Essential thrombocythemia (ET) is a clonal disorder19 characterized by panmyelosis of the bone marrow with a predominance of megakaryocytes resulting in marked thrombocyto-sis.112-114 Megakaryocyte progenitors may be hypersensitive to thrombopoietin. The platelet count usually exceeds 106/|l. Mild leukocytosis (20 to 30,000/|l) is often present, but the red cell mass is normal (unless iron deficiency occurs due to bleeding) and must be so to distinguish this entity from polycythemia vera. Bone marrow reticulin may be mildly increased, but an excessive degree of fibrosis excludes ET and suggests the entity of agnogenic myeloid metaplasia with myelofibrosis. Although the bone marrow examination is not diagnostic, clusters of atypical and dysplastic megakaryocytes are generally noted115 (Plate 3-1K). These cells may contain cytoplasmic blebs and may also be observed in the peripheral blood. Large aggregates of platelets including giant and bizarre forms are seen in peripheral blood (Plate 3-1L). The Polycythemia Vera Study Group has established specific criteria for the diagnosis of ET, which have included the absence of overt fibrosis, the absence of the Philadelphia chromosome, a normal red cell mass, and the absence of any explanation for secondary thrombocytosis.116
Both bleeding and clotting diathesis have been reported in patients with ET.117>118 Such problems are believed to be due to associated platelet functional abnormalities rather than to the absolute magnitude of the thrombocytosis, which does not bear a direct relationship to the severity of clinical problems.119,120 Splenomegaly is unusual: Although transition to acute leukemia is not unknown, the incidence is enhanced by the use of cytotoxic therapeutic agents.121,122 The clinical course is quite protracted in most patients with an 80 percent survival at 8 years.123
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