|Classification and external resources|
Blood smear from a patient with polycythemia vera
Polycythemia vera (PV, PCV) (also known as erythremia, primary polycythemia and polycythemia rubra vera) is a myeloproliferative blood disorder in which the bone marrow makes too many red blood cells. It may also result in the overproduction of white blood cells and platelets.
Most of the health concerns associated with polycythemia vera are caused by the blood being thicker as a result of the increased red blood cells. It is more common in the elderly and may be symptomatic or asymptomatic. Common signs and symptoms include itching (pruritus), and severe burning pain in the hands or feet that is usually accompanied by a reddish or bluish coloration of the skin. Patients with polycythemia vera are more likely to have gouty arthritis. Treatment consists primarily of phlebotomy.
Polycythemia vera occurs in all age groups, although the incidence increases with age. One study found the median age at diagnosis to be 60 years, while a Mayo Clinic study in Olmsted County, Minnesota found that the highest incidence was in people aged 70–79 years. The overall incidence in the Minnesota population was 1.9 per 100,000 person-years, and the disease was more common in men than women. A cluster around a toxic site was confirmed in northeast Pennsylvania in 2008. 
Polycythemia vera (PCV), being a primary polycythemia, is caused by neoplastic proliferation and maturation of erythroid, megakaryocytic and granulocytic elements to produce what is referred to as panmyelosis. In contrast to secondary polycythemias, PCV is associated with a low serum level of the hormone erythropoietin (EPO). Instead, PCV cells have a mutation in the tyrosine kinase (JAK2), which acts in signaling pathways of the EPO-receptor, rendering those cells hypersensitive to EPO.
Patients with polycythemia vera can be asymptomatic. A classic symptom of polycythemia vera is pruritus or itching, particularly after exposure to warm water (such as when taking a bath), which may be due to abnormal histamine release or prostaglandin production. Such itching is present in approximately 40% of patients with polycythemia vera. Gouty arthritis may be present in up to 20% of patients. Peptic ulcer disease is also common in patients with polycythemia vera; most likely due to increased histamine from mast cells, but may be related to an increased susceptibility to infection with the ulcer-causing bacterium H. pylori. Another possible mechanism for the development for peptic ulcer is increased histamine release and gastric hyperacidity related with polycythemia vera.
A rare but classic symptom of polycythemia vera (and the related myeloproliferative disease essential thrombocythemia) is erythromelalgia. This is a sudden, severe burning pain in the hands or feet, usually accompanied by a reddish or bluish coloration of the skin. Erythromelalgia is caused by an increased platelet count or increased platelet "stickiness" (aggregation), resulting in the formation of tiny blood clots in the vessels of the extremity; it responds rapidly to treatment with aspirin.
Patients with polycythemia vera are prone to the development of blood clots (thrombosis). A major thrombotic complication (e.g. heart attack, stroke, deep venous thrombosis, or Budd-Chiari syndrome) may sometimes be the first symptom or indication that a person has polycythemia vera.
Headaches, lack of concentration and fatigue are common symptoms that occur in patients with polycythemia vera as well.
Physical exam findings are non-specific, but may include enlarged liver or spleen, plethora, or gouty nodules. The diagnosis is often suspected on the basis of laboratory tests. Common findings include an elevated hemoglobin level or hematocrit, reflecting the increased number of red blood cells; the platelet count or white blood cell count may also be increased. The erythrocyte sedimentation rate (ESR) is decreased due to an increase in zeta potential. Because polycythemia vera results from an essential increase in erythrocyte production, patients have a low erythropoietin (EPO) level.
In primary polycythemia, there may be 8 to 9 million and occasionally 11 million erythrocytes per cubic millimeter of blood (a normal range for adults is 4-6), and the hematocrit may be as high as 70 to 80%. In addition, the total blood volume sometimes increases to as much as twice normal. The entire vascular system can become markedly engorged with blood, and circulation times for blood throughout the body can increase up to twice the normal value. The increased numbers of erythrocytes can cause the viscosity of the blood to increase as much as five times normal. Capillaries can become plugged by the very viscous blood, and the flow of blood through the vessels tends to be extremely sluggish.
As a consequence of the above, people with untreated polycythemia vera are at a risk of various thrombotic events (deep venous thrombosis, pulmonary embolism), heart attack and stroke, and have a substantial risk of Budd-Chiari syndrome (hepatic vein thrombosis), or myelofibrosis. The condition is considered chronic; no cure exists. Symptomatic treatment (see below) can normalize the blood count and most patients can live a normal life for years.
The disease appears more common in Jews of European extraction than in most non-Jewish populations. Some familial forms of polycythemia vera are noted, but the mode of inheritance is not clear.
Recently, in 2005, a mutation in the JAK2 kinase (V617F) was found by multiple research groups  to be strongly associated with polycythemia vera. JAK2 is a member of the Janus kinase family and makes the erythroid precursors hypersensitive to erythropoietin (EPO). This mutation may be helpful in making a diagnosis or as a target for future therapy.
Untreated, polycythemia vera can be fatal. Research has found that the "1.5-3 years of median survival in the absence of therapy has been extended to at least 10-20 years because of new therapeutic tools."
As the condition cannot be cured, treatment focuses on treating symptoms and reducing thrombotic complications by reducing the erythrocyte levels.
Bloodletting (called venesection or phlebotomy) is one form of treatment, which often may be combined with other therapies. The removal of blood from the body reduces the blood volume and brings down the hematocrit levels; in patients with polycythemia vera, this reduces the risk of blood clots. Venesection is typically performed in people with polycythemia vera to bring their hematocrit (red blood cell percentage) down below 45 for men or 42 for women. It has been observed that phlebotomy also improves cognitive impairment.
Low dose aspirin (75–81 mg daily) is often prescribed. Research has shown that aspirin reduces the risk for various thrombotic complications.
Chemotherapy for polycythemia may be used, either for maintenance, or when the rate of bloodlettings required to maintain normal hematocrit is not acceptable, or when there is significant thrombocytosis or intractable pruritus. This is usually with a "cytoreductive agent" (hydroxyurea, also known as hydroxycarbamide).
The tendency of some practitioners to avoid chemotherapy if possible, especially in young patients, is a result of research indicating possible increased risk of transformation to acute myelogenous leukemia (AML). While hydroxyurea is considered safer in this aspect, there is still some debate about its long-term safety.
In the past, injection of radioactive isotopes (principally phosphorus-32) was used as another means to suppress the bone marrow. Such treatment is now avoided due to a high rate of AML transformation.
Bone marrow transplants are rarely undertaken in polycythemia patients; since this condition is non-fatal if treated and monitored, the benefits rarely outweigh the risks involved in such a procedure.
See also 
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- 11-141d. at Merck Manual of Diagnosis and Therapy Professional Edition