Myelodysplastic Syndrome

• Clonal expansion with altered stem cell kinetics: A single mutated hematopoietic stem cell acquires selective growth advantage over normal hematopoietic stem cells through dysregulation of apoptosis, differentiation, and self-renewal. This contrasts with AML where proliferation is explosive; in MDS, expansion is gradual but relentless. The initiating events frequently involve mutations in genes regulating DNA methylation (DNMT3A, TET2), chromatin remodeling (ASXL1, EZH2), transcription factors (TP53, RUNX1), or RNA splicing (SF3B1, SRSF2). These early mutations create a pre-leukemic clone that expands over months to years before secondary mutations accumulate, driving progression toward leukemia.

• Ineffective hematopoiesis and enhanced apoptosis: Despite increased marrow cellularity (hypercellular MDS in >50% of cases), MDS patients paradoxically develop cytopenias due to excessive intramedullary apoptosis of dysplastic cells. This ineffective hematopoiesis results from dysregulation of pro-apoptotic signals (particularly p53 pathway activation and TNF-α upregulation) and impaired differentiation. Dysplastic cells are often larger and more abnormal, lacking mature morphology even as they accumulate. The bone marrow can be hypercellular yet fail to produce adequate peripheral blood cells—a pathophysiologically unique scenario distinguishing MDS from aplastic anemia or nutritional cytopenias.

• Genetic instability and multi-hit carcinogenesis: MDS progression to AML follows a multi-step model requiring sequential acquisition of mutations. Early mutations typically affect epigenetic regulators and RNA splicers (relatively "stable" early hits), while later transformations require TP53, RUNX1, or TP53-like mutations. Chromosomal instability emerges through loss of chromosome 5 (del5q), monosomy 7, trisomy 8, or complex karyotypes, each conferring additional genomic instability and accelerating progression. Patients with high-grade dysplasia and multiple mutations (especially TP53) have 5-year leukemic transformation rates exceeding 50%, while isolated del5q may never transform.

• Altered bone marrow microenvironment and cytokine dysregulation: The microenvironment in MDS is fundamentally altered, with increased TNF-α, TGF-β, and other inflammatory mediators promoting both apoptosis in progenitors and extramedullary hematopoiesis. Angiogenesis is paradoxically increased despite cytopenias, with elevated VEGF and FGF supporting the abnormal clone while promoting apoptosis of normal cells. Immunologic dysfunction allows dysplastic clones to escape immune surveillance through altered HLA expression and T-cell exhaustion, while simultaneously immune hyperactivation contributes to apoptosis. This creates a permissive microenvironment for clonal evolution.

• Impaired myeloid differentiation: Mutations affecting transcription factors (RUNX1, CEBPA) and epigenetic modifiers block normal myeloid lineage maturation at various stages, resulting in accumulation of immature cells with dysplastic features. The dysplasia may manifest as abnormal granulation (hypogranulation or ring sideroblasts), abnormal nuclear morphology (multilobation, hyposegmentation), or cytoplasmic abnormalities. In MDS with ring sideroblasts specifically, mutations in SF3B1 (splicing factor) cause aberrant splicing of ABCB7, impairing heme synthesis and causing iron accumulation around the nucleus. This pathophysiology directly drives the morphologic findings on bone marrow examination.

• Primary MDS (de novo): The majority of MDS cases (>80%) occur without prior cytotoxic exposures and appear spontaneously, likely from age-related accumulation of mutations in long-term hematopoietic stem cells. Age itself is the dominant risk factor, with incidence rising exponentially after age 60. Genetic predisposition through germline mutations in genes like GATA2, DDX41, or RUNX1 accounts for familial MDS and younger-onset disease, though these are rare.

• Secondary MDS (therapy-related MDS or t-MDS): Prior exposure to cytotoxic chemotherapy (alkylating agents like cyclophosphamide, melphalan; topoisomerase II inhibitors like etoposide) or radiation therapy increases MDS risk 5-10 fold with latency typically 4-10 years post-exposure. Patients treated for breast cancer, lymphoma, or other malignancies account for the majority of t-MDS cases. These patients often have worse outcomes than de novo MDS, particularly if TP53 mutations are present.

• Chronic inflammatory states and environmental exposures: Chronic immune stimulation (HIV, chronic hepatitis C, and other autoimmune conditions) creates a pro-inflammatory milieu predisposing to clonal hematopoiesis. Smoking and benzene exposure are established environmental risk factors increasing MDS risk through genotoxic stress. Occupational exposures (pesticides, organic solvents) and prior treatment with immunosuppressive agents (for aplastic anemia) also increase risk.

