No distinct level of thrombocytosis correlates well with clinical manifestations. In polycythemia vera, thromboembolic complications can generally be explained by the increased red blood cell mass, but in the other myeloproliferative diseases, the poorly functioning platelets evidently predispose to hemorrhage and, paradoxically, to thrombosis. Neither platelet number nor measurements of platelet function predict the degree of thrombosis or hemorrhage. Platelet lipoxygenase activity is always normal in patients with reactive thrombocytosis, but it is deficient in some patients with the thrombocythemia of myeloproliferative disorders. There may be splenic vein thrombosis, portal or mesenteric vein thrombosis, and recurrent deep vein thrombosis with or without pulmonary embolism.
Arterial thrombosis is less common.
In thrombocythemia, the key problem is defective platelet function, although the absolute increase in platelet numbers may play an additional, though poorly defined, role. Some investigators believe that chemotherapy could suppress the more dysplastic megakaryocyte clones, thereby allowing the more normally differentiated clones to deliver platelets. This symptom complex appears to be caused by occlusion and inflammation of arterioles by platelet aggregates. Aspirin or indomethacin produces relief within hours. Aspirin given at a dosage of 0.3 to 0.6 g every other day can produce lasting benefit.