Acquired aplastic anemia: Pathogenesis, clinical manifestations, and diagnosis

INTRODUCTION — 

Aplastic anemia (AA) refers to pancytopenia in association with bone marrow hypoplasia. The term "aplastic anemia" is a misnomer because this disorder manifests pancytopenia rather than anemia alone. AA is a life-threatening condition that, if untreated, is associated with very high mortality.

Most cases of AA are associated with an autoimmune attack on hematopoietic stem cells, but the trigger is often unidentified. These cases are described as immune AA or idiopathic AA, to distinguish them from other causes of bone marrow hypoplasia. Unless otherwise specified, the term AA is used throughout this topic to describe immune/idiopathic AA.

AA is a clinicopathologic diagnosis based on the presence of ≥2 cytopenias and bone marrow hypoplasia. AA is graded according to the severity of the cytopenias and the degree of bone marrow hypoplasia.

The diagnostic evaluation must distinguish AA from transient cytopenias (eg, from infections or drugs) and other causes of persistent pancytopenia, including megaloblastic anemia, various malignancies, bone marrow infiltrative disorders, and inherited bone marrow failure syndromes (IBMFS).

This topic reviews the causes, clinical manifestations, evaluation, diagnosis, and differential diagnosis of AA.

Related topics include:

●(See "Approach to the adult with pancytopenia".)

●(See "Treatment of acquired aplastic anemia in children and adolescents".)

●(See "Treatment of aplastic anemia in adults".)

It is especially important to distinguish AA from IBMFS, which generally require distinctly different management:

●(See "Clinical manifestations and diagnosis of Fanconi anemia" and "Shwachman-Diamond syndrome" and "Telomere biology disorders, including Dyskeratosis congenita" and "Diamond-Blackfan anemia" and "Familial disorders of acute leukemia and myelodysplastic syndromes".)

OVERVIEW — 

AA refers to pancytopenia in association with hypocellular bone marrow without an abnormal infiltrate or fibrosis.

Most cases of AA are caused by immune destruction or suppression of hematopoietic stem cells and/or progenitor cells. (See 'Pathophysiology' below.)

Most patients present with symptoms related to cytopenias (eg, infections, bleeding, and/or fatigue), but some are asymptomatic at diagnosis. (See 'Clinical manifestations' below.)

The urgency of evaluation and referral to a hematologist is guided by the severity of the cytopenias and associated complications. Evaluation requires history, physical examination, routine laboratory studies, bone marrow examination, and various specialized tests. (See 'Evaluation' below.)

AA should be suspected in a child or adult with pancytopenia. It is a clinicopathologic diagnosis characterized by cytopenias in ≥2 blood lineages (ie, anemia, thrombocytopenia, neutropenia) and bone marrow hypoplasia. The depth of cytopenias, degree of bone marrow hypoplasia, and reticulocyte count are used to grade the severity of AA. (See 'Diagnosis and grading' below.)

AA must be distinguished from other causes of bone marrow hypoplasia (table 1), including megaloblastic anemia, various malignancies, bone marrow infiltration (table 2), and inherited bone marrow failure syndromes. (See 'Differential diagnosis' below.)

EPIDEMIOLOGY — 

AA is a rare disorder with a bimodal age distribution.

AA is rare in Western Europe and the United States (<2 cases per million population per year), while the incidence is estimated to be two- to threefold higher in Asia [1,2]. There is no clear association with race or ethnicity, but the higher incidence in some settings has been associated with environmental exposures, such as chemicals or infections. An increased incidence of AA has been associated with the inheritance of certain human leukocyte antigen (HLA) class I alleles that vary among populations [3].

One-half of AA cases occur in the first three decades of life, and there is a second peak of incidence in patients ≥60 years [2]. The sex ratio of AA is close to 1:1 in almost all population-based studies.

PATHOPHYSIOLOGY — 

AA results from autoimmune destruction or suppression of immature hematopoietic cells.

The loss of hematopoietic stem cells (HSCs) and progenitor cells in AA is a result of T lymphocyte-mediated attack and cytokine-influenced growth inhibition and apoptosis [4].

Hematopoietic stem and progenitor cells — HSCs and progenitor cells in bone marrow are the source of all mature cells in peripheral blood and tissues. HSCs are multipotent (ie, give rise to diverse cellular lineages), they are generally quiescent, and they have the capacity for self-renewal (thereby sustaining a lifelong store of HSCs). HSCs give rise to committed progenitor cells, which have reduced lineage potential but high proliferative capacity; progenitor cells ultimately produce fully mature blood cells by successive mitotic divisions.

HSCs are not identifiable by morphology, but they can be recognized and isolated based on their characteristic immunophenotype. They constitute a small population within the CD34-positive/CD38-negative fraction of bone marrow cells. HSCs are also present in very small amounts in peripheral blood, from which they can be isolated for use in hematopoietic cell transplantation.

