MYTHS - MYocarditis THerapy With Steroids

Inclusion Criteria: * Patients admitted to hospital for suspected AM * Age 18 years or older and below 70 years (18-69 years) * Acute HF with clinically suspected acute myocarditis based on 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; * Left ventricular ejection fraction (LVEF)\<41% and left ventricular end diastolic diameter (LV-EDD)\<56 mm (parasternal long-axis view) on echocardiogram; * Increased troponin (3x upper reference limit \[URL\]) 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. 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 giant cell myocarditis (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 of a systemic autoimmune disorder, or cardiac sarcoidosis or GCM; * Patie…