Aplastic Anemia
Contents (8)
Aplastic anemia is a life-threatening disorder characterized by pancytopenia (reduction in all three myeloid lineages) resulting from severe hypoplasia or aplasia of the bone marrow. The condition represents a failure of hematopoietic stem cell production rather than increased peripheral destruction, distinguishing it from immune-mediated hemolytic anemias or consumptive coagulopathies. Aplastic anemia has an incidence of 2-3 cases per million annually in Western countries, with higher incidence in Asia (particularly Southeast Asia where rates reach 5-10 per million), and can occur at any age with slight female predominance in younger populations. The disease carries significant morbidity and mortality without treatment, making rapid diagnosis and initiation of therapy critical in clinical practice. Approximately 50% of cases are idiopathic, while the remainder have identifiable triggering agents including drugs, chemicals, infections, and radiation exposure. Understanding aplastic anemia is essential for Step 2 CK as it frequently appears in case scenarios involving unexplained cytopenias and requires knowledge of both diagnostic criteria and contemporary immunosuppressive/transplant approaches.
Aplastic anemia results from quantitative or qualitative failure of hematopoietic stem cells within the bone marrow microenvironment, leading to diminished production of red blood cells, white blood cells, and platelets. The fundamental defect involves either loss of functional stem cells or creation of a hostile marrow environment that suppresses hematopoiesis.
Key Mechanism 1: T-Cell-Mediated Immune Attack on Hematopoietic Stem Cells
In the majority of acquired aplastic anemia cases (~80%), abnormal T lymphocytes mount an immune-mediated attack against hematopoietic progenitors and stem cells through mechanisms involving both cytotoxic CD8+ T-cell infiltration and secretion of inhibitory cytokines. Activated T cells produce excessive interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), and transforming growth factor-beta (TGF-β), which directly suppress stem cell proliferation and promote apoptosis. This T-cell dysfunction is often triggered by antigenic stimulation from infectious agents, drugs, or environmental toxins that serve as molecular mimicry targets. The suppressive cytokine milieu prevents normal hematopoietic recovery even though sufficient stem cell numbers may remain; this explains why immunosuppressive therapy can restore hematopoiesis in 30-40% of patients. Clonal expansion of autoreactive T cells suggests that aplastic anemia exists on a spectrum with T-cell lymphoproliferative disorders, with some patients demonstrating evolution to overt lymphoma or myelodysplasia over time.
Key Mechanism 2: Direct Hematopoietic Stem Cell Injury and Loss
Chemical and physical agents cause direct damage to the hematopoietic microenvironment and stem cell compartment through generation of reactive oxygen species, DNA damage, and mitochondrial dysfunction. Benzene, a classic culprit, undergoes hepatic metabolism to phenol and hydroquinone, which alkylate cellular macromolecules and cause dose-dependent stem cell killing; lower-dose chronic exposure paradoxically carries higher aplasia risk than acute high-dose exposure. Ionizing radiation damages hematopoietic stem cells proportional to marrow dose, with recovery generally following if stem cell reserves are not completely exhausted. Some drugs (notably chloramphenicol, phenylbutazone, and certain anticonvulsants) cause idiosyncratic reactions independent of dose, suggesting genetic predisposition to metabolism of toxic intermediates or enhanced apoptotic sensitivity. In contrast, other medications show dose-dependent effects (as seen with methotrexate or certain chemotherapy agents) that are reversible upon discontinuation as surviving stem cells regenerate. The severity of aplasia correlates with the extent of stem cell damage, explaining why severe aplastic anemia (SAA) represents a critical threshold where fewer than 25,000 CD34+ progenitors remain per microliter of blood.
Key Mechanism 3: Defective Hematopoietic Stem Cell Intrinsic Function
Inherited bone marrow failure syndromes such as Fanconi anemia, dyskeratosis congenita, and Shwachman-Diamond syndrome demonstrate that germline mutations in DNA repair genes (particularly nucleotide excision repair and interstrand crosslink repair pathways), telomere maintenance genes, and ribosomal biogenesis genes predispose to aplastic anemia through accumulation of genetic damage. These conditions feature increased chromosomal fragility (detectable by DEB/MMC chromosome breakage tests in Fanconi anemia) and progressive stem cell exhaustion. Even in acquired aplastic anemia, somatic mutations in genes involved in DNA repair and cell cycle regulation (TP53, PTEN, ASXL1) accumulate in hematopoietic clones, suggesting that surviving stem cells face genomic stress. Recent evidence demonstrates that approximately 50% of aplastic anemia patients harbor clonal hematopoiesis defined by expanded populations of mutated hematopoietic cells; this "preleukemic" state explains higher rates of clonal evolution to myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) during long-term follow-up (5-10 year cumulative risk of 10-15%).
