Abatacept for steroid-refractory fulminant immune checkpoint inhibitor myocarditis with triple M overlap syndrome: a case report with serial endomyocardial biopsy assessment.

Background: Triple M overlap syndrome is a rare, but often fatal, immune-related adverse event. Evidence on the effectiveness of immunosuppressive treatments, including abatacept, and disease monitoring in complex cases remains limited. Case Summary: A man in his 60s developed fulminant immune checkpoint inhibitor (ICI)-related myocarditis after nivolumab plus ipilimumab for recurrent lung adenocarcinoma, complicated by triple M overlap syndrome (myocarditis, myositis, and myasthenia gravis). Despite high-dose corticosteroids and intravenous immunoglobulin, cardiac biomarkers worsened, with progressive conduction abnormalities and a rapid decline in the left ventricular ejection fraction to 23%. Upon transfer to our institution, he developed sustained ventricular tachycardia, requiring direct current cardioversion and intra-aortic balloon pump support. Endomyocardial biopsy (EMB) at admission showed CD8 Discussion: This case illustrates that abatacept-based multidisciplinary therapy may help stabilize fulminant steroid-refractory ICI-related myocarditis with triple M overlap syndrome. Serial EMB provided a histopathological assessment of residual myocardial inflammation and guided immunosuppressive therapy, suggesting a potential role for serial biopsy in complex cases.