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Chapter 3: Aplastic Anemia: Acquired and Inherited

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DEFINITION

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  • Pancytopenia with markedly hypocellular marrow and normal marrow cell cytogenetics.

  • Incidence worldwide is 2 to 5 cases/million population per year and 5 to 12 cases/million population per year in the United States (and in other industrialized countries). Incidence is approximately twice as high in Asian countries.

  • Peak incidence between ages 15 and 25 and 65 to 69 years.

  • The definitions for range of severity of aplastic anemia are shown in Table 3–1.

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Table 3–1Degree of Severity of Acquired Aplastic Anemia
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Table 3–1 Degree of Severity of Acquired Aplastic Anemia
Diagnostic Categories Hemoglobin Reticulocyte Concentration Neutrophil Count Platelet Count Marrow Biopsy Comments
Moderately severe <100 g/L <40 × 109/L <1.5 × 109/L <50 × 109/L Decrease of hematopoietic cells. At the time of diagnosis at least 2 of 3 blood counts should meet these criteria.
Severe <90 g/L <30.0 × 109/L <0.5 × 109/L <30.0 × 109/L Marked decrease of hematopoietic cells. Search for a histocompatible sibling should be made if age permits.
Very Severe <80 g/L <20.0 × 109/L <0.2 × 109/L <20.0 × 109/L Marked decrease or absence of hematopoietic cells. Search for a histocompatible sibling should be made if age permits.

These values are approximations and must be considered in the context of an individual patient's circumstances. (In some clinical trials, the blood count thresholds for moderately severe aplastic anemia are higher, e.g., platelet count <100 × 109/L and absolute reticulocyte count < 60,000 × 109/L.) The marrow biopsy may contain the usual number of lymphocytes and plasma cells; Rare "hot spots," focal areas of erythroid cells, may be seen. No fibrosis, abnormal cells, or malignant cells should be evident in the marrow. Dysmorphic features of blood or marrow cells are not features of acquired aplastic anemia. Ethnic differences in the lower limit of the absolute neutrophil count should be considered (see Chap. 1).

Source: Williams Hematology, 8th ed, Chap. 34, Table 34–1, p. 464.

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ETIOLOGY AND PATHOGENESIS

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Pathogenesis

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  • Immune suppression of marrow by autoreactive T lymphocytes.

  • Toxic injury to stem and/or progenitor cells (e.g., certain chemotherapy or drugs) (see Table 3–2).

  • Inherited intrinsic stem cell defect (e.g., Fanconi anemia).

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Table Graphic Jump Location
TABLE 3–2SOME DRUGS ASSOCIATED WITH MODERATE RISK OF APLASTIC ANEMIA*
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TABLE 3–2 SOME DRUGS ASSOCIATED WITH MODERATE RISK OF APLASTIC ANEMIA*
Acetazolamide
Carbamazepine
Chloramphenicol
Gold salts
Hydantoins
Oxyphenbutazone
Penicillamine
Phenylbutazone
Quinacrine

*Drugs with 30 or more reported cases.

Source: Williams Hematology, 8th ed, Chap. 34, Table 34–3, p. 467.

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Etiologic Classification

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Acquired
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  • Acquired T lymphocyte mediated autoimmune suppression of hematopoietic stem cells and/or progenitor cells in most cases (~70%).

  • Paroxysmal nocturnal hemoglobinuria (PNH) (frequently associated with hypoplastic marrow).

  • Chemicals, e.g., benzene. Rare today in countries with workplace regulations limiting exposure.

  • Drugs, e.g., chloramphenicol (see Table 3–2 for most frequent offenders; see also ...

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