Mechanisms underlying acquired von Willebrand syndrome associated with an IgM paraprotein.

Acquired von Willebrand (vW) syndrome is a rare bleeding disorder which is frequently associated with immunological, malignant or cardiovascular disorders. The underlying pathomechanisms, particularly in patients with IgM monoclonal gammopathies, often remain unknown. We report a patient with indolent small B-cell lymphoma (immunocytoma) and plasmacytic differentiation with an IgM kappa paraprotein who was admitted with retroperitoneal haematoma. Medical history and coagulation testing were consistent with acquired vW syndrome. vW immunohistochemistry showed normal cytoplasmic labelling of endothelial cells and megakaryocytes, whereas the lymphomatous infiltrate was negative. Acquired vW syndrome due to adsorption of vW factor on malignant cells was thus excluded. In the multimeric analysis, all multimers were present similar to that in type 1 vW syndrome, but the triplet structures were blurred. The bands on serum immunofixation electrophoresis were also atypically broadened, which suggested complex formation between the IgM and vW factor. Immunoprecipitation studies showed that the 176-kDa proteolytic fragment of vW factor co-precipitated with the IgM paraprotein in the patient but not in the controls, suggesting a specific interaction between vW factor and the paraprotein in the patient. The patient required surgery and was successfully managed by chemotherapy consisting of rituximab and fludarabin as well as plasma exchange.