Aplastic anemia (AA) is a disorder of bone marrow failure caused by various potential etiologies and mechanisms. It is characterized primarily by reduced bone marrow hematopoietic activity, pancytopenia, and the resulting syndromes of anemia, bleeding, and infection.
According to the widely used Camitta criteria (established in 1976), AA is classified into severe aplastic anemia (SAA) and non-severe aplastic anemia (NSAA) based on disease severity. AA can be further categorized by etiology into congenital (hereditary) and acquired forms. Acquired AA is divided into secondary AA, which has a specific identifiable cause, and primary AA, which lacks a clearly defined cause. Recent studies suggest that aberrant hyperactivation of T-cell function, mediated through direct cytotoxic T-cell attack and/or lymphokine-driven excessive apoptosis of hematopoietic stem cells, is the primary pathogenic mechanism of acquired AA.
Etiology and Pathogenesis
The majority of cases have unclear etiologies, but possible contributing factors include:
- Viral Infections: Certain viruses, particularly hepatitis viruses and parvovirus B19, are implicated in the development of AA.
- Chemical Factors: Chemicals such as chloramphenicol, sulfonamides, antineoplastic agents, and benzene have been associated with AA. While the bone marrow suppression caused by antineoplastic agents and benzene is dose-dependent, the development of AA related to antibiotics, sulfonamides, and pesticides appears to be less dose-dependent and more linked to individual sensitivity.
- Ionizing Radiation: Prolonged exposure to X-rays, radium, and radioactive isotopes can disrupt DNA replication, inhibit cellular mitosis, and interfere with bone marrow cellular production, leading to a reduction in hematopoietic stem cell quantity.
Conventional theories suggest that, under certain genetic predispositions, AA may represent a heterogeneous "syndrome" resulting from exposure to specific pathogenic factors. Pathogenesis may involve three mechanisms: primary or secondary defects in hematopoietic stem/progenitor cells ("seeds"), abnormalities in the hematopoietic microenvironment ("soil"), and immune dysregulation ("pests"). In primary acquired AA, abnormal T-cell activation and hyperfunction are considered central to bone marrow damage. Recent research highlights a potential role of genetic predisposition in AA pathogenesis, including mutations in telomerase genes and other somatic mutations.
Defects in Hematopoietic Stem/Progenitor Cells
Abnormalities in both quantity and quality of stem/progenitor cells have been identified in AA. Bone marrow CD34+ cells are significantly reduced in patients with AA, and the degree of reduction correlates with disease severity. Studies report that the ability of AA hematopoietic stem/progenitor cells to form colonies is severely impaired, with poor responsiveness to hematopoietic growth factors (HGFs) in vitro. Even after immunosuppressive therapy, hematopoietic recovery remains incomplete in certain cases. Some patients exhibit clonal hematopoiesis, which may progress to hematopoietic stem cell abnormalities such as paroxysmal nocturnal hemoglobinuria (PNH), myelodysplastic syndromes (MDS), or even leukemia.
Abnormalities in the Hematopoietic Microenvironment
Bone marrow biopsies in AA patients reveal not only reduced hematopoietic cells but also characteristics such as bone marrow "fatty transformation," venous sinus wall edema, hemorrhaging, and capillary necrosis. Some AA patients exhibit poor growth of bone marrow stromal cells in vitro, with altered secretion of various hematopoietic regulatory factors compared to healthy individuals. Hematopoietic stem cell transplantation is less likely to succeed in cases where AA is associated with impaired bone marrow stromal cells.
Immune Dysregulation
AA patients often display an increased proportion of lymphocytes in both peripheral blood and bone marrow, along with an imbalance in T-cell subsets. Elevated levels of type-1 helper T cells (Th1), CD8+ T cells, and γδT cells have been reported. Hematopoietic inhibitory cytokines secreted by T cells, including interleukin-2 (IL-2), interferon-γ (IFN-γ), and tumor necrosis factor (TNF), are significantly increased, contributing to excessive apoptosis of myeloid cells. Immunosuppressive therapy has been effective in the majority of patients with AA.
Clinical Manifestations
Severe Aplastic Anemia (SAA)
The onset is abrupt, the progression is rapid, and the condition is severe. Some cases may develop from non-severe aplastic anemia (NSAA).
Anemia
Anemia is typically progressive and severe, with symptoms such as pallor, fatigue, dizziness, palpitations, and shortness of breath being prominent.
Infection
Most patients experience fever, often with a body temperature exceeding 39°C. In rare cases, patients may remain in a state of uncontrollable high fever from onset to death. Respiratory tract infections are the most common, with pathogens primarily being Gram-negative bacilli, Staphylococcus aureus, and fungi. Sepsis is often a concurrent condition.
