Myelodysplastic Syndrome (MDS)

At a Glance

Myelodysplastic syndrome (MDS) is a group of diseases that are due to ineffective hematopoiesis. The primary clinical findings in these patients are related to the sequelae of cytopenias (i.e., fatigue/dyspnea, bruising/bleeding, and infections). The primary morphologic finding is that of dysplasia in a cytopenic patient and, typically, a hypercellular bone marrow. Because there are many causes of both dysplasia and cytopenias and because there are no single diagnostic tests for MDS, the diagnosis of MDS relies on the appropriate constellation of findings in a clinically suspicious patient with no other apparent cause for the symptoms.

There are multiple subtypes of myelodysplasia that can generally be classified into two grades. The low-grade myelodysplasias include refractory cytopenia with unilineage dysplasia (RCUD), refractory anemia with ringed sideroblasts (RARS), and myelodysplastic syndrome with isolated deletion of 5q (5q- MDS). There are four high-grade myelodysplastic subtypes: refractory anemia with excess blasts (RAEB)1, refractory anemia with excess blasts 2, therapy-related myelodysplastic syndrome and refractory cytopenias with multilineage dysplasia (RCMD).

The high-grade MDS subtypes have a propensity to develop increased blasts and may eventually transform into acute myeloid leukemia. Importantly, MDS-associated Acute Myeloid Leukemia (AML) is a separate World Health Organization (WHO) category clinically notable for its low response rate to standard anti-proliferative chemotherapy and poor prognosis. Even in those high-grade MDS patients who do not transform to AML, severe cytopenias can significantly shorten lifespan. High-grade MDS patients have a mean overall survival, ranging from 9 months to 3 years. Conversely, low-grade MDS may respond well to supportive therapy, hypomethylating agents, or immunomodulating agents, giving these patients mean overall survivals of 6-12 years.

Because therapies vary depending on MDS subtype, a careful examination of all the laboratory data by a qualified pathologist is mandatory.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

A complete blood count (CBC) to identify the cytopenias is the first step in diagnosing Myelodysplasia. In particular, anemic MDS patients often present with a low reticulocyte count and a low immature platelet fraction reflecting their ineffective hematopoiesis. Moreover, the anemia in MDS is typically macrocytic. A morphologic review of the peripheral blood smear often reveals dysplasia in the neutrophils, including hypogranularity or atypically large granules in the cytoplasm (pseudoChediak-Higashi). Neutrophil nuclei can show hypolobation (pseudo-Pelger-Huet anomaly) or hyperlobation. Platelets may be large or agranular. Red cell morphology in the peripheral blood may be normal or macrocytic with anisopoikilocytosis.

Because MDS is a clinicopathologic diagnosis without pathognomonic findings, it is crucial that other causes of cytopenias and dysplasia be excluded. Once these other causes have been ruled out, a bone marrow biopsy and aspirate with karyotype is recommended for patients for whom no other cause of cytopenias can be identified. If the aspirate contains increased blasts, flow cytometry may be useful in characterizing the blasts. In some flow cytometry laboratories, evaluation of myeloid or erythroid maturation patterns can support the diagnosis of MDS in the appropriate context.

Blasts must comprise less than 20% of total cells on morphologic review of blood and bone marrow aspirate. If 20% or more blasts are seen in either blood or bone marrow, the best diagnosis is acute leukemia. There are two subcategories of MDS associated with increased blasts less than 20%. In RAEB2, blasts comprise 5-19% of total cells in blood or 10-19% in marrow. In RAEB1, blasts and promonocytes comprise less than 5% of cells in the blood and 5-9% in bone marrow.

Abnormal cytogenetics are detected in approximately 40% of MDS patients at presentation. There is no single pathognomonic cytogenetic finding in MDS. Karyotype has recently been shown in several studies to be as sensitive as fluorescent in situ hybridization (FISH) for the diagnosis of MDS. The karyotypic changes most often seen are deletion of 5q, monosomy 7, trisomy 8, deletion 20q, loss of Y. However, there are many more karyotypic changes also consistent with MDS but are of low frequency.

Of note, the findings in flow cytometry, karyotype, and histomorphology are all nonspecific and must be considered together in the appropriate clinical context before a diagnosis of MDS can be made.

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