MDS and MPS

• The myelodysplastic syndromes (MDS) are characterized by dysplastic bone marrow hyperplasia or even occasional hypoplasia, associated with variable degrees of peripheral blood cytopenia with or without monocytosis [4]. The paradox between the proliferative bone marrow and the cytopenic peripheral blood picture in the MDS may be explained by increased intramedullary myeloid precursor cell apoptosis [5]. (See "Treatment and prognosis of the myelodysplastic syndromes").

• The myeloproliferative disorders (MPDs), unlike the MDS, usually exhibit terminal myeloid cell expansion in the peripheral blood [6]. The MPDs are operationally classified into "classic" and "atypical" MPDs. Classic MPDs include polycythemia vera, essential thrombocytosis, chronic myelogenous leukemia (CML), and chronic idiopathic myelofibrosis (AMM, agnogenic myeloid metaplasia).

FAB classification — Patients with MDS have been classified into subgroups based, in part, upon the percentage of bone marrow (BM) blasts, according to a 1982 French, American, and British (FAB) consensus conference (show table 1) [4]:

• Refractory anemia (RA) — less than 5 percent BM blasts.

• Refractory anemia with ringed sideroblasts (RARS) — less than 5 percent BM blasts with >15 percent ringed sideroblasts

• Refractory anemia with excess blasts (RAEB) — 5 to 19 percent BM blasts. RAEB has been further subdivided, with RAEB-I and RAEB-II having BM blast counts of 5-9 and 10-19 percent, respectively [5]

• Chronic myelomonocytic leukemia (CMML) — up to 20 percent BM blasts plus peripheral blood monocyte count >1000/microL. CMML has been divided into two subtypes: patients with a white blood cell count (WBC) 12,000/microL who are considered to have MDS, and the proliferative-type CMML with a WBC >12,000/microL, who are considered to have a chronic myeloproliferative disorder [5]. Based on retrospective data, a separate prognostic scoring system for CMML has been proposed [6]. Unlike the other categories of MDS, an absolute lymphocyte count >2,500/microL was found to be an adverse prognostic factor in CMML [6,7].

• Refractory anemia with excess blasts in transformation (RAEB-T) — 21 to 30 percent BM blasts

Using the FAB classification system and a meta-analysis of a large number of available studies, patients with RAEB and RAEB-T were shown to have relatively poor prognoses, with median survivals ranging from 5 to 12 months (show table 2). In contrast, those with RA and RARS had median survivals of approximately three to six years.

The proportion of individuals who transformed to AML varied similarly, being 40 to 50 percent for patients with RAEB and RAEB-T, compared to 5 to 15 percent in patients with RA and RARS (show table 2). In a separate study evaluating the time to disease evolution, the incidence of transformation to AML at one and two years was zero in RARS, 5 and 10 percent in RA, 25 and 35 percent in RAEB, and 55 and 65 percent in RAEB-T [8].