The decrease in hematopoietic tissue impairs the patients ability to generate new blood cells, resulting in progressive pancytopenia, a shortage of all blood cell types. However, the proliferation of fibroblasts and deposition of collagen is a secondary phenomenon, and the fibroblasts themselves are not part of the abnormal cell clone.In primary myelofibrosis, progressive scarring, or fibrosis, of the bone marrow occurs, for the reasons outlined above. The result is extramedullary hematopoiesis, i.e. blood cell formation occurring in sites other than the bone marrow, as the hemopoietic cells are forced to migrate to other areas, particularly the liver and spleen. This causes an enlargement of these organs. In the liver, the abnormal size is called hepatomegaly. Enlargement of the spleen is called splenomegaly, which also contributes to causing pancytopenia, particularly thrombocytopenia and anemia. Another complication of extramedullary hematopoiesis is poikilocytosis, or the presence of abnormally shaped red blood cells.Myelofibrosis can be a late complication of other myeloproliferative disorders, such as polycythemia vera, and less commonly, essential thrombocythemia. In these cases, myelofibrosis occurs as a result of somatic evolution of the abnormal hematopoietic stem cell clone that caused the original disorder. In some cases, the development of myelofibrosis following these disorders may be accelerated by the oral chemotherapy drug hydroxyurea.
Primary myelofibrosis can begin with a blood picture similar to that found in polycythemia vera or chronic myeloid leukemia. Most people with myelofibrosis have moderate to severe anemia. Eventually thrombocytopenia, a decrease of blood platelets develops. When viewed through a microscope, a blood smear will appear markedly abnormal, with presentation of pancytopenia, which is a reduction in the number of all blood cell types: red blood cells, white blood cells, and platelets. Red blood cells may show abnormalities including bizarre shapes, such as teardrop-shaped cells, and nucleated red blood cell precursors may appear in the blood smear (leukoerythroblastic reaction). Normally, mature red blood cells in adults do not have a cell nucleus, and the presence of nucleated red blood cells suggests that immature cells are being released into the bloodstream in response to a very high demand for the bone marrow to produce new red blood cells. Immature white cells and platelets (large megakaryocytes) are also seen in blood samples, and basophil counts are increased. When late in the disease progression an attempt is made to take a sample of bone marrow by aspiration, it may result in a dry tap, meaning that where the needle can normally suck out a sample of semi-liquid bone marrow, it produces no sample because the marrow has been replaced with collagen fibers. A bone marrow biopsy will reveal collagen fibrosis, replacing the marrow that would normally occupy the space. Treatment
The one known curative treatment is allogeneic stem cell transplantation, but this approach involves significant risks.