• Inherited bone marrow failure syndromes: Patients with Fanconi anemia, dyskeratosis congenita, Shwachman-Diamond syndrome, and neurofibromatosis type 1 have significantly elevated MDS risk due to inherited genetic instability or impaired hematopoietic stem cell maintenance. These patients develop MDS at younger ages with more aggressive natural history.

• Clonal hematopoiesis of indeterminate potential (CHIP): Individuals with age-related somatic mutations in hematopoietic cells but without cytopenias or dysplasia carry increased risk of MDS progression (approximately 0.5-1% annual risk). DNMT3A, TET2, and ASXL1 mutations are frequently identified in CHIP and represent early steps in MDS evolution.

• Anemia-related symptoms (most common presenting complaint): Fatigue, dyspnea on exertion, and reduced exercise tolerance result from tissue hypoxia due to chronic red blood cell deficiency. Symptoms vary with severity and acuity of anemia; patients with chronic mild anemia (Hgb 9-10 g/dL) may be asymptomatic or minimally symptomatic due to compensatory mechanisms (increased cardiac output, rightward oxygen-hemoglobin curve), while sudden-onset or severe anemia causes dyspnea at rest and angina. Exertional chest pain or syncope in elderly MDS patients with anemia warrants evaluation for coronary disease exacerbation.

• Bleeding and thrombotic manifestations: Thrombocytopenia results in mucocutaneous bleeding (petechiae, epistaxis, gingival bleeding) and risk of severe hemorrhage (intracranial, gastrointestinal). Paradoxically, MDS patients also have elevated VTE risk despite thrombocytopenia, particularly those with monosomy 7 or high-grade dysplasia, attributed to tissue factor expression on dysplastic cells and activation of coagulation. Spontaneous ecchymoses and bleeding gums may be presenting signs.

• Infections from neutropenia: Absolute neutropenia (ANC <500/μL) increases infection susceptibility, manifesting as fever, mouth ulcers, perirectal abscesses, or atypical presentations of opportunistic infections. Unlike AML patients where neutropenia develops acutely, MDS neutropenia typically develops gradually, allowing some immune adaptation. However, functional defects in MDS neutrophils (impaired respiratory burst, chemotaxis) compound the problem even when ANC appears adequate.

• Constitutional symptoms and splenomegaly: Patients may report fever, night sweats, and weight loss, particularly in advanced MDS approaching AML transformation. Splenomegaly occurs in 10-25% of MDS cases due to extramedullary hematopoiesis and marrow infiltration, and hepatomegaly is less common but suggests systemic involvement or prior iron overload.

• Asymptomatic cytopenias discovered on routine evaluation: Increasingly, MDS is discovered incidentally on laboratory evaluation for other reasons, particularly in elderly patients undergoing preoperative workup or evaluation for unrelated conditions. These patients may have indolent MDS with single cytopenia or minimal dysplasia that remains stable for years.

• Specific clinical variants:
• MDS with isolated del(5q): Often presents with severe anemia with relatively preserved platelets and neutrophils; often involves fewer dysplastic changes; associated with favorable prognosis and specific response to lenalidomide
• MDS with ring sideroblasts: Predominantly involves red blood cell dysplasia with ring sideroblasts; may present with isolated anemia refractory to iron supplementation
• MDS with excess blasts: Higher frequencies of blasts (10-19%) present with cytopenias and constitutional symptoms; higher transformation risk approaching acute leukemia presentation

• Peripheral blood smear and complete blood count: Cytopenias are virtually universal; anemia is most common (>80%), followed by thrombocytopenia and neutropenia. Morphologic abnormalities (dysplasia) should be documented: anisopoikilocytosis, hypogranular neutrophils, hyposegmented nuclei, or macrocytosis. The diagnosis requires dysplasia in at least one cell line (morphologic criterion) AND cytopenia OR demonstrates diagnostic cytogenetic/molecular abnormality. Circulating blasts should be quantified (normal <2%); presence of Auer rods or >20% blasts indicates AML, not MDS.

• Bone marrow aspiration and biopsy: Mandatory for MDS diagnosis and classification. Cellularity assessment (hypercellular vs normocellular vs hypocellular) provides prognostic information; most MDS are hypercellular (>50%) despite peripheral cytopenias. Dysplasia is assessed in myeloid, erythroid, and megakaryocytic lineages; dysplasia in ≥10% of cells in a lineage is considered significant. Blast percentage is critical: <5% blast in marrow = lower-grade MDS; 5-19% = higher-grade MDS (previously called MDS with excess blasts-2); ≥20% = AML. Ring sideroblasts (iron-laden mitochondria forming ring around nucleus) are seen in 15% of cases and define a specific subtype.

