Acquired aplastic anemia.

In aplastic anemia, hematopoiesis fails: Blood cell counts are extremely low, and the bone marrow appears empty. The pathophysiology of aplastic anemia is now believed to be immune-mediated, with active destruction of blood-forming cells by lymphocytes. The aberrant immune response may be triggered by environmental exposures, such as to chemicals and drugs or viral infections and, perhaps, endogenous antigens generated by genetically altered bone marrow cells. In patients with post-hepatitis aplastic anemia, antibodies to the known hepatitis viruses are absent; the unknown infectious agent may be more common in developing countries, where aplastic anemia occurs more frequently than it does in the West. The syndrome paroxysmal nocturnal hemoglobinuria (PNH) is intimately related to aplastic anemia because many patients with bone marrow failure have an increased population of abnormal cells. In PNH, an entire class of proteins is not displayed on the cell surface because of an acquired X-chromosome gene mutation. The PNH cells may have a selective advantage in resisting immune attack. In contrast, the disease myelodysplasia can be confused with aplasia and can also evolve from aplastic anemia. The occurrence of cytogenetic abnormalities in patients years after presentation implies that genomic instability is a feature of this immune-mediated disease. Aplastic anemia can be effectively treated by stem-cell transplantation or immunosuppressive therapy. Transplantation is curative but is best used for younger patients who have histocompatible sibling donors. Antithymocyte globulin and cyclosporine restore hematopoiesis in approximately two thirds of patients. However, recovery of blood cell count is often incomplete, recurrent pancytopenia requires retreatment, and some patients develop late complications (especially myelodysplasia).