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At a glance

Chronic myeloproliferative disorder caused by a clonal increase in platelets resulting in thromboembolic and hemorrhagic complications. It is a rare chronic blood condition characterized by overproduction of platelets by megakaryocytes in the bone marrow. It is believed to eventually develop into acute myeloid leukemia or myelofibrosis. It is one of four myeloproliferative neoplasms, defined as hematological cancers due to inappropriate amount of one of its constituents, ie, erythrocytes, leukocytes, or platelets.

Synonyms

Primary Thrombocytosis; Idiopathic Thrombocytosis; Hemorrhagic Thrombocytosis.

History

Dr William Dameshek, a pioneering hematologist, placed ET within a family of blood diseases called myeloproliferative disorders in 1950. His observations were based on the fact that affected individuals were producing abnormal amounts of various blood cells. In Dr Dameshek’s words, it was perceived “due to a hitherto undiscovered stimulus.” At the time, it was unclear if these “proliferations” represented a natural response to some external cause, or were the result of an internal defect. Over time, it became obvious that the myeloproliferative disorders were caused by genetic mutations and passed along to the entire progeny of that cell, even as they mature into platelets, erythrocytes, or leukocytes. In acknowledgement of this new understanding, myeloproliferative disorders were renamed myeloproliferative neoplasms (MPN). For this reason, ET is best thought of as a chronic type of leukemia—albeit one with an overall excellent prognosis and often requiring minimal or no treatment.

Incidence

The incidence is estimated at 0.6 to 2.5:100,000 per year. The median age at the onset of the disease is 65 to 70 years. However, the age range is wide and includes women in their childbearing years, which creates an important subset of ET patients with special therapeutic considerations. Overall, both genders are approximately equally affected in older age but there is for an unknown reason a predilection for females to be affected more frequently than males at younger ages. Although believed to be extremely rare in children and transmitted genetically, the estimated incidence reported in recent years established it at 0.9:100,000 per year. Approximately 20% of the patients are younger than 40 years at the time of onset.

Genetic inheritance

Rare familial form with autosomal dominant transmission has been described. However, in the vast majority of patients, no genetic background is detectable.

Pathophysiology

Not fully elucidated yet. ET is caused by proliferation of megakaryocytes, of which the exact pathogenesis remains to be elucidated. However, evidence now indicates that a mutation of the thrombopoietin gene (located on 3q26.3-q27) may play a major role. Thrombopoietin is the key hormone in the regulation of megakaryocyte differentiation and proliferation. The mechanisms by which ET leads to thromboembolic and hemorrhagic complications remain unclear, although different laboratory findings have been used to explain them (intrinsic abnormality of the platelets resulting in hypoaggregation ...

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