Chapter 44: Myelodysplastic Syndromes (Clonal Cytopenias and Oligoblastic Myelogenous Leukemia)

DEFINITION

• Myelodysplasia or myelodysplastic syndrome (MDS) is the term used, as a generalization, to encompass a diverse group of myeloid neoplasms that have in common (1) their origin in a somatically mutated multipotential hematopoietic cell, (2) ineffective hematopoiesis (late precursor apoptosis) leading to cytopenias despite a normocellular or hypercellular marrow, and (3) a propensity to undergo clonal progression to acute myelogenous leukemia (AML).

• The spectrum ranges from (1) indolent disorder with mild or moderate anemia to (2) more troublesome multicytopenias without morphologic evidence of leukemic cells to (3) oligoblastic (subacute) myelogenous leukemia with leukemic blast cells in the marrow and blood.

• Clonal cytopenia refers to disorders without increased leukemic blast cells in the marrow or blood. The manifestations vary from isolated cytopenia (eg, anemia) with a marrow with erythroid dysmorphia to severe multicytopenias with hypercellular marrow and with dysmorphia of marrow precursors in each major lineage and of blood neutrophils, red cells, and platelets.

• Because the neoplasm originates in a multipotential hematopoietic cell (eg, the lymphohematopoietic stem cell), careful inspection of blood and marrow will usually identify mild involvement of all three major lineages (eg, low normal blood cell counts) or subtle morphological abnormalities.

• Oligoblastic or subacute myelogenous leukemia (synonym: refractory anemia with excess blasts) refers to patients with cytopenias and with marrow containing 3% to 19% leukemic blast cells. (The World Health Organization [WHO] currently uses the range of 5% to 19% blast cells.)

• If the marrow blast cell count is 20% or higher, the disease is considered AML and so treated.

• The use of less than 5% marrow blasts as a demarcation between a normal and pathological blast percentage is an anachronism dating from a decision made in 1955, at which time the first definition of a remission in childhood acute lymphoblastic leukemia used less than 5% 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,” however, is not relevant at the time of diagnosis, especially in the case of myeloid neoplasms, has not been validated, but it has been ensconced.

• The use of less than or greater than or equal to 20% blasts to distinguish oligoblastic myelogenous leukemia (refractory anemia with excess blasts) from AML is arbitrary and without pathobiologic foundation. Studies have shown no difference in phenotypic manifestations or survival between populations with 10% to 19% as compared to 20% to 30% leukemic myeloblasts. The physician should determine management of the case by several factors (eg, physiologic age, severity of cytopenias, cytogenetic or oncogene risk category, transfusion requirements, frequency and severity of infections), not principally whether a patient with myelogenous leukemia has 10%, 15%, 20%, or 25% blast cells.

• More rarefied diagnostic categories are in the WHO classification shown in Table 44–1 and are currently being ...