Pathogenic mechanisms — The decrease in mature blood cells in AA is caused by the loss of HSCs or suppression of HSC differentiation and the loss of progenitor cells. When the HSC pool falls below a critical level, the conflicting demands of self-renewal and differentiation can lead to pancytopenia.

Autoimmune effects on HSCs and/or progenitor cells are thought to contribute to most or all cases of AA, whether another underlying cause is identified. Drug, chemical, and infectious exposures may serve as triggers that lead to autoimmune destruction or suppression of stem and progenitor cells. This hypothesis is supported by clinical observations, laboratory correlative studies, animal models, and the responsiveness of AA to immune suppression.

Pathophysiologic processes that lead to the loss of HSCs and cause acquired AA include [4,5]:

Autoimmune mechanisms — At least two autoimmune mechanisms lead to the loss of HSCs in AA:

●Pathogenic T cell responses – Idiopathic or immune-mediated AA is associated with pathogenic T cell responses.

The best evidence of the importance of autoimmune mechanisms is the ability to cure AA in some patients using T cell-directed therapies, such as antithymocyte globulin (ATG) and cyclosporine. Preclinical and translational studies have identified T cell oligoclonality, decreased regulatory T cell function, and increased T cell cytotoxicity toward HSCs [6-8]. However, the precise nature of HSC and progenitor cell loss/suppression is uncertain.

No autoantigens have been identified, and the dominant mechanism of autoimmune action may differ among patients. In some patients, viral exposure may lead to the activation of virus-reactive CD8-positive T cells that eliminate HSC and/or progenitor cells through molecular mimicry [9]. A CD8-positive T cell-mediated, viral antigen-specific mechanism is supported by the observation that certain human leukocyte antigen (HLA) class I alleles (eg, HLA-B*14:02; HLA-B *40:02) are enriched in patients with AA and are frequently targeted for clonal loss in patients with immune-mediated AA [3,10]. Deficient and/or dysfunctional T regulatory cells may also contribute to HSC/progenitor cell disruption [9,11,12].

In the classic form of postviral bone marrow failure, hepatitis-associated AA (HAAA), severe hepatitis is followed, usually within six months, by the onset of AA [13]. The mechanism of hepatitis is autoimmune (rather than infectious), and it typically improves spontaneously or with steroid therapy. HAAA has an immune signature that is distinct from idiopathic AA, including oligoclonality, and very low levels of circulating CD4-positive T cells [14].

●Inflammatory cytokines – Aberrant production of proinflammatory cytokines in the bone marrow microenvironment, including interferon-gamma and tumor necrosis factor (TNF)-alpha, may suppress (but not necessarily eliminate) hematopoiesis [15].

Hematopoietic suppression may account for the clinical observation that some patients can fully recover blood counts in response to immune suppression therapy, which would not be expected if HSCs had been destroyed.

Other mechanisms — Some cases of AA are caused by other pathogenic mechanisms (table 1).

●Direct injury – HSCs and/or progenitor cells can be directly injured by various drugs, chemicals, or ionizing radiation.

It is important to distinguish idiosyncratic drug reactions from predictable, transient, and dose-dependent causes of bone marrow suppression, which are not considered AA. Examples of such predictable bone marrow suppression include cytotoxic medications, ionizing radiation, immunosuppressive agents (eg, azathioprine), anti-inflammatory medications (eg, phenylbutazone, gold), and some antibiotics.

Cytotoxic drugs generally cause a peripheral blood count nadir 7 to 10 days after drug administration, with recovery to near baseline values within 14 to 28 days. By contrast, idiosyncratic reactions to drugs are associated with less predictable patterns of bone marrow hypoplasia. In some cases, cytopenias arise while the patient is still taking the medication, while in other cases, the effects are not recognized until days or weeks after exposure. The unpredictable response in idiosyncratic reactions can make it challenging to indict a particular drug as the cause of AA.

Many drugs, including sulfonamides, antiseizure medications (eg, felbamate, carbamazepine, valproic acid, phenytoin), and nifedipine may be associated with cytopenias (table 1), but the vast majority of exposed patients do not develop AA. The reasons for idiosyncratic reactions are not well understood, but mutations in genes encoding cellular efflux pumps (eg, P-glycoprotein 1) or drug-metabolizing enzymes (eg, glutathione-S-transferase) have been described [16-19].

Chloramphenicol is associated with both idiosyncratic and predictable bone marrow suppression. An idiosyncratic reaction to chloramphenicol causes irreversible bone marrow aplasia in approximately 1 of every 20,000 individuals, with a sudden onset several months after therapy [20]. Chloramphenicol is also associated with predictable, reversible dose-related bone marrow suppression in virtually all patients.

Solvents/degreasing agents, industrial chemicals, insecticides (eg, lindane), and pesticides are considered significant risk factors for the development of severe AA, based on case-control and other population-based studies [1,21-24]. Prolonged exposure to benzene is particularly notorious in this regard, but benzene and pesticides account for only a small number of AA cases [1,25].

