Von Willebrand disease (VWD) is a group of genetically heterogenous disorders resulting in abnormal function of the Von Willebrand factor (VWF). More than 100 mutations have been described. Symptoms include mucocutaneous bleeding (epistaxis, easy bruising, prolonged bleeding after minor trauma, menorrhagia and gastrointestinal bleeding) of varying severity. Hemarthrosis is relatively uncommon. Unlike hemophilia, the mode of inheritance is predominantly autosomal dominant (some autosomal recessive variants have been described). Although it is the most common inherited disorder of coagulation with a prevalence of 1-2%, only a small fraction of individuals inheriting the gene suffer from a clinically significant diathesis.
FUNCTION OF VON WILLEBRAND FACTOR IN HEMOSTASIS
Several different mutations in the Von Willebrand factor gene lead to quantitative and qualitative changes in the von Willebrand factor. Different management strategies in the various types of VWD underlie the importance of classification. Traditionally, the disease is phenotypically classified into three broad categories:
Based on specific structural abnormalities, type 2 VWD has been further subdivided into four subtypes (2A, 2B, 2M, 2N). Figure 1 shows the structure of VWF and the distinct subtypes produced by changes in specific domains.
Since currently available screening tests (PTT and bleeding time) are non-specific and not highly sensitive, in the presence of clinical symptoms, specific tests including quantitative assay (VWF AG), functional assay (VWF ristocetin cofactor/ collagen binding capacity), structural analysis (multimer gel analysis), platelet count and plasma levels of factor VIII are recommended.
The VWF RCo test utilizes formalin fixed platelets in the presence of platelet activator, ristocetin. In quantitative disorders, it parallels the VWF antigen while in qualitative disorders resulting in decreased affinity for platelets (ie types 2A and 2M), VWF RCo is more severely affected compared to VWF AG. Although specific, the test has a high degree of interassay and interlaboratory variability. Collagen binding capacity is a newer commercially available test utilizing collagen binding adhesive function of VWF. Since this test depends on the presence of high molecular weight multimers, it allows good discrimination between types 1 and 2, in 90% of cases. One strong point against this test is that it utilizes a physiologic property of VWF and no defect in collagen binding has been proven so far in patients with VWD.
Patients with type 2B VWD may inially present with thrombocytopenia and normal or borderline APTT, giving the erroneous impression of a primary platelet disorder. VWF antigen maybe normal but VWF RCO is often decreased. Low dose RIPA is helpful in that it detects abnormally increased platelet aggregation which is only seen in type 2B VWD and platelet type VWD. These two disorders may further be distinguished by repeating the test using normal fixed washed platelets and incubating them with the patient's plasma and varying doses of ristocetin. In this case, only type 2B shows enhanced binding of VWF to platelets at low dose ristocetin.
In type 2N, VWF AG and VWF RCO may be normal but factor VIII levels are severely reduced (often less than 10%) giving a laboratory profile of hemophilia. Since platelet binding is unaffected, the bleeding time and platelet function studies are normal. Hence this subtype has been referred as autosomal hemophilia. In cases of apparent hemophilia A with poor factor VIII recovery and survival in vivo and normal mixing studies, VWD type 2N should be considered in the differential. VWF:FACTOR VIII binding assay is necessary to confirm the diagnosis.
Multimer gel electrophoresis is used for identifying qualitative defects in VWF primarily in cases with abnormal ratios of structural and functional VWF assays. Even in the absence of abnormal ratios, multimeric analysis has been shown to detect at least 10% more patients with type 2 VWD and improves the overall accuracy of diagnosis of VWD. Increased susceptibilty of certain VWF molecules (types 2A, 2B) to proteolysis in the plasma forms the basis of this test.
Figure 2 shows the multimer pattern from a normal person (lane 1) and our patient, before and after treatment with DDAVP(lane 3 and 4). Lane 3 shows a normal multimer pattern but weak staining. Lane 4 shows a similar pattern but with increased intensity of staining, due to release of endogenous VWF secondary to DDAVP (see management below). This pattern and response to DDAVP is most consistent with type 1 VWD. Lane 2 represents a patient with type 3 VWD (absence of VWF).
Which of the following types of VWD can be represented by lane 5?
What additional test what you do to help in your diagnosis?
Blood group O
Interestingly, the blood group of an individual affects the plasma concentration of VWF, with plasma concentration being 25-35% lower in blood group O individuals. This difference is observed in plasma VWF and not in platelet VWF, suggesting increased plasma turnover rather than decreased synthesis. This "disease modifying" gene may explain the variable clinical expression of VWD amongst individuals within a family who have identical VWF alleles.
PITFALLS IN DIAGNOSIS
The biggest problem is in distinguishing mild VWD from normal persons with low concentration of VWF because of a broad normal range, variable clinical symptoms, blood group dependency of VWF and its behavior as an acute phase protein. Recently, three determinations in the morning at different time intervals have been recommended for exclusion of mild VWD. In women of child bearing age, effects of hormones need to be taken into account. First three days of menstrual cycle are the best time for testing. The International Society on Thrombosis and Hemostasis (in its provisional consensus criteria) recommends using blood type adjusted means of VWF. Ultralarge multimers of VWF (similar to those found in thrombotic thrombocytopenic purpura) are present for a few months in newborns. Hence there may be false negative results during this time. In the absence of a family history of VWD, a diagnosis of possible VWD is preferred. In our case, the patient's father was later also found to have type 1 VWD.
Acquired abnormalities of VWF have been described in multiple myeloma, leukemia, autoimmune disorders, certain solid tumors, thrombocytosis, congenital heart disease and use of certain drugs such as valproate and ciprofloxacin. The underlying mechanisms are variable and include autoantibodies, selective adsorption of VWF, decreased synthesis or increased clearance. Multimer analysis often shows type 2 pattern suggesting a greater effect on high molecular weight multimers. Platelet VWF may be useful in distinguishing these cases from congenital VWD. At present, lack of consensus on expression of results, limits the utility of this test. VW AGII is another test that may become useful in this scenario. The test measures VWF propeptide that is cleaved from the precursor protein during assembly of multimers and is secreted in equimolar amounts with mature VWF. Preliminary studies show that it is not affected by the ABO blood type and does not undergo increased degradation in patients with acquired VWS.
The therapeutic strategies depend on accurate diagnosis and subtyping of VWD. A clinical trial with DDAVP is recommended for increasing endogenous VWF in type 1 disease, although the response is variable. Type 3 patients are unresponsive to DDAVP and exogenous VWF is the treatment of choice.
In patients with type 2 disease, DDAVP is not always effective. Specifically, in type 2B, DDAVP may worsen the thrombocytopenia and also cause spontaneous platelet aggregation (although no cases of thrombosis have been reported). Thus treatment with VWF concentrates is required for maintaining hemostasis in most type 2 VWD.
In summary, the diagnosis of VWD may be difficult as the screening tests (APTT and/or bleeding time) may be normal or only marginally prolonged. Distinguishing the variants is also challenging and necessary as the management strategies differ.
Contributed by Hina A. Sheikh, MD and Franklin Bontempo, MD