Abnormal myocardial perfusion-fibrosis pattern in sickle cell disease assessed by cardiac magnetic resonance imaging

Evaluation of cardiac fibrosis and subclinical cardiac changes in children with sickle cell disease using magnetic resonance imaging, echocardiography, and serum galectin-3

BACKGROUND

Myocardial fibrosis has recently been proposed as one of the contributing factors to the diverse pathogenicity of cardiomyopathy in sickle cell disease.

OBJECTIVE

In this study, cardiac fibrosis and subclinical cardiac changes in children with sickle cell disease were evaluated using cardiac magnetic resonance imaging (MRI), tissue Doppler echocardiography and serum galectin-3.

MATERIALS AND METHODS

The study included 34 children with sickle cell disease who were compared with a similar number of healthy controls. Cardiac MRI was used to evaluate late gadolinium enhancement, native T1 mapping, extracellular volume, and T2* for estimation of iron load. Cardiac function and myocardial performance index (MPI, evaluated by tissue Doppler echocardiography) and serum galectin-3 were compared to controls.

RESULTS

The mean age of the included patients was 13.3 ± 3.2 years. Myocardial iron load by T2* was normal. The mean level of extracellular volume (35.41 ± 5.02%) was significantly associated with the frequency of vaso-occlusive crises (P = 0.017) and negatively correlated with hemoglobin levels (P = 0.005). Galectin-3 levels were significantly higher among cases than controls (P = 0.00), at a cutoff value on the receiver operating characteristic curve of 6.5 ng/ml, sensitivity of 82.5% and specificity of 72.8%. The extracellular volume was significantly higher in cases, with a MPI > 0.4.

CONCLUSION

Diffuse interstitial myocardial fibrosis can be detected early in children with sickle cell disease using T1 mapping and is associated with a high frequency of vaso-occlusive crisis. MPI of the left ventricle and serum galectin-3 are recommended screening tools for subclinical cardiac abnormalities.

Cardiomyopathy in Sickle Cell Disease

Sickle cell disease (SCD) is an inherited disorder that occurs due to point mutation in the beta-globin chain resulting in the production of hemoglobin S that tends to become rigid and sickle-shaped under low oxygen concentration. These sickle-shaped red blood cells (RBCs) obstruct the blood vessels leading to reduced blood flow to the organs, causing ischemia and tissue fibrosis. These sickle RBCs being abnormal in shape are frequently sequestered by the spleen, creating a state of chronic anemia in the body. This chronic anemia leads to a high cardiac output state causing cardiac remodeling. To tackle chronic anemia, patients are frequently treated with blood transfusions that makes them more prone to the risk of iron overload (from newly transfused RBCs and iron release from the RBCs that just got sequestered as well as from volume overload) and volume overload causing left ventricular (LV) dilation. The above-mentioned mechanism of cardiac hypertrophy, along with LV dilation together, makes SCD-related cardiomyopathy unique cardiomyopathy with features of restrictive cardiomyopathy with LV dilation. It is interesting to note here that even though there is a presence of LV dilatation, Systolic dysfunction is very uncommon in SCD-related cardiomyopathy.

Impact of baseline calibration on semiquantitative assessment of myocardial perfusion reserve by adenosine stress MRI

In this study, we sought to investigate the impact of baseline calibration, which is used in quantitative cardiac MRI perfusion analysis to correct for surface coil inhomogeneity and noise, on myocardial perfusion reserve index (MPRI) and its contribution to previously reported paradoxical low MPRI < 1.0 in patients with unobstructed coronary arteries. Semiquantitative perfusion analysis was performed in 20 patients with unobstructed coronary arteries undergoing stress/rest perfusion CMR and in ten patients undergoing paired rest perfusion CMR. The following baseline calibration settings were compared: (1) baseline division, (2) baseline subtraction and (3) no baseline calibration. In uncalibrated analysis, we observed ~ 20% segmental dispersion of signal intensity (SI)-over-time curves. Both baseline subtraction and baseline division reduced relative dispersion of t0-SI (p < 0.001), but only baseline division corrected for dispersion of peak-SI and maximum upslope also (p < 0.001). In the assessment of perfusion indices, however, baseline division resulted in paradoxical low MPRI (1.01 ± 0.23 vs. 1.63 ± 0.38, p < 0.001) and rest perfusion index (RPI 0.54 ± 0.07 vs. 0.94 ± 0.12, p < 0.001), respectively. This was due to a reversed ratio of blood-pool and myocardial baseline-SI before the second perfusion study caused by circulating contrast agent from the first injection. In conclusion, baseline division reliably corrects for inhomogeneity of the surface coil sensitivity profile facilitating comparisons of regional myocardial perfusion during hyperemia or at rest. However, in the assessment of MPRI, baseline division can lead to paradoxical low results (even MPRI < 1.0 in patients with unobstructed coronary arteries) potentially mimicking severely impaired perfusion reserve. Thus, in the assessment of MPRI we propose to waive baseline calibration.