Bleeding
Bleeding of varying degrees is observed in the skin, mucosa, and internal organs. The skin may show petechiae or large ecchymoses, while the oral mucosa may present with blood blisters. Common manifestations include epistaxis, gum bleeding, and conjunctival hemorrhage. Internal organ bleeding may manifest as hematemesis, hemoptysis, melena, hematuria, vaginal bleeding, fundus hemorrhage, or intracranial hemorrhage, the latter often being life-threatening.
Non-Severe Aplastic Anemia (NSAA)
The onset and progression are slower, and the severity is milder compared to SAA.
Anemia
The condition follows a chronic course, with symptoms such as pallor, fatigue, dizziness, palpitations, and exertional dyspnea being common. Symptoms improve after blood transfusion, but the effect is temporary.
Infection
High fever is less common than in SAA, and infections are relatively easier to control, rarely persisting for more than a week. Upper respiratory tract infections are frequent, followed by gingivitis, bronchitis, and tonsillitis. Severe infections such as pneumonia and sepsis are less common. The typical pathogens include Gram-negative bacilli and various cocci.
Bleeding
The bleeding tendency is mild, primarily involving the skin and mucosa, with internal organ bleeding being rare. Common presentations include petechiae on the skin, gum bleeding, and vaginal bleeding in females. Bleeding is often easier to control; however, intracranial hemorrhage may occur in patients refractory to long-term treatment.
Laboratory Investigations
Peripheral Blood Findings
SAA is characterized by severe pancytopenia:
- Marked normocytic, normochromic anemia with a reticulocyte percentage below 0.5% and an absolute count of less than 15×109/L.
- Leukocyte count is often below 2×109/L, with neutrophils less than 0.5×109/L. Lymphocyte proportion is significantly increased.
- Platelet count is typically below 20×109/L.
NSAA also exhibits pancytopenia but to a lesser degree than SAA.
Bone Marrow Findings
In SAA, severe hypoplasia is observed in bone marrow specimens from multiple sites. There is a notable reduction in granulocyte, erythroid, and megakaryocyte lineages, yet their morphology remains largely normal. The lymphocyte and non-hematopoietic cell proportions are increased, and the bone marrow particles appear nearly empty.
In NSAA, bone marrow hypoplasia is also present, with an increased presence of fat droplets. Granulocyte, erythroid, and megakaryocyte counts are reduced. Lymphocyte, reticular cell, and plasma cell proportions are elevated, and many marrow particles appear empty. Bone marrow biopsy reveals overall hypocellularity, reduced hematopoietic tissue, and increased fat or non-hematopoietic cells, without evidence of abnormal cells.
Other Relevant Findings
The CD4:CD8 ratio is reduced, while the Th1:Th2 ratio is elevated. Proportions of CD8+ T cells and γδ T cells are increased. Serum levels of IL-2, IFN-γ, and TNF are elevated.
Bone marrow cytogenetics indicate a normal karyotype. Bone marrow iron staining shows increased iron storage. Neutrophil alkaline phosphatase staining is strongly positive.
Hemolysis tests are all negative.
Diagnosis
Diagnostic criteria for AA:
- Pancytopenia, with a reticulocyte percentage below 1% and an increased lymphocyte proportion.
- An absence of significant liver or spleen enlargement.
- Bone marrow from multiple sites shows reduced (<50% of normal) or severely reduced (<25% of normal) proliferation, decreased hematopoietic cells, increased non-hematopoietic cells, and empty marrow particles. Bone marrow biopsy, if available, demonstrates uniformly decreased hematopoietic tissue.
- Other causes of pancytopenia, such as paroxysmal nocturnal hemoglobinuria (PNH), Fanconi anemia, Evans syndrome, and immune-related pancytopenia, must be excluded.
Classification of AA:
- SAA Type I: Also referred to as AAA; the onset is acute, anemia is progressively severe, and serious infections and/or bleeding are typically present. Peripheral blood meets at least two of the following criteria: reticulocyte absolute count <15×109/L, neutrophil count <0.5×109/L, and platelet count <20×109/L. Bone marrow exhibits widespread severe hypoplasia. Cases with neutrophil counts <0.2×109/L are classified as very severe aplastic anemia (VSAA).
- NSAA: Also known as CAA, refers to cases not meeting the diagnostic criteria for SAA Type I. If NSAA progresses and meets the clinical, peripheral blood, and bone marrow criteria for SAA Type I, it is classified as SAA Type II.
Differential Diagnosis
Paroxysmal Nocturnal Hemoglobinuria (PNH)
Typical cases present with hemoglobinuria, which aids in diagnosis. However, atypical cases lack hemoglobinuria, exhibit pancytopenia, and may show reduced bone marrow proliferation, making them prone to misdiagnosis as AA. PNH patients often have CD55- and CD59- deficient blood cells in various lineages detectable in bone marrow or peripheral blood.
