Polycythaemia
Polycythaemia refers to a group of disorders in which there is elevation of the haemoglobin, packed cell volume and red cell count (Table 96). In true polycythaemia there is an absolute increase in the red cell mass, whereas in apparent or relative polycythaemias the red cell mass is normal and the rise in packed cell volume is secondary to a reduced plasma volume. The clinical significance of these disorders is their significant association with thrombotic disorders, for example myocardial infarction, stroke, peripheral vascular disease and deep vein thrombosis. This results from the increase in whole blood viscosity, which leads to reduced blood flow.
The most important disease is primary proliferative polycythaemia (polycythaemia rubra vera), which, like myelofibrosis and essential thrombocythaemia, is a myeloproliferative disorder. The defect seems to be clonal and originate in a stem cell that produces defects in the erythroid, granulocyte and megakaryocyte components of the marrow.
Often, the diagnosis is reached by chance, with the finding of an elevated haemoglobin on a routine blood sample, or the chance finding of splenomegaly on clinical examination; it may be recognized only after an acute thrombotic event. The major clinical clues are facial plethora, conjunctival suffusion and splenomegaly, present in 70% of patients. Other associated features are acne rosacea, urticaria, leg ulcers, retinal changes and loss of vision because of retinal haemorrhage. The liver is enlarged in up to 50% of patients, and hypertension is present in about 20%. Evidence should be sought of previous stroke, peripheral vascular disease or deep vein thrombosis. Itch is found in about 15% of patients, and there may be signs of chronic excoriation. The condition is also associated with peptic ulcer and acute gout.