Additional Mechanisms
- Abnormal Bone Marrow Microenvironment: Disrupted stromal cell function, altered growth factor signaling (especially thrombopoietin and stem cell factor pathways), and excessive apoptotic signals from damaged niche cells perpetuate stem cell suppression even after the initial injury resolves
- Infections Triggering Immune Dysregulation: Epstein-Barr virus (EBV), hepatitis C, and parvovirus B19 can initiate aplastic anemia through superantigen activation of T cells or direct viral infection of hematopoietic cells; seronegative hepatitis (non-A, non-B, non-C) accounts for 5-10% of fulminant aplasia cases
- Complement Activation and Cellular Destruction: Some patients show evidence of complement-mediated destruction of hematopoietic cells, explaining why anticomplement therapies (eculizumab) have shown efficacy in small trials
Approximately 50% of aplastic anemia cases are idiopathic, while the remainder have identifiable triggers; it is critical to perform thorough history and physical examination to identify modifiable risk factors.
Medications and Chemicals (Most Common Identifiable Cause)
- Benzene: High-risk occupational exposure causes dose-dependent stem cell suppression; history of paint, adhesive, or petroleum product exposure is essential to elicit. Even sub-clinical exposures over years can trigger aplasia, making occupational history crucial.
- Chloramphenicol: Historically a major culprit, now rare in developed countries due to restricted use; causes dose-independent idiosyncratic reactions in genetically susceptible individuals
- Phenytoin and other anticonvulsants (phenylbutazone, carbamazepine): Idiosyncratic reactions unrelated to dose; hypersensitivity patterns (fever, rash, lymphadenopathy) may precede aplasia
- Sulfonamides, trimethoprim-sulfamethoxazole: Dose-dependent or idiosyncratic depending on agent
- Immunosuppressive agents: Methotrexate, azathioprine, and mycophenolate cause dose-dependent reversible suppression
- NSAIDs and acetaminophen: Rare but documented triggers, particularly with chronic high-dose use
- Chemotherapy and radiation: Dose-dependent effects; recovery typically occurs within 2-4 weeks if stem cells survive initial injury
Infections
- Seronegative hepatitis: Non-A, non-B, non-C hepatitis accounts for 5-10% of fulminant aplastic anemia; diagnosis is retrospective (exclusion of A, B, C, D, E); high mortality without treatment
- Epstein-Barr virus and cytomegalovirus: Particularly in immunocompromised hosts; may present with viral prodrome
- Parvovirus B19: Causes selective red cell aplasia rather than true aplastic anemia in most cases, but can trigger comprehensive marrow failure
- HIV infection: Associated with higher aplasia risk, potentially through chronic immune activation and direct viral effects
Radiation and Environmental Exposure
- Ionizing radiation (occupational, accidental, or therapeutic) causes dose-dependent stem cell killing; atomic bomb survivors from Japan showed markedly elevated aplasia incidence
- Pesticide and organic solvent exposure (beyond benzene) carries documented risk
Inherited Bone Marrow Failure Syndromes (Account for ~10-15% of Pediatric Cases)
- Fanconi anemia: Autosomal recessive disorder with chromosomal fragility; positive DEB or MMC test confirms diagnosis; increased AML/MDS risk
- Dyskeratosis congenita: Premature telomere shortening leads to progressive bone marrow failure; oral leukoplakia and increased cancer risk are cardinal features
- Shwachman-Diamond syndrome: Pancreatic insufficiency with bone marrow failure; SBDS gene mutations
- Severe congenital neutropenia (Kostmann syndrome): Early-onset neutropenia with aplasia risk; some cases show evolution to MDS/AML
Autoimmune and Other Systemic Conditions
- Systemic lupus erythematosus and Sjögren syndrome: Immune-mediated mechanisms similar to idiopathic aplastic anemia
- Eosinophilic fasciitis: Shulman syndrome associated with aplastic anemia in 5-10% of cases
- Thymoma and thymic disorders: Thymoma-associated aplastic anemia occurs in 5% of thymoma patients; thymectomy may allow hematopoietic recovery
Pregnancy-Associated Aplastic Anemia
Rare but well-documented; hematopoietic recovery typically occurs post-partum, though immunosuppressive therapy may be needed urgently during pregnancy due to risk of hemorrhage and infection.
