Acquired sideroblastic anemia is a heterogeneous group of disorders characterized by excessive accumulation of amorphous iron deposits in the mitochondria of erythrocyte precursors. Ring sideroblasts, the sine qua non of sideroblastic anemia, are erythroblasts in Prussian blue-stained marrow aspirates that have numerous iron-positive mitochondria that occur in a perinuclear distribution. In sideroblastic anemia, typical ring sideroblasts have a full or partial ring and usually comprise more than 5% of all erythroblasts (Chapter 25, Fig. 25.1a, b). Other erythroblasts in the same specimens may have markedly increased numbers of iron-positive mitochondria that do not form a perinuclear halo. Acquired sideroblastic anemias encompass two main categories: clonal hematopoietic stem cell disorders classified as myelodysplastic syndromes (MDS); and reversible types of sideroblastic anemia due to drugs, chemicals, or other factors. Heritable types of sideroblastic anemia are described in Chapter 25.

Acquired sideroblastic anemia due to myelodysplastic syndromes

Classification

Refractory anemia with ring sideroblasts (RARS) is characterized by erythroid dysplasia, mitochondrial accumulation of mitochondrial iron-containing ferritin (mitoferrin), defective erythroid maturation, and anemia; some patients also have elevated platelet counts. Other patients with MDS and ring sideroblasts are classified as having refractory cytopenias with multilineage dysplasia and ring sideroblasts (RCMD-RS). MDS in patients with either RARS or RCMD-RS may progress to excess marrow blasts (refractory anemia with excess blasts, RAEB) or acute leukemia.

Pathophysiology

Non-germline mutations in marrow stem cells underlie the pathogenesis of MDS, including MDS subcategories typically associated with ring sideroblasts.