The diagnosis of type 3 and the variants of type 2 von Willebrand disease (VWD) is generally straightforward. Significant bleeding problems in these patients are associated with severe quantitative (type 3) or qualitative/quantitative (type 2) defects in the von Willebrand factor (VWF) protein that mediates the deposition of platelet thrombi at sites of injury and, through non-covalent binding, also serves to protect plasma factor (F) VIII from proteolysis .
In contrast, the diagnosis of VWD type 1, which may account for about 75% of the total , may be difficult to make, particularly in patients with little or no bleeding symptoms. For example, while the lower limit (95% confidence limit) of the plasma VWF in the general population had been estimated to be about 50% , asymptomatic normal subjects with the group O blood type may have values as low as 35% . In addition, there appears to be little correlation between the VWF level and bleeding symptoms, even in pedigrees of families with clear evidence of VWD type 1 , and the percentage of ‘normal’ controls with at least one bleeding symptom has been conservatively estimated to be about 25% . Sadler has recently suggested that bleeding and low VWF often will be associated by chance, and that most subjects whose VWF levels are within the range 15–50% might be better advised that they have ‘low VWF’ rather than VWD type 1 . In this way, a low VWF level might be considered one of many risk factors for bleeding, much as a high cholesterol and increased blood pressure are considered risk factors for cardiovascular events .
The possibility that the presence of other risk factors might account for the variable degree of bleeding in patients with low VWF was studied by screening a database (Microsoft Excel) constructed from a review of the charts of 564 patients referred to me for a bleeding workup. The bases for the referrals (66% from hematologists, 24% from internists) were a history of bleeding, an abnormal laboratory value, or a family history of bleeding. Patients referred because of thrombocytopenia were not included. Among the data entered for each patient were the values obtained for the plasma VWF antigen (VWF:Ag) by the Laurell method , VWF:ristocetin cofactor (VWF:RCo) as originally described , and a platelet aggregation response (normal or abnormal) to graded doses of either a collagen-containing tissue extract  or ‘Horm’ collagen . The degree of bleeding (bleeding score) was determined from the chart notes on each patient as follows: the magnitude of the clinical problem was assigned a value (0–3) for each of the following six symptoms: bleeding after cuts, surgical procedures or dental extractions, easy bruising, epistaxis, and gastrointestinal bleeding. The bleeding score was determined by adding the weighted values of these symptoms (weighted 2, 3, 2, 1, 1 and 1, respectively). Bleeding scores varied from 0 to 16, the former value observed in some patients referred because of abnormal studies in another laboratory, or who were members of a family with a positive history of bleeding.
Data were extracted on 94 patients whose VWF value (either the VWF:Ag or VWF:RCo) was 15–50%. In 87 patients, studies of collagen aggregation had also been performed. These patients were then grouped according to the bleeding score (those with scores of 8 or higher were combined into a single group, 8 +) and, for each bleeding score group, the percentage of patients in whom collagen aggregation was abnormal was determined. The results are shown in Fig. 1. The percentage of patients with abnormal collagen aggregation was much higher in the groups with the highest bleeding scores (7 and 8 +) than in those with lower scores (0–6). For the entire group of 87 patients, abnormal collagen aggregation was observed in 62% of those in groups 7 and 8 + (n = 21), compared with 21% of those in groups 0–6 (n = 66).
To evaluate further the factors that determine the risk of bleeding among patients with the various VWD phenotypes, data were also extracted on 12 patients with very low VWF values (<15%), all of whom had either VWF type 3 or some variant of VWD type 2. The 87 patients with low VWF (15–50%) were classified as collagen abnormal or collagen normal, as above. Results are shown in Table 1. The highest bleeding score (6.5 ± 0.6) was observed in the group of patients with VWD type 2 or 3, whose average VWF value was 3%. In all but one patient, collagen aggregation was normal. Thus, excessive bleeding in these patients may reasonably be attributed to their very low VWF values. In patients with ‘low VWF’, the bleeding score was significantly greater in the group with abnormal collagen aggregation (5.8 ± 0.8 vs. 3.9 ± 0.4, P= 0.016). The VWF values in the two groups were entirely similar (42 vs. 39%); the only difference was an impairment of collagen aggregation in the group with the greater bleeding tendency.
|Von Willebrand factor (VWF)†||Collagen |
|Descriptive‡||Phenotype||Range (%)||Mean (%)|
|Very low VWF (n = 12)||2 or 3||1–13||3||Normal*||6.5 ± 0.6|
|Low VWF (n = 27)||1||19–50||42||Abnormal||5.8 ± 0.8|
|Low VWF (n = 60)||1§||16–50||39||Normal||3.9 ± 0.4**|
The results support the suggestion of Sadler that a low VWF value (15–50%) be considered a risk factor whose significance may be enhanced by the presence of other risk factors for bleeding . The additional risk factor in the patients described above was an abnormality in platelet aggregation. In 19 patients, the defect was specific for collagen; in eight others, an additional impairment of epinephrine-induced aggregation was also observed. The specific defect in collagen aggregation observed in most of the patients is fully consistent with recent observations  on the frequency, in VWD subtypes, of the 807C allele within the α2 gene that is associated with a low density of the α2β1 collagen receptor in platelets that is required for normal collagen aggregation . Thus, Di Paola et al. observed a higher frequency of the 807C allele, compared with normals, in patients with VWD type 1, and the presence of the allele in these patients was strongly correlated with longer closure times in the Platelet Function Analyzer (PFA-100). Interestingly, there was not a significant difference in the frequency of the 807 allele in patients with VWD type 2 or 3 , possibly related to the normal collagen aggregation I observed in most patients with these phenotypes (Table 1). More recently, it has been shown that inheritance of the 807C allele is also associated with a more extensive history of bleeding in patients with VWD type 1 . Further studies on polymorphisms in other receptors for collagen, such as GP VI , or in receptors for other platelet agonists, may identify other risk factors that increase the likelihood of bleeding in subjects with low VWF.