ECGs are virtually always abnormal in children with myocarditis, but a normal ECG does not rule out the possibility of the disease. Nonspecific and nonsensitive findings include low QRS voltages throughout (<5 mm), ST segment and T wave changes (e.g., decreased T wave amplitude), prolongation of the QT interval, evidence of ventricular or atrial enlargement, and arrhythmias (especially premature contractions or first- or second-degree AV block).

MRI can accurately assess left ventricular ejection fraction, chamber size, and wall thickness. It also can document the location and extent of inflammation, areas of inflammatory hyperemia and edema, myocyte necrosis, and scarring. T2-weighted imaging (Fig. 7.2) has been shown to demonstrate a significant increase in the mean T2 myocardial/skeletal muscle signal intensity ratio when compared with patients who did not have evidence of myocarditis on biopsy. The quantitative analysis of signal intensity is more sensitive with MRI, but sensitivity is limited in less severe cases.

Cardiac MR perfusion exams can show hyperemia and capillary leak that allow gadolinium contrast enhancement in the first minutes of T1-weighted turbo spin-echo imaging (Fig. 7.3a). Delayed hyperenhancement in the subepicardial or transmural areas suggests the presence of myocardial fibrosis and irreversible injury (Fig. 7.3b). This finding can differentiate from ischemic injury, which is always subendocardial. Delayed hyperenhancement of the midwall is also seen with dilated cardiomyopathy, which is frequently associated with myocarditis.

Large pediatric studies are not yet available to determine the sensitivity and specificity of MR. Though MRI is noninvasive, infants and small children usually require sedation and intubation, a consideration for patients with significant myocardial dysfunction who have minimal hemodynamic reserves.

Transthoracic echocardiogram is often the first test and the most common test utilized to assess left ventricular structure and function, but it has low sensitivity and specificity for diagnosing myocarditis. The most common finding associated with myocarditis is a dilated cardiomyopathy phenotype of left ventricular dilatation, impaired left ventricular function, and reduced ejection fraction.

Other findings may include changes in left ventricular geometry, including dilatation or thickening, segmental wall motion abnormalities, and pericardial effusions suggestive of myopericarditis. Color Doppler may demonstrate mitral regurgitation. An echocardiogram may help rule out noninflammatory cardiac disease such as anomalous origin of the left coronary artery from the pulmonary artery (ALCAPA), which may have a similar presentation to heart failure .

Initial testing is generally focused on determining the presence and severity of cardiac dysfunction, and often includes ECG, cardiac biomarkers, chest radiography, and echocardiography. The best imaging tool is cardiac MR. We advise a protocol that evaluates cardiac function with T2-weighted myocardial imaging, with early and late gadolinium imaging, looking for increased T2 signal of the myocardium and delayed gadolinium enhancement of the midwall and possibly the epicardium (Fig. 7.4). Echocardiography can be used for frequent reassessment of the left ventricular structure and function.

In patients with symptoms of acute heart failure , echocardiogram or cardiac MR can be used to differentiate between myocarditis and these disorders:

The differential diagnosis for patients with symptoms of respiratory distress includes pneumonia and bronchiolitis. Chest radiograph and normal ECG can confirm the diagnosis.

Pericarditis (especially of bacterial origin) can be an associated diagnosis.