be released from endothelium in an indirect manner.
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