Published online by Cambridge University Press: 07 August 2009
Introduction
Myelodysplastic syndromes (MDS) are a clinically heterogeneous group of clonal hematopoietic disorders characterized by ineffective hematopoiesis associated with dysplastic changes in one or more marrow lineages together with progressive cytopenias. MDS may occur as primary diseases, or may follow toxic exposures or therapy (Jaffe et al., 2001). Primary MDS is more often seen in elderly patients and frequently progresses to marrow failure, or evolves to acute leukemia, usually myeloid. The overall rate of transformation to acute leukemia depends on the subtype of MDS, ranging from 10% to over 60%. As a rule, the degree of trilineage dysplasia and percentage of blast cells correlate with the aggressiveness of the disease. It should be stressed that dysplastic features of hematopoietic elements is not unique to MDS and may occur in many reactive conditions, such as inflammatory states, HIV infection, endocrine dysfunctions, autoimmune disorders, and may also be associated with certain medications.
MDS have been traditionally subdivided according to the French–American–British (FAB) system (1982) into five major categories, primarily based on the percentage of blast cells in the peripheral blood and bone marrow (Table 7.1), the presence of ringed sideroblasts in the marrow, and the absolute count of monocytes in peripheral blood (Bennett et al., 1982). These subgroups show different rates of progression to acute myeloid leukemia (AML) and overall survival. In particular, refractory anemia and refractory anemia with ringed sideroblasts are associated with much longer median survival and a lower incidence of progression to acute leukemia than the other three FAB subtypes and can be considered “low-grade” MDS.
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