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Thrombocythemia (primary thrombocythemia) is a disorder in which excess platelets are produced, leading to abnormal blood clotting or bleeding.

Platelets (thrombocytes) are normally produced in the bone marrow by cells called megakaryocytes. In thrombocythemia, megakaryocytes increase in number and produce too many platelets.

Thrombocythemia is rare, affecting about 2 to 3 of 100,000 people. It usually occurs in people older than 50 and more frequently in women. The cause of thrombocythemia is unknown.

Other Causes of a High Platelet Count When the cause of thrombocythemia is known, the disorder is called secondary thrombocythemia. Bleeding, removal of the spleen, infections, rheumatoid arthritis, certain cancers, and sarcoidosis can cause secondary thrombocythemia. People with secondary thrombocythemia may have no symptoms related to a high number of platelets; symptoms of the underlying condition usually dominate. When symptoms from a high number of platelets do occur, they are similar to those of primary thrombocythemia. Secondary thrombocythemia is diagnosed—and distinguished from primary thrombocythemia—when a person with a high platelet count has a condition that readily accounts for the high platelet count. Treatment is aimed at the cause. If the treatment is successful, the platelet count usually returns to normal.

Symptoms

Often, thrombocythemia does not produce symptoms. However, an excess of platelets can cause blood clots to form spontaneously, blocking the flow of blood through blood vessels, especially smaller ones. Older people with thrombocythemia are much more likely to form clots than are younger people.

Symptoms are due to the blockage of blood vessels and may include tingling and other abnormal sensations in the hands and feet (paresthesias), cold fingertips, headaches, weakness, and dizziness. Bleeding, usually mild, may occur, often consisting of nosebleeds, easy bruising, slight oozing from the gums, or bleeding in the digestive tract. The spleen and liver may enlarge.

Diagnosis

A doctor makes a diagnosis of thrombocythemia on the basis of the person's symptoms or after finding increased platelets during routine screening of the blood. Blood tests may be used to confirm the diagnosis. In thrombocythemia, the platelet count is usually 2 to 4 times higher than normal. In addition, a microscopic examination of the blood reveals abnormally large platelets, clumps of platelets, and fragments of megakaryocytes.

To distinguish primary thrombocythemia, whose cause is unknown, from secondary thrombocythemia, which has a known cause, a doctor looks for signs of other conditions that could increase the platelet count. Removal of a sample of bone marrow for examination under a microscope (bone marrow biopsy (see Symptoms and Diagnosis of Blood Disorders: Bone Marrow Examination) is sometimes helpful and can exclude chronic myelocytic leukemia as a cause of an increased platelet count.

Treatment

Thrombocythemia may require treatment with a drug that decreases platelet production. Such drugs include hydroxyureaSome Trade Names
HYDREA
, anagrelideSome Trade Names
AGRYLIN
, and interferon-alpha. Treatment with one of these drugs is typically started when the platelet count becomes exceedingly high or when bleeding or clotting complications develop. The age of the person, the other risks present, and previous history of thrombosis determine the need for such treatment. The drug is continued until the platelet count falls into a safe range. The dose must be adjusted to maintain an adequate number of platelets and other circulating cells. Small doses of aspirinSome Trade Names
ECOTRIN
ASPERGUM
, which makes platelets less sticky and impairs clotting, may also be used.

If drug treatment does not slow platelet production quickly enough, it may be combined with or replaced by plateletpheresis, a procedure reserved for emergency situations. In this procedure, blood is withdrawn, platelets are removed from it, and the platelet-depleted blood is returned to the person.

Last full review/revision February 2003