MIS-C is a new entity that develops 2–4 weeks after COVID-19. Cardiovascular system manifestations have been reported in as high as 80% of MIS-C patients . Echocardiography is the first-line imaging for the detection of cardiac dysfunction. Depressed left ventricular systolic function and decreased ejection fraction, CAD, aneurysm, and pericarditis have been reported [32, 33]. Capone et al.  evaluated 50 MIS-C patients of which 33 (66%) presented with cardiac manifestations which were LVD in 26 (52%) patients, and CAD in 10 (20%) patients. Theocharis et al.  showed that out of 11 patients with cardiac involvement, 8 (40%) had LV systolic dysfunction, 2 (10%) had left anterior descending coronary artery dilatation, and 1(5%) had right coronary artery dilatation on admission. We observed cardiovascular involvement in 13 (38.2%) patients on admission, which was lower than in the study by Capone et al. . However, in our study, LVD was less frequently observed (12.9%) than in previous studies, with varying degrees of reduction in ejection fraction.
In previous studies, pericardial effusion has been shown in 9–72% of MIS-C patients [19, 35, 36]. Valverde et al.  reported 80 (27.9%) patients with pericardial effusion at admission, 66 (25%) of them being mild, 8 (3%) of them moderate. Pericardial effusion persisted in 20.6% of patients during hospitalization. We detected 4 (11.8%) patients with pericardial effusion on admission; all of them were mild. In the follow-up period, echocardiographic examination showed that pericardial effusion completely recovered in all patients.
A previous study determined 50% mitral regurgitation and 72% pericardial effusion via echocardiography among MIS-C patients on admission, while only 20% had small pericardial effusion and 18% had mild mitral regurgitation at discharge . Valverde et al.  reported the rate of mitral regurgitation and tricuspid regurgitation in a large European MIS-C cohort as 42.5% and 5.9%, respectively. In this study, the initial echocardiographic evaluation showed mild mitral regurgitation in 12 (35.3%) patients and tricuspid regurgitation in 4 (11.8%) patients, and all those patients recovered within 6 months. Mitral regurgitation was less, and tricuspid regurgitation was more frequent in our cohort than in previous studies [19, 36].
The incidence of coronary artery abnormalities varies considerably across studies. Usually, mild or moderately sized coronary artery abnormalities have been reported in 9% to 26.7% of the MIS-C cases, and large/giant coronary artery aneurysms have been reported in several studies [11, 12, 14, 37, 38]. Valverde et al.  showed coronary artery abnormalities in 69 (24.1%) of the 286 patients and giant aneurysm in 1 (0.3%) patient. Coronary artery ectasia (Z score: 2.53 and 2.6 in the right coronary artery) was detected in only two patients (4%) in one of the previous studies and they recovered until discharge . Similar to these studies, coronary artery abnormalities were detected in 14.7% of patients in our study.
Previous studies generally evaluated acute and early cardiac involvement of MIS-C cases [39,40,41,42,43,44,45,46]. Capone et al.  reported early midterm results of cardiac MRI performed in the convalescence phase 2–4 weeks after discharge in 11 of 50 MIS-C patients with initial LVD. They did not determine persistent edema or fibrosis at 8 weeks, despite higher troponin levels during hospitalization (median:37, IQR:9–109, reference < 14 ng/L) in these patients . In two other studies, cardiac MRI was performed in MIS-C patients, usually within the first 1 month, and fibrosis was not reported in the acute phase [20, 47]. Bermejo et al.  performed cardiac MRI (between 12 and 72 days) in 20 of 44 MIS-C patients with a median age of 8 years. They detected small areas of LGE in 2 patients, abnormal mean T1 levels in 1 patient, and normal mean global T2 levels of basal, midventricular, and apical slices in all patients. Higher T2 levels in the apical lateral segment in 1 patient, and basal septal levels were abnormal in 1 patient. Blondiaux et al.  performed MRI 14 days after discharge in four children and found diffuse myocardial edema and hyperemia without evidence of focal myocardial necrosis/fibrosis. Dominguez et al.  performed cardiac MRI in 12 of 37 MIS-C patients with a median age of 8 years and detected myocardial edema in 7 patients, pericardial effusion in 5 patients, and decreased left ventricular function in 3 patients. Valverde et al.  reported T2 hyperintensity in 14 (33.3%) of 42 patients, pericardial effusion in 10 (23.8%), early gadolinium enhancement in 1 (2.4%) and 6 (14.3%) LGE. Studies of cardiac MRI in MIS-C patients are summarized in Table 4. However, cardiac MRI was generally performed in the acute phase, and the number of patients was, respectively, small in these studies [20, 32, 34, 39,40,41,42,43,44,45,46]. Theocharis et al.  detected myocardial edema in 10/20 (50%) patients. Matsubara et al.  reported the cardiac outcomes of 14 of 60 patients with MIS-C. Cardiac MRI was performed in five patients during the subacute phase and nine patients during the following period. Only one of nine patients had residual edema on cardiac MRI . The recent study by Dove et al.  detected late gadolinium enhancement, T1 mapping abnormalities, and abnormal or borderline extracellular volume calculations suggesting myocardial fibrosis in two of 51 patients, and no patient had T2 mapping abnormalities corresponding with edema. In our study, echocardiography did not show any abnormality at the 6th month (except coronary dilatation), whereas cardiac MRI demonstrated cardiac involvement, particularly pericardial effusion. Sixty-one percentage of the patients still had at least one of the following findings: pericardial effusion (45.2%), right ventricular dysfunction (19.4%), and LVEF abnormality (16.7%).
Bermejo et al.  detected abnormal T2 levels correlated with elevated D-dimer on admission. We did not observe any correlations between T2 levels with WBC, ANC, ALC, PLT, monocyte, CRP, procalcitonin, ESR, D-dimer NT-proBNP, and troponin-T (p > 0.05).
Several studies have reported the short-term outcomes of cardiac complications in children with MIS-C. Cardiac complications persisted during firsty and second months at follow-up in two studies [48, 49]. Caro-Paton et al.  showed cardiac complications in 3 (25%) patients. Pouletty et al.  demonstrated mild LV dysfunction persisted in only two of 7 patients admitted to the intensive care unit. A study focused on follow-up of patients with MIS-C determined only one patient with a medium CA aneurysm (Z score 9.8) was stable at the 6th month of initial diagnosis . These reports highlighted that the majority of the cardiac manifestations resolved during the short-term follow-up period. In our study, CAD persisted in 2 (40%) of 5 patients with CAD detected by echocardiography which was higher than the literature.
MIS-C seems to be a severe acute disease with minor complications on the midterm. If we are not going to start treatment for cardiac involvement, performing cardiac MRI in general in young children who need sedation for cardiac MRI data, and in patients with mild and asymptomatic pericardial effusions or ventricular dysfunction may be a topic of discussion. However, we showed abnormalities with cardiac MRI even in asymptomatic patients or without biochemical abnormality. Therefore, we suggest performing cardiac MRI on all MIS-C patients, even echocardiography does not reveal any abnormality.