IgG4-related disease as an uncommon cause of myocarditis: a case report.

Background: IgG-4-related disease (IgG4-RD) is a systemic fibroinflammatory disease that can affect any organ system. Myocarditis is a rare manifestation. Case summary: A 65-year-old woman with clinically diagnosed IgG4-RD, who had recently completed corticosteroid taper, presented with multiple admissions for recurrent chest pain and myocardial injury. Coronary angiography repeatedly showed preserved coronary anatomy, and initial transthoracic echocardiography showed preserved left ventricular systolic function. Subsequent transthoracic echocardiography showed rapid decline in ejection fraction, and cardiac magnetic resonance imaging (MRI) showed myocardial oedema, infarct scar, and patchy non-ischaemic fibrosis. A probable diagnosis of IgG4-RD myocarditis was made clinically based on medical history, multimodal imaging findings, and exclusion of alternative diagnoses, although tissue confirmation was not obtained. The patient improved clinically and radiographically with corticosteroid re-initiation followed by maintenance with mycophenolate mofetil (MMF). Conclusion: This case underscores the importance of considering IgG4-RD in the differential diagnosis of unexplained myocardial inflammation and highlights the potential reversibility of cardiac injury with early immunosuppression.