Severe aplastic anemia (SAA) is a potentially fatal disease characterized by peripheral pancytopenia, coupled with hypoplastic or aplastic bone marrow. If untreated, most patients die within 1-2 years after diagnosis¹⁾. The principal therapeutic interventions for SAA include repeated red blood cell transfusion, immunosuppressive therapy, and hematopoietic stem cell transplantation (HSCT) as the definitive treatment. In children and young adults, HSCT is the initial treatment of choice if a matched sibling donor is available. A recent large retrospective study showed that the 5-year survival rate after HSCT in patients under 20 years old was around 80%²⁾.

There have been few reports regarding the late effects that are associated with systemic organ dysfunction after HSCT in SAA survivors^3,4). Late cardiovascular complications are supposed to be uncommon in this population because SAA patients are rarely exposed to well-known cardiotoxic agents such as anthracyclines, which are widely used in pediatric cancer patients. However, SAA patients require multiple transfusions as a pre-HSCT supportive care, which may inevitably lead to systemic organ iron overload^5,6). Iron overloading in the cardiac tissues can progress to iron overload cardiomyopathy and heart failure, which are the major causes of late death in patients with transfusion-dependent hematologic disease⁷⁾. In addition, HSCT conditioning-related toxicity and HSCT-related complications might adversely affect cardiac function⁴⁾. Long-term HSCT survivors are likely to have an increased risk of premature cardiovascular accidents^8,9). A recent study in children who underwent HSCT showed that subclinical changes in systolic and diastolic function are evident at one year after HSCT, and suggested that serial cardiac function assessment is necessary in all HSCT patients¹⁰⁾.

Left ventricular ejection fraction (LVEF) or fractional shortening (FS) measured by 2-dimensional (2D) echocardiography have been generally used to monitor left ventricular (LV) function and included in long-term follow-up protocols of pediatric cancer survivors. Recently, studies using tissue Doppler imaging (TDI) and 2D speckle-tracking echocardiography (STE) have shown that subclinical LV dysfunctions were not uncommon in this population with normal EF^11,12,13,14). However, there have been no reports on the use of TDI or 2D STE for the LV functional assessments of children who underwent HSCT for SAA.

The purpose of this study was to assess the LV functions in patients who had undergone HSCT for SAA using TDI and STE, and compare them to those of age-matched normal controls. In addition, we evaluated the changes in LV functional parameters from pre- to post-HSCT echocardiographic findings in order to examine the influence of HSCT on cardiac function. We hypothesized that iron overloading associated with multiple transfusion and HSCT-related complications may adversely affect cardiac function.

The demographic and HSCT characteristics of the study cohort are summarized in Table 1. Forty-six patients (28 male patients) who had their first HSCT for SAA at the age of 9.9±3.0 years (range, 1.5-18 years) were included in this study. All patients had the same treatment for pretransplant conditioning. Echocardiography was performed within a median of 23 days (range, 1-61 days) before and 11 months (range, 6-20 months) after HSCT, respectively. No patients had symptomatic LV dysfunction needing medication. After HSCT, acute GVHD developed in 20 patients (43%), and were treated with systemic corticosteroids. The serum hemoglobin level increased significantly after HSCT (8.1±1.5 g/dL before vs. 13.0±1.9 g/dL after, P<0.001). The serum ferritin levels did not change significantly after HSCT (2,068±3,875 before vs. 1,595±2,165 after, P=0.30), and were >1,000 µg/L in 21 patients (46%). No patients had other systemic organ dysfunction needing a special treatment associated with iron overload. Patients with acute GVHD after HSCT had significantly higher serum ferritin levels at the time of echocardiography compared to patients without acute GVHD (2,693±2,898 µg/L vs. 751±585 µg/L, P<0.05).

For the 40 control subjects, the mean (±SD) age and weight at study were 9.6±3.0 years and 36.1±15.8 kg, respectively, and did not differ significantly from the patient group (Table 1).

In this study, we firstly report the LV functional changes measured by STE in patients who underwent HSCT for SAA. Our results showed that LV systolic and diastolic functions based on the STE-derived LV deformation parameters were decreased in children who received HSCT, even with the normal range of LVEF or FS, compared to the control group. Early markers of diastolic dysfunction, the mitral inflow Doppler E velocity, and the E/A ratio also decreased in the patient group. In addition, decreased LV deformation parameters were associated with increased iron load in this population. In the patient group, some LV systolic and diastolic function parameters decreased after HSCT when compared with the pretransplantation assessments.

Patients with SAA suffer from chronic anemia, which can chronically cause a decreased systemic organ oxygen supply. In order to compensate for this, the cardiovascular system increases the myocardial workload and circulatory volume, resulting in LV remodeling including LV dilatation and hypertrophy¹⁵⁾. In this study, patients had larger LV end-diastolic dimensions but the same LV end-systolic dimensions before compared to post-HSCT. After the HSCT, correction of chronic anemia mignt cause reverse remodeling of the LV, resulting in a decrease in the LV end-diastolic dimensions along with a decrease in LVEF¹⁶⁾. Therefore, in terms of LV volumetric functional indices such as LVEF and FS, decreases in the values after HSCT do not necessarily indicate a true decline in the LV function. This might be supported by the fact that LVEF and LVFS were within normal range in all patents after HSCT and were similar to those of normal controls.

However, it is note worthy that patients had decreased STE-derived LV deformation paremeters such as myocardial strain and SR, despite having normal LVEF and SF. The exact mechanism of myocardial dysfunction in this population is quite uncertain. The HSCT involves pre-HSCT conditioning chemotherapy, immunosuppresive therapy, and corticosteroids for the treatment of GVHD, which might adversely affect myocarial function and be potential risk factors for the development of late cardiac complications after HSCT¹⁷⁾. In addition, SAA patients need multiple transfusions for supportive management, which inevitably leads to iron overload in the systemic organs^5,6). Since humans have no mechanism for iron excretion, cumulative iron overload leads to iron toxicity with organ dysfunction and damage. Chronic iron overload in the cardiac tissue can cause myocyte apoptosis, interstitial fibrosis, and mitochondrial dysfunction, which are responsible for cardiac failure¹⁸⁾. Iron overload cardiomyopathy and heart failure is the major cause of late death in patients with transfusion-dependent hematologic disease⁷⁾. In this study, almost half of all patients (46%) had serum ferritin levels of >1,000 µg/L more than 6 months after HSCT. Moreover, serum ferritin levels showed weak but significant correlations with the STE LV myocardial strain and SR. These findings suggest that iron overload in the cardiac tissue is a possible mechanism of myocardial dysfunction in our patients. Therefore, continuous monitoring of cardiac function is necessary in this population.

At our institution, in order to reduce complications associated with iron overload, earlier HSCT is considered in severe cases needing frequent transfusion. After HSCT, a phlebotomy is usually done in patients with iron overload. Otherwise in patients who had not received HSCT, iron-chelating therapy is started if serum ferritin level is higher than 1,000 µg/L. In this study, there were no cardiac complication in need of treatment due to iron overload, but unfortunately we could not accurately check for the other systemic organ involvement.

Echocardiogram is the commonly used imaging tool for assessment of cardiac function. Traditionally, echocardiographic LVEF and/or FS were frequently included in follow-up protocols for monitoring LV function in pediatric cancer survivors, despite limitations such as geometric assumption and load dependency^19,20,21). Recently, 2D STE has been introduced in order to quantify complex cardiac motions. It can assess global and regional ventricular myocardial deformations such as strain, SR, displacement, and velocity in longitudinal, radial, and circumferential directions without angle dependency²²⁾. Additionally, it has been known as clinically useful for evaluating cardiac systolic and diastolic functions, providing new insights for understanding cardiac physiology and mechanics, and identifying early subclinical changes in various pathologies²²⁾. In a study using STE, Cheung et al.¹¹⁾ reported that, despite having LVFS values that were within normal limits, children had decreased LV myocardial deformation after anthracycline therapy. They suggested that the new echocardiographic techniques can detect early subclinical LV systolic dysfunction in patients who are considered having normal LV systolic function using conventional M-mode techniques¹¹⁾. Our previous study involving HSCT among children with acute leukemia also showed that subclinical cardiac dysfunction is obvious in post-HSCT children with normal LVEF and FS¹⁴⁾. In this study, all patients had similar LVEF, SF, and TDI derived MV annular velocities after HSCT, but had significantly decreased STE-derived LV deformation parameters (strain and SR) compared to controls. It is not clear if this early subclinical change in LV deformation function detected by STE may have long-term cardiovascular consequences. Further long-term longitudinal studies are needed to elucidate this. Furthermore, it should be considered to use 2D STE as a routine imaging techniques to assess cardiac function and detect early myocardial dysfunction in this population.

The present study has several limitations. First, this is a retrospective observational study with a relatively small number of patients, which may affect the power of the study. However, considering that SAA is a very rare disease, this number is not so small for a single center study. In addition, a case control study design may partly overcome this limitation. Second, we only used the serum ferritin level as a marker of iron overload. In these patients, the cumulative transfusion units and/or the results of liver magnetic resonance imaging may provide accurate information on iron overloading. Third, a single observer performed the off-line STE analyses, and the intra- or interobserver reliability was not assessed. However, our previous study that involved STE showed excellent, intra- and interobserver reliability with high intraclass correlation coefficients²³⁾. This suggests that STE is a reliable imaging modality for assessing complex LV deformation functions in children.

In conclusion, this is the first study that evaluated LV function in patients who received HSCT for SAA. This study showed that early subclinical myocardial systolic and diastolic dysfunctions, as assessed by STE LV deformation parameters, were evident in this population, despite the presence of normal LVEF or FS. In addition, LV dysfunction is associated with increased serum ferritin levels, which is a marker of iron overload. The 2D STE is a sensitive and practical imaging modality for detecting early subclinical myocardial dysfunction. In order to examine the effects of this early cardiac dysfunction on long-term outcomes, serial quantitative echocardiography using new imaging techniques should be considered for all post-HSCT patients.