Diagnostic Dilemma Between Probable Cardiac Sarcoidosis and Non-Granulomatous Myocarditis: A Grand Round Case Report

Abstract Background Cardiac sarcoidosis can mimic other inflammatory cardiomyopathies, and true steroid resistance is uncommon. Distinguishing granulomatous myocardial involvement from active non-granulomatous myocarditis remains clinically challenging. Case Summary A 43-year-old woman with biopsy-proven systemic sarcoidosis and dilated cardiomyopathy presented with recurrent decompensated heart failure. Fluorodeoxyglucose positron emission tomography–computed tomography demonstrated diffuse myocardial uptake and was interpreted as probable cardiac sarcoidosis. Despite guideline-directed heart failure therapy and immunosuppression, she developed progressive heart failure complicated by renal failure and refractory shock, culminating in death. Post-mortem myocardial biopsy revealed active non-granulomatous myocarditis without granulomas. Rare viral inclusion–like structures consistent with cytomegalovirus were identified in lung tissue, with elevated cytomegalovirus titres; however, immunohistochemistry was negative. Discussion This grand round highlights a diagnostic dilemma between probable cardiac sarcoidosis and biopsy-demonstrated non-granulomatous myocarditis. While histology showed myocarditis, co-existing sarcoidosis cannot be definitively excluded due to post-mortem sampling limitations. This case underscores the limitations of imaging-based diagnosis, the potential risks of escalating immunosuppression in the setting of diagnostic uncertainty, and the importance of timely myocardial tissue reassessment, particularly when clinical deterioration occurs despite 2–4 weeks of intensive therapy, within a multidisciplinary framework.