Polycythemia, a chronic myeloproliferative disorder, affects approximately 2.3 per 100,000 persons per year. It is prevalent in all age groups with the highest incidence occurring in the seventh and eight decades. This disorder, arising from a mutation in the JAK2 gene, gives rise to the proliferation of all hematopoietic precursors, with erythrocytosis being the most common manifestation, and leukocytosis and thrombocytosis more rarer presentations. This resultant erythrocytosis leads to abnormally high hematocrit, predisposing patients to hyperviscosity of the blood and a multitude of associated complications.
The diagnosis of polycythemia is considered when the hematocrit is greater than 48% in women and greater than 52% in men. The elevated hematocrit seen in polycythemia results from either increase in red blood cell (RBC) mass or from reduction in plasma volume (i.e., relative polycythemia). An acute decrease in plasma volume converts an asymptomatic polycythemia into one in which hyperviscosity threatens tissue perfusion. This is particularly the case when hematocrit rises to greater than 55%–60%; in this range, blood viscosity increases exponentially (Figure 95-1). Small blood vessels (i.e., capillaries with low flow/shear rates), including those in the cerebral circulation, are especially vulnerable to reductions in blood flow secondary to hyperviscosity, risking ischemia.
Viscosity of heparinized normal human blood as a function of hematocrit. (Reproduced with permission from Lichtman MA, Kipps TJ, Seligsohn U, et al. William’s Hematology, 9th ed. McGraw-Hill Education, Inc., 2016.)
Primary Polycythemia (Polycythemia Vera)
The criterion for the diagnosis of primary polycythemia, also known as “Polycythemia Vera” (PV), involves an elevated hemoglobin level (>18.5 g/dL in men; >16.5 g/dL in women) and either the presence of a JAK2 mutation or two of the following: hypercellularity of the bone marrow, a subnormal serum erythropoietin (EPO) level, or endogenous erythroid colony formation. The most common presentations includes Budd–Chiari syndrome (hepatic vein thrombosis), generalized pruritus, Mitchell’s disease (erythromelalgia), coronary or cerebral thrombosis, and pulmonary hypertension.
Secondary polycythemia most commonly arises as a result of hypoxia or from excess EPO production. An increase in RBC mass along with evidence of changes in other hematopoietic cell lines distinguishes polycythemia from an appropriate physiologic response to hypoxia—in which an increase in RBC mass is an isolated finding. Examples of secondary polycythemia due to hypoxia include the following:
Acute and chronic mountain sickness
Significant cardiopulmonary disease, that is, congenital heart diseases with left to right shunting leading to cyanosis
Chronic pulmonary disease, for example, Pickwickian syndrome (extreme obesity) leading to the development of hypoventilation
Inherited defects in hemoglobin
Disorders or drugs producing methemoglobinemia
Secondary polycythemia due to increased EPO production is often found in patients with EPO-secreting tumors including renal cell carcinoma, uterine ...