Endomyocardial Biopsy and Clinical Outcomes in Acute Myocarditis ― Multicenter Cohort Study ―

Background: Endomyocardial biopsy (EMB) remains the gold standard for diagnosing acute myocarditis (AM); however, its prognostic value has not been fully established. We investigated the prognostic impact of EMB, with a focus on fulminant myocarditis (FM). Methods and Results: We included 841 AM patients across 7 hospitals who were categorized having undergone EMB (n=268) or not (n=573). The primary outcome was in-hospital death; the secondary outcome was a composite of all-cause death, cardiac transplantation, or left ventricular assist device implantation. In-hospital mortality was lower in the EMB group (6.3% vs. 11.5%; P=0.019), with a similar reduction in the EMB group among patients with FM (11.3% vs. 23.7%; P=0.002), despite a higher rate of extracorporeal membrane oxygenation. During a median follow-up of 447 days, the composite outcome was also lower in the EMB group among patients with FM (P=0.031). In multivariable analyses, EMB was independently associated with lower in-hospital mortality in both the overall cohort and the FM subgroup. Outcomes did not differ substantially across histopathologic subtypes, although patients with giant cell myocarditis had a worse prognosis. Conclusions: In patients with AM, EMB was associated with lower in-hospital mortality and improved long-term outcomes, particularly among those with FM. Further well-designed prospective studies are warranted to evaluate the clinical impact of EMB and its potential role in improving outcomes.