Diagnosis and exclusion

IIMs were defined and classified into subtypes of PM and DM based on the 2017 European League Against Rheumatism American College of Rheumatology classification criteria (14). Muscle biopsies were assessed in the neuropathological laboratory of PUMCH with accepted histopathological diagnostic criteria (15). To identify the myocardial injury, the strategy for early detection of heart involvement in patients with IIMs was obeyed (16), followed by excluding other common cardiovascular etiologies. CMR and/or EMB was recommended for certification in uncertain cases. The criteria of EMB to diagnose myocarditis is in accordance with the “Dallas criteria”, which means “inflammatory cells such as lymphocytes infiltrate in myocardium, accompanying with nearby myocardial fiber necrosis or degeneration, or interstitial edema”. In all 31 cases, 5 of them underwent EMB, for uncertain results in CMR and clinical characteristics. These pathologic characteristics were seen in all of the 5 patients with EMB. Meanwhile, to diagnose myocarditis related to IIMs, patient suspected of virus infection was excluded (N=1), then patients who encountered the 2013 ESC diagnostic criteria for clinically suspected myocarditis were involved (N=34) (6). Finally, myocarditis was defined by CMR in accordance with the Lake Louise criteria (at least meeting two of the following criteria): (I) myocardial edema indicated by regional or global myocardial signal intensity increase in T2-weighted images; (II) hyperemia and capillary leak indicated by increased global myocardial early gadolinium-enhancement (EGE) ratio between myocardium and skeletal muscle in gadolinium-enhanced T1-weighted images; (III) necrosis and fibrosis indicated by at least one focal lesion with non-ischemic regional distribution in LGE (7) (e.g., Figures S1,S2​S1,S2).). Thirty-one patients with myocarditis accommitant with IIMs were enrolled in our study (Figure 1).

Schematic workflow of involving patients with IM. Patients underwent CMR for suspected myocarditis. Identification of autoimmune myocarditis related to IM was based on the 2013 ESC Diagnostic criteria and the Lake Louise criteria of CMR values for myocarditis. CMR, cardiovascular magnetic resonance; IM, inflammatory myopathy.

A 16-year-old female with DM and myocarditis. (A) is T2-weighted image with high signal intensity indicating diffuse endocardial edema of the left ventricular wall (arrows); (B) is diffuse sub-endocardial edema of the ventricular and atrial walls in the four-chamber section (arrows); (C) is late enhancement image indicated sub-endocardial layer of high signal intensity in the inferolateral wall (arrow); (D) is late enhancement image indicated sub-endocardial layer of high signal intensity in the ventricular and atrial septum, and lateral, apical and right ventricular walls (arrow). DM, dermatomyositis.

CMR of the patients with PM and myocarditis. (A) is a 23-year-old female; (a) is sub-endocardial edema of the lateral and septal wall (arrow); (b) is no obvious early enhancement; (c) is diffuse sub-endocardial late enhancement of the septum, inferolateral and left ventricular free walls (arrow). (B) is a 43-year-old female; (a) is diffuse transmural edema of the left ventricle in T2-weighted image (arrow); (b) is early enhancement of the middle-layer myocardium in the inferolateral wall in T1-weighted image (arrow); (c) is diffuse late enhancement of the sub-endocardial and middle-layer myocardium in the septum, inferolateral and left ventricular free wall (arrow). CMR, cardiovascular magnetic resonance; PM, polymyositis.

The following concomitant diseases were also excluded: inclusion-body myositis, necrotizing autoimmune myopathy, dystrophies, metabolic myopathy, long-term excessive alcohol consuming, mild to serious kidney dysfunction (estimated GFR <60 mL/min), malignant tumors, coronary heart disease, flu-like syndrome in the past 6 months.