Aplastic anemia

Overview

• Definition: aplastic anemia is an acquired hematopoietic stem cell disorder in which stem cells are destroyed, usually by autoimmune T-cell attack, producing severely decreased bone marrow cellularity and pancytopenia (anemia, neutropenia, and thrombocytopenia together).
• Marrow signature: hypocellular marrow replaced by fat, with no dysplastic cells, no excess blasts, and no fibrosis, distinguishing it from the marrow failure syndromes it mimics.
• Trigger spectrum: most cases are idiopathic (presumed autoimmune), but recognized causes include drugs, toxins, radiation, viral infections, and rare inherited syndromes such as Fanconi anemia.
• Why it is dangerous: untreated severe disease has 80% mortality within 2 years from infection or hemorrhage, so severity grading and treatment selection happen promptly.
• Curative versus palliative pathway: allogeneic hematopoietic stem cell transplantation (HSCT) can cure the disease in patients with a matched donor, whereas immunosuppression controls but does not eliminate the autoimmune process.

Epidemiology

• Rare disease: uncommon in Western populations, with a bimodal age distribution peaking in adolescents/young adults and again in older adults.
• Idiopathic majority: most acquired cases have no identifiable cause and are presumed autoimmune.
• Inherited minority: Fanconi anemia (autosomal recessive DNA cross-link repair defect) presents with aplastic anemia, often with short stature, café-au-lait spots, and thumb or radial ray defects, and carries increased lifetime risk of MDS, AML, and solid tumors.
• Post-hepatitis association: a subset of idiopathic aplastic anemia follows an episode of acute seronegative hepatitis.

Risk factors

• Drug exposure: chloramphenicol (dose-independent idiosyncratic reaction), carbamazepine and other anticonvulsants, antithyroid medications (methimazole, propylthiouracil), NSAIDs, sulfonamides, beta-lactam antibiotics, and gold salts.
• Chemotherapy and radiation: cytotoxic chemotherapy and ionizing radiation directly injure hematopoietic stem cells.
• Toxin exposure: benzene.
• Viral infections: hepatitis viruses (a seronegative hepatitis syndrome), Epstein-Barr virus, cytomegalovirus, HIV, and human parvovirus B19 (more classically causes isolated pure red cell aplasia than pancytopenia).
• Inherited marrow failure syndromes: Fanconi anemia and other congenital DNA repair or telomere maintenance defects, suspected in children or young adults with characteristic dysmorphic features.
• Pregnancy: an uncommon precipitant.

Pathophysiology

• Autoimmune stem cell destruction: in most acquired disease, activated cytotoxic T cells destroy hematopoietic stem and progenitor cells, driving the marrow toward failure.
• Loss of marrow cellularity: as stem cells are destroyed, marrow is progressively replaced by fat, so a biopsy specimen normally cellular for age becomes markedly hypocellular.
• No abnormal clone in classic disease: unlike MDS or leukemia, residual marrow shows no dysplastic maturation, no increase in blasts, and no fibrosis; the defect is a quantitative loss of normal stem cells, not a clonal disorder.
• Emergence of a PNH clone: the autoimmune attack spares stem cells lacking glycosylphosphatidylinositol (GPI)-anchored surface proteins, so a paroxysmal nocturnal hemoglobinuria clone (loss of complement-regulatory CD55 and CD59) is commonly selected for, though its presence does not usually change initial treatment.
• Drug/toxin mechanisms vary: chloramphenicol causes a rare, dose-independent idiosyncratic marrow toxicity distinct from its common dose-dependent reversible anemia; benzene and radiation are directly cytotoxic to stem cells; Fanconi anemia results from a defective DNA cross-link repair pathway causing progressive stem cell attrition.

Clinical features

• Symptoms reflect the three cytopenias, not one dominant complaint: fatigue, malaise, and pallor from anemia; fever and recurrent or severe infections from neutropenia; petechiae, mucosal bleeding, and easy bruising from thrombocytopenia.
• No lymphadenopathy or organomegaly: hepatosplenomegaly and lymphadenopathy are notably absent, helping distinguish aplastic anemia from leukemia, lymphoma, or myelofibrosis at the bedside.
• Insidious onset in most acquired disease: symptoms develop gradually over weeks as counts fall, though presentation can be abrupt with severe infection or hemorrhage.
• Physical stigmata of Fanconi anemia in a child or young adult: short stature, café-au-lait spots, and thumb or radial ray abnormalities.
• Preceding illness: a recent seronegative hepatitis-like illness, new drug exposure, or occupational solvent exposure should be sought.

