Key points

Introduction

Acquired aplastic anemia (AA) in childhood remains an uncommon, life-threatening disorder. Because of major advances in diagnosis and therapeutic approaches, AA in children is today a disease that results in long-term survival in more than 90% of cases. This article reviews current practices in the diagnosis and therapy for patients with acquired AA, discusses alternative approaches, and assesses the evidence base.

Introduction

Acquired aplastic anemia (AA) in childhood remains an uncommon, life-threatening disorder. Because of major advances in diagnosis and therapeutic approaches, AA in children is today a disease that results in long-term survival in more than 90% of cases. This article reviews current practices in the diagnosis and therapy for patients with acquired AA, discusses alternative approaches, and assesses the evidence base.

Definitions

AA is characterized by peripheral blood pancytopenia and a hypocellular bone marrow without dysplasia or fibrosis ( Fig. 1 ). The degree or severity of AA is defined by peripheral blood cell counts in the presence of a hypocellular bone marrow ( Box 1 ). AA in children is distinct from that in adults; inherited AA is more frequently found in children and the human leukocyte antigen (HLA) association differs, suggesting an age-specific immune pathogenesis. Moreover, there are age-specific differences in treatment, response to treatment, treatment outcome, and late manifestations.

Bone marrow aspirate and biopsy from a patient with acquired AA. Hematopoietic elements are greatly reduced, and there is replacement of marrow space with adipose tissue. Focal islands of left-shifted erythropoiesis (Fig 1: H&E stain, original magnification ×20; Inset: H&E stain, original magnification ×200).

( Courtesy of Dr Michele E. Paessler, DO, Pathology, The Children’s Hospital of Philadelphia.)

Acquired AA must be distinguished from inherited bone marrow failure syndromes (IBMFS) and hypoplastic myelodysplastic syndrome (MDS). IBMFS are more frequent in the pediatric population and comprise roughly 25% to 30% of cases of bone marrow aplasia in children. Distinguishing between acquired AA and IBMFS can be difficult in patients with inherited conditions lacking classic congenital anomalies or in patients without a supporting family history, which can be due to a de novo mutation or a mutation with low disease penetrance. Fig. 2 shows the interrelationship of acquired AA and IBMFS from a genetic viewpoint.

Relationship between genetic mutations, disease penetrance, and gene-environment interaction in the pathogenesis of bone marrow failure. Mutations with a high disease penetrance almost always cause disease; that is, mutations in the Fanconi anemia genes ( FANC ), in the SBDS gene causing Shwachman-Diamond syndrome, or in DKC1 causing X-linked dyskeratosis congenita. By contrast, mutations in genes with low disease penetrance may not manifest as clinically apparent bone marrow failure; examples include mutations in the TERT gene responsible for autosomal dominant dyskeratosis congenita or in certain DBA genes responsible for Diamond-Blackfan anemia. Genetic polymorphisms associated with AA do not cause disease in the majority of carriers but, in combination with other modifier genes and the appropriate environmental insult, may contribute to the development of AA. Examples are HLA-DR2 in adult AA and HLA-B14 in pediatric AA or GSTT1 gene deletions.

( Data from Refs. )

Similarly, hypoplastic MDS can be difficult to differentiate from acquired AA (and IBMFS), especially in children. The new World Health Organization (WHO) classification for myeloid neoplasms distinguishes refractory cytopenia of childhood (RCC) from AA and considers it as a provisional entity of childhood MDS ( Box 2 ). This new WHO classification is becoming increasingly established in Europe and Japan, but its application in North America is still limited. Of clinical importance is that RCC, though classified as a low-risk childhood MDS entity, differs from the current broader concept of MDS in older adults, which is associated with a poor prognosis. Current diagnosis, care, and treatment of AA and RCC are largely the same, so this review does not distinguish between AA and RCC. Box 2 summarizes the histologic and morphologic criteria that differentiate AA from RCC. Prospective future and ongoing studies will determine the clinical significance of the RCC MDS entity.

Epidemiology

Acquired AA is a rare disorder with an incidence of about 2 in 1 million children per year in North America and Europe and a 2- to 3-fold higher incidence in Asia. The peak incidence is in adolescents and young adults as well as in the elderly, with a roughly equal male to female ratio. A classification of AA based on etiology is summarized in Table 1 .

Pathogenesis

For many years, an immune-mediated pathogenesis has been postulated for AA because immunosuppressive therapy (IST) is often successful in the treatment of AA, and bone marrow lymphocytes from AA patients can suppress normal bone marrow in vitro. Results from numerous laboratories have demonstrated increased cytokine expression, low CD4 T regulatory cells, oligoclonal CD8 cytotoxic T cells, and, to a lesser extent, expansion of specific CD4 cell populations in the bone marrow of AA patients. Coupled with the recent finding of acquired copy number–neutral loss of heterozygosity of the short arm of chromosome 6 (6pLOH), representing a likely genetic signature of immune escape, these findings have strengthened the belief that bone marrow aplasia in acquired AA is immune-mediated, replacing the conventional term “idiopathic AA” with “immune-mediated AA” ( Fig. 3 ).

