Corticoid Therapy in Acute Myocarditis

Inclusion Criteria: * Age ≥ 18 years * Written signed informed consent * Affiliation to the French health care system or to another social protection scheme with the exception of State Medical Aid * Active myocarditis defined by (all items are required): * Acute chest pain and/or unexplained heart failure and/or syncope and/or sustained ventricular arrhythmias and/or aborted sudden death and/or cardiogenic shock and/or ECG modification (atrioventricular block or bundle branch block or sinus arrest or ST or T waves change or ventricular arrhythmia or atrial fibrillation or abnormal Q waves) * And troponin rise (1,5 times the normal range) * And diagnosis of active myocarditis on Cardiac Magnetic Resonance (according to Lake-Louise criteria) or by histological evidence on endomyocardial biopsy (Dallas's criteria) * Left-ventricular dysfunction defined as LVEF \< 50% and/or GLS \< -16% assessed with 2D-TTE * Normal coronary angiography or CT Scan (without stenosis \> 50%) during the previous year Exclusion Criteria: * Active coronary disease * Other causes of chronic heart failure (coronary artery disease, primary valvular heart disease, congenital heart disease) * Other etiology of myocarditis requiring corticosteroids treatment as giant cells myocarditis, eosinophilic myocarditis and cardiac sarcoidosis or immune checkpoint inhibitor myocarditis * Other auto-immune or inflammatory disease requiring corticosteroids treatment within 6 months before enrolment * Pregnancy or breastfeeding * Woman of childbearing potential without effective method of birth control (included contraceptive methods with a failure rate of \< 1% per year include bilateral tubal ligation, male sterilization, established proper use of hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices) * Patient deprived of liberty or under Curatorship/Tutorship, safeguard of justice, according to French law * Foreseeable inability, according to the investigator, to participate in all the visits, treatments and measures provided for in the protocol * Patient not speaking or understanding French * Concomitant participation in another clinical trial on medical product for human use, to a clinical investigation on a medical device, to interventional study involving human participants or in the exclusion period at the end of a previous clinical trial on medical product for human use, a clinical investigation on a medical device, or study involving human participants. Participation in non-interventional research is permitted. * Any medical and/or cognitive condition which limits the ability of participant to participate in study * Contra-indication linked to steroids (Methylprednisolone and Prednisone) according to summary of product characteristics: * Any infectious condition excluding the specified therapeutic indications of Methylprednisolone and Prednisone * Certain evolving viruses (notably hepatitis, herpes, chickenpox, shingles) * Psychotic states not yet controlled by treatment * Recent live vaccines or live attenuated vaccines in patients receiving dosages greater than 20 mg/day of prednisone equivalent for more than two weeks and during the 3 months following the cessation of corticosteroid therapy (risk of generalized vaccine disease possibly fatal) * Hypersensitivity to the active substances or to any of the excipients * Contra-indication linked to auxiliary drugs according to respective summary of product characteristics: * Beta-blockade * Angiotensin-converting-enzyme inhibitor (ACE-I) * Angiotensin receptor blockers (ARB) * Mineralocorticoid antagonists (MRA) * Angiotensin receptor-neprilysin inhibitor (ARNi)