Essential thrombocythemia (ET) is a myeloproliferative disorder defined as a persistent isolated platelet count above 600,000/mL. It is frequently accompanied by a marked increase in megakaryocytes in the bone marrow, and on physical examination splenomegaly is common. Patients frequently develop complications with both thrombosis and hemorrhage.

The exact incidence of ET is unknown, although it is not a rare disorder. There is no known etiology. The major clinical manifestations are related to the elevated platelet count, resulting in either hemorrhagic or thrombotic episodes. Elderly patients can be particularly prone to these additional thrombohemorrhagic complications because of other comorbid conditions. The platelets in patients with ET are qualitatively abnormal, and this may explain why the frequency of thrombohemorrhagic complications is not directly related to the absolute number of circulating platelets.

The diagnosis is made on the basis of a sustained and unexplianed increase in the platelet count. It is important to distinguish primary thrombocythemia from reactive thrombocytosis, as well as to consider the possibility of another myeloproliferative disorder associated with an elevated platelet count. Some of the important causes of secondary thrombocytosis that should be considered in a differential diagnosis include iron deficiency anemia, occult carcinoma, chronic inflammatory conditions such as rheumatoid arthritis and inflammatory bowel disease, and acute or chronic infections. Treatment for patients who have reactive thrombocytosis should be directed at the underlying etiology regardless of the platelet count because these patients rarely develop thrombohemorrhagic complications despite markedly elevated platelet counts.

Treatment
The treatment of ET is usually with hydroxyurea, and the goal is to control the platelet count to around 500,000/mL. The important point is that many patients can be observed without therapy until the platelet count approaches 1 million. Older patients may need to be treated somewhat earlier because of their otherwise increased risk for thrombohemorrhagic complications. Previous studies using busulfan and P have demonstrated increased toxicities, and these therapies are no longer standard for ET. The prognosis is similar to that of PV, with prolonged survival possible. Hemorrhage or thrombotic complications frequently contribute to morbidity and mortality. Some patients will evolve to myelofibrosis or acute leukemia.

Future Directions
The new antiplatelet agent anagrelide can inhibit platelet aggregation and platelet production. Interferon-alpha has some activity in this disorder. Combinations of these agents, with or without hydroxyurea, may represent an improvement in the management of patients with ET. Carefully controlled trials are needed to assess these issues.