Current evidence suggests that acquired AA results from the aberrant activation of one or more autoreactive T-cell clones caused by alteration of antigens presented by the major histocompatibility complex (MHC) on the surface of antigen-presenting cells (APC). This antigen alteration is triggered by viral infection, chemical exposure, or genetic mutation, and leads to the inappropriate activation of antigen-specific effector T cells and decreased activity of regulatory T cells, which normally serve to prevent autoimmunity. T-cell activation leads to interleukin (IL)-2–driven expansion and differentiation of T cells into effector and memory T cells. These proinflammatory T cells produce a variety of cytokines, including FAS ligand (FASL), interferon-γ (IFN-γ), and tumor necrosis factor α (TNFα), which (1) induce hematopoietic stem cell (HSC) apoptosis and (2) alter gene regulation and decrease protein synthesis to prevent HSC cycling, ultimately leading to bone marrow failure. Immunosuppressive therapy disrupts T-cell–driven HSC destruction by inhibiting T-cell responses at several points along this pathway.

( Data from Young NS, Calado RT, Scheinberg P. Current concepts in the pathophysiology and treatment of aplastic anemia. Blood 2006;108:2509–19; and Shin SH, Lee JW. The optimal immunosuppressive therapy for aplastic anemia. Int J Hematol 2013;97:564–72.)

Clinical presentation

Most children with AA present with signs and symptoms resulting from advanced pancytopenia, with others being diagnosed by incidental laboratory findings. Thrombocytopenia may manifest as easy bruising or petechiae. Epistaxis and menorrhagia in postmenarchal girls are other common complaints at presentation. Anemia may manifest as pallor, fatigue, or exercise intolerance. Neutropenia may predispose to infections and, thus, fever or focal signs of infection can occur as initial complaints. Hepatosplenomegaly and lymphadenopathy are typically absent. A history of jaundice often occurring 2 to 3 months before discovery of pancytopenia is consistent with hepatitis-associated AA.

Establishing the diagnosis

A comprehensive history should include exposure to medications, recreational drugs, and chemicals as well as preceding infectious symptoms. The family history needs to be detailed, and assessed for diseases and signs suggestive of IBMFS ( Table 2 ). A comprehensive laboratory panel is requested to establish the diagnosis of AA, classify its severity, and screen for potential causative factors ( Table 3 ). A bone marrow aspirate and biopsy are needed to establish the diagnosis (see Fig. 1 ). A complete blood count, reticulocyte count, and review of the peripheral blood smear confirm existing cytopenias and exclude signs of dysplasia. Review of the biopsy is required to grade the severity of disease (see Box 1 ). AA can present with a hematopoietic cell clone that already lacks glycosyl phosphatidylinositol (GPI)-anchored proteins characteristic of paroxysmal nocturnal hemoglobinuria (PNH) at the time of diagnosis. Flow cytometry for the absence of GPI-anchored proteins is used to detect an early PNH cell clone. However, because of leukopenia, testing for PNH at diagnosis has poor sensitivity and needs to be repeated when neutrophil counts are recovering. Patients with acquired AA have deceased numbers of regulatory T cells (Treg, CD4 ⁺ CD25 ⁺ T cells). The number of Treg negatively correlates with the severity of disease, and low Treg numbers have been found to be associated with treatment failure. At the Comprehensive Bone Marrow Failure Center of the Children’s Hospital of Philadelphia (CHOP), the authors routinely assess peripheral lymphocytes with chromosome breakage studies to rule out Fanconi anemia (FA), and telomere-length analysis is performed to rule out dyskeratosis congenita (DC). Because these tests may take several days to complete, and because results have a great impact on treatment decisions, it is recommended that these tests be performed early in the evaluation. The initial clinical presentation of patients with either FA or DC may be indistinguishable from that seen in patients with acquired AA. Genetic testing to confirm the diagnosis of DC is ordered when a patient with an aplastic marrow has a telomere length in peripheral blood lymphocytes that is far below the first percentile of that in healthy controls (of note, different criteria are used for dyskeratosis in a nonaplastic patient).

Bone marrow examination usually includes cytogenetic studies, including a karyotype and fluorescence in situ hybridization (FISH) analysis for monosomy 7, trisomy 8, and others as indicated by the karyotype results. The utility of genome-wide single-nucleotide polymorphism (SNP) arrays performed in addition to conventional cytogenetics currently remains investigational, but may be helpful for the early detection of clonal hematopoiesis.

The authors routinely obtain high-resolution HLA typing, including a preliminary donor search at diagnosis in all patients who present with severe AA (SAA) and very severe AA (vSAA) (see Box 1 ), even in the absence of a potential sibling donor. Early HLA typing will expedite an unrelated donor search for patients who are refractory to IST, and will be beneficial for patients in whom the initial investigations reveal an underlying IBMFS diagnosis.

Early therapeutic interventions (IST <4 weeks and bone marrow transplantation [BMT] <12 weeks from presentation) are associated with significantly improved outcomes in acquired AA. Thus, every effort should be made to complete diagnostic evaluations and initiate therapy within 3 to 4 weeks of the initial diagnosis.

Supportive care

Gains in survival for patients with acquired AA are due in part to the improvement in supportive therapy. However, infections and bleeding still remain a major cause of morbidity and mortality in this patient population. In an afebrile patient with a good performance status (Eastern Cooperative Oncology Group/WHO/Zubrod 0–2), the evaluation of AA may be performed in the outpatient setting in a center and by a care team experienced in treating AA patients.

Although there is a lack of strong evidence for most strategies in preventing neutropenic infections, the authors prescribe basic neutropenic precautions for patients with neutrophil counts lower than 500/μL ( Table 4 ). Prophylactic antifungals are routinely used for AA patients with prolonged (>7 days) neutrophil counts of less than 500/μL or for AA patients on IST ( Fig. 4 ). In AA patients with lymphopenia (<500/μL) or those receiving IST, Pneumocystis jirovecii pneumonia (PJP) prophylaxis is provided. Trimethoprim/sulfamethoxazole (cotrimoxazole, TMP/SMX) three times weekly has been shown to be superior to oral dapsone, aerosolized pentamidine, or oral atovaquone in individuals with lymphopenia from human immunodeficiency virus or chemotherapy, although owing to its potential bone marrow toxicity TMP/SMX is frequently abandoned when recovery of AA is delayed. The authors therefore use aerosolized pentamidine as a first-line PJP prophylaxis in children with AA, because it has good PJP protection in this patient population and has very good therapy-patient compliance rates attributable to its monthly dosing.

Flow diagram for antimicrobial prophylaxis and empiric management of fever for patients with severe aplastic anemia, currently used at the Comprehensive Bone Marrow Failure Center, CHOP/University of Pennsylvania. ANC, absolute neutrophil count; ATG, antithymocyte globulin; CBC, complete blood count; CRP, C-reactive protein; CSA, cyclosporine; ID, infectious disease specialist; IV, intravenous; PJP, Pneumocystis jirovecii pneumonia.

( Courtesy of Drs Talene Metjian, PharmD, Brian T. Fisher, DO MSCE, Infectious Diseases, and Shefali Parikh, MD, Hematology, The Children’s Hospital of Philadelphia.)

Granulocyte-colony stimulating factor (G-CSF) alone is not a treatment of AA, and its routine use for patients with AA is controversial. At the authors’ center, G-CSF is given to pediatric AA patients with neutrophil counts lower than 500/μL in combination with IST. Prolonged use of high doses of G-CSF may increase the risk of clonal hematopoiesis and malignant transformation to MDS/acute myelogenous leukemia.

Neutropenic fever requires immediate attention and hospitalization with the initiation of antibiotic therapy according to preestablished hospital guidelines (see Fig. 4 ). For persistent fever or suspected fungal infection, galactomannan testing and a computed tomography scan of the chest are performed, and empiric antifungal agents are started. In life-threatening situations, the use of granulocyte infusions may be considered to provide a bridge between treatment response and neutrophil recovery.

Platelet transfusions should be considered to prevent bleeding in asymptomatic patients with platelet counts lower than 10,000/μL. Higher thresholds for platelet transfusions are reserved for patients with either active bleeding or a history of significant bleeding complications. Higher thresholds (<20,000/μL) are also recommended in patients at risk for worsening thrombocytopenia (eg, febrile patients or those receiving IST).

Transfusion policies in patients with AA are, in general, restrictive. Institutional policies vary; in their practice the authors transfuse red blood cell concentrates for hemoglobin less than 8 g/dL or if symptomatic. Leukodepleted and irradiated blood products should be given to reduce the risk of transfusion-associated graft-versus-host disease (GVHD) and HLA sensitization. Iron chelation is initiated for patients who remain transfusion dependent over a prolonged period. Iron chelation is performed with desferrioxamine or deferasirox. Deferiprone is not recommended for AA patients with iron overload because of the associated risk of agranulocytosis.

Institutional policies and recommendations for vaccinations vary. The authors do not recommend vaccination until 1 year after the cessation of IST, at which time age-appropriate vaccines may be resumed. The use of inactivated vaccines is recommended. Because of potential infectious complications from live attenuated vaccines and the potential risk of AA relapse, live attenuated vaccines are not recommended in this population, although in each case the immunologic benefit of using attenuated live viruses has to be weighed against the potential risk of AA relapse.

Psychosocial support of patients and families and an age-appropriate explanation of disease, treatment, and prognosis are important at diagnosis and during the course of disease, and improve therapy adherence and disease outcomes. Psychosocial support is particularly important at the time of transition from pediatric to adult health care. Often there is a lack of ancillary support services to assist in care transition in the adult-care settings, which may result in a lack of continuity of care and poor therapy compliance.