Platelet suppressive therapy in clinical medicine

The use of aspirin in polycythaemia vera and primary thrombocythaemia

In polycythaemia vera (PV; polycythaemia rubra vera, primary proliferative polycythaemia) and primary thrombocythaemia (PT; essential thrombocythaemia), occlusive complications in the microvasculature and larger vessels are a significant cause of morbidity and mortality. Central to the pathogenesis of these complications are the quantitative and qualitative platelet changes present in these myeloproliferative disorders. Aspirin irreversibly inactivates cyclo-oxygenase in platelets. This leads to a reduced production of platelet thromboxane A2 which has vasoconstricting and platelet aggregatory properties. In haematologically normal individuals, aspirin has been shown to reduce thrombo-embolic complications in populations at risk of these events. In PV and PT, aspirin has been shown to specifically eliminate the micro-circulatory and vasomotor manifestations and there is some evidence of a reduction in larger vessel occlusion. Low-dose aspirin has been shown to substantially reduce the raised thromboxane A2 production of platelets in PV and PT patients. The incidence of haemorrhagic side-effects of aspirin are minimized by the use of low doses. Haemorrhagic events are particularly found in patients with platelet counts > 1000 x 10(9)/l and these events are enhanced by aspirin therapy in these patients. Aspirin should be used with caution in patients with dyspeptic symptoms or a history of peptic ulceration or bronchospasm. Precise PCV control (< 0.45) and cytoreduction (platelets < 400 x 10(9)/l) should be used in patients with PV to minimize the vascular occlusion risk but routine cytoreduction is proposed only for those at particular risk of vascular occlusion in PT. In the acute presentation of patients with vascular occlusion, cytoreduction and an aspirin dose of 300 mg a day is proposed, reducing to 75 mg a day with the control of symptoms and signs, while 75 mg a day may play a role as prophylactic therapy in the prevention of thrombosis. However, there are no prospective studies in PT to demonstrate the benefit/risk profile and to confirm these recommendations, while a randomized prospective placebo-controlled study of low-dose aspirin in PV has only recently been initiated.

The effect of sulphinpyrazone and alpha-tocopherol on platelet activation and function in haemodialysed patients

In 30 patients on chronic haemodialysis treatment the platelet activity and function were studied before and during antiplatelet therapy with alpha-tocopherol and sulphinpyrazone. In both kinds of treatment a significant decrease of ADP-induced and spontaneous aggregation was observed. Sulphinpyrazone exerts an inhibitory effect not only on platelet aggregation but also on platelet factor 3 and provokes a significant prolongation of the bleeding time.

Platelets and antiplatelet therapy in ischemic heart disease

Blood platelets have been shown to play an important role not only in thrombosis, but also in the pathogenesis of coronary artery disease and its complications. Drugs that affect platelets have been shown to reduce mortality in survivors of acute myocardial infarction, to reduce the risk of myocardial infarction in patients with unstable angina, and to preserve the potency of saphenous venous grafts used to bypass obstructed coronary arteries. The drugs may also play a role in the primary prevention of arteriosclerosis and in preventing thrombotic complications following coronary angioplasty.

Polycythaemia and thrombocythaemia in the elderly

The investigation of elderly patients presenting with raised PCV values has been described. Suitable clinical and laboratory investigation enables the separation of those with a raised red cell mass (RCM) into three groups: primary proliferative polycythaemia (PPP), secondary polycythaemia and idiopathic erythrocytosis. Those patients with a raised PCV but normal RCM either have apparent polycythaemia (normal plasma volume) or relative polycythaemia (low plasma volume). PPP is a clonal disorder with a peak incidence in the elderly. It commonly presents with vascular occlusive symptoms/signs involving larger vessels, both arterial and venous. The microvasculature may also be involved, particularly when there is associated thrombocythaemia. Effective treatment is required to minimize the future vascular occlusive incidence and diminish the complication rate of surgery if it is ever required. Both the PCV and the platelet count, if elevated, should be adequately controlled. 32P is probably the simplest treatment and is very effective, but venesection and intermittent low-dose busulphan is equally satisfactory in the co-operative patient with good peripheral veins. Secondary polycythaemia may arise from a variety of causes, particularly from arterial hypoxaemia and renal lesions. Occasionally, more than one pathology is identified in the elderly patient. Lung disease is the most common cause of hypoxaemia. Venesection may be indicated in those patients with excessively raised PCV values. The term idiopathic erythrocytosis should only be used for patients who have been adequately investigated. These patients most commonly present with ischaemic or vascular occlusive symptoms/signs. Relative polycythaemia may be caused by fluid loss, but generally the origin of the low plasma volume is not established. Apparent polycythaemia may represent a physiological variant or a stage before the development of a definitely raised RCM. The management of idiopathic erythrocytosis, and relative and apparent polycythaemia, should initially involved removal of known risk factors if present (e.g. hypertension) with the addition of venesection in selected patients. Reactive thrombocytosis in the elderly is most commonly due to malignant disease of chronic infection. The high platelet count is usually asymptomatic, and antiplatelet therapy is rarely required. Primary thrombocythaemia (PT) is a clonal myeloproliferative disorder similar to PPP. The finding of splenomegaly, abnormal platelet morphology or function helps to separate PT from reactive thrombosis. PT most commonly presents with digital or transient cerebral ischaemia or haemorrhage.(ABSTRACT TRUNCATED AT 400 WORDS)