Polycythemia Vera

Polycythemia vera (PV) is a myeloproliferative disorder with excessive proliferation of red cells, megakaryocytes, and myelocytes. It is usually an insidious disease with gradual onset and can be associated with prolonged survival if the cytosis is controlled. Splenomegaly is common and is frequently one of the presenting features. The clini-cal course of PV can be quite varied. With uncontrolled proliferation of red cells and platelets, an increase in whole blood viscosity can occur, leading to impairment of blood flow to critical organs. Central nervous system and cardiovascular symptoms, including confusion, angina, and claudication, can occur as a result of insufficient oxygen delivery. Thrombotic complications are a significant cause of morbidity and mortality, with strokes, myocardial infarctions, arterial occlusions, and thromboembolic complications being common. There is also an increased risk of hemorrhage.

The long-term course of PV can include distinct phases. Initially, an asymptomatic phase may be associated with only mild elevation of the red cell mass and mild splenomegaly; this gives way to an erythrocytic phase associated with pancytosis, with complications such as thrombosis and hemorrhage. An inactive phase may ensue in which no therapy is required to control the proliferative cells, and eventually a postpolycythemic myeloid metaplasia phase can develop with evidence of marrow fibrosis and extramedullary hematopoiesis. A significant percentage of patients with PV will culminate in acute nonlymphocytic leukemia. The ongoing management of patients during the erythrocytic phase affects the risks of developing this devastating outcome.

Although PV increases in incidence with advancing age, there is a wide range of ages, including some childhood cases. There is no known cause, and surprisingly there is no link to radiation exposure. In mice, the friend erythroleukemia virus causes a similar disease, but no viral infection has been found associated with PV in humans.

The diagnosis of PV rests on a combination of clinical and laboratory findings. The polycythemia study group established the diagnostic criteria shown in Table 38.1. The most important step is to confirm that the red cell mass is actually increased using chromium-51 red cell labeling and a plasma volume study. In many patients with apparently elevated hematocrit, the red cell mass is normal and the plasma volume is below normal or reduced, giving a spuriously increased hematocrit without true polycythemia; this effect can be seen in patients on a diuretic or patients known to have spurious erythrocytosis.