[Vascular complications of essential thrombocythemia]

Vascular manifestations are the main clinical complication of essential thrombocythemia (ET). They include arterial thrombosis (30-40% of patients), venous thrombosis (5%), and ischemia due to microcirculatory disorders. Their incidence is highest at disease onset and diminishes gradually thereafter. The pathophysiology of ET involves dysmegakaryocytopoiesis, leading to platelet, leukocyte and vascular endothelial cell activation. The recent discovery of the V617F mutation of the JAK2 tyrosine kinase in 50-60% of patients with ET defined a new subgroup resembling polycythemia vera. This review examines biological findings and their correlation with the risk of thrombosis. Until now, stratification of the vascular risk has relied on a clinical case-control study showing that thrombotic and vascular complications are more frequent in patients over 60 and those with a history of thrombosis. These criteria, along with a rapid increase in thrombocytosis (>1500 G/L) leading to an increase in the bleeding risk, define a high-vascular-risk subgroup of patients warranting cytoreductive therapy. Although many biological markers of dysmegakaryocytopoiesis and cellular hyperactivation have been linked to an increase in the thrombotic risk, none is available for large-scale prediction of an intermediate vascular risk. The role of the JAK2 V617F mutation itself is controversial. Whatever the ET subgroup, antiplatelet therapy is largely used, based on the results of the ECLAP prospective controlled trial that showed a statistically significant reduction in thrombotic complications in patients receiving aspirin for polycythemia vera, a very similar myeloproliferative disorder.