Dear Doctor: I've been diagnosed with polycythemia vera, which I understand is very rare. I'm currently being treated with phlebotomies. Please explain this diagnosis and what science has learned about it.
Dear Reader: To understand polycythemia vera (PV), you must first understand how bone marrow works. For starters, the marrow is full of stem cells that eventually become red blood cells (which carry oxygen), platelets (which help the blood to clot) and white blood cells (which fight infection). Initially, however, the stem cells become either lymphoid cells (which develop into lymphocytes, a specific type of white blood cell) or myeloid cells (which develop into red blood cells, platelets or white blood cells that are not lymphocytes). PV occurs when the myeloid cells reproduce preferentially, producing more of their type of cells.
This causes a significant elevation in the number of red blood cells and, to a lesser degree, platelets and white blood cells. The origin of this proliferation is linked to a mutation in the JAK2 gene, which is only occasionally linked to a genetic susceptibility within families. More often than not, it's a random mutation.
The greatest complications of PV are due to the increased concentration of red blood cells and platelets within the blood stream. This increases the risk of blood clots, which can lead to strokes, heart attacks and a sudden loss of blood flow to other organs or to the extremities. About 16 percent of people with PV develop an arterial thrombosis, or blood clot in an artery. About 7 percent develop blood clots in the veins, which can lead to clots in the legs or lungs or in the vein to the liver and spleen.
Speaking of the spleen, 36 percent of people with PV develop an enlarged spleen. Because this organ stores red blood cells and filters out old ones, the high concentration of red blood cells mean that these cells become sequestered in the spleen, causing it to enlarge and compress surrounding structures.
One of the more irritating symptoms of PV is itching, which worsens with exposure to warm water. A more troublesome symptom is a burning sensation in the hands or feet due to alterations of blood flow in the small blood vessels in the feet or hands. This condition, called erythromelalgia, affects 29 percent of patients.
Once a practitioner is clued into the possibility of PV, he or she may order a bone marrow biopsy as well as further blood work, including checking for the mutation in the JAK2 gene.
Phlebotomy is often necessary to lower the red blood cell count, which significantly decreases the risk of blood clots. Low-dose aspirin (81 milligrams) can provide benefit too, not only by reducing the chance of clotting, but by reducing the itching of PV. And, for people over 60, or those with a history of clots, the medication hydroxyurea can help by lowering the red and white blood cell and platelet counts.
As you mention, PV is rare, occurring in 1 out of every 50,000 people per year on average. Although the condition can be life-threatening -- for all the reasons explained above -- the phlebotomies (inconvenient though they are) can help keep the disease under control.