Aplastic Anemia Overview
Introduction to Aplastic Anemia
- Definition: Aplastic anemia is classified among a group of disorders known as hypoproliferative disorders. These disorders are characterized by a reduced growth or production of blood cells.
Conditions Easily Confused with Aplastic Anemia
| Condition | Type | Characteristics |
|---|---|---|
| Fanconi's anemia | Constitutional | Expressed in young patients with physical anomalies |
| Dyskeratosis congenita | Constitutional | Expressed in young patients with physical anomalies |
| Aleukemic leukemia | Acquired | Very young or very old patients; presents with blasts in buffy coat and bone marrow spicules |
| Large granular lymphocytosis | Acquired | Older patients; noted for neutropenia |
| Myelodysplasia | Acquired | Older patients; bone marrow demonstrates normal cellularity or hypercellularity |
| Myelofibrosis | Acquired | Noted for hepatosplenomegaly; leukoerythroblastic appearance on peripheral blood smear |
| Paroxysmal nocturnal hemoglobinemia (PNH) | Acquired | Associated with RBC hemolysis |
- Note: Phenotypic abnormalities of these conditions may be subtle or absent, making differentiation difficult.
Etiology of Aplastic Anemia
- Iatrogenic Causes: Factors leading to aplastic anemia due to medical treatment or environmental exposure include:
- Benzene and benzene derivatives
- Trinitrotoluene (TNT)
- Insecticides and weed killers
- Inorganic arsenic
- Antimetabolites, including antifolate compounds and analogs of purines and pyrimidines
- Certain Antibiotics
Etiologic Classification of Aplastic Anemia
Direct Toxicity:
- Iatrogenic causes: Radiation, chemotherapy
- Other: Benzene exposure, intermediate metabolites of various common drugs
Immune-Mediated Causes:
- Iatrogenic causes: Can include transfusion-associated graft versus host disease, eosinophilic fasciitis, hepatitis-associated diseases, and pregnancy effects
- Other: Intermediate metabolites from common drugs, and idiopathic aplastic anemia
Note: Mechanisms indicated in boldface type are described as relatively well-established.
Pathophysiology of Aplastic Anemia
Immune-Mediated Pathophysiology
- Key Points:
- Involves activation of type 1 cytotoxic T cells.
- Mutations in telomere repair genes affect target cells, leading to dysregulated T-cell activation pathways.
- Cellular immune suppression may occur transiently with certain viral infections, such as parvovirus, or due to certain medications.
Hematopoietic Failure Pathophysiology
- Key Points:
- Characterized by insufficient or defective pluripotent stem cells, progenitor stem cells, or committed stem cells.
- The hematopoietic microenvironment might fail to support the normal development of hematopoietic cells.
- There is often an absence of humoral or cellular cytokines, which typically serve as stimulators of blood cell production.
- There may be excessive suppression of hematopoiesis, often mediated by resident T lymphocytes or macrophages.