• Cytogenetics and FISH: Standard karyotyping is abnormal in 50-70% of MDS cases; common abnormalities include del(5q), del(7q), monosomy 7, trisomy 8, del(20q), and complex karyotype (≥3 abnormalities). FISH is useful for detecting small deletions and is sensitive for common abnormalities. Cytogenetic findings are among the strongest prognostic indicators: del(5q) and del(20q) are favorable; monosomy 7 and complex karyotype are unfavorable; TP53 deletion/mutation portends very poor prognosis.

• Molecular testing and mutations: Next-generation sequencing (NGS) identifying somatic mutations is increasingly standard, though not required for diagnosis. Common mutations include DNMT3A (15-25%), TET2 (10-20%), ASXL1 (10-15%), TP53 (5-15%), RUNX1, SRSF2, and SF3B1. TP53 mutation is particularly prognostically significant, associated with complex karyotype and rapid progression. SF3B1 mutations are associated with ring sideroblasts and indolent course. Mutation burden (number of mutations and variant allele frequency) correlates with transformation risk.

• Revised International Prognostic Scoring System (IPSS-R): This scoring system stratifies patients into 5 risk groups (Very Low, Low, Intermediate, High, Very High) based on:
• Blast percentage (0-2%, 2-5%, 5-10%, 10-19%, ≥20%)
• Karyotype (favorable, intermediate, unfavorable, very unfavorable based on specific abnormalities)
• Cytopenia degree (0-1, 1-2, ≥2 cell lines affected)
• The 5-year leukemic transformation rate ranges from <3% (Very Low) to >70% (Very High)

• WHO 2016/2022 Classification: Diagnostic categories include:
• MDS with single-lineage dysplasia: Dysplasia in one cell line only
• MDS with multi-lineage dysplasia: Dysplasia in ≥2 cell lines
• MDS with ring sideroblasts: ≥15% ringed sideroblasts (now divided into MDS-RS-SLD and MDS-RS-MLD)
• MDS with excess blasts-1 and -2: 5-19% blasts (combined in 2016 revision)
• MDS with isolated del(5q): Specific category with favorable prognosis
• MDS, unclassifiable: Dysplasia present but doesn't meet other criteria

• Differential diagnosis considerations:
• Aplastic anemia: Hypocellular marrow with cytopenias but WITHOUT dysplasia; treated with immunosuppression rather than MDS-directed therapy
• AML: >20% blasts or Auer rods present; requires intensive chemotherapy approach
• Vitamin B12/folate deficiency: Macrocytic anemia with megaloblastic changes; correctable with supplementation; lacks dysplasia in non-erythroid lines
• Copper deficiency: Can mimic MDS with myelopathy; responds to copper repletion
• Chronic myelomonocytic leukemia (CMML): Monocytes >1000/μL; separate WHO category with different prognostic model

• Supportive care foundation: All MDS patients require baseline therapy including transfusion support for symptomatic anemia (target Hgb 7-9 g/dL; avoiding higher targets due to iron overload and transfusion refractoriness risk), platelet transfusion for bleeding or prophylaxis (count <10,000 if asymptomatic, <20,000 if symptomatic), and prophylactic antibiotics for neutropenic fever. Iron chelation therapy (deferasirox, deferoxamine) is instituted after 20-30 transfusions (~5-10 grams iron) to prevent secondary hemochromatosis, cardiac arrhythmias, cirrhosis, and other iron-related complications. Growth factors (G-CSF, GM-CSF, erythroid-stimulating agents) are generally reserved for documented deficiency states and have limited efficacy in MDS due to the clonal nature of the disease.

• First-line treatment for lower-risk MDS (IPSS-R Low or Very Low risk): Observation with supportive care is appropriate for many patients, as median survival can exceed 5 years and transformation risk is <10% at 5 years. For symptomatic anemic patients with MDS with isolated del(5q), lenalidomide (5 mg daily,