Myelodysplastic Syndromes (MDS)
Refractory anemia (RA), a subtype of MDS, can present with pancytopenia. The reticulocyte count may be low or not elevated, and bone marrow may show hypoproliferation, leading to confusion with AA. However, RA typically exhibits dyshematopoiesis, increased early myeloid progenitor cell (CD34) expression, and may present with chromosomal karyotype abnormalities, which help differentiate it from AA.
Pancytopenia Mediated by Autoantibodies
This category includes Evans syndrome and immune-related pancytopenia. In Evans syndrome, autoantibodies against mature peripheral blood cells can be detected, whereas in immune-related pancytopenia, autoantibodies against immature bone marrow cells are present. Patients from both groups may exhibit pancytopenia and reduced bone marrow proliferation. However, reticulocyte or neutrophil percentages in the peripheral blood are often not markedly reduced and may even be elevated. Bone marrow shows a preserved erythroid progenitor ratio and the presence of "erythroid islands." Moreover, these patients often exhibit a decreased Th1:Th2 ratio (higher Th2 cell proportion), elevated CD5+ B cells, and increased serum IL-4 and IL-10 levels. Treatment responses to corticosteroids, high-dose intravenous immunoglobulin (IVIG), CD20 monoclonal antibodies, or cyclophosphamide are generally favorable.
Acute Leukemia (AL)
Particularly in leukopenic or hypoproliferative subtypes of AL, early stages may present without significant liver, spleen, or lymph node enlargement, along with bi- or pancytopenia in peripheral blood, making it challenging to distinguish from AA. Close examination of blood smears and bone marrow from multiple sites often reveals an increase in blast cells, including myeloblasts, monoblasts, or lymphoblasts. Some cases of acute promyelocytic leukemia (APL) may present with pancytopenia but can be differentiated via bone marrow morphology, chromosomal translocation t(15;17), and the presence of the PML::RARA gene.
Transient Aplastic Crisis
This condition is commonly induced by infections or medications and is more frequently observed in children or individuals with malnutrition. The onset is often accompanied by high fever, severe anemia, and rapid progression, making it susceptible to misdiagnosis as AA. However, the condition tends to be self-limiting and resolves within 2–6 weeks without the need for specialized treatment.
Bone Marrow Suppression Due to Radiation or Chemotherapy for Neoplastic Disease
Patients typically have a clear history of malignancy and prior exposure to radiation or chemotherapy. Hematopoiesis often recovers following hematopoietic-stimulating treatments.
Other Conditions
Hemophagocytic syndrome may also present with pancytopenia and hemophagocytosis in the bone marrow. However, it is often associated with infectious triggers, high fever, hepatosplenomegaly, and occasionally jaundice or ascites. There is a marked proliferation of mature histiocytes in the bone marrow, often accompanied by hemophagocytosis, which aids in distinguishing it from AA.
Treatment
Supportive Care
Protective Measures
Infection prevention measures, including maintaining dietary and environmental hygiene, are necessary. Protective isolation is required for SAA patients, with antifungal prophylactic treatment administered when appropriate. Measures to prevent bleeding, such as avoiding trauma and vigorous activities, are recommended. Contact with risk factors, including drugs harmful to bone marrow or those that inhibit platelet function, should be avoided. Psychological counseling should be provided as necessary.
Symptomatic Treatment
Correction of Anemia
Blood transfusion may be considered when hemoglobin levels fall below 60 g/L and patients show poor tolerance to anemia. However, transfusion should not be excessive.
Control of Bleeding
Hemostatic agents, such as ethamsylate, may be used. For cases complicated by increased plasma fibrinolytic activity, antifibrinolytic drugs such as aminocaproic acid can be administered, except in patients with genitourinary bleeding. Intramuscular testosterone may be considered for uterine bleeding in females. Severe bleeding caused by thrombocytopenia responds well to transfusions of concentrated platelets. If transfusions of random donor platelets prove ineffective, HLA-matched platelets should be used. Coagulopathy due to insufficient clotting factors, such as in hepatitis cases, should be corrected.
Control of Infection
In cases of febrile infection, samples from suspected infection sites, including secretions, urine, stool, or blood, should undergo bacterial culture and antibiotic sensitivity testing. Broad-spectrum antibiotics are typically used initially, and narrow-spectrum antibiotics sensitive to the pathogen should be administered once culture results become available. Long-term use of broad-spectrum antibiotics may lead to fungal infections or disruption of the intestinal flora; antifungal agents may be used to address fungal infections.
