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Abstract
Microvascular circulation disturbances including erythromelalgia, its microvascular ischemic complications, and migraine-like atypical or typical transient ischemic cerebral, ocular, and coronary ischemic attacks are specific clinical manifestations in patients with essential thrombocythemia (ET), and polycythemia vera (PV) associated with thrombocythemia. Thrombocythemia (ET and PV) patients with microvascular disturbances have shortened platelet survival, increased β-thromboglobulin (β-tg), platelet factor 4 (PF4), and thrombomoduline (TM) levels, and increased urinary thromboxane B2 (TxB2) excretion indicating platelet-mediated processes in vivo. Inhibition of platelet cyclooxygenase (COX 1) by aspirin is followed by relief of microvascular disturbances, correction of shortened platelet survival, and return of plasma levels of β-tg, PF4, TM levels and TxB2 excretion to normal. The transient ischemic attacks and thrombotic complications in thrombocythemia are very likely caused by hypersensitive platelets produced by spontaneously proliferating enlarged megakaryocytes in the bone marrow of ET and PV patients. In contrast to normal platelets in healthy individuals the circulating hypersensitive thrombocythemic platelets spontaneously activate and secrete their products, thus forming aggregates that transiently plug the microcirculation, or result in occlusive platelet thrombi in arterioles or small arteries. Clear evidence is presented that the microvascular transient ischemic and occlusive thrombotic complications in thrombocythemia patients are relieved by treatment with aspirin and by reduction of platelet counts to normal (<400 × 109/l), but not by coumadin. In patients with thrombocythemia associated with PV, increased hematocrit and whole blood viscosity aggravate the platelet-mediated microvascular ischemic and thrombotic syndrome of thrombocythemia to major arterial and venous thrombotic complications. Correction of hematocrit and blood viscosity by phlebotomy significantly reduces the major arterial and venous thrombotic complications, but fails to prevent the platelet-mediated microvascular circulation disturbances in PV patients because thrombocythemia persists. Complete relief and prevention of microvascular and major thrombosis in PV patients are obtained by treatment with low-dose aspirin on top of phlebotomy or by treatment with the platelet lowering agents, anagrelide, interferon or hydroxyurea.