The clinical manifestations of aplastic anemia result directly from the degree and duration of pancytopenia, with symptoms reflecting anemia, neutropenia, and thrombocytopenia rather than marrow infiltration (as occurs in leukemia or lymphoma).
Cardinal Symptoms from Anemia
- Progressive dyspnea on exertion and fatigue: Result from decreased hemoglobin and reduced oxygen-carrying capacity; may begin insidiously with exertional dyspnea weeks before diagnosis
- Palpitations and tachycardia: Compensatory responses to anemia; occur particularly with rapid development of anemia or in patients with underlying cardiac disease
- Pallor: Visible in conjunctivae, palms, and nail beds when hemoglobin falls below 7-8 g/dL; physical exam finding reflecting degree of anemia
- Headache and dizziness: From cerebral hypoxia and decreased oxygen delivery to brain; may be exacerbated by infections
Symptoms from Thrombocytopenia (Platelet Count <50,000/μL)
- Bleeding manifestations: Spontaneous petechiae (typically on lower extremities and areas of pressure), purpura, and ecchymoses usually apparent when platelets <20,000/μL
- Mucosal bleeding: Epistaxis, gum bleeding, hemoptysis, hematemesis, melena, and hematuria develop with severe thrombocytopenia; intracranial hemorrhage is a life-threatening risk when platelets <10,000/μL
- Menorrhagia: Often first symptom recognized by female patients; may worsen anemia if severe
- Bleeding with minor trauma: Disproportionate bleeding from minor cuts or dental procedures
Symptoms from Neutropenia (Neutrophil Count <500/μL)
- Recurrent infections: Fever, chills, and documented infections (pneumonia, bacteremia, fungemia) become prominent with severe neutropenia; oral ulcers and pharyngitis from bacterial superinfection of damaged mucosa
- Perirectal infections and abscesses: Classic presentation in severe neutropenia; rectal examination must be performed cautiously (avoid digital examination in severe cases)
- Absence of lymphadenopathy and splenomegaly: Key distinguishing feature from leukemia; lymph node enlargement or organomegaly should prompt consideration of other diagnoses
Physical Examination Findings
- Absence of lymphadenopathy, hepatomegaly, and splenomegaly: Aplastic anemia does NOT typically cause organomegaly; their presence suggests alternative diagnosis (acute leukemia, lymphoma, infection, myelofibrosis)
- Oral mucosal ulceration and pharyngitis: From neutropenia permitting secondary bacterial infection of mucosa
- Petechiae, purpura, and ecchymoses: Reflect thrombocytopenia; distribution on lower extremities and dependent areas typical
- Pallor: Conjunctival, palmar, and nail bed pallor indicating anemia
- Tachycardia and flow murmurs: Compensatory responses to anemia; high-output cardiac state with "innocent" systolic murmur from increased flow
Important Clinical Variants
- Fulminant presentation with hemorrhage: Some patients present acutely with severe thrombocytopenia (<10,000/μL) and life-threatening hemorrhage; requires urgent hospitalization and transfusion support
- Insidious onset with fatigue prominence: Other patients experience weeks to months of progressive fatigue before seeking care; anemia may be dominant feature
- Infection as presenting symptom: Severe neutropenic patients may present with fever, sepsis, or documented infection (fungal pneumonia, bacteremia) before anemia or thrombocytopenia become apparent
Diagnosis of aplastic anemia requires integration of clinical presentation, peripheral blood findings, and bone marrow examination (biopsy and aspiration), as diagnosis cannot be established from blood counts alone. The Cavendish Classification (revised) and FAB (French-American-British) criteria categorize disease severity, which determines initial treatment approach.