Hepatoprotective Treatment
Considering the frequent occurrence of liver function impairment in AA, hepatoprotective drugs should be selected based on the severity of liver dysfunction.
Iron Chelation Therapy
For patients undergoing repeated blood transfusions exceeding 20 units and/or those with serum ferritin levels above 1,000 µg/L, assessments such as liver and cardiac MRI may be conducted to evaluate the degree of iron overload. Chelation therapy may be utilized based on the patient’s hematological status and organ function.
Vaccination
Reports suggest that vaccination may trigger bone marrow failure or AA relapse. Vaccination is therefore not advised unless absolutely necessary. Routine vaccinations are not recommended for AA patients, except for specific cases such as after hematopoietic stem cell transplantation.
Mechanism-Targeted Therapies
Immunosuppressive Therapy
Antilymphocyte/Antithymocyte Globulin (ALG/ATG)
ALG or ATG, primarily used in SAA patients, includes rabbit-derived ATG (3–5 mg/kg/day for five days) or pig-derived ALG (20–30 mg/kg/day for five days). A hypersensitivity test is necessary before administration. Corticosteroids may be used concurrently to prevent allergic reactions during treatment. The intravenous infusion rate of ATG should not be too fast, with daily doses infused over 12–16 hours. ALG/ATG may be combined with cyclosporine (CsA) to form an intensified immunosuppressive protocol.
Cyclosporine
This is applicable to all forms of AA, with a daily dose of 3–5 mg/kg. Treatment duration typically exceeds one year. Dosage and treatment duration should be adjusted based on hematopoietic function recovery, T-cell immune status, adverse drug reactions (e.g., liver and kidney damage, gingival hyperplasia, and gastrointestinal symptoms), and blood drug concentration.
Other Options
Therapies for SAA may include CD3 monoclonal antibodies, mycophenolate mofetil (MMF), cyclophosphamide, and methylprednisolone.
Hematopoietic Stimulation Therapy
Androgens
This is applicable to all forms of AA. Common options include stanozolol (2 mg, three times daily), testosterone undecanoate (40–80 mg, three times daily), danazol (0.2 g, three times daily), and intramuscular testosterone propionate (100 mg/day). Treatment duration and dosage should be adjusted according to efficacy and side effects (e.g., virilization or liver dysfunction).
Hematopoietic Growth Factors
These are applicable to all types of AA, particularly SAA. Frequently used agents include granulocyte-macrophage colony-stimulating factor (GM-CSF) or granulocyte colony-stimulating factor (G-CSF) at doses of 5 µg/kg/day, and erythropoietin (EPO) at doses of 50–100 U/kg/day. Growth factors are typically introduced after immunosuppressive therapy for SAA, with doses reduced gradually and treatment maintained for at least three months. Thrombopoietin receptor agonists, such as eltrombopag and avatrombopag, are approved for refractory SAA. Initial doses are 75 mg/day for eltrombopag and 7.5 mg/day for avatrombopag, with dose increments every two weeks, up to 150 mg/day and 15 mg/day, respectively. Recombinant human thrombopoietin (rhTPO) has shown efficacy in single-center studies, administered at 15,000 U three times weekly after ATG to enhance remission rates and promote hematopoietic recovery. Interleukin-11 (IL-11) has also been reported to show efficacy when combined with immunosuppressive therapy.
Hematopoietic Stem Cell Transplantation (HSCT)
Allogeneic HSCT may be considered as a first-line treatment for SAA patients under 40 years of age presenting without infection or other complications and having HLA-matched sibling donors.
Efficacy Criteria for AA Treatment
Complete Remission (CR): Hemoglobin levels ≥100 g/L, neutrophil counts ≥1.5 × 109/L, platelet counts ≥100 × 109/L, with no relapse during one year of follow-up.
Partial Remission (PR): Independent of transfusion of blood components, or at least a twofold increase in one blood cell lineage or normalization of one lineage. Hemoglobin >30 g/L, neutrophil counts >0.5 × 109/L, and platelet counts >20 × 109/L.
No Response (NR): Failure to achieve the above efficacy criteria.
Prevention
Improvement of occupational and environmental safety is crucial. Avoidance of radiation exposure and overexposure to toxic chemicals (e.g., benzene derivatives) should be prioritized. Usage of medications with potential bone marrow toxicity should be minimized or avoided when possible.
Prognosis
With proper treatment, most NSAA patients achieve remission or even a cure, with only a few progressing to SAA type II. SAA has a rapid onset and severe course, historically associated with high mortality rates (>90%). However, advancements in treatment strategies over the past decade have significantly improved SAA outcomes, although approximately 1/3 of patients still succumb to infection or bleeding.