Due to a damage in pluripotent stem cells, bone marrow aplasia results in a complete breakdown of hematopoiesis which leads to pancytopenia in the peripheral blood. Chemicals (e.g. benzene), drugs (e.g. chloramphenicol), and viruses (e.g. hepatitis viruses, parvoviruses) are associated with aplastic anemia. In about 2/3 of the patients, the cause remains unknown (= idiopathic aplastic anemia). The pathomechanism is an autoimmune disease directed at hematopoietic stem cells.
Bone marrow aplasia that occurs unexpectedly must be distinguished from the expected bone marrow aplasia induced by chemotherapy.
As in the case of the acute leukemias, symptoms result from pancytopenia. Weakness, fatigue and palpitations result from anemia, fevers and mucositis from severe neutropenia, and a bleeding diathesis (e.g. petechiae) from thrombocytopenia.
All three cell lines are decreased or lacking in the peripheral blood film. The erythrocytes tend to be macrocytic. The reticulocytes are clearly diminished (reticulopenia). The few neutrophils that are present may show toxic abnormalities. No additional specific abnormalities are present.
The bone marrow is hypocellular or aplastic. In the aspirate, there are empty crumbles with a net of fibroblasts, but without hematopoietic elements. he few remaining cells are macrophages, and mast cells, as well as lymphocytes and plasma cells. Since after cytostatic therapy, also the lymphocytes are absent, they are relatively increased in aplastic anemia as sign of the autoimmune process. Therefore, it can sometimes be difficult to differentiate these cases from lymphoproliferative neoplasms. This makes immunophenotyping (biopsy, flow cytometry) indispensable. The differentiation from a hypocellular myelodysplastic syndrome is difficult. While mild signs of dysplasia may be present in acquired bone marrow aplasia, marked signs of dysplasia are important indicators for a myelodysplastic syndrome.