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DEFINITION

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  • Myelodysplasia is the term used, as a generalization, to encompass a diverse group of myeloid neoplasms that have in common (a) their origin in a somatically mutated multipotential hematopoietic cell, (b) ineffective hematopoiesis (late precursor apoptosis) leading to cytopenias despite a normocellular or hypercellular marrow, and (c) a propensity to undergo clonal progression to acute myelogenous leukemia (AML).

  • Spectrum ranges from (a) indolent disorder with mild or moderate anemia to (b) more troublesome multicytopenias without morphologic evidence of leukemic cells to (c) oligoblastic (subacute) myelogenous leukemia with leukemic blast cells in the marrow and blood (Table 42–1).

  • Clonal cytopenia refers to disorders without increased leukemic blast cells in the marrow. The manifestations vary from isolated anemia with a nearly normal marrow to severe multicytopenias with hypercellular marrow and with dysmorphia of marrow precursors and blood cells.

  • Oligoblastic or subacute myelogenous leukemia (synonym: refractory anemia with excess blasts) refers to patients with cytopenias and with marrow containing 2 to 19 percent leukemic blast cells. (The World Health Organization [WHO] uses the range 5 to 19%.)

  • If the marrow blast cell count is 20 percent or higher, the disease is considered acute myelogenous leukemia and so treated.

  • The use of <5 percent marrow blasts as a demarcation is an anachronism dating from a decision made in 1955, at which time the first definition of remission in childhood acute lymphoblastic leukemia used <5 percent marrow blasts (and other salutary changes) to avoid additional cytotoxic treatment in an era without platelet transfusion, potent wide-spectrum antibiotics, or other support for children treated with multidrug regimens. Also, only light microscopy was available to distinguish nonleukemic lymphocytes in treated marrows from residual leukemic lymphoblasts. This, so-called, "5 percent rule" is irrelevant at the time of diagnosis, especially in the case of myeloblasts, but it has been ensconced.

  • The use of 19 versus 20 percent blasts to distinguish oligoblastic myelogenous leukemia (refractory anemia with excess blasts) from AML is, of course, arbitrary and without pathobiologic foundation. The physician should determine management of the case by several factors (e.g., physiologic age, severity of cytopenias, transfusion requirements, frequency and severity of infections) not principally whether a patient with myelogenous leukemia has 15, 20, or 25 percent blast cells.

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Table Graphic Jump Location
TABLE 42–1CLASSIFICATION OF THE MYELODYSPLASTIC SYNDROMES (CLONAL CYTOPENIAS AND OLIGOBLASTIC LEUKEMIA)
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ETIOLOGY AND PATHOGENESIS

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  • The fundamental alteration is a somatically mutated multipotential hematopoietic cell, resulting in trilineage blood cell abnormalities in most cases. Even in clonal anemia in which the neutrophils count and platelet count are within the normal range, evidence of dysmorphia in neutrophil and megakaryocyte-platelet lineages may be evident on careful examination.

  • Epigenetic gene ...

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