●Viral infection – Epstein-Barr virus (EBV), other herpes viruses, human immunodeficiency virus (HIV), and seronegative hepatitis (non-A through -G) can cause marrow aplasia/hypoplasia.

Viral infections are often implicated as triggers for the development of autoimmune AA by activation of cytotoxic T cell clones or excessive inflammatory cytokine production. As an example, viral hepatitis may cause the release of cytokines or activation of a cytotoxic T cell clone that recognizes similar target antigens on both liver and bone marrow cells [26,27].

Some cases of anorexia nervosa are associated with pancytopenia that mimics AA; the bone marrow findings are distinct (ie, gelatinous degeneration and serous fat atrophy), but the mechanisms are not well understood. Similarly, the mechanisms that cause AA in association with pregnancy or as a complication of fulminant hepatic failure after orthotopic liver transplantation are poorly understood [28-30]. (See "Anorexia nervosa in adults and adolescents: Medical complications and their management", section on 'Hematologic'.)

Clonal evolution — AA may coexist with or evolve into another hematologic disorder, such as paroxysmal nocturnal hemoglobinuria (PNH), myelodysplastic syndromes/neoplasms (MDS), or acute myeloid leukemia (AML). Clonal evolution in AA can be detected by the acquisition of somatic mutations or karyotypic abnormalities.

Clonal evolution of AA is most often associated with the following disorders.

●PNH – There is a close relationship between AA and PNH, a clonal disorder in which acquired mutations of the PIG-A gene can lead to the global absence of certain proteins (eg, CD59) on the surface of blood cells. (See "Pathogenesis of paroxysmal nocturnal hemoglobinuria".)

PNH is inextricably linked to the development of immune-mediated bone marrow failure. The association may become evident when hemolysis or thrombosis suggestive of PNH is seen in a patient with AA, or when PNH evolves into bone marrow hypoplasia characteristic of AA.

Expanded populations of blood cells with the PNH defect can be detected by flow cytometry in many patients with AA [31]. In one study, flow cytometry detected a population of PNH-type cells (ranging from 0.005 to 23 percent) in 68 percent of 122 adults with newly diagnosed AA [32]. Other studies also reported small PNH clones in up to two-thirds of patients with AA at diagnosis [33,34].

PNH clones are thought to arise and propagate by escaping immune destruction, rather than being excessively proliferative. The loss of GPI (glycosyl phosphatidylinositol) anchors in PNH facilitates cell survival by one of several potential mechanisms, including loss of costimulatory molecules needed to trigger CD8-positive T cell-mediated stem/progenitor cell death, impairment of natural killer (NK) cell- and NK/T cell-mediated immune destruction and reduced MHC (major histocompatibility complex) class I and II antigen presentation.

●MDS/AML – Cytogenetic, molecular, or immunophenotypic changes in HSCs and/or progenitor cells that are common in MDS and AML may cause hypoplasia by immune destruction/suppression by T cells. A subset of patients with MDS has hypoplastic bone marrow, which shares some features with AA. (See "Treatment of lower-risk myelodysplastic syndromes/neoplasms (MDS)", section on 'Likely to respond to immunosuppression'.)

Chromosomal abnormalities or mutations that are characteristic of MDS are observed in a minority of patients with AA (5 to 15 percent) [35]. Dysplastic HSCs of MDS may be subject to immune destruction/suppression by T lymphocytes and lead to bone marrow hypoplasia that is characteristic of AA.

Examples of cytogenetic abnormalities associated with AA include:

●Mutations – The most commonly mutated genes in adults with acquired AA include DMNT3A, ASXL1, BCOR, BCORL1, and PIGA [35].

Some of these mutations are the same as those seen in hematopoietic cells of healthy older individuals with clonal hematopoiesis of indeterminate potential (CHIP) [36]. The pattern in pediatric patients is similar, although mutations in genes such as DNMT3A (that are more closely associated with CHIP) are not seen in pediatric patients. Up to 10 percent of pediatric patients acquire specific gene mutations that lead to loss of expression of specific HLA-A or HLA-B alleles [37]. (See "Clonal hematopoiesis of indeterminate potential (CHIP) and related disorders of clonal hematopoiesis", section on 'Clonal hematopoiesis of indeterminate potential (CHIP)'.)

●Cytogenetic abnormalities – The most common karyotypic abnormality is acquired uniparental disomy with loss of heterozygosity in the short arm of chromosome 6 (6pUPD; also called copy neutral loss of heterozygosity of chromosome 6p), which occurs in 10 to 15 percent of patients [38]. Notably, 6pUPD clones are distinct from clones that predispose to the development of MDS/AML. 6pUPD leads to the loss of specific MHC/HLA class I alleles on the surface of HSC/progenitor cells in patients with AA. HLA haplotype loss that occurs through 6pUPD is nonrandom and selectively targets the elimination of HLA A and HLA B alleles (eg, HLA B*14:02 and HLA B*40:02) that have been defined as high risk for both the development of AA and for development of clonal hematopoiesis involving HLA loss. 6pUPD occurs in patients with AA of all races/ethnicities, but the alleles targeted for loss vary among race/ethnic backgrounds [3]. Cytogenetic findings associated with evolution to MDS/AML include abnormalities of chromosomes 7, 8, and/or 13 [35].

