Pathophysiology and treatment of platelet-mediated microvascular disturbances, major thrombosis and bleeding complications in essential thrombocythaemia and polycythaemia vera.

Essential thrombocythaemia (ET) is associated with a broad spectrum of microvascular circulation disturbances including erythromelalgia and its ischaemic complications, episodic neurological symptoms of atypical and typical transient ischaemic attacks (TIAs), transient ocular ischaemic attacks, acute coronary syndromes, and superficial 'thrombophlebitis'. The microvascular circulation disturbances are caused by spontaneous activation and aggregation of hypersensitive thrombocythaemic platelets at high shear stress in the endarterial microcirculation involving the peripheral, cerebral and coronary circulation. As this microvascular syndrome is a pathognomonic feature of essential thrombocythaemia and of thrombocythaemia associated with polycythaemia vera (PV) in complete remission with normal haematocrit, we have labelled these two variants of thrombocythaemia as thrombocythaemia vera. The arterial thrombophilia of microvascular circulation disturbances in thrombocythaemia vera already occur at platelet counts in excess of 400 x 10(9)/l. Complete relief of microvascular circulation disturbances in thrombocythaemia vera is obtained with the platelet cyclooxygenase inhibitor aspirin 50-100 mg/day, but not with dipyridamole, ticlopidine, coumarin or heparin. Haemorrhagic thrombocythaemia (HT) is a clinical syndrome of recurrent spontaneous mucocutaneous and secondary haemorrhages associated with extremely high platelet counts far in excess of 1000 x 10(9)/l. The paradoxical occurrence of microvascular circulation disturbances and mucocutaneous bleeding is usually seen at platelet counts between 1000 and 2000 x 10(9)/l. At increasing platelet counts from below 1000 to in excess of 2000 x 10(9)/l, the arterial thrombophilia of thrombocythaemia vera changes into a spontaneous bleeding tendency of HT as a consequence of platelet-mediated increased proteolysis of the large von Willebrand factor multimers leading to a type 2 acquired von Willebrand syndrome. As PV is usually associated with thrombocythaemia, the vascular complications in PV patients are microvascular circulation disturbances typical of thrombocythaemia. On top of this, major arterial and venous thrombotic events and haemorrhages are related to increased haematocrit, red cell mass and its concomitant increased blood viscosity. Correction of increased blood viscosity and haematocrit to normal values (0.40-0.44) by bloodletting alone will significantly reduce the risk of major thrombotic complications, but does not prevent the microvascular circulation disturbances because thrombocythaemia persists. The microvascular syndrome associated with thrombocythaemia in PV patients in remission after bloodletting is best controlled by low-dose aspirin (50-100 mg/day) or by reduction of platelet count to normal (< 350 x 10(9)/l).