Diagnostic dilemma between probable cardiac sarcoidosis and non-granulomatous myocarditis: a grand round case report.

Background: Cardiac sarcoidosis (CS) 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 (HF). Fluorodeoxyglucose positron emission tomography-computed tomography demonstrated diffuse myocardial uptake and was interpreted as probable CS. Despite guideline-directed HF therapy and immunosuppression, she developed progressive HF 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 CS 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.