Polycythemia vera is a primary bone marrow disorder. Bone marrow is found in the centre of most bones and normally produces all red blood cells and platelets, and most white blood cells. In polycythemia vera, a mutation occurs in a bone marrow cell, resulting in overproduction of the bone marrow cells. The overproduced cells include the red blood cells, but can also include white blood cells and platelets. Although the exact cause of polycythemia vera is unknown, researchers have found that a specific gene mutation in the JAK2 gene is present in over 90% of cases. Polycythemia vera is an uncommon condition It occurs more often in men than in women. It is rarely seen in people under the age of 40.

Secondary polycythemia is different from polycythemia vera in several ways. Secondary polycythemia occurs when the body is not getting enough oxygen or as a response to certain drugs or hormones. Red blood cell production increases in response to low oxygen concentration in the air. Because there's less oxygen in the blood, the body attempts to overcome the lack by making more red blood cells. It doesn't stop, however, and keeps producing them until there are too many. The bone marrow can also be overstimulated by testosterone replacement therapy.

Living for long periods at high altitudes where there's less oxygen may lead to polycythemia. It also may occur in people with chronic lung conditions and certain kidney tumours and cysts. Heavy smoking is associated with an increase in carbon monoxide in the blood and may also lead to higher red cell and hemoglobin levels. Low blood oxygen levels (hypoxia) due to congenital heart disease also appear to be a factor in the development of polycythemia.

Polycythemia may not cause any symptoms. It's often discovered only if a hemoglobin test or a red blood cell count is done. Some people do experience symptoms that appear gradually. These may include itching following bathing, dizziness, and a flushing of the face and hands. Weakness, headaches, visual disturbances, and a sense of "fullness" in the head and in the left upper abdomen may also be associated with the condition. Some people may have high blood pressure. Polycythemia is associated with an increased risk of blood clots (venous thrombosis, stroke, heart attack) and leukemia.

Without treatment, the  risk of death from stroke, blood clots, or heart attack increases. Blood clots are the most common cause of death, followed by complications of myelofibosis (a condition in which the bone marrow is replaced by scar tissue),  hemorrhage, and development of acute leukemia.

To diagnose polycythemia, a doctor will begin by discussing the patient's health history and doing a physical examination. The doctor will also look for physical signs of increased blood volume, such as dilated veins and a ruddy complexion. If the skin is flushed or itchy, particularly after a hot bath, it may indicate a diagnosis of polycythemia.

The doctor may also check for a history of smoking or alcohol abuse. A careful physical examination might also be done to check for high blood pressure, obesity, lack of oxygen (cyanosis), heart murmurs, or an enlarged spleen. Various blood tests will likely be done, including a complete blood cell count, erythropoietin level, and an assay to detect a JAK2 mutation. Additional tests will then be done to find out whether it is polycythemia vera or secondary polycythemia. These tests will probably include a history and physical examination, measurements of oxygen saturation. For some people, the doctor may suggest a bone marrow test.

Treatment will vary according to the person's age, symptoms, and blood test results.

To reduce the risk of blood clots, treatment with low-dose acetylsalicylic acid* (ASA) is recommended for most people with polycythemia (unless there is a reason they should not take it).
Phlebotomy (removal of small amounts of blood) used to be the most common type of therapy for polycythemia vera.

When phlebotomy is used, blood will be withdrawn in amounts of 300 mL to 500 mL every few days at first, then every few weeks, and then every few months. The treatment goal will be to keep the blood hemoglobin level within the low-to-normal range. Often, the process of blood withdrawal can be stopped for months at a time if the hemoglobin level stays in the required range. For seniors or for people with heart or brain blood vessel disease, less blood is usually taken (i.e., only 200 mL to 300 mL twice a week). Once a person's hemoglobin levels are normal, they will probably have monthly doctor's appointments.

If the blood has high white blood cell and platelet counts in addition to a high content of red blood cells, the physician may prescribe a medication that cuts down blood cell production by the bone marrow (e.g., hydroxyurea, ruxolitinib).

Complications such as high uric acid blood levels and itchy skin may be treated with allopurinol or antihistamines, respectively.

In rare cases, the spleen can become extremely enlarged. The physician may recommend its surgical removal (splenectomy).

Anyone who has had a splenectomy must get vaccinations to prevent future infections.

Unfortunately, there's no way to prevent polycythemia vera. However, you may reduce your risk of secondary polycythemia by not smoking.

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