Initial Laboratory Evaluation - Complete Blood Count (CBC)
- Hemoglobin: Typically <10 g/dL at diagnosis (range 5-10 g/dL); progressive decline over weeks to months
- Absolute neutrophil count (ANC): <1500/μL defines neutropenia; severe aplastic anemia specifically defined by ANC <500/μL
- Platelet count: <50,000/μL defines thrombocytopenia; severe aplastic anemia requires platelets <20,000/μL
- Reticulocyte count: Inappropriately LOW or low-normal despite anemia; this is the critical distinguishing feature from hemolytic anemias (which show high reticulocyte count). A reticulocyte count >60,000/μL would argue against aplastic anemia
- Mean corpuscular volume (MCV): Often normal or elevated (macrocytic) reflecting younger RBC population in circulation; microcytic anemia suggests iron deficiency or thalassemia, not aplastic anemia
- White blood cell differential: Shows relative lymphocytosis and absence of left shift (immature forms); presence of blasts or abnormal cells suggests acute leukemia
Bone Marrow Examination - Gold Standard
Both aspiration and biopsy are required (aspiration alone may miss focal areas of residual hematopoiesis or abnormal cytology):
- Cellularity: Severely decreased (<25% overall cellularity defines aplastic anemia; hypoplasia typically shows 10-20% cellularity)
- Absence of dysplastic changes: Rules out myelodysplastic syndrome; dysplasia would suggest MDS rather than aplastic anemia
- Normal karyotype (usually): Most cases show 46,XX or 46,XY; cytogenetic abnormalities (monosomy 7, trisomy 8, complex karyotype) suggest MDS or transformation risk
- Absence of increased blasts: >5% blasts raises concern for acute leukemia rather than aplastic anemia
- Absence of myelofibrosis or marrow infiltration: Rules out primary myelofibrosis, malignancy infiltration, or granulomatous diseases
- Flow cytometry: Usually normal; abnormal populations suggest MDS or leukemia
- Iron stain: May show ringed sideroblasts if present (MDS marker)
Severity Classification - Cavendish/International Prognostic Scoring System (IPSS) Criteria
Severe Aplastic Anemia (SAA) is defined by bone marrow cellularity <25% PLUS ≥2 of the following
- ANC <500/μL
- Platelets <20,000
Immediate stabilisation
- Withdraw the culprit exposure: stop any implicated drug (chloramphenicol, anticonvulsant, sulfonamide) and remove benzene/solvent exposure — some drug-induced cases recover spontaneously within weeks.
- Transfusion support: give leukoreduced and irradiated cellular products to reduce HLA alloimmunisation and prevent transfusion-associated GVHD in a profoundly lymphopenic, potentially transplant-bound host. AABB guidance supports prophylactic platelets at a threshold near 10,000/μL in hypoproliferative thrombocytopenia and a restrictive red cell strategy. Avoid directed donations from family members, who may become the marrow donor (risk of sensitisation to donor minor antigens).
- Febrile neutropenia is a medical emergency: cultures then empiric antipseudomonal beta-lactam (cefepime or piperacillin-tazobactam) without waiting for a source, per IDSA neutropenic fever guidance; add mould-active antifungal therapy for persistent fever.
Definitive therapy — decided by age, severity, and donor availability (British Society for Haematology aplastic anaemia guideline; EBMT Severe Aplastic Anaemia Working Party)
- Allogeneic hematopoietic stem cell transplant: first-line for severe/very severe disease in younger patients (roughly under 40–50 years) with a matched sibling donor; it replaces the failing stem cell compartment and is potentially curative. Conditioning is cyclophosphamide-based with ATG; total body irradiation is avoided given secondary malignancy risk.
- Immunosuppressive therapy (IST): for older patients or those without a matched sibling — horse antithymocyte globulin plus cyclosporine plus eltrombopag. Horse ATG outperformed rabbit ATG for response and survival in the NIH randomized comparison, so horse ATG is the standard first-line preparation. Eltrombopag, a thrombopoietin-receptor agonist, is FDA-approved both for refractory disease and in combination with standard first-line IST.
- Non-severe, transfusion-independent disease: observation with supportive care is acceptable.
Escalation and adjuncts
- Refractory after one IST course: matched-unrelated or haploidentical transplant (post-transplant cyclophosphamide platforms), or a second IST course.
- Androgens (danazol): reserved for telomere biology disorders.
- G-CSF: may shorten neutropenia but does not improve survival; not routine.