Differential diagnosis

• Myelodysplastic syndrome (hypoplastic variant): hypocellular marrow, but MDS shows dysplastic maturation and often a clonal cytogenetic abnormality, absent in aplastic anemia (myelodysplastic syndromes).
• Acute leukemia ("aleukemic" presentation): pancytopenia with a deceptively hypocellular-appearing marrow, but biopsy shows increased blasts and immature leukocytes rather than fat replacement; disseminated intravascular coagulation at presentation favors acute promyelocytic leukemia over aplastic anemia.
• Hairy cell leukemia: pancytopenia with massive splenomegaly and a "dry tap" on aspiration, but biopsy is hypercellular with diffuse infiltration by TRAP-positive hairy cells rather than fat replacement (hairy cell leukemia).
• Primary myelofibrosis: pancytopenia with massive splenomegaly, teardrop RBCs, and a "dry tap," but biopsy shows marrow fibrosis and atypical megakaryocyte hyperplasia rather than hypocellularity (Myelofibrosis).
• Paroxysmal nocturnal hemoglobinuria: pancytopenia with hemolysis, dark morning urine, and thrombosis; flow cytometry shows loss of CD55/CD59, and PNH can coexist with or evolve from aplastic anemia (paroxysmal nocturnal hemoglobinuria).
• Vitamin B12 or folate deficiency: macrocytic anemia with hypersegmented neutrophils and oval macrocytes; severe deficiency can affect other lineages, but the marrow is hypercellular and megaloblastic, and cytopenias correct with repletion (pernicious anemia).
• Pure red cell aplasia: isolated anemia with reticulocytopenia and absent erythroid precursors but preserved granulocytes and platelets, often from parvovirus B19, thymoma, or autoimmune disease, unlike the trilineage failure of aplastic anemia.
• Marrow infiltration (metastatic tumor, granulomatous disease): hypercellular marrow replaced by tumor or granulomas rather than fat.
• Hypersplenism from chronic liver disease: cytopenias from peripheral sequestration with splenomegaly and a normal or hypercellular marrow.
• HIV or other viral marrow suppression: confirmed by serology, typically less profound than classic aplastic anemia.

Diagnosis

• Establish the diagnosis with pancytopenia plus a hypocellular marrow biopsy showing no alternative explanation, then grade severity by the Camitta criteria: requires peripheral pancytopenia with reticulocytopenia and a marrow biopsy showing markedly decreased cellularity (less than 25% of expected for age, or 25% to 50% with fewer than 30% residual hematopoietic cells) with fat replacement, and absence of dysplasia, excess blasts, abnormal infiltrate, or fibrosis that would instead indicate MDS, acute leukemia, or myelofibrosis.
  • Severe: marrow cellularity less than 25% (or 25% to 50% with less than 30% hematopoietic cells) plus at least 2 of: ANC less than 0.5 x 10^9/L, platelet count less than 20 x 10^9/L, or corrected reticulocyte count less than 1%.
  • Very severe: meets severe criteria with ANC less than 0.2 x 10^9/L.
  • Moderate (non-severe): marrow hypocellularity and cytopenias present but not meeting severe thresholds.
• First-line labs: CBC with differential showing pancytopenia, an inappropriately low reticulocyte count (reticulocytopenia), and a peripheral smear otherwise unremarkable, without blasts, dysplastic forms, or teardrop cells.
• Bone marrow aspirate and biopsy is the confirmatory test: aspirate is often difficult to obtain because of hypocellularity, and biopsy is required to assess cellularity and exclude dysplasia, blasts, infiltrate, and fibrosis.
• Cytogenetic and molecular testing on the marrow excludes a clonal disorder (MDS, acute leukemia) that can mimic hypocellular aplastic anemia.
• Test for a PNH clone by peripheral blood flow cytometry for CD55 and CD59 (or FLAER): common in aplastic anemia and supports an immune-mediated process, though it does not routinely change initial treatment.
• Evaluate for Fanconi anemia with chromosomal breakage testing (diepoxybutane or mitomycin C stress testing) in children, young adults, or anyone with suggestive stigmata, because it changes transplant conditioning and excludes standard immunosuppression as definitive therapy.
• HLA typing of the patient and siblings at diagnosis in any potential transplant candidate, because donor availability drives the choice between HSCT and immunosuppression.

Clinical pearl. Aplastic anemia and acute leukemia can both present with pancytopenia and a seemingly "empty" marrow, but disseminated intravascular coagulation at presentation points to acute promyelocytic leukemia, not aplastic anemia, which does not itself cause a coagulopathy.

