Characteristics and Outcomes of Multisystemic Inflammation in Patients Clinically Diagnosed With Non-COVID-19-Related Fulminant Myocarditis.

Background: In patients with COVID-19-related fulminant myocarditis (COVID-FM), multisystem inflammatory syndrome (MIS) influences clinical outcomes. Multisystemic inflammation (MSI) based on clinical symptoms correlates with the prognosis of non-COVID-FM diagnosed by endomyocardial biopsy. The effect of MSI on clinically diagnosed non-COVID-FM regarding age and prognosis remains unclear. Methods and Results: This retrospective cohort study included 736 patients clinically diagnosed with non-COVID-FM. MSI was defined by clinical symptoms at admission aligned with MSI criteria. We compared all-cause death according to MSI status. Baseline balance was adjusted using inverse probability treatment weighting (IPTW). Overall, 275 (37.4%) non-COVID-FM patients met the MSI status. The MSI (+) group was younger, with fewer male patients and complications such as hypertension, diabetes, chronic kidney disease, and dyslipidaemia. When stratified per age, MSI was highest in patients aged <50 years (48.7%). Overall, the probability of all-cause death was lower in the MSI (+) group (30-day: 25.1% vs. 32.1%, P=0.053, 3-year: 36.1% vs. 42.5%, P=0.004). After IPTW, the MSI (+) group, especially in patients <50 years, was associated with significantly lower risk difference and risk ratio for 30-day all-cause death compared to the MSI (-) group, but not 3-year all-cause death. Conclusions: In patients with non-COVID-FM, MSI often developed at a younger age, and was associated with lower 30-day mortality rate, particularly in patients <50 years.