The myelodysplastic syndromes are a group of disorders with a varying number of features that may raise the question of early, borderline, or atypical acute leukemia but that do not satisfy FAB criteria for leukemia (especially, the FAB cutoff level of 30% blasts in the bone marrow). The disorders included in this category by the FAB group have certain common features: insufficient blasts in bone marrow to be diagnosed as acute leukemia; some degree of abnormality in at least two cell lines (RBC, WBC, or platelets), a high incidence of pancytopenia or cytopenia of less than three cell lines, frequent normocellular or hypercellular bone marrow in spite of peripheral blood pancytopenia, and a relatively high rate of progression to acute nonlymphocytic leukemia (roughly 15%-45%, depending on the particular subgroup of the FAB classification). Some or all of these disorders were previously considered a subgroup of Di Guglielmo’s syndrome by some investigators and were called “preleukemia” by others. Some believe that these disorders represent a leukemic cell clone or clones that for some reason have a relatively slow or variable progression. These syndromes can be idiopathic or preceded (and presumably induced) by bone marrow injury from toxins or radiation. The myelodysplastic syndrome occurs predominantly in persons over age 50 (reported mean ages varying between 60 and 80). Men are affected more frequently than women in some studies but not in others. Chromosome abnormalities have been reported in 35%-50% of cases, with the most common being monosomy 7 (loss of one chromosome 7) or 7q- (loss of a chromosome 7 long arm).

The peripheral blood frequently contains some abnormal cells such as oval macrocytes; moderate anisocytosis and poikilocytosis; a few nucleated RBCs; myeloid cells with Pelger-Huлt nuclear changes, abnormal granulation, or abnormal granules; and abnormal platelets. There may be a few blasts, but less than 5%. Bone marrow shows various combinations of myeloid immaturity (but <30% blasts), increased monocytes, megaloblastic, or megaloblastoid RBC precursor change, ring sideroblasts, and abnormal megakaryocytes in addition to changes similar to those of the peripheral blood. Table 7-2 lists the major differences between subgroups of the myelodysplastic syndrome.

French-American-British classification of myelodysplastic syndromes

Table 7-2 French-American-British classification of myelodysplastic syndromes