Essential Thrombocythemia

Essential thrombocythemia, by contrast, is a myeloproliferative disease (see p. 114) in which the main feature of increased thrombocytes is accompanied by other signs of this group of diseases that may vary in severity, such as leukocytosis and an enlarged spleen. Severe thrombocythemia may also be seen in osteomyelosclerosis, polycythemia vera, and chronic myeloid leukemia, and for this reason the following specific diagnostic criteria have been suggested:

Diagnostic criteria for essential thrombocytopenia (according to Murphy et al.)

> Thrombocytes > 600 x 109/l (with control)

> Normal erythrocyte mass or Hb < 18.5 g/dl 16.5 g/dl 5

> No significant bone marrow fibrosis

> No splenomegaly

> No leukoerythroblastic CBC

> Absence of morphological or cytogenetic criteria of myelodysplasia

> Secondary thrombocytosis (iron deficiency, inflammation, neoplasia, trauma, etc.) excluded

Large thrombocytes are found in the peripheral blood smear. However, these also occur in polycythemia vera and osteomyelosclerosis.

Bone marrow cytology will show markedly elevated megakaryocyte counts, with the cells often forming clusters and often with hyper-segmented nuclei.

Fig. 61 Essential thrombocythemia. a Increased thrombocyte density and mar- ► ked anisocytosis in essential thrombocythemia. b Large thrombocytes (1) and a micro(mega)karyocyte nucleus (2) in essential thrombocythemia. Micro(me-ga)karyocytes are characterized by a small, very dense and often lobed nucleus with narrow, uneven cytoplasm, the processes of which correspond to thrombocytes (arrow).

I Thrombocyte proliferation with large megakaryocytes: essential thrombocythemia, a chronic myeloproliferative disease

Polycythemia Vera Blood Smear

Fig. 61 c and d Bone marrow cytology in essential thrombocythemia: there is a striking abundance of very large, hyperlobulated megakaryocytes (c); such megakaryocytes may also be seen in polycythemia vera. d Size comparison with basophilic erythroblasts (arrow). The cloudy cytoplasm of the megakaryocyte is typical of effective thrombocyte production.

Fig. 61 c and d Bone marrow cytology in essential thrombocythemia: there is a striking abundance of very large, hyperlobulated megakaryocytes (c); such megakaryocytes may also be seen in polycythemia vera. d Size comparison with basophilic erythroblasts (arrow). The cloudy cytoplasm of the megakaryocyte is typical of effective thrombocyte production.

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