Diagnostic Dilemmas

Certain patients with MDS may present with unusual or uncharacteristic features and represent a diagnostic challenge. These cases usually represent a small but difficult subset of patients in which to confidently establish the diagnosis. Patients may present with isolated thrombocytopenia that may antedate the diagnosis of MDS by months or years.59 The bone marrow may reveal increased megakaryocytes, and an antiplatelet antibody may be identified in the serum. However, as noted above, this may be seen in conjunction with MDS, and may not represent a true case of immune thrombocytopenic purpura (ITP). Often, treatments typically used for ITP are ineffective. In these patients, careful examination of peripheral blood and bone marrow morphology may reveal the correct diagnosis. As in other patients in whom establishment of a diagnosis may be problematic, cytoge-netic analysis may be helpful in leading to the correct diagnosis of MDS. Finding a cytogenetic abnormality frequently associated with MDS can often lead to the appropriate diagnosis. This applies both to these patients and to others described below.

Some patients with MDS present with features also suggestive of a myeloproliferative disorder (MPD), representing an "overlap syndrome."6061 In these patients, cytopenias may present simultaneously with elevated white blood cell or platelet counts. In some patients, an increased leukocyte count may be accompanied by a monocytosis. Under current classification systems, some of these patients will be clearly defined as having an MPD, while others are still categorized as having MDS depending on the upper limit of the WBC count permitted in the classification system (see Chapter 38). Others may have myelofibrosis with or without marked splenomegaly and peripheral blood cytopenias,59,61-63 yet also have dysplastic features suggesting MDS. These patients are more difficult to classify. Those with myelofibrosis with markedly enlarged spleens and a leukoerythroblastic peripheral smear are more likely to have classical myelofibrosis with myeloid metaplasia, while patients without significant splenomegaly and/or the peripheral leukoerythroblastic picture may be considered to have primary MDS with fibrosis.

The hypoplastic MDS variant is often indistinguishable from aplastic anemia, and may have many features in common.5364-67 Those patients with increased expression of HLA-DR15 and the paroxysmal nocturnal hemoglobinuria (PNH) phenotype (decreased expression of CD59), whether having MDS or aplastic anemia, may respond to immunomodulatory treatments. Cytogenetic abnormalities, if present, involving chromosomes frequently abnormal in MDS may suggest the diagnosis of MDS, but does not completely exclude aplastic anemia.68

Finally, there are patients who present with severe pancytopenia and bone marrow findings that are nondiagnostic (i.e., minimal if any dysmorphic changes, no increase in myeloblasts) and without any cytogenetic abnormalities. Some of these patients

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