CLINICAL MANIFESTATIONS — 

Patients with AA most often present with recurrent infections due to neutropenia, mucosal hemorrhage or menorrhagia due to thrombocytopenia, and/or fatigue and cardiopulmonary findings associated with progressive anemia. Some patients are asymptomatic and present with abnormal blood counts.

Infections in patients with AA are typically bacterial and may include sepsis, pneumonia, skin infections (cellulitis, abscess), and a urinary tract infection [39]. Invasive fungal infection is a common cause of death, especially in subjects with prolonged, severe neutropenia and antibiotic administration.

Some patients present with hemolytic anemia or thrombosis that may suggest coexistent paroxysmal nocturnal hemoglobinuria. (See "Clinical manifestations and diagnosis of paroxysmal nocturnal hemoglobinuria".)

AA must be distinguished from inherited bone marrow failure syndromes (IBMFS). Most patients with an IBMFS are diagnosed in childhood, but these conditions are also diagnosed in adults. Patients with an IBMFS often, but not always, manifest short stature, microcephaly, developmental delay, skin and nail lesions, unexplained organ dysfunction, and/or a family history of hematologic disorders. Distinguishing an IBMFS from AA is discussed below. (See 'Inherited/germline disorders' below.)

EVALUATION — 

Patients suspected of AA because of cytopenias and/or related clinical findings should be evaluated to establish the diagnosis, seek an underlying cause, and distinguish AA from other causes of pancytopenia. A bone marrow examination is required to establish the diagnosis of AA.

Urgency — The urgency of the clinical evaluation and bone marrow biopsy is guided by the depth of cytopenias and the patient's clinical status.

●For critical cytopenias and/or potentially life-threatening complications, such as infections, bleeding, or cardiorespiratory compromise (table 3), the patient should undergo immediate hematology consultation (including a bone marrow biopsy) and hospitalization. Further discussion of the assessment and management of such conditions is provided separately. (See "Approach to the adult with pancytopenia", section on 'Emergencies'.)

●There is less urgency to hospitalize, obtain a hematology consultation, and/or perform an urgent bone marrow examination for patients with milder cytopenias and no clinical complications. In such a setting, close observation and monitoring of blood counts over days or a few weeks may reveal transient cytopenias (eg, due to recent viral infection). However, evaluation should proceed promptly if no improvement is observed and/or cytopenia-associated complications arise.

Clinical evaluation

●History – The history should evaluate the patient for cytopenia-related findings (eg, infections, bleeding, fatigue), and it may provide clues to an underlying cause for cytopenias (eg, exposure to drugs/chemicals, hepatitis, or other viral infections).

In both children and adults, the family history may reveal relatives with cytopenias and/or somatic findings suggestive of an inherited disorder. However, autoimmune diseases also tend to cluster in families of patients with AA; thus, a compelling family history may also be consistent with an autoimmune etiology [40].

●Examination – Physical findings are generally consistent with pancytopenia, especially pallor and petechiae. Physical examination should also assess potential complications of the cytopenias (eg, cardiovascular manifestations, infections). The liver, spleen, and lymph nodes are not typically enlarged in AA; when present, they may suggest an alternative diagnosis.

Unexplained abnormalities of skin, nails, eyes, ears; short stature; or skeletal or genitourinary abnormalities may suggest inherited bone marrow failure syndromes (IBMFS), as discussed below. (See 'Inherited/germline disorders' below.)

Laboratory studies — Routine laboratory studies can provide clues to the cause of pancytopenia.

●Hematology – Complete blood count and differential reveals cytopenias in ≥2 lineages (ie, neutropenia, thrombocytopenia, and/or anemia with reticulocytopenia).

The blood smear typically reveals normocytic red blood cells, but they may be macrocytic or mildly microcytic. Abnormal cells (eg, myeloblasts, atypical lymphoid cells) are not present unless there is an associated hematologic disorder.

●Chemistries – Serum electrolytes, kidney function tests, and liver function tests, including lactate dehydrogenase, may identify complications (eg, hemolysis) or potential causes or associated conditions that account for pancytopenia.

Serum vitamin B12 and red blood cell folate levels should be performed to exclude megaloblastic anemia. (See "Approach to the adult with pancytopenia".)

●Infectious diseases – Neutropenic patients with fever or other clinical manifestations of an infection must be promptly evaluated and empirically treated with antimicrobial agent(s), as discussed separately. (See "Overview of neutropenic fever syndromes".)

Testing for infectious causes of pancytopenia should be guided by the clinical evaluation and laboratory findings. Examples include testing for HIV, hepatitis, and herpes viruses, as discussed above. (See 'Other mechanisms' above.)

Bone marrow examination — Bone marrow aspiration and biopsy are required to diagnose AA, determine its severity, and exclude other causes of pancytopenia.

