Inheritance and prevalence of von Willebrand's disease severe form in a Brazilian population.

Reviewed data suggest that the prevalence of severe von Willebrand's disease is influenced by ethnic and geographic factors. In the State of Rio Grande do Sul, Brazil, seven genealogies in which 11 individuals had a severe expression of von Willebrand's disease were localized. These affected subjects had similar laboratory results and all of them seemed to have resulted from double genetic defects, but the genealogic examination revealed that four of them probably resulted from combinations of autosomal recessive genes, while in the remaining the presence of dominant genes was likely and the involvement of genes for types I or II of von Willebrand's disease was possible. All of their examined relatives were asymptomatic but some of them presented unusual laboratory results, indicative of heterozygosis. The prevalence of severe cases in the surveyed population was higher than expected even when only the recessive forms were considered. It entered the expected values when it was presumed that these were all the cases currently living in the State. Genetic heterogeneity of the severe form was confirmed and it is suggested that the designations 'severe von Willebrand's disease' and 'type III von Willebrand's disease' should not be used as synonyms.