Diagnostic Considerations
The two major diagnostic challenges are distinguishing myelodysplastic syndrome (MDS) with a low blast count due to aplastic anemia and other nonclonal bone marrow disorders and differentiating MDS with excess blasts from acute myeloid leukemia (AML). Additional diagnoses to consider include autoimmune cytopenias, Diamond-Blackfan anemia, vitamin and mineral deficiencies, zinc toxicity, viral infections, and other inherited bone marrow failure syndromes.
Refractory cytopenia may be difficult to diagnose because bone marrow cellularity is often reduced (as in aplastic anemia), impeding the identification of the often subtle dysplastic changes that may be present. In the absence of a cytogenetic marker, the clinical course must be carefully monitored with repeated bone marrow examinations and biopsies at least 2-3 weeks apart. The bone marrow morphology in RCC has a characteristic appearance, with patchy, left-shifted erythropoiesis with increased mitosis, markedly decreased and left-shifted myelopoiesis, and markedly decreased megakaryocytes with dysplastic micromegakaryocytes. [21]
Differentiating MDS with increased blast count from de novo AML remains challenging, and thresholds of blast counts (set at 20% or 30%) are arbitrary and may not reflect the biology of these transitional states. De novo AML is chemotherapy-sensitive and is characterized by balanced translocations, such as t(8;21), t(15;17), t(9;11). The usual genetic changes in MDS, typically markers of chemoresistance, are aneuploidy and aberrations in chromosome numbers (eg, monosomy 7).
Thus, individuals with typical cytogenetic abnormalities should be treated as having de novo AML, regardless of the blast count. Note that most patients with MDS have a blast count of less than 20%, whereas the vast majority of children with de novo AML have frankly leukemic marrow. For patients with borderline blast counts, other clinical signs (eg, organomegaly, chloroma, spinal fluid blasts) suggest a diagnosis of de novo AML.
Differential Diagnoses
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Herpesvirus 6 Infection
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Pediatric myelodysplastic syndrome. A = binucleate megaloblastoid erythroid precursor; B = megaloblastoid erythroid precursor; C = small megakaryocyte with monolobate nucleus.
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Pediatric myelodysplastic syndrome. A = multinucleate erythroid precursor; B = binucleate megaloblastoid erythroid precursor; C = dysplastic erythroid nuclei.
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Pediatric myelodysplastic syndrome. A = vacuolated erythroblasts; B = hypogranular band.
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Pediatric myelodysplastic syndrome. Internuclear bridge between erythroid precursors (arrow).
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Pediatric myelodysplastic syndrome. Hypogranular Pelger-Huet neutrophils and dimorphic hypochromic and normochromic red blood cells.
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Pediatric myelodysplastic syndrome. Micromegakaryocytes with single or multiple, small, round nuclei.
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Pediatric myelodysplastic syndrome. Bone marrow section, hematoxylin and eosin. Note the megakaryocytes (arrows) with a peripheral ring of nuclei (resembling Touton giant cells) and central eosinophilic inclusions displacing the nuclei.
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Pediatric myelodysplastic syndrome. Bone marrow aspirate, Wright-Giemsa stain. Note the megakaryocyte with a central mass displacing the nuclei peripherally.
