Recommendation 158: We recommend that myocarditis should be suspected in the following clinical scenarios:

• Cardiogenic shock due to LV systolic dysfunction (global or regional), where etiology is not apparent.
• Acute or subacute development of LV systolic dysfunction (global or regional), where etiology is not apparent.
• Evidence of myocardial damage not attributable to epicardial CAD or another cause (Strong Recommendation, Low Quality Evidence).

Recommendation 159: We recommend referral to a center with experience and expertise in the assessment and management of myocarditis should be considered for patients with suspected myocarditis (Strong Recommendation, Low Quality Evidence).

Recommendation 160: We recommend urgent referral for evaluation/consideration for cardiac transplantation or MCS be considered for patients with myocarditis associated with HF, progressive clinical deterioration or end-organ dysfunction despite standard HF therapy (Strong Recommendation, Low Quality Evidence).

Recommendation 161: We recommend that all patients with suspected myocarditis have CMR where available and in the absence of contraindications (Strong Recommendation, High Quality Evidence).

Recommendation 162: We suggest EMB be considered for patients presenting with a) new-onset (less than two weeks duration) heart failure of undetermined etiology with hemodynamic compromise, b) heart failure and high-grade heart block, c) heart failure with recurrent ventricular arrhythmias or d) heart failure unresponsive to medical therapy (Weak Recommendation, Low Quality Evidence).

Recommendation 163: We recommend best medical therapy, including supportive care for the treatment of myocarditis (Strong Recommendation, Low Quality Evidence).

Recommendation 164: Routine use of general or specific immunological therapies directed toward myocarditis are not recommended, as this has not been shown to alter outcomes, and may lead to side effects or complications (Strong Recommendation, Moderate Quality Evidence).

Recommendation 165: We suggest that treatment with immunosuppressive therapy should be considered in subgroups of patients with myocarditis due to specific underlying etiologies such as giant cell myocarditis, sarcoidosis, myocarditis due to systemic autoimmune disease or biopsy proven myocarditis with undetectable viral infection by PCR (Weak Recommendation, Low Quality Evidence).

Recommendation 166: We recommend that the antiviral therapy should not routinely be used in patients with myocarditis (Strong Recommendation, Low Quality Evidence).

Recommendation 167: We recommend that expert clinical follow-up is required until myocarditis is determined to be resolved or until a chronic management plan is in place (Strong Recommendation, Low Quality Evidence).

• Clinical signs and symptoms of myocarditis may be highly variable.
• Other potential causes of cardiac dysfunction must be ruled out before a diagnosis of myocarditis can be made; additional tests may include cardiac catheterization or CMR, with or without RV biopsy.
• Biomarker and 12-lead ECG findings in patients with myocarditis may mimic those of acute myocardial infarction or acute pericarditis.
• Patients with suspected myocarditis should have troponin I or T and BNP or NT-proBNP measured.
• Treatment with immunosuppressive agents should be implemented by centers/physicians with considerable experience in managing these cases
• Patients with persistence of heart failure symptoms or ventricular dysfunction should be followed in a multidisciplinary heart failure/ function clinic, and referred to specialized centres when appropriate.
• Precise diagnostic criteria for acute myocarditis have not been prospectively validated; however, the criteria consider four major elements in determining the potential for the presence of acute myocarditis. They are:
  1. Symptoms and clinical findings consistent with acute or recent myocardial damage.
  2. Evidence of myocardial injury in the absence of a demonstrable epicardial coronary cause.
  3. Evidence of hyperemia, edema or irreversible injury on CMR images.
  4. Presence of inflammatory cell infiltrate or positive viral genome signal on examination of EMB specimens.
• Evaluation of EMB samples should be performed by an experienced cardiac pathology laboratory. Evaluation of EMB for myocarditis should include the use of histopathological markers of inflammation and necrosis, immunohistochemical markers, and assessment for viral particles.