Prognosis impact of pericardial effusion in patients with acute myocarditis

Background Recent Guidelines of the European Society of Cardiology have introduced the concept of Inflammatory Myocardial and Pericardial Syndrome (IMPS), a unified entity encompassing both myocarditis and pericarditis (1). This framework reflects the continuum between these conditions in term of anatomy, shared etiologies, and a common diagnostic approach (2,3). This study sought i) to describe factors associated with pericardial involvement; ii) to compare the outcomes of patients hospitalized for proven acute myocarditis (AM) with or without pericardial effusion; iii) to identify the independent correlates of clinical outcome. Methods This is an observational ambispective cohort study based on the AMPHIBIA registry, including patients hospitalized for proven AM between august 2008 and December 2024 at a French hospital. Patients were stratified into two groups based on the presence of pericardial effusion on admission transthoracic echocardiography, ranging from minimal effusions to tamponade. The main composite outcome was defined as the occurrence of all-cause death, temporary mechanical circulatory support, severe ventricular arrhythmia and heart transplantation, at longest follow-up, up to 7 years. Results A total of 581 patients with AM were included (follow-up 3.7 [1.3 – 7.3] years). Among them, 134 (23%) patients presented with pericardial effusion and 447 (77%) patients without. Four independent variables were associated with pericardial effusion at admission: left ventricular (LV) dysfunction, immunological myocarditis, elevated CRP, and fever. The occurrence of the composite event was significantly higher in patients with pericardial effusion compared than in those without (38.1% vs. 15.7%, hazard ratio [HR] 2.69, 95% confidence interval [CI] 1.88-3.86, p < 0.001). This difference was observed regardless of pericardial effusion size. After multivariate analysis, pericardial effusion was independently associated with the main composite outcome (HR 1.99 [1.32-3.00], p = 0.001). Conclusion This study characterizes the phenotypes and outcomes of patients hospitalized for an AM, with and without pericardial effusion. Pericardial involvement was associated with LV dysfunction, immunological myocarditis, elevated CRP, and fever; and was linked to a markedly higher risk of the composite outcome, remaining an independent predictor of adverse events. Differentiating these two phenotypes may provide physicians with valuable guidance for managing therapy in these patients.