Myelodysplastic syndrome (MDS) is a rare clonal hematopoietic stem cell disorder characterized by abnormal (dysplastic) blood cells, ineffective bone marrow function, and decreased blood cell counts (cytopenias). There are various types of MDS, classified based on peripheral blood, bone marrow, and cytogenetic features. The risk of developing MDS increases with age, and the median age at diagnosis is 65 years or older, with a male predominance. Some types of MDS are related to prior chemotherapy.

The peripheral blood smear usually demonstrates low cell counts (ie, anemia, thrombocytopenia, leukocytopenia) and dysplastic features in RBCs (basophilic stippling, megaloblastic changes, etc) and/or WBCs (hyposegmented or hypersegmented granulocytes, hypogranularity, etc).

The bone marrow aspirate and biopsy will usually show hypercellularity, as well as single- or multi-lineage dysplasia. Maturing RBCs may show nuclear budding, irregularity, or bridging. Ring sideroblasts may be seen. Megakaryocytes may show hypolobulation of the nucleus. Myeloblasts may be present; however, they must comprise less than 20% of cells. Certain cytogenetic abnormalities may also be present that can determine the subtype of MDS.

Depending on MDS subtype, patients may be at higher risk of developing acute myeloid leukemia (AML).