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 affects 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.
Often, thrombocythemia does not cause symptoms. However, an excess of platelets can cause blood clots to form spontaneously, blocking the flow of blood through blood vessels, especially smaller ones but also in large vessels, including vessels in the brain, liver, and heart. 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, chest pain, vision changes, 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.
Doctors make a diagnosis of thrombocythemia on the basis of the symptoms or after finding increased platelets during routine screening of the blood. Blood tests may be used to confirm the diagnosis. In addition, microscopic examination of the blood may reveal 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, doctors look 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.
Thrombocythemia may require treatment with a drug that decreases platelet production. Such drugs include hydroxyurea, anagrelide, and interferon-alpha. Treatment with one of these drugs is typically started when clotting complications develop. The age of the person, the other risks present, and previous history of forming blood clots (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 aspirin, 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 July 2008 by Josef T. Prchal, MD