The risk of thrombosis in essential thrombocythemia and polycythemia vera.

Thrombotic events are present in 20% to 50% of patients with polycythemia vera (PV) and essential thrombocythemia (ET) at diagnosis and involve major vessels and the microcirculation. Establishing the precise risk of thrombosis in ET and PV is difficult. However, estimates may be obtained from retrospective and prospective studies, and some risk factors have been defined. In ET, increasing age and previous thrombosis emerge as major factors. The level of the platelet count per se does not correlate with thrombotic incidence. However, there is substantial evidence that adequate control of the platelet count reduces the frequency of thrombosis. While aspirin is usually effective at relieving vasomotor and microvascular occlusive symptoms or signs, there is only limited evidence that its use reduces the risk of larger vessel thrombosis. Other suggested risk factors include apparent clonal hematopoiesis in younger patients, hypercholesterolemia, and cigarette smoking. In PV, the overall impression is that the incidence of thrombosis is greater than in ET. As in ET, increasing age and previous thrombosis are significant risk factors. In addition, adequate reduction of the hematocrit reduces the incidence of thrombosis. Identifying the risk associated with thrombocytosis in PV is confounded by the overall myeloproliferation usually found in these patients and the fact that the thrombocytosis is not usually as marked as in ET. However, by analogy with ET, one could argue that the risk associated with the thrombocytosis in ET is equally applicable to PV. In support of this, there is overwhelming evidence that patients treated with adequate myelosuppression with complete normalization of their blood counts have the lowest thrombotic incidence. Essential thrombocythemia and PV patients presenting with thrombosis should be investigated for other congenital or acquired prothrombotic conditions. Establishment of the presence of one or the other of these may alter the long-term management of these patients with the use of either aspirin or anticoagulation in addition to cytoreduction. Thrombotic risk has emerged as a major factor in stratifying patients with myeloproliferative disease. While some clear thrombotic risk factors have been defined, there is a need to identify further additive risk factors in patients considered to be at low risk of thrombosis. Copyright 2002, Elsevier Science (USA). All rights reserved.