Findings from a bone marrow examination in a patient with AA include:

●Profound hypoplasia affects all hematologic elements; the marrow space is composed mostly of fat cells and marrow stroma (picture 1).

●Residual hematopoietic cells are morphologically normal, and hematopoiesis is not megaloblastic, but there may be some erythroid dysplasia.

●There is no infiltration of the bone marrow with malignant cells or fibrosis.

Repeat bone marrow examinations may be needed to establish the diagnosis, with the interval between studies informed by the severity of cytopenias (eg, for patients with AA at the time of diagnosis) or when there are major changes in clinical status or blood counts.

Specialized testing — Specialized tests on bone marrow and peripheral blood provide valuable information about AA and possible coexistent disorders or complications.

●Bone marrow testing

•Cytogenetics – Cytogenetic testing evaluates the patient for clonal evolution and coexistent myelodysplastic syndromes/neoplasms (MDS) or acute leukemia. (See 'Clonal evolution' above.)

Cytogenetic testing may yield a nondiagnostic result because of insufficient metaphases. In such cases, fluorescence in situ hybridization (FISH) can be performed to find abnormalities that might be diagnostic for MDS (eg, abnormalities of chromosome 5 and/or 7) [41]. Single-nucleotide polymorphism microarrays can also be used as an alternative method to detect most cytogenetic aberrations [42].

Most patients with AA have normal cytogenetics, but some of the cytogenetic abnormalities seen in AA are not considered adverse or indicative of MDS (eg, chromosomes 7, 8, and/or 13; loss of heterozygosity of chromosome 6p) [43].

•Mutation analysis – Next-generation sequencing using targeted gene panels can identify mutations commonly associated with various myeloid malignancies [44].

The reported incidence of mutations in AA specimens ranges widely (ie, 5 to >70 percent) [40], and the mutations do not distinguish AA from other hematologic disorders. Mutations in DNMT3A, BCOR, BCORL1, and ASXL1 are seen in AA, but they are also seen with MDS, aging populations, and IBMFS [45]. AA generally has smaller clonal populations, with variable allele frequencies (VAF) <10 percent [46], but the VAF for DNMT3A and ASXL1 may increase over time with clonal evolution [47].

•Immunophenotyping – Flow cytometry can also identify clonal evolution, MDS, or acute leukemia.

While paroxysmal nocturnal hemoglobinuria (PNH) is rare in children [48], identifying a PNH clone is important for long-term monitoring and treatment decisions; it indicates immune-mediated AA, thereby excluding an IBMFS.

●Peripheral blood testing

•All children and young adults should have telomere length analysis and a chromosomal stress/breakage test performed from peripheral blood to rule out the diagnoses of telomere biology disorders and Fanconi anemia, respectively.

•Flow cytometry for cell surface CD59 on peripheral blood red blood cells and neutrophils should be performed to detect PNH clones in all patients with AA.

DIAGNOSIS AND GRADING — 

AA should be suspected in a child or adult with unexplained cytopenias.

The diagnosis of AA is confirmed by blood counts and a bone marrow examination, as described above. (See 'Laboratory studies' above.)

AA is graded for disease severity, as described below, to guide management. (See 'Severity grading' below.)

Diagnosis — AA is a clinicopathologic diagnosis that requires both of the following:

●Cytopenias – A cytopenia in ≥2 blood lineages.

•The level of cytopenias varies with the severity of AA. (See 'Severity grading' below.)

•The duration of cytopenias is not defined, but if a potential cause is identified (eg, drug, viral infection), blood counts should be monitored closely over days to a few weeks to determine if the insult is reversible.

●Bone marrow hypoplasia/aplasia without an infiltrative process – The degree of hematopoietic cell hypoplasia varies with disease severity, but no abnormal infiltrate or marrow fibrosis is present.

Decreased hematopoiesis may initially affect one or two lineages disproportionately, but AA is ultimately associated with trilineage hypoplasia [49].

Severity grading — The severity of AA is graded according to blood counts and a bone marrow examination [50,51].

●Severe AA (SAA) – Both of the following:

•Bone marrow – Cellularity <25 percent (or 25 to 50 percent if <30 percent of residual cells are hematopoietic)

•Peripheral blood – ≥2 of the following:

-Absolute neutrophil count (ANC) <500/microL (<0.5 x 10⁹/L) (calculator 1)

-Platelet count <20,000/microL

-Reticulocyte count <60,000/microL; some centers use a threshold of <40,000/microL

●Very severe AA (vSAA) – Fulfills criteria for SAA with ANC <200/microL

●Nonsevere AA – Criteria include:

•Bone marrow – Cellularity as described for SAA.

•Peripheral blood – Cytopenias not fulfilling criteria for SAA or vSAA.

DIFFERENTIAL DIAGNOSIS — 

The differential diagnosis includes other causes of pancytopenia, hypersplenism, leukemias and lymphomas, bone marrow infiltration/fibrosis, inherited/germline bone marrow failure, and other causes.