A reappraisal of the mechanisms underlying the cardiac complications of sickle cell anemia

Anemia, hemolysis-driven vasculopathy, and intrinsic myocardial injury have been proposed as predisposing factors to cardiac disease in sickle cell anemia (SCA). The individual impact of these mechanisms on the cardiac features of SCA and the way they influence complications such as sudden death and dysrhythmias have been unclear. Recent findings of an acquired restrictive SCA-related cardiomyopathy, driven by myocardial fibrosis, may explain some of these cardiac features. Given the complexity of cardiac pathology in SCA, using additional parameters to tricuspid regurgitant jet velocity (left atrial volume, diastolic parameters, NT-proBNP) may improve the accuracy of noninvasive screening for cardiopulmonary complications in SCA.

Applications of cardiac magnetic resonance imaging in sickle cell disease

Cardiac magnetic resonance imaging (CMR) has evolved from an effective research tool to a non-invasive clinical modality with versatile applications. The accuracy of volume measurements and functional assessment and the ability to identify unique myocardial tissue characteristics non-invasively are the primary advantages of CMR. The use of CMR in sickle cell disease (SCD) has been limited clinically to myocardial iron assessment. The use of other CMR applications to characterize the cardiac pathology in SCD is slowly emerging but remains limited to research level. In this review, we discuss some of the applications of CMR in studying cardiovascular diseases and its potential uses in SCD for research and clinical purposes.

Sickle cell anemia mice develop a unique cardiomyopathy with restrictive physiology

Cardiopulmonary complications are the leading cause of mortality in sickle cell anemia (SCA). Elevated tricuspid regurgitant jet velocity, pulmonary hypertension, diastolic, and autonomic dysfunction have all been described, but a unifying pathophysiology and mechanism explaining the poor prognosis and propensity to sudden death has been elusive. Herein, SCA mice underwent a longitudinal comprehensive cardiac analysis, combining state-of-the-art cardiac imaging with electrocardiography, histopathology, and molecular analysis to determine the basis of cardiac dysfunction. We show that in SCA mice, anemia-induced hyperdynamic physiology was gradually superimposed with restrictive physiology, characterized by progressive left atrial enlargement and diastolic dysfunction with preserved systolic function. This phenomenon was absent in WT mice with experimentally induced chronic anemia of similar degree and duration. Restrictive physiology was associated with microscopic cardiomyocyte loss and secondary fibrosis detectable as increased extracellular volume by cardiac-MRI. Ultrastructural mitochondrial changes were consistent with severe chronic hypoxia/ischemia and sarcomere diastolic-length was shortened. Transcriptome analysis revealed up-regulation of genes involving angiogenesis, extracellular-matrix, circadian-rhythm, oxidative stress, and hypoxia, whereas ion-channel transport and cardiac conduction were down-regulated. Indeed, progressive corrected QT prolongation, arrhythmias, and ischemic changes were noted in SCA mice before sudden death. Sudden cardiac death is common in humans with restrictive cardiomyopathies and long QT syndromes. Our findings may thus provide a unifying cardiac pathophysiology that explains the reported cardiac abnormalities and sudden death seen in humans with SCA.

Cardiomyopathy With Restrictive Physiology in Sickle Cell Disease

OBJECTIVES

The aim of this study was to identify a unifying cardiac pathophysiology that explains the cardiac pathological features in sickle cell disease (SCD).

BACKGROUND

Cardiopulmonary complications, the leading cause of adult death in SCD, are associated with heart chamber dilation, diastolic dysfunction, elevated tricuspid regurgitant jet velocity (TRV), and pulmonary hypertension. However, no unifying cardiac pathophysiology has been identified to explain these findings.

METHODS

In a 2-part study, we first examined patients with SCD who underwent screening echocardiography during steady state at our institution. We then conducted a meta-analysis of cardiac studies in SCD.

RESULTS

In the 134 patients with SCD studied (median age 11 years), significant enlargement of the left atrial volume was present (z-score 3.1, p = 0.002), shortening fraction was normal (37.6 ± 4.7%), and lateral and septal ratios of mitral velocity to early diastolic velocity of the mitral annulus (E/e') were severely abnormal in 8% and 14% of patients, respectively, indicating impaired diastolic function. Both TRV and lateral E/e' correlated with enlarged left atrial volume in SCD (p = 0.003 and p = 0.006, respectively). Meta-analysis of 68 studies confirmed significant left atrial diameter enlargement in patients with SCD compared with controls, evidence of diastolic dysfunction and enlarged left ventricular end-diastolic dimension with normal shortening fraction. The majority of patients with catheter-confirmed pulmonary hypertension had mild pulmonary venous hypertension consistent with restrictive cardiac physiology.

CONCLUSIONS

Patients with SCD have a unique form of cardiomyopathy with restrictive physiology that is superimposed on hyperdynamic physiology and is characterized by diastolic dysfunction, left atrial dilation, and normal systolic function. This combination results in mild, secondary, pulmonary venous hypertension and elevated TRV. Sudden death is common in other forms of restrictive cardiomyopathy. Our finding of this unique restrictive cardiomyopathy may explain the increased mortality rates and sudden death seen in patients with SCD with mildly elevated TRV.