Clinical Features And Natural History

Thrombosis is the major cause of morbidity and mortality in untreated PV. The increased risk of thrombosis relates to the hemorheological effects of the erythrocytosis and may be further exacerbated by accompanying thrombocythemia and possibly by abnormal platelet function. In a large retrospective study of over 1000 patients, 20 percent of patients presented with a thrombotic complication, with arterial and venous events comprising two-thirds and one-third of these thromboses, respectively.14 Twenty-one percent of these thromboses proved to be fatal. There is an increased risk of thrombosis in treated patients also with an incidence of 3.4 percent per year in the above study,14 and thrombosis remains the most common cause of mortality in PV. The patients in this study were treated heterogeneously with venesection and myelosuppression and, as is discussed later, the ongoing thrombotic risk relates to treatment modality. Less commonly patients may also present with hemorrhagic complications such as significant gastrointestinal, uterine, or cerebral bleeding.14 Hemorrhage may also occur with follow-up of treated patients, accounting for 2.6 percent of deaths in the above Italian study14 and 6.8 percent of recorded fatalities in the Polycythaemia Vera Study Group PVSG-01 protocol.15 This hemorrhagic tendency may be exacerbated by higher doses of antiaggregating agents as was suggested in the PVSG-05 trial (see below).16

Patients with PV are often plethoric and may complain of pruritus, which is classically related to bathing or showering in warm water (aquagenic pruritus) and may be intractable. They may also describe headaches and blurred vision, night sweats, and gout, attributable to hyperviscosity, hypervolemia, and hypermetabolism. Splenomegaly is present in around two-thirds of PV patients at diagnosis and its prevalence increases with disease duration.17 However, in most cases the diagnosis of PV is first suspected following an abnormal full blood count performed to investigate nonspecific or unrelated symptoms.

There is variable risk of disease transformation to acute myeloid leukemia (AML) or post polycythemic myelofibrosis, also described as the "spent" phase of polycythemia. The percentage of patients progressing to a myelofibrotic state is reported to be around 10 percent at 12.5 years follow-up4 and as high as 25 to 40 percent at 20 years depending on treat-ment.18,19 There is an increased risk of development of AML in patients who have developed marrow fibrosis. This overall risk of leukemic transformation also relates to previous treatment modalities with reports of the incidence varying between 1.5 percent, for patients treated only with phlebotomy, to 14 percent, for patients treated with chlorambucil, after almost 20 years of maximal follow-up.15 The figure of 1.5 percent probably underestimates the inherent tendency of this disease to transform into AML since the phlebotomy patients represented a selected subgroup with less aggressive disease.20 The incidence of AML in patients treated with hydroxyurea is between 5 and 10 percent at 9 to 10 years of follow-up.9'18'21

In the absence of treatment PV is a fatal disease. Cheivitz and Theide22 found a median survival of less than 2 years in symptomatic patients who received no treatment. This compares with a median survival of between 9 and 15 years quoted for treated patients.14,15 In older patients there is controversy over the effect of the disease on life expectancy. Some have found that this does not differ from an age-matched control population23,24 whereas in one large study the age- and sex-standardized mortality rate was 1.7 times that of the normal control population.14 In younger patients with PV, however, although the median survival is significantly longer than older patients,23,25 their survival is significantly less than an age- and sex-matched normal population (p < .01).25 In one study of PV patients less than 50 years of age the observed mortality rate was 4.1 times that of the normal age- and sex-matched population.25

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