AbstractBackgroundAlthough rare, myocarditis in the pediatric population is a disease process that carries significant morbidity and mortality. Prior to the SARS-CoV-2 related (COVID-19) pandemic, enteroviruses were the most common cause of classic myocarditis. However, since 2020, myocarditis linked to multisystem inflammatory syndrome in children (MIS-C) is now common. In recent months, myocarditis related to COVID-19 vaccines has also been described. This study aims to compare these three different types of myocarditis with regards to clinical presentation, course, and outcomes.MethodsIn this retrospective cohort study, we included all patients <21 years of age hospitalized at our institution with classic viral myocarditis from 2015-2019, MIS-C myocarditis from 3/2020-2/2021 and COVID-19 vaccine-related myocarditis from 5/2021-6/2021. We compared demographics, initial symptomatology, treatment, laboratory data, and echocardiogram findings.ResultsOf 201 total participants, 43 patients had classic myocarditis, 149 had MIS-C myocarditis, and 9 had COVID-19 vaccine-related myocarditis. Peak troponin was highest in the classic myocarditis group, whereas the MIS-C myocarditis group had the highest recorded brain natriuretic peptide (BNP). There were significant differences in time to recovery of normal left ventricular ejection fraction (LVEF) for the three groups: nearly all patients with MIS-C myocarditis (n=139, 93%) and all patients with COVID-19 vaccine-related myocarditis (n=9, 100%) had normal LVEF at the time of discharge, but a lower proportion of the classic myocarditis group (n=30, 70%) had a normal LVEF at discharge (p<0.001). Three months post-discharge, 18 of 40 children (45%) in the classic myocarditis group still required heart failure treatment, whereas only one of the MIS-C myocarditis patients and none of the COVID-19 vaccine-associated myocarditis patients did.ConclusionsCompared to those with classic myocarditis, those with MIS-C myocarditis had more significant hematologic derangements and worse inflammation at presentation, but had better clinical outcomes, including rapid recovery of cardiac function. Patients with COVID-19 vaccine-related myocarditis had similar clinical presentation to patients with classic myocarditis, but their pattern of recovery was similar to those with MIS-C, with prompt resolution of symptoms and improvement of cardiac function. Long-term follow-up should focus on cardiac and non-cardiac consequences of myocarditis associated with COVID-19 illness and vaccination.