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Acquired aplastic anemia is a clinical syndrome in which there is a deficiency of red cells, neutrophils, monocytes, and platelets in the blood, and fatty replacement of the marrow with a near absence of hematopoietic precursor cells. Reticulocytopenia, neutropenia, monocytopenia, and thrombocytopenia, when severe, are life-threatening because of the risk of infection and bleeding, complicated by severe anemia. Most cases occur without an evident precipitating cause and are the result of the expression of autoreactive cytotoxic T lymphocytes that suppress or destroy primitive CD34+ multipotential hematopoietic cells. The disorder also can occur after (1) prolonged high-dose exposure to certain toxic chemicals (e.g., benzene), (2) after specific viral infections (e.g., Epstein-Barr virus), (3) as an idiosyncratic response to certain pharmaceuticals (e.g., ticlopidine, chloramphenicol), (4) as a feature of a connective tissue or autoimmune disorder (e.g., lupus erythematosus), or, (5) rarely, in association with pregnancy. The final common pathway may be through cytotoxic T-cell autoreactivity, whether idiopathic or associated with an inciting agent since they all respond in a similar fashion to immunosuppressive therapy. The differential diagnosis of acquired aplastic anemia includes the hypoplastic marrow that can accompany paroxysmal nocturnal hemoglobinuria or hypoplastic oligoblastic (myelodysplastic syndrome) or polyblastic myelogenous leukemia. Allogeneic hematopoietic stem cell transplantation is curative in approximately 80 percent of younger patients with high-resolution human leukocyte antigen (HLA)-matched sibling donors although the posttransplant period may be severely complicated by graft-versus-host disease. The disease may be significantly ameliorated or occasionally cured by immunotherapy, especially a regimen coupling antithymocyte globulin with cyclosporine. However, after successful treatment with immunosuppressive agents, the disease may relapse or evolve into a clonal myeloid disorder, such as paroxysmal nocturnal hemoglobinuria, a clonal cytopenia, or oligoblastic or polyblastic myelogenous leukemia. Several uncommon inherited disorders, including Fanconi anemia, Shwachman-Diamond syndrome, dyskeratosis congenita and others have as a primary manifestation aplastic hematopoiesis.

Acronyms and Abbreviations

Acronyms and abbreviations that appear in this chapter include: A, adenine; ALG, antilymphocyte globulin; ALL, acute lymphocytic leukemia; AML, acute myelogenous leukemia; ATG, antithymocyte globulin; ATR, ataxia-telangiectasia mutated and rad3-related kinase; BFU–E, erythroid burst-forming units; CD, cluster of differentiation; CFU-GM, colony-forming unit–granulocyte-macrophage; CMV, cytomegalovirus; EBV, Epstein-Barr virus; G, guanine; G-CSF, granulocyte colony-stimulating factor; HHV, human herpes virus; HIV, human immunodeficiency virus; HLA, human leukocyte antigen; IL, interleukin; LDH, lactic dehydrogenase; NMRI, nuclear magnetic resonance imaging; PCP, pentachlorophenol; PNH, paroxysmal nocturnal hemoglobinuria; SCF, stem cell factor; T, thymine; TNF, tumor necrosis factor; TNT, trinitrotoluene; TPO, thrombopoietin.

Definition and History

Aplastic anemia is a clinical syndrome that results from a marked diminution of marrow blood cell production. The latter results in reticulocytopenia, anemia, granulocytopenia, monocytopenia, and thrombocytopenia. The diagnosis usually requires the presence of pancytopenia with a neutrophil count fewer than 1500/μL (1.5 × 109/L), a platelet count fewer than 50,000/μL (50 × 109/L), a hemoglobin concentration less than 10 g/dL (100 g/L), and an absolute reticulocyte count fewer than 40,000/μL (40 × 109...

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