Reversible marrow suppression — AA must be distinguished from reversible bone marrow suppression.

As examples, predictable, dose-dependent effects of cytotoxic chemotherapy or radiation therapy, overwhelming sepsis, or acute viral infection can cause transient, reversible pancytopenia with hypoplastic bone marrow. Other reversible causes of cytopenias include immunosuppressive agents, anti-inflammatory medications, certain antibiotics, and excessive alcohol consumption.

Sepsis syndromes are usually associated with leukocytosis, but severe inflammation can also cause cytopenias by transient suppression of bone marrow and other mechanisms. (See "Sepsis syndromes in adults: Epidemiology, definitions, clinical presentation, diagnosis, and prognosis".)

Reversible causes of cytopenias are distinguished from AA by history, laboratory studies (eg, microbiologic and serologic testing), and serial blood counts. Reversible conditions should demonstrate improved blood counts in days to weeks. Bone marrow examination is generally not required.

Megaloblastic anemia — Megaloblastic anemia due to pernicious anemia or malnutrition can cause profound pancytopenia and bone marrow hypoplasia, most often due to a deficiency of vitamin B12 and/or folate.

Megaloblastic anemia is distinguished from AA by the presence of hypersegmented neutrophils and macro-ovalocytes on the blood smear and megaloblastic changes in the bone marrow. Serum levels of vitamin B12, methylmalonic acid, and/or folate confirm these diagnoses. (See "Clinical manifestations and diagnosis of vitamin B12 and folate deficiency".)

Hypersplenism — Hypersplenism refers to cytopenias due to blood sequestration/redistribution to an enlarged spleen.

Splenomegaly may be caused by liver cirrhosis, portal vein thrombosis, bone marrow infiltrative disorders, and hairy cell leukemia (HCL). Splenomegaly alone rarely causes the degree of cytopenias seen with AA or infiltrative bone marrow disorders.

Causes of hypersplenism are assessed by clinical evaluation and imaging. A bone marrow examination is not routinely required, but if performed, it is expected to reveal adequate or increased hematopoietic activity.

Malignancies — Pancytopenia in children and adults can be caused by various malignancies and infiltrative disorders.

These conditions are generally distinguished from AA by the clinical evaluation, screening blood studies, and bone marrow examination. Details of the evaluation and diagnosis of these disorders are discussed in disease-specific topics.

Malignant causes of pancytopenia include:

●Myeloid malignancies – Myelodysplastic syndromes/neoplasms (MDS) and acute myeloid leukemia (AML) often present with pancytopenia.

Many cases of myeloid malignancies demonstrate characteristic abnormalities in the blood smear, such as dysplastic cells with MDS and circulating blasts or other immature forms with AML. These malignancies are distinguished from AA by morphology, immunophenotype, and cytogenetic/molecular abnormalities.

Details of the diagnosis and classification of myeloid malignancies are presented separately:

•(See "Acute myeloid leukemia: Clinical manifestations, pathologic features, and diagnosis".)

•(See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)".)

●Lymphoid malignancies – Lymphoid leukemias and lymphomas can manifest cytopenias, but they are distinguished from AA by clinical evaluation and findings from the blood smear and bone marrow examination.

Examples include:

•Acute lymphoblastic leukemia (ALL)/lymphoblastic lymphoma (LBL) may reveal blasts on the blood smear; bone marrow morphology, immunophenotype, and cytogenetic/molecular studies confirm the presence of lymphoblasts. Imaging may reveal lymphadenopathy, organomegaly, or a tumor mass. ALL/LBL is more common in children but is also encountered in adults. Details of the diagnosis of ALL/LBL are presented separately. (See "Overview of the clinical presentation and diagnosis of acute lymphoblastic leukemia/lymphoma in children" and "Classification, cytogenetics, and molecular genetics of acute lymphoblastic leukemia/lymphoma".)

•Various lymphomas can cause cytopenias by infiltration of bone marrow, hypersplenism, and/or immune mechanisms. Clinical evaluation; imaging; and biopsy of a lymph node, bone marrow, or other involved sites distinguish lymphoma-associated pancytopenia from AA, as discussed separately. (See "Clinical presentation and initial evaluation of non-Hodgkin lymphoma" and "Overview of Hodgkin lymphoma in children and adolescents".)

•Large granular lymphocyte (LGL) leukemia is a clonal disorder of LGLs in blood and bone marrow, splenomegaly, and cytopenias (usually neutropenia but occasionally pancytopenia) [52,53]. LGLs are larger than most circulating lymphocytes, and they usually have characteristic azurophilic granules (picture 2). The evaluation and diagnosis of LGL leukemia are discussed separately. (See "Clinical manifestations, pathologic features, and diagnosis of T cell large granular lymphocyte leukemia".)

•HCL is a rare, indolent lymphoid malignancy that often presents with cytopenias. It is characterized by the accumulation in blood, bone marrow, and spleen of neoplastic B cells with abundant cytoplasm and "hairy" projections. The evaluation and diagnosis of HCL are discussed separately. (See "Clinical features and diagnosis of hairy cell leukemia".)