Avoid: aspirin/NSAIDs and intramuscular injections while thrombocytopenic; live vaccines during IST; and full-dose alkylator/irradiation conditioning in Fanconi anemia, where crosslink hypersensitivity mandates dose reduction.
Complications of the disease
- Intracranial or major hemorrhage (emergency): profound thrombocytopenia with no marrow reserve; signalled by new headache, vomiting, focal deficit, or altered mental status — obtain non-contrast head CT and transfuse platelets immediately.
- Neutropenic sepsis and invasive fungal infection (emergency): absent phagocytes mean minimal inflammatory signs, so fever may be the only finding. Persistent fever despite broad-spectrum antibacterials, pleuritic chest pain, or a halo sign on chest CT suggests invasive aspergillosis.
- Clonal evolution to MDS/AML: surviving stem cells carry genomic stress and immune-escape clones; heralded by rising blasts, new dysplasia, or acquired monosomy 7 or trisomy 8 on karyotype.
- Paroxysmal nocturnal hemoglobinuria (PNH) clone expansion: PIGA-mutant cells escape immune attack and lose GPI-anchored CD55/CD59, producing complement-mediated intravascular hemolysis. Look for elevated LDH, low haptoglobin, hemoglobinuria, and unexplained thrombosis in unusual sites (Budd–Chiari syndrome) — thrombosis here is an emergency.
- Transfusional iron overload: each unit delivers iron with no excretory route; rising ferritin, cardiomyopathy, cirrhosis, and endocrinopathy follow chronic support. Cardiac MRI T2* quantifies myocardial loading.
- Platelet refractoriness from HLA alloimmunisation: failure of the count to rise after transfusion; mitigated by leukoreduction.
Complications of therapy
- Serum sickness after ATG: immune complexes from equine protein cause fever, urticarial rash, and arthralgias about a week after infusion; corticosteroids are given prophylactically. Anaphylaxis during infusion is the acute emergency.
- Cyclosporine toxicity: calcineurin inhibition causes nephrotoxicity, hypertension, hyperkalemia, hypomagnesemia, tremor, hirsutism, and gingival hyperplasia; monitor levels and creatinine.
- Eltrombopag hepatotoxicity: transaminase and bilirubin elevation warrant monitoring.
- Transplant complications: graft failure/rejection (higher with prior heavy transfusion), acute and chronic GVHD, sinusoidal obstruction syndrome, and CMV reactivation.
- Transfusion-associated GVHD: fatal if unirradiated cellular products are given — pancytopenia with rash, diarrhea, and hepatitis.
- The triad that defines the stem: pancytopenia + inappropriately low reticulocyte count + a hypocellular, fat-replaced marrow with no infiltrate, no fibrosis, and no dysplasia. Low retics separate production failure from hemolysis or blood loss.
- Single best next step after confirming pancytopenia: bone marrow aspiration AND biopsy. Aspiration alone can give a dry or diluted sample; the biopsy core is what establishes cellularity.
- The association examiners test: aplastic anemia and PNH sit on a continuum. Send peripheral blood flow cytometry for loss of GPI-anchored proteins (CD55/CD59, FLAER) in every new case — the tested clue is unexplained thrombosis or hemolysis in a patient with marrow failure.
- Any child or young adult with marrow failure needs a chromosomal breakage test (DEB/MMC) for Fanconi anemia; look for short stature, café-au-lait macules, and radial ray/thumb anomalies. This changes conditioning intensity before transplant.
- Treatment decision rule: young patient with a matched sibling donor → allogeneic transplant first. Older patient or no sibling donor → horse ATG + cyclosporine + eltrombopag. Rabbit ATG is the classic wrong answer for first-line therapy.
- Blood products must be leukoreduced and irradiated, and family members should not be used as donors in a potential transplant candidate — a favourite management question.
- Common distractors to avoid:
- B12/folate deficiency also causes pancytopenia, but the marrow is hypercellular with megaloblastic changes and hypersegmented neutrophils.
- Hairy cell leukemia gives pancytopenia and a dry tap, but with massive splenomegaly and TRAP-positive/CD103+ cells; aplastic anemia has no organomegaly or lymphadenopathy.
- Aleukemic leukemia and MDS show blasts or dysplasia; aplastic anemia does not.
- Classic exposures: chloramphenicol, benzene, phenytoin, and seronegative hepatitis in a young man weeks after an acute hepatitic illness.