Bleeding and thrombosis in myeloproliferative disorders.

Patients with polycythemia vera and essential thrombocythemia have a hemostatic imbalance, the mechanisms of which are still elusive. Hemorrhagic tendency usually manifests in patients with high platelet count and reveals the clinical and laboratory features of acquired von Willebrand's disease. In most patients, however, the increased risk of arterial and venous thromboses constitutes the main therapeutic challenge. Data from large retrospective studies have recently allowed us to reassess the clinical epidemiology of these manifestations. Erythromelalgia and other peculiar microcirculatory disturbances are selectively sensitive to aspirin. This finding, and the thromboxane A2 hyperproduction existing in polycythemic and thrombocythemic subjects, provide the rationale for the use of low-dose aspirin in these patients. The efficacy and safety of this treatment in patients with polycythemia vera is being tested in a large scale, randomized trial. Appropriately designed clinical studies are also needed for defining the neoplastic risk of commonly used chemotherapic agents and the antithrombotic efficacy of alternative cytoreductive strategies.