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Thrombocytosis, Essential

Background: Essential thrombocytosis (ET), first described by Epstein and Goedel in 1934, is a nonreactive, chronic myeloproliferative disorder. ET is associated with sustained megakaryocyte proliferation that increases the number of circulating platelets. Traditionally, ET was considered a clonal disorder that involved pluripotent stem cells; however, recent studies indicate that some patients may have polyclonal hematopoiesis.

ET is characterized by a platelet count greater than 600,000/mL, megakaryocytic hyperplasia, splenomegaly, and a clinical course complicated by hemorrhagic and/or thrombotic episodes.

Pathophysiology: Platelet survival is normal in ET. Megakaryocytes increase the production of platelets, causing thrombocytosis. The cause of this increase in platelet production remains unclear, though it may be a result of autonomous production, increased sensitivities to cytokines (eg, interleukin-3), decreased inhibition to platelet-inhibiting factors (eg, transforming growth factor beta), or defects in accessory cell microenvironment.

The mechanism by which thrombocythemia produces hemorrhage or thrombosis is not well defined. Several defects have been described, including a decrease in aggregation, hyperaggregation, and intracellular concentration of various chemicals. In addition, reports show a decrease in von Willebrand ristocetin cofactor activity and high molecular weight von Willebrand factor multimers. Some reports show patients with an acquired deficiency of antithrombin III, protein C, and protein S.

Frequency:

* In the US: Clinicians diagnose approximately 6000 cases each year. Some researchers speculate that the incidence rate may be several times higher. A study from southeastern Minnesota reports an incidence of 2.38 cases per 100,000 population per year (Mesa, 1999).

Mortality/Morbidity:

* Patients with ET have a 10-year survival rate of 64-80%, which may not be significantly different from that of the age-matched general population. Death occurs from thrombotic complications. Transformation to acute myelogenous leukemia (AML) occurs in 0.6-5% of patients and may accelerate if the patient takes chemotherapeutic agents.

* Patients may experience symptoms relating to large vessel or microvascular thrombosis and bleeding.

Sex: In older patients, the frequency is similar in both sexes; however, the disease occurs more often in young women than in young men.

Age: ET is more frequent in older patients, although younger patients may develop the disease. The median age at diagnosis is 60 years, and perhaps up to 20% of patients are younger than 40 years. The disease is rare in children.

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