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Chronic idiopathic myelofibrosis (CIMF) is a chronic myeloproliferative disorder first described in 1879 by Gustav Heuck1 with a report of two patients with massive splenomegaly, constitutional symptoms, and intramedullary fibrosis. This disorder was further defined as a distinct entity in the twentieth century as agnogenic myeloid metaplasia2 (AMM). AMM was subsequently categorized as a subset of myelofibrosis with myeloid metaplasia (MMM).3 The diagnosis of MMM includes patients with a de novo presentation (i.e., AMM) as well as those who progressed from a previous chronic myeloproliferative disorder (specifically essential thrombocythemia or polycythemia vera4) to the disorders of postthrombocythemic (PTMM) and post-poly cythemic myeloid metaplasia (PPMM), respec-tively.3 As the clinical presentation, prognosis, and management are indistinguishable for both de novo CIMF and the secondary states of PTMM and PPMM, we will refer to MMM as one disease.

MMM is a clonal5 hematopoietic stem disorder, resulting in two major processes. The first process is a myeloproliferation that is manifest by an intramedullary expansion of one or more myeloid lineages, resulting frequently in either leukocytosis and/or thrombocy-tosis. Additionally, an increase in circulating immature myeloid cells is observed which can accumulate in the spleen, liver, or other organs and lead to extramedullary hematopoiesis. The second is a prominent, polyclonal reactive process within the marrow, which leads to fibroblast proliferation, deposition of collagen (types I and III) and other connective tissues (i.e., fibronectin and proteoglycans), neoangiogene-sis,6 osteosclerosis, reticulin fibrosis, and ineffective hematopoiesis.

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