Disease-related complications

• Progression to acute myeloid leukemia (oncologic emergency when accompanied by leukostasis or DIC): sequential mutation acquisition (often TP53, RUNX1, or evolving complex karyotype) drives blasts past the 20% marrow/blood threshold. Signal: rapidly falling counts, new circulating blasts or Auer rods, B symptoms, or a rising blast percentage on repeat marrow.
• Febrile neutropenia (medical emergency): quantitative neutropenia plus qualitative neutrophil defects (impaired chemotaxis and respiratory burst) means fever may be the only sign of gram-negative bacteremia. IDSA febrile neutropenia guidance calls for blood cultures and empiric antipseudomonal beta-lactam monotherapy (e.g., cefepime) without waiting for localizing findings.
• Major hemorrhage (emergency): severe thrombocytopenia with platelet dysfunction from dysplastic megakaryocytes; intracranial or GI bleeding is the feared event. Signal: new headache or focal deficit, or melena, in a patient with a low platelet count.
• Symptomatic anemia unmasking coronary disease: chronic tissue hypoxia in an elderly population precipitates angina, decompensated heart failure, or syncope.
• Venous thromboembolism: thrombotic risk is increased despite thrombocytopenia, reflecting inflammation and tissue factor expression by the clonal myeloid population; lenalidomide adds further thrombotic risk, so a rising leg swelling or pleuritic dyspnea on therapy warrants imaging rather than reassurance.
• Autoimmune and paraneoplastic phenomena: clonal myeloid cells drive systemic inflammation; Sweet syndrome, cutaneous vasculitis, and relapsing polychondritis are the classic associations.

Treatment-related complications

• Transfusional iron overload: no physiologic iron excretion route, so chronic red cell support deposits iron in myocardium, liver, and endocrine organs. Signal: markedly rising ferritin with transfusion burden, new cardiomyopathy/arrhythmia, or hepatic dysfunction; cardiac T2* MRI quantifies myocardial loading.
• Alloimmunization and transfusion refractoriness: repeated exposure generates HLA/red cell antibodies; signal is failure of the post-transfusion platelet increment.
• Hypomethylating agent myelosuppression: azacitidine and decitabine deepen cytopenias during the first several cycles before response — a predictable effect, not treatment failure, and a common trigger for infection.
• Lenalidomide toxicity: dose-limiting neutropenia and thrombocytopenia; teratogenic and dispensed under a REMS program.
• Chelation toxicity: deferasirox causes renal and hepatic injury and GI bleeding; deferoxamine causes ocular and auditory toxicity.
• Allogeneic transplant: graft-versus-host disease, graft failure, and sinusoidal obstruction syndrome — the price of the only curative option.

• The classic stem: an elderly patient with fatigue and a macrocytic anemia, normal B12/folate, normal reticulocyte count, and a smear showing pseudo-Pelger-Huët (hyposegmented, bilobed) neutrophils, hypogranular granulocytes, and occasional nucleated RBCs.
• Single best next step: bone marrow aspiration and biopsy with cytogenetics — never anchor on the smear alone. Karyotype carries heavy prognostic weight in the IPSS-R; the molecular IPSS-M (2022) refines IPSS-R by adding somatic mutation data and is increasingly incorporated into NCCN risk stratification.
• The 20% rule, stated correctly: ≥20% blasts in blood or marrow defines AML. Auer rods with <20% blasts do not make it AML — they place the case in the highest-grade MDS category (MDS-EB-2 under WHO 2016; MDS-IB2 under WHO 2022). Conversely, certain genetically defined AMLs — t(8;21), inv(16)/t(16;16), PML::RARA, *NPM1*-mutated, *KMT2A*-rearranged — are diagnosed regardless of blast percentage under current WHO/ICC criteria (BCR::ABL1 and CEBPA being exceptions that still require a blast threshold).
• The one association to know: isolated del(5q) — macrocytic anemia with preserved or elevated platelets, hypolobated megakaryocytes, favorable prognosis, and response to lenalidomide (5q− syndrome). SF3B1 mutation pairs with ring sideroblasts and an indolent course.
• Ring sideroblasts are not specific: iron-laden mitochondria ringing the nucleus on Prussian blue also occur in copper deficiency (zinc excess or post-bariatric surgery, with myelopathy and neutropenia), lead poisoning, alcohol, isoniazid, and congenital sideroblastic anemia. Check copper before committing to MDS.
• Only curative therapy: allogeneic hematopoietic stem cell transplant, per NCCN — reserved for higher-risk disease in fit candidates. Hypomethylating agents (azacitidine, decitabine) improve survival in higher-risk disease but are not curative.
• Common distractor — giving iron: the anemia of MDS is a production defect; these patients are usually iron overloaded from transfusions.
• Distinguishing mimic: aplastic anemia gives a hypocellular marrow without dysplasia and is treated with immunosuppression; MDS is typically hypercellular despite cytopenias.
• Erythropoiesis-stimulating agents work best in lower-risk, low-transfusion-burden patients with a low endogenous serum erythropoietin level; check the EPO level first.