Polycythemia

Polycythemia is a condition in which there is a net increase in the total circulating erythrocyte (red blood cell) mass of the body. There are several types of polycythemia.

Types

Primary polycythemia (also known as polycythemia vera)

Primary polycythemia, often called polycythemia vera (PCV), polycythemia rubra vera (PRV), erythremia, or just PV, occurs when excess erythrocytes are produced as a result of a proliferative abnormality of the bone marrow. This can also be brought on by abnormalities (tumors) in the kidneys or other growths since the kidney helps to regulate erythrocytes production. Often, excess white blood cells (leukocytosis) and platelets (thrombocytosis) are also produced. It is, therefore, classified as a myeloproliferative disease.

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 4-5), 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 increase the viscosity of the blood to 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.

Recently, in 2005, a mutation in the JAK2 kinase (V617F) was found by multiple research groups (Baxter et al., 2005; Levine et al., 2005) to be strongly associated with polycythemia vera. JAK2 is a member of the Janus kinase family. This mutation may be helpful in making a diagnosis or as a target for future therapy.

As a consequence of the above, people with untreated PV 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). 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.

Secondary polycythemia

Secondary polycythemia is caused by either appropriate or inappropriate increases in the production of erythropoietin that result in an increased production of erythrocytes. In secondary polycythemia, there may be 6 to 8 million and occasionally 9 million erythrocytes per cubic millimeter of blood. A type of secondary polycythemia in which the production of erythropoietin increases appropriately is called physiologic polycythemia. Physiologic polycythemia occurs in individuals living at high altitudes (4275 to 5200 meters), where oxygen availability is less than at sea level. Many athletes train at higher altitudes to take advantage of this effect -- a legal form of blood doping.

Other causes of secondary polycythemia include smoking, renal or liver tumors, or heart or lung diseases that result in hypoxia. Endocrine abnormalities, prominently including pheochromocytoma and adrenal adenoma with Cushing's syndrome are also secondary causes. Athletes and bodybuilders who abuse anabolic steroids or erythropoietin may develop secondary polycythemia.

Relative polycythemia

Relative polycythemia is an apparent rise of the erythrocyte level in the blood; however, the underlying cause is reduced blood plasma. Relative polycythemia is often caused by fluid loss eg. burns, dehydration and stress polycythemia.

Treatment

As the condition cannot be cured, treatment focuses on treating symptoms and reducing thrombotic complications reducing the erythrocyte levels.

Bloodletting 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.

Low dose aspirin is often prescribed. Research has shown that aspirin reduces the risk for various thrombotic complications.

Chemotherapy for polycythemia may be used sparingly, when the rate of bloodlettings required to maintain normal hematocrit is not acceptable. This is usually with a "cytoreductive agent" (hydroxyurea, also known as hydroxycarbamide).

The tendency to avoid chemotherapy if possible, especially in young patients, is due to research indicating increased risk of transformation to AML, and 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 was used as another means to suppress the bone marrow. Such treatment is now avoided due to a high rate of AML transformation.

Other therapies include interferon injections, and in cases where secondary thrombocytosis (high platelet count) is present, anagrelide may be prescribed.

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.

References

External links

For more information, see:

Polycythaemia vera | Maladie de Vaquez | פוליציטמיה | Eritrocitozė | 多血症

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