MYTHS-MR Trial (MYocarditis THerapy With Steroids in Patients With Mildly Reduced Ejection Fraction)

Inclusion Criteria: * LVEF\<50% and LV-EDD\<56 mm (parasternal long-axis view) on echocardiogram; * Increased troponin (3x URL) at the time of randomization; * Clinical onset of cardiac symptoms within 3 weeks from randomization; * Excluded coronary artery disease by coronary angiogram in subjects ≥46 years of age, in case myocarditis is not histologically proven; * Randomization within 120 hours from hospital admission. * Endomyocardial biopsy (EMB) is not considered necessary before randomization and performing EMB is based on the decision of the local team. Exclusion Criteria: * Known systemic autoimmune disorder or other conditions at the time of randomization where immunosuppression is assumed useful. Patients in whom a systemic autoimmune disorder will be diagnosed during hospitalization will be included in the study if randomized, including patients with a diagnosis of cardiac sarcoidosis or GCM). Both patients included in the corticosteroids-treatment arm or in the placebo-treatment arm can receive the standard immunosuppressive therapy used in the center since the diagnosis; * Patients already on oral/IV chronic corticosteroid therapy or other chronic immunosuppressive therapies (colchicine or nonsteroidal anti-inflammatory drugs \[NSAIDs\] are not considered immunosuppressive drugs); * Contraindication to corticosteroids, including allergies to this medication and its excipients; * Patients with persistent peripheral eosinophilia (persistent eosinophil count \>7% of the leukocytes) or known hypereosinophilic syndrome at the time of randomization. Patients in whom eosinophilic myocarditis will be diagnosed on EMB will be included in the study if already randomized. Both patients included in the corticosteroids-treatment arm or in the placebo-treatment arm can receive the standard immunosuppressive therapy used in the center since the diagnosis; * Myocarditis associated with the ongoing administration of anti-cancer immune checkpoint inhibitor (ICI) agents; * Previously known chronic cardiac (i.e., previous cardiomyopathy, that does NOT include previous myocarditis if there is a functional recovery at the time of screening); * Evidence of active bacterial or fungal infectious disease (presence of fever or increased C-reactive protein are not considered exclusion criteria), or suspected bacterial/fungal infection associated with increased levels of procalcitonin (cut-off \>10 ng/mL), if the laboratory exam is available in the center; * Known chronic infective disease, such as HIV infection or tuberculosis; * Out-of-hospital cardiac arrest; * Echocardiographic presence of images suggestive of other cardiac diseases (i.e. endocarditis) * Participants involved in another clinical trial; * Pregnant women (known pregnancy) or POSITIVE human chorionic gonadotropin (HCG) test measures (urine/blood) for women of 18-50 years of age. * Any other significant disease with expected life expectancy \<12 months (i.e., evidence of irreversible severe brain injury) or disorder which, in the opinion of the Investigator, may either put the participants at risk because of participation in the trial, or may influence the result of the trial, or the participant's ability to participate in the trial. * If LVEF\<41%, an N-terminal pro-B-type natriuretic peptide (NT-proBNP) concentration of 1600 pg/mL or more or a B-type natriuretic peptide (BNP) concentration of 400 pg/mL or more; (if LVEF 41%-\<50% any NT-proBNP or BNP concentration is allowed).