Management

• Remove the offending exposure first: stop any implicated drug (chloramphenicol, carbamazepine, antithyroid drugs, NSAIDs, gold, sulfonamides) or eliminate toxin/radiation exposure; recovery can take weeks, so definitive therapy is not delayed awaiting this response in severe disease.
• Risk-stratify by age, donor availability, and severity:
  • Severe or very severe disease, age under about 40, with an HLA-matched sibling donor: allogeneic HSCT, curative and preferred first-line, with survival exceeding 80% in good-risk young patients.
  • Severe disease without a matched sibling donor, or older age: first-line immunosuppression, because transplant-related mortality and graft-versus-host disease risk rise with age and with unrelated/mismatched donors.
  • Moderate (non-severe) disease without transfusion dependence or infection: observe with close monitoring before committing to transplant or immunosuppression.
• First-line immunosuppression is triple therapy: antithymocyte globulin (equine) 40 mg/kg IV once daily for 4 days, plus cyclosporine 5 to 6 mg/kg/day PO divided every 12 hours (trough approximately 200 to 400 ng/mL) continued at least 6 to 12 months with a slow taper, plus eltrombopag 150 mg PO once daily (50 mg in patients of East Asian ancestry) added upfront to improve trilineage response.
• If there is no response by 3 to 6 months, proceed to matched unrelated donor HSCT if a donor is available, or a second course of antithymocyte globulin (rabbit ATG, since equine ATG is not repeated) for patients who are not transplant candidates.
• Supportive care runs throughout: transfuse irradiated, leukoreduced red cells and platelets (to prevent transfusion-associated graft-versus-host disease and preserve transplant eligibility), treat neutropenic fever empirically with broad-spectrum antibiotics, give antifungal/antiviral prophylaxis during severe neutropenia, and avoid intramuscular injections while thrombocytopenic.
• Fanconi anemia requires reduced-intensity conditioning HSCT rather than standard immunosuppression, because these patients are hypersensitive to the DNA cross-linking chemotherapy and radiation used in conventional conditioning.
• Growth factors (G-CSF) are not standard first-line therapy and are reserved for specific infectious complications, since they do not address the underlying stem cell failure.

Complications

• Life-threatening infection: the leading cause of death in untreated or refractory severe disease, driven by profound neutropenia.
• Hemorrhage: intracranial or gastrointestinal bleeding from severe thrombocytopenia, the other major early cause of death.
• Clonal evolution to MDS or acute myeloid leukemia, particularly after prolonged immunosuppression, monitored for with periodic marrow cytogenetics.
• Expansion of a PNH clone with hemolysis and thrombosis requiring complement-inhibitor therapy in a subset of patients.
• Graft-versus-host disease and transplant-related mortality: principal risks of HSCT, particularly with unrelated/mismatched donors and older recipient age (graft-versus-host disease).
• Iron overload from chronic transfusion dependence in patients without durable remission.

Prognosis

• Untreated severe disease is rapidly fatal: approximately 80% die within 2 years from infection or hemorrhage.
• HSCT offers the best long-term outcome when available: overall survival exceeds 80% in young, good-risk patients undergoing matched sibling HSCT, and this option is curative.
• Immunosuppression is effective but not curative: most patients respond, but relapse on cyclosporine taper and late clonal evolution to MDS or AML remain risks, so surveillance continues after response.
• Response predicts outcome: patients achieving a hematologic response have substantially better survival than nonresponders, who require transplant or additional therapy.

Prevention

• No primary prevention exists for idiopathic autoimmune disease; prevention targets avoidable causes and minimizes complications once diagnosed.
• Avoid re-exposure to an implicated drug or toxin: document the culprit (chloramphenicol, carbamazepine, antithyroid drugs, gold, NSAIDs) and avoid rechallenge indefinitely.
• Use irradiated and leukoreduced blood products for all transfusions, limiting transfusion volume to what is clinically necessary in any potential transplant candidate, to prevent transfusion-associated graft-versus-host disease and preserve future HSCT eligibility.
• Screen young patients (age under about 40) and those with suggestive stigmata for Fanconi anemia with chromosomal breakage testing before committing to standard-intensity conditioning, since unrecognized Fanconi anemia leads to fatal toxicity with conventional transplant regimens.
• Give meningococcal vaccination before starting a complement inhibitor (eculizumab or ravulizumab) if a PNH clone progresses to symptomatic PNH.
• Repeat marrow cytogenetics at intervals (typically every 6 to 12 months) in patients maintained on long-term immunosuppression to detect early clonal evolution to MDS or AML.

Key points

• Aplastic anemia is pancytopenia from immune-mediated (or toxic, viral, or inherited) destruction of hematopoietic stem cells, producing a hypocellular, fatty marrow with no dysplasia, blasts, or fibrosis.
• Diagnose with marrow biopsy showing less than 25% cellularity and grade severity by the Camitta criteria (ANC, platelets, corrected reticulocyte count thresholds), which sets the treatment pathway.
• Test for a PNH clone (CD55/CD59 flow cytometry) and consider Fanconi anemia testing in young patients, since either changes surveillance or conditioning strategy.
• Young, fit patients with an HLA-matched sibling donor and severe disease go to curative allogeneic HSCT; everyone else gets antithymocyte globulin, cyclosporine, and eltrombopag.
• Untreated severe aplastic anemia kills about 80% of patients within 2 years.
• Use irradiated, leukoreduced blood products to prevent transfusion-associated graft-versus-host disease and preserve future transplant eligibility.