Infiltrative disorders — Pancytopenia can be caused by the infiltration of bone marrow by malignancies or infectious diseases.

●Myelofibrosis – Myelofibrosis refers to the infiltration of bone marrow by fibrosis that interferes with hematopoiesis.

Bone marrow fibrosis may be due to primary myelofibrosis (PMF), a BCR::ABL1-negative myeloproliferative neoplasm (MPN) that often presents with abnormalities on the blood smear, thrombotic events, splenomegaly, and extramedullary hematopoiesis. The evaluation and diagnosis of PMF are discussed separately. (See "Clinical manifestations and diagnosis of primary myelofibrosis".)

Myelofibrosis can also be caused by the progression of another subtype of MPN, rare cases of AML, HCL, metastatic cancers, and autoimmune disorders (eg, systemic lupus erythematosus, sarcoidosis, scleroderma, mixed connective tissue disease, polymyositis). The evaluation and diagnosis are discussed separately. (See "Clinical manifestations and diagnosis of primary myelofibrosis", section on 'Differential diagnosis'.)

●Infectious causes – Various infectious diseases can cause cytopenias by infiltration of bone marrow.

Examples of infections that may cause cytopenias include tuberculosis and other mycobacterial disorders, brucellosis, leishmaniasis, and others.

Bacterial sepsis can transiently suppress bone marrow, as discussed above. (See 'Reversible marrow suppression' above.)

Inherited/germline disorders — AA must be distinguished from various inherited (germline) bone marrow failure syndromes (IBMFS).

IBMFS are mostly seen in children, but they are increasingly recognized in adults. More than 25 percent of children and 5 to 15 percent of adults ≤40 years who present with AA have an inherited etiology [40].

Suspicion for an IBMFS should be increased in patients with a personal or family history of somatic abnormalities (eg, short stature, skeletal abnormalities, skin/nail lesions, idiopathic pulmonary fibrosis) or relatives with unexplained blood disorders or myeloid malignancies [54,55]. Many patients with an IBMFS manifest somatic findings, but these may be subtle or entirely absent in some individuals. Characteristic somatic findings and family history of an IBMFS are absent in up to 40 percent of cases [56].

A history and physical examination suggestive of an IBMFS warrant a dedicated work-up by an expert clinician or genetic counselor [40].

Patients with an IBMFS require distinctly different management from AA. Many require special surveillance for hematologic and/or nonhematologic malignancies, organ dysfunction, and other complications. They may require reduced doses or different treatments for malignancies or hematopoietic cell transplantation (HCT). An evaluation of potential related transplant donors must ensure that the donor does not have the same genetic condition. In most cases, patients with an IBMFS are expected to have poor responses to immune suppression therapy.

The evaluation and diagnosis of patients with an IBMFS are discussed separately. (See "Familial disorders of acute leukemia and myelodysplastic syndromes".)

Specific inherited or germline disorders that cause pancytopenia are discussed separately:

●Fanconi anemia (FA) – FA is characterized by pancytopenia, physical abnormalities (eg, short stature, microcephaly, developmental delay, café-au-lait skin lesions, other characteristic malformations), and predisposition to malignancies. Diagnosis is usually made in childhood, but some individuals are not diagnosed until adulthood because of variable disease manifestations [57].

Numerous inherited or germline pathogenic gene variants can cause FA, and the specific subtype may affect the phenotype, complications, and management. The evaluation, diagnosis, and classification of FA are discussed separately. (See "Clinical manifestations and diagnosis of Fanconi anemia", section on 'Clinical features'.)

●Telomere biology disorders (TBD) – TBDs, including dyskeratosis congenita (DC), are associated with short telomeres that can lead to bone marrow failure. TBDs can present with cytopenias, somatic abnormalities, organ dysfunction, and/or various malignancies.

DC is associated with characteristic skin and nail findings (picture 3), somatic abnormalities (table 4), pulmonary fibrosis, and cancer predisposition, but many affected individuals manifest only some of these findings. The age of presentation is variable. Other TBDs are associated with specific clinical manifestations and/or complications.

The evaluation, diagnosis, and classification of patients with TBDs are discussed separately. (See "Telomere biology disorders, including Dyskeratosis congenita".)

●Shwachman-Diamond syndrome (SDS) – SDS classically presents in childhood with neutropenia, exocrine pancreatic dysfunction, and skeletal anomalies. However, many affected patients present with only one or two of these features, or they may be asymptomatic and detected only by genetic testing [58].

SDS can manifest pancytopenia, with neutropenia usually the most prominent hematologic manifestation. The pathogenesis, evaluation, and diagnosis of SDS are discussed separately. (See "Shwachman-Diamond syndrome".)

●Other categories of IBMFS – Other inherited or germline disorders can present with pancytopenia and/or various myeloid malignancies or lymphoid leukemias.

