The myelodysplastic syndromes (MDS) are a group of disorders characterized by one or more peripheral blood cytopenias secondary to bone marrow dysfunction. The MDS are diagnosed in slightly more than 10,000 people in the United States yearly for an annual age-adjusted incidence of 3.4/100,000 people.[1] The MDS are more common in men and whites. The syndromes may arise de novo, or secondarily after treatment with chemotherapy and/or radiation therapy for other diseases. Secondary myelodysplasia usually has a poorer prognosis than does de novo myelodysplasia. Prognosis is directly related to the number of bone marrow blast cells and to the amount of peripheral blood cytopenias. The MDS transform to acute myeloid leukemia (AML) in about 30% of patients after various intervals from diagnosis and at variable rates. (Refer to the Cellular Classification section for more information.)

The acute leukemic transformation is much less responsive to chemotherapy than is de novo AML. Prognosis is also related to the type of myelodysplastic syndrome. Supportive care has been the mainstay of treatment. Judicious use of platelet and blood transfusions and iron chelation may prevent or delay alloimmunization and iron overload and favorably affect prognosis.

The MDS are characterized by abnormal bone marrow and blood cell morphology. Megaloblastic erythroid hyperplasia with macrocytic anemia, which is associated with normal vitamin B₁₂ and folate levels, is frequently observed. Circulating granulocytes are frequently severely reduced, often hypogranular or hypergranular, and may display the acquired pseudo-Pelger-Huët abnormality. Early, abnormal myeloid progenitors are identified in the marrow in varying percentages, depending on the type of myelodysplastic syndrome. Abnormally small megakaryocytes (micromegakaryocytes) may be seen in the marrow and hypogranular or giant platelets may appear in the blood.

The MDS occur predominantly in older patients (usually >60 years), though patients as young as 2 years have been reported.[2] Anemia, bleeding, easy bruising, and fatigue are common initial findings. (Refer to the PDQ summary on Fatigue for more information.) Splenomegaly or hepatosplenomegaly may occasionally be present in association with an overlapping myeloproliferative disorder. Approximately 50% of the patients have a detectable cytogenetic abnormality, most commonly a deletion of all or part of chromosome 5 or 7, or trisomy 8.[3] Although the bone marrow is usually hypercellular at diagnosis, 15% to 20% of patients present with a hypoplastic bone marrow.[4] Hypoplastic myelodysplastic patients tend to have profound cytopenias and may respond more frequently to immunosuppressive therapy.

A variety of risk classification systems have been developed to predict the overall survival of patients with MDS and the evolution from MDS to AML. These classification systems include the French-American-British classification,[5] the Bournemouth score,[6] the Sanz score,[7] the Lille score,[8] and the World Health Organization classification.[9] Clinical variables in these systems have included bone marrow and blood myeloblast percentage, specific cytopenias, age, lactate dehydrogenase level, and bone marrow cytogenetic pattern.