Complete resolution of severe secondary mitral regurgitation accompanying eosinophilic myocarditis due to immunosuppressive treatment

Eosinophilic myocarditis (EM) is an under-diagnosed inflammatory heart disease that often leads to severe left ventricular (LV) dysfunction. Meanwhile, severe secondary mitral regurgitation (MR) with valve disruption, possibly requiring mitral valve repair, is rarely concomitant with EM. We present the case of a 64-year-old female diagnosed with heart failure with severe LV dysfunction and localized asynergy. Echocardiography revealed severe secondary MR with mitral valve disruption. Cardiac magnetic resonance imaging (CMR) showed transmural late-gadolinium enhancement localized in the anterior wall and diffuse high-signