Introduction
Sickle cell disease is the most common hereditary hemolytic anemia in the United States of America and one of the prevalent hereditary hemolytic anemias in sub-Saharan Africa, the Mediterranean region and central India [
1]. Cardiovascular complications are a notable cause of premature deaths in adults with sickle cell disease, and are responsible for approximately 32% of deaths of individuals in their mid-thirties to mid-fifties [
1,
2]. In children, the disease profile has been changed, mostly due to screening programs, vaccine therapy and antibiotic prophylaxis. Consequently, acute severe infections are no longer the main cause of death in children [
3].
The predominant manifestations of the disease are related to vaso-occlusive crises. Repeated episodes of tissue ischemic reperfusion injury, in addition to the effect of hemolysis, result in a chronic vasculopathy with an inflammatory state, which leads to chronic end-organ damage [
1,
3]. Multiple pathological mechanisms might contribute to sickle cell cardiomyopathy, including chronic anemia, recurrent vaso-occlusive crises and iron overload with excess production of reactive oxygen species, in addition to pulmonary, renal and hepatic damage [
4‐
6].
Recent studies have reported cardiac fibrosis in adult patients with sickle cell disease as a novel mechanism for cardiac dysfunction [
7]. Cardiac fibrosis is a complex heterogeneous process that is characterized by excessive accumulation of extracellular matrix in response to pathological stimuli [
8]. Cardiac fibrosis may be replacement fibrosis, which develops in response to myocardial infarction, or reactive fibrosis which develops in response to pressure and volume overload [
9,
10].
The noninvasive detection of myocardial fibrosis is challenging. Cardiac magnetic resonance imaging (MRI) studies using late gadolinium enhancement are currently the gold standard noninvasive method to detect focal myocardial fibrosis. However, focal myocardial fibrosis has only been reported in a few patients with sickle cell disease [
4,
11‐
13]. Late gadolinium enhancement is insensitive to diffuse myocardial fibrosis [
13,
14]. T1 mapping is a new technique that has emerged to overcome the limitations of late gadolinium enhancement in detecting diffuse myocardial fibrosis by quantifying the expansion of the extracellular matrix [
15]. Cardiac MRI measurement of myocardial T1 relaxation times before and after gadolinium administration has been used to quantify the myocardial extracellular volume [
14].
Other noninvasive imaging modalities have been employed to detect myocardial fibrosis including tissue Doppler echocardiography, nuclear imaging and cardiac computed tomography [
16‐
18]. In addition, some laboratory biomarkers can aid in both detection and risk stratification of cardiac fibrosis [
19]. Galectin-3 has been suggested to be a marker for cardiac inflammation and fibrosis as it is readily expressed on the cell surface and secreted by the injured inflammatory cells [
20,
21].
This study evaluated cardiac fibrosis in children with sickle cell disease using cardiac MRI and also aimed to detect the effectiveness of tissue Doppler echocardiography and serum galectin-3 in identifying subclinical cardiac abnormalities indicative of myocardial fibrosis.
Discussion
In the current study, diffuse myocardial fibrosis was found to be a common subclinical feature in children with sickle cell disease. Using late gadolinium enhancement, increased extracellular volume was found in all patients, even in the absence of focal fibrosis. Our results highlight an important and underrecognized mechanism of heart pathology in sickle cell disease that appears to predate the development of cardiac dysfunction. A few studies have reported this abnormality in adult patients and this phenomenon has not been fully studied in children [
7]. Niss et al. used T1 mapping to measure extracellular volume in a cohort study that included 25 adult patients with sickle cell disease. They reported markedly increased extracellular volume in all patients when compared to a control group (mean 44% ± 8% versus 26 ± 2%,
P < 0.0001), indicating diffuse myocardial fibrosis [
13]. Our results are consistent with this, as we found that the mean extracellular volume among children with sickle cell disease was 35.41 ± 5.02%. This value is higher than most previously published extracellular volume values in other fibrotic heart diseases, such as hypertrophic cardiomyopathy [
37,
38]. Similarly, Morin et al. conducted a study that included 26 adolescents with sickle cell disease with a median age of 17.5 years; 96% of them had increased extracellular volume (median 30.5%) despite the absence of focal myocardial fibrosis by late gadolinium enhancement [
39]. Native T1 values in the present study were significantly increased in children with sickle cell disease compared to control group, which matches with the findings reported by Niss et al. (patients, 1,008 ± 67ms vs control subjects, 942 ± 25ms;
P < 0.001) [
7]. The role of anemia in the development of myocardial fibrosis in sickle cell disease is unknown. However, a negative correlation between the extracellular volume and hemoglobin level was observed (
r = − 0.5,
P = 0.005), which is in agreement with the findings of Niss et al. [
13]. Despite high extracellular volume being detected in all of our patients, none had late gadolinium enhancement, which suggests that patients with sickle cell disease may have myocardial fibrosis and ischemia in the absence of major coronary artery disease. In an adult study, Bratis et al. [
40] postulated that repeated vaso-occlusive crises resulted in a reduced vascular bed by impairment of vasoreactivity and reperfusion-induced vascular injury at the microvascular level. In agreement with this hypothesis, extracellular volume was significantly associated with the frequency of vaso-occlusive crises in our patients (
t = − 2.536,
P = 0.017)
. Although blood ferritin levels were high in all patients, the T2* values were normal, which minimized the possibility that cardiac siderosis was the underlying cause of the myocardial alterations in children with sickle cell disease. This is in line with the findings of Junqueira et al. [
41] who reported that patients with sickle cell disease rarely experience cardiac siderosis. This value was normal in our patients. However, theoretically, if there is myocardial iron overload, this will result in a decrease, “not increase,” in native T1 values, as reported by Sado et al. [
42]. In the present study, the extracellular volume was highest among TD patients who required more than six transfusions per year, and lowest for non-TD patients or those who required fewer than three transfusions per year. In the present study, the extracellular volume was significantly associated with the frequency of vaso-occlusive crises, supporting the ischemic-reperfusion hypothesis.
We were interested in evaluating left ventricular function among the patient group using echocardiography as myocardial fibrosis might affect cardiac function at a subclinical level. Combined systolic and diastolic function of the left ventricle was evaluated using the MPI, which was significantly higher in the children with sickle cell disease than in the control group. However, all patients showed normal systolic function on conventional echocardiography. Similarly, Caldas et al. [
43] studied 107 children with sickle cell disease (mean age 10.1 ± 4.7years) and reported higher MPI in the left ventricle for patients than for the control group (
P = 0.00). Other parameters of systolic and diastolic function of the left ventricle on tissue Doppler echocardiography were significantly decreased in patients compared to healthy control children. Eddine et al. [
44] conducted a study of 55 children and young adults with sickle cell disease (6–21 years old) and reported similar results. Ghaderian et al. [
45] conducted a study of 64 Iranian children with sickle cell disease (mean age 11.7 ± 5.5 years) compared to 50 healthy control subjects and reported no significant differences in tissue Doppler echocardiographic systolic, diastolic or MPI of the left ventricle. Surprisingly, some studies have reported no significant difference in echocardiographic variables between adult patients and control subjects, e.g., Dabirian et al. conducted a study of 30 asymptomatic patients ages 18–40 years. Age is not the only risk factor for myocardial injury, there are many other confounding factors, such as vaso-occlusive crises, anemia, transfusion dependence and even variations in genotype–phenotype patterns [
45‐
47]. Interestingly, 44% of the children with sickle cell disease in the current study showed impaired left ventricular relaxation with a restrictive pattern (E/A > 2), which was positively correlated with high values of extracellular volume versus the subgroup with E/A < 2 (36.12 ± 5.08% versus 32.1 ± 3.86%,
P = 0.038). This clarifies the impact of interstitial myocardial fibrosis on diastolic dysfunction and the restrictive pattern of sickle cell disease cardiomyopathy early in childhood. Similar findings were reported by Niss et al. [
15] and Alsaied et al. [
48] in adults with sickle cell disease.
The current study has demonstrated a correlation between the MPI and diffuse cardiac fibrosis, which has not previously been reported. Additionally, increased extracellular volume was correlated with a higher MPI. The mean extracellular volume was significantly higher for the subgroup with MPI > 0.4 (impaired) than the subgroup with MPI < 0.4 (38.1 ± 4.25% versus 34.43 ± 5.02%, respectively, P = 0.03).
Galectin-3, a novel biomarker, has been predicted to play an important regulatory role in cardiac fibrosis and remodeling. In fact, galectin-3 expression appears to begin prior to the onset of heart failure [
49,
50]. There are limited data on serum galectin-3 levels in children and most studies have been conducted on congenital heart diseases. Kotby et al. [
51] studied galectin-3 among children with heart failure and found increased serum galectin-3 in patients compared to control subjects (
P > 0.001) at a galectin-3 cutoff value of 3.5 ng/ml. In the current study, galectin-3 levels were significantly higher among children with sickle cell disease than among healthy control subjects (
P < 0.001). At a cutoff value for galectin-3 of 6.5 ng/ml, the ROC curve showed a sensitivity of 82.5% and a specificity of 72.8%, and patients were discriminated from control subjects. These finding are in agreement with the results of Wagdy et al. [
5], who reported a similar finding among Saudi Arabian children with sickle cell disease at a cut off of 5.1 ng/ml. To our knowledge, no other studies have investigated galectin-3 levels in chronic hemolytic anemia. Of note, the patient subgroup with left ventricular MPI > 0.4 exhibited higher galectin-3 levels than the subgroup with left ventricular MPI < 0.4. However, this difference did not reach statistical significance and needs further study. Finally, it was interesting to assess the effect of the chronicity of sickle cell disease on the heart through subgrouping the patients based on median age. A significantly higher level of galectin-3 was found in subgroup II (age> 13.5 years) than in subgroup I (age< 13.5 years). Higher extracellular volume and MPI were also found in the older subgroup; however, differences were not statistically significant. This may indicate that cardiac alterations linked to sickle cell disease worsen with age, and this might need further wide-scale studies [
52].The tissue Doppler echocardiographic-derived left ventricular MPI played an essential role in the present study in reflecting the process of myocardial fibrosis through its relation to the extracellular volume and serum level of galectin-3.
Our study has some limitations. First, not all controls were subjected to cardiac MRI and we did not administer contrast to the controls who did have cardiac MRI as there is no ethical justification in our institute. This was a single-center study and studies with larger cohorts and longer follow-up are recommended. Children under the age of eight years were not included in our study as younger children usually require anesthesia for MRI scans. Finally, not investigating patients during their vaso-occlusive crises may be considered a limitation.
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