Congenital amegakaryocytic thrombocytopenia (CAMT), which is caused by pathogenic gene variants of thrombopoietin or its receptor (MPL) [59,60], manifests thrombocytopenia and bleeding at birth and often progresses to bone marrow failure.

Other inherited or germline disorders that may be associated with cytopenias and other hematologic manifestations and cancers are discussed separately. (See "Familial disorders of acute leukemia and myelodysplastic syndromes".)

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Bone marrow failure syndromes".)

INFORMATION FOR PATIENTS — 

UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient education" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Aplastic anemia (The Basics)")

SUMMARY

●Description – Aplastic anemia (AA) is a bone marrow failure (BMF) disorder characterized by pancytopenia with bone marrow hypoplasia/aplasia caused by the loss or suppression of hematopoietic stem and progenitor cells (HSPCs). Transient, reversible cytopenias from medications, infections, or other causes are not AA.

●Epidemiology – AA is rare; there is a bimodal age distribution at presentation, with one-half of cases in the first three decades of life and a second peak at age ≥60 years. (See 'Epidemiology' above.)

●Pathophysiology – AA is associated with immune destruction or the suppression of HSPCs by T cell-mediated effects and/or inflammatory cytokines. Some cases are triggered by infections, medications, chemicals, or mutations that lead to autoimmune destruction. (See 'Pathophysiology' above.)

●Presentation – Most patients present with cytopenia-related symptoms (eg, infections, bleeding, fatigue), but some are initially asymptomatic. (See 'Clinical manifestations' above.)

●Evaluation – Severe cytopenias and/or clinical complications (table 3) require prompt hospitalization, hematology consultation, and management. Others may be closely monitored before definitive evaluation. (See 'Urgency' above.)

•Clinical – History and physical examination for findings of cytopenias and comorbidities. (See 'Clinical evaluation' above.)

•Laboratory – Complete blood count/differential, reticulocyte count, review of blood smear, and serum liver and kidney function tests are routine. Testing for vitamin B12, folate, infectious diseases, and others are performed as clinically indicated. (See 'Laboratory studies' above.)

•Bone marrow – Bone marrow aspirate and biopsy are required to diagnose AA, determine its severity, and exclude other causes of pancytopenia. (See 'Bone marrow examination' above.)

•Specialized testing – Cytogenetic and molecular testing of a bone marrow specimen to evaluate clonal evolution and coexistent myelodysplastic syndromes/neoplasms (MDS) or acute leukemia. Immunophenotyping by flow cytometry evaluates for cell clones of paroxysmal nocturnal hemoglobinuria (PNH), which may arise from AA. (See 'Specialized testing' above.)

●Diagnosis – AA should be suspected in a child or adult with unexplained cytopenias.

AA is a clinicopathologic diagnosis that requires both of the following (see 'Diagnosis' above):

•Cytopenias in ≥2 blood lineages

•Bone marrow hypoplasia/aplasia without an infiltrative process

●Severity grading – Severity is graded by blood counts, reticulocytes, and bone marrow hypoplasia, as follows (see 'Severity grading' above):

•Severe AA (SAA)

-Bone marrow cellularity <25 percent (or 25 to 50 percent if <30 percent of residual cells are hematopoietic)

Plus ≥2 of the following:

-Absolute neutrophil count (ANC) <500/microL (<0.5 x 10⁹/L)

-Platelets <20,000/microL

-Reticulocyte count <60,000/microL

•Very severe AA (vSAA) – Fulfills criteria for SAA with ANC <200/microL.

•Nonsevere AA – Bone marrow cellularity as described for SAA, with cytopenias not fulfilling criteria for SAA or vSAA.

●Differential diagnosis – Clinical evaluation, laboratory studies, bone marrow examination, and/or specialized testing are used to exclude transient cytopenias and other disorders, including:

•Transient effects of drugs, radiation, infections, and other agents. (See 'Reversible marrow suppression' above.)

•Vitamin B12 or folate deficiency. (See 'Megaloblastic anemia' above.)

•Splenomegaly due to liver cirrhosis, portal vein thrombosis, bone marrow infiltrative disorders, and hairy cell leukemia. (See 'Hypersplenism' above.)

•MDS, acute myeloid leukemia, acute lymphoblastic leukemia, other leukemias, and lymphomas. (See 'Malignancies' above.)

•Bone marrow infiltration from myelofibrosis, cancers, and certain infectious diseases. (See 'Infiltrative disorders' above.)

•Inherited or germline disorders that cause cytopenias are seen mostly in children, but also in adults. There may be somatic abnormalities (eg, short stature, skeletal abnormalities, skin/nail lesions), unexplained blood disorders, organ dysfunction, and/or cancers in the patient or relatives. (See 'Inherited/germline disorders' above.)

ACKNOWLEDGMENT — 

The editors of UpToDate acknowledge the contributions of Stanley L Schrier, MD as author on this topic, his tenure as the founding Editor-in-Chief for UpToDate in Hematology, and his dedicated and longstanding involvement with the UpToDate program.