Chapter 450 Other Microcytic Anemias
Sideroblastic anemias result from acquired and hereditary disorders of heme synthesis. The anemias are characterized by hypochromic microcytic red blood cells (RBCs) mixed with normal RBCs, thus giving an overall picture of a dimorphic population of erythrocytes, and the complete blood cell count indicates an extremely high RBC distribution width (RDW). The serum iron concentration usually is elevated, and the transferrin saturation of iron is increased. In all cases of sideroblastic anemia, regardless of the specific cause, impaired heme synthesis leads to retention of iron within the mitochondria. Morphologically, this is seen in marrow nucleated RBCs with iron granules (aggregates of iron in mitochondria) that have a perinuclear distribution. These unusual cells, known as ringed sideroblasts (see 
Fig. 450-1 on the Nelson Textbook of Pediatrics website at www.expertconsult.com), are found only in pathologic states and are distinct from the sideroblasts (RBC precursors that contain diffuse cytoplasmic ferritin granules) in the marrow of normal subjects. Sideroblastic anemias most commonly occur in adulthood, and these acquired disorders can be idiopathic or secondary to drugs, alcohol, or myelodysplastic disorders. A few sideroblastic anemias are seen in children.
Figure 450-1 Ring sideroblast in myelodysplastic syndrome (refractory anemia with ring sideroblasts)—iron stain.
(From Ryan DH, Cohen HJ: Bone marrow examination. In Hoffman R, Benz EJ Jr, Shattil SJ, et al, editors: Hematology, ed 4, Philadelphia, 2005, Churchill Livingstone.)
