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.