Systemic Inflammation Response Index Is Associated with the Presence and Extent of Late Gadolinium Enhancement in Acute Myocarditis

Background: Acute myocarditis is characterized by heterogeneous inflammatory myocardial involvement, and cardiac magnetic resonance (CMR)-derived late gadolinium enhancement (LGE) is an important marker of myocardial injury. However, the relationship between systemic inflammatory burden and quantitative CMR-defined myocardial involvement remains insufficiently characterized. We aimed to evaluate the association between inflammatory indices, particularly the systemic inflammation response index (SIRI), and the presence and extent of LGE in patients with acute myocarditis. Methods: This retrospective single-center observational cohort study included clinically suspected acute myocarditis patients who underwent CMR within 4 weeks of symptom onset. Patients were classified according to the presence or absence of LGE on CMR. Inflammatory indices, including the SIRI, systemic immune–inflammation index (SII), multi-inflammatory indices (MII-I and MII-II), and stress hyperglycemia ratio (SHR), were calculated from admission laboratory parameters. Receiver operating characteristic (ROC) analysis, logistic regression, linear regression, and hurdle Poisson regression analyses were performed. Results: LGE was present in 62 patients (36.5%). Patients with LGE demonstrated significantly higher inflammatory markers and inflammatory index values. Among all evaluated biomarkers, the SIRI demonstrated the highest discriminatory performance for LGE detection (AUC: 0.944, 95% CI: 0.897–0.973; cut-off > 1.33; sensitivity 93.5%; specificity 85.2%; p < 0.001). In multivariable logistic regression analysis, the SIRI remained independently associated with LGE presence (OR: 4.538, 95% CI: 2.570–8.011; p < 0.001). The SIRI was also independently associated with both log-transformed LGE percentage and the number of involved myocardial segments. Conclusions: Higher SIRI levels were associated with both the presence and extent of CMR-defined myocardial involvement in patients with acute myocarditis and may reflect the burden of early inflammatory myocardial injury. However, these findings should be interpreted cautiously given the retrospective single-center design and require confirmation in larger prospective multicenter studies before potential clinical application.