Sindroma Mielodisplastik (MDS)

01Pathophysiology

MDS originates from acquired somatic mutations in a hematopoietic stem cell that gains a clonal proliferative advantage over normal stem cells. These mutations disrupt normal differentiation and maturation across one or more cell lineages (erythroid, myeloid, megakaryocytic), producing cells that are morphologically abnormal and functionally defective. The central concept to understand is ineffective hematopoiesis: the bone marrow is working hard (typically hypercellular), but the products it releases into the peripheral blood are reduced in number and quality. This explains the paradox that appears in vignettes: a patient with a packed marrow on biopsy who nonetheless presents with pancytopenia.

In early-stage MDS, the dominant mechanism driving cytopenias is excessive intramedullary apoptosis. The dysplastic progenitor cells undergo programmed cell death before they can mature and exit the marrow. This is why the marrow appears hypercellular or normocellular while the peripheral counts remain low. As the disease progresses, the clone accumulates additional mutations, the rate of apoptosis decreases, and the blast percentage rises. Once blasts reach 20% or higher, the disease is reclassified as acute myeloid leukemia (AML). This transformation to AML is a defining risk of MDS and is the reason blast percentage is central to both classification and prognosis.

The mutations involved in MDS cluster into several functional categories. Mutations in epigenetic regulators (TET2, DNMT3A, IDH1/2) alter DNA methylation patterns and gene expression. Mutations in RNA splicing machinery (SF3B1) are strongly associated with the formation of ringed sideroblasts, which are erythroblasts containing iron-laden mitochondria arranged in a perinuclear ring visible on Prussian blue stain. The iron accumulates because abnormal splicing disrupts mitochondrial iron utilization in heme synthesis. TP53 mutations indicate genomic instability and carry a particularly poor prognosis with high risk of leukemic transformation.

Therapy-related MDS (t-MDS) deserves separate attention because it appears frequently in vignettes. Alkylating agents (cyclophosphamide, melphalan, busulfan) cause t-MDS after a latency period of 5 to 7 years and are associated with deletions of chromosomes 5 and 7. Topoisomerase II inhibitors (etoposide, doxorubicin) cause disease after a shorter latency of 2 to 3 years and are associated with balanced translocations involving 11q23 (KMT2A/MLL gene). When a vignette describes a patient with a prior cancer history now presenting with new-onset cytopenias, think t-MDS immediately.

02Classification and Clinical Manifestation

The WHO classification stratifies MDS primarily by the blast percentage, the number of dysplastic lineages, the presence of ringed sideroblasts, and key cytogenetic or molecular findings. The blast percentage is the single most important prognostic variable within the classification.

The Revised International Prognostic Scoring System (IPSS-R) is used for risk stratification and treatment decisions. It incorporates five variables:

IPSS-R risk categories range from Very Low to Very High, and the category determines whether a patient receives supportive care versus aggressive therapy including transplant.

03Diagnostic Workup

The evaluation begins when a CBC reveals unexplained cytopenias, most commonly a macrocytic anemia in an elderly patient that does not respond to B12, folate, or iron supplementation. The first step is to examine the peripheral blood smear carefully. The hallmark findings are pseudo-Pelger-Huet cells (neutrophils with only two nuclear lobes, resembling sunglasses) and hypogranular neutrophils (neutrophils with pale or absent cytoplasmic granules). These morphologic abnormalities point directly toward a myeloid disorder. Oval macrocytes may also be present, but unlike megaloblastic anemia, you will not see hypersegmented neutrophils.

Before proceeding to marrow biopsy, it is essential to exclude reversible causes of dysplasia and cytopenias. Check serum B12, folate, copper, and iron studies. B12 and folate deficiency can mimic MDS by causing megaloblastic changes with dysplastic-appearing cells. Copper deficiency can cause sideroblastic changes and neutropenia. These must be ruled out before the diagnosis of MDS is established.

The bone marrow biopsy with aspirate is the confirmatory test. You are looking for three things: (1) evidence of dysplasia in at least 10% of cells in one or more lineages, (2) the blast percentage (which determines the subtype and prognosis), and (3) the overall cellularity of the marrow. In MDS, the marrow is typically hypercellular, which is the key distinction from aplastic anemia.

Cytogenetic analysis is performed on the marrow aspirate and is critical for both classification and prognosis. Isolated del(5q) identifies a distinct subtype with favorable prognosis and a targeted treatment (lenalidomide). Monosomy 7 or complex karyotype indicates poor prognosis.

04Management and Treatment

The first decision point is risk stratification using the IPSS-R. This determines the entire treatment approach.

For lower-risk MDS (IPSS-R Very Low, Low, or Intermediate), the goal is to manage cytopenias and maintain quality of life. If the patient has symptomatic anemia and a serum EPO level below 500 mU/mL, the best initial step is to start an ESA (epoetin alfa or darbepoetin). If the patient has del(5q), the next best step is lenalidomide regardless of EPO level, because this subtype has an exceptionally high response rate to this drug. For patients with the ring sideroblast subtype (MDS-RS) who fail ESA therapy, luspatercept is now an established option. Patients who become transfusion-dependent require iron chelation therapy with deferasirox to prevent iron overload and end-organ damage.

For higher-risk MDS (IPSS-R High or Very High), the disease behaves more aggressively and carries a substantial risk of transformation to AML. The standard of care is azacitidine, a hypomethylating agent that has been shown to improve overall survival compared to conventional care. Azacitidine must be given for a minimum of 4 to 6 cycles before response is assessed, because it works by gradually reprogramming the epigenetic landscape of the malignant clone. Stopping the drug prematurely for apparent lack of response is a common management error.

Allogeneic stem cell transplant is the only therapy that can cure MDS. It is reserved for patients with higher-risk disease who are young enough and fit enough to tolerate the procedure, typically those under 70 with good performance status. For vignette purposes, when a question asks about the only curative treatment for MDS, the answer is always allogeneic transplant.

Contraindications and cautions: Lenalidomide is teratogenic (pregnancy category X) and requires enrollment in a risk management program. Azacitidine can cause myelosuppression; dose adjustments are needed for renal impairment.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on MDS questions is by confusing the direction of nuclear lobation. MDS produces pseudo-Pelger-Huet cells with bilobed or hypolobated nuclei, while megaloblastic anemia (B12/folate deficiency) produces hypersegmented neutrophils with five or more lobes. These are opposite findings, and a vignette will often present macrocytic anemia with one of these morphologic clues to see whether you can distinguish the two.

A second common trap involves the del(5q) subtype. This subtype classically presents in older women with macrocytic anemia and a normal or elevated platelet count. The elevated platelet count is counterintuitive in a disease defined by cytopenias and can trick students into looking elsewhere. The treatment is lenalidomide, not azacitidine, and this pairing (del(5q) plus lenalidomide) is a high-frequency tested association.

Students also commonly err by jumping to AML too quickly. If a vignette describes a patient with refractory cytopenias and blasts in the marrow, always check the blast percentage. Below 20% is MDS; at or above 20% is AML. The management, prognosis, and treatment intent differ significantly between the two.

Another trap involves therapy-related disease. When a vignette describes a patient who received chemotherapy for a prior malignancy and now presents years later with new cytopenias, the expected answer is therapy-related MDS, not relapse of the original cancer. The latency period provides the clue: 5 to 7 years for alkylating agents, 2 to 3 years for topoisomerase II inhibitors.

Finally, remember that a hypercellular marrow with peripheral cytopenias is the defining paradox of MDS. If the marrow is hypocellular, reconsider aplastic anemia. If the cytopenias correct with vitamin supplementation, reconsider nutritional deficiency. The core competency being tested is the ability to recognize ineffective hematopoiesis as the unifying mechanism, connect it to the dysplastic morphology on smear, and sequence the workup correctly from CBC to smear to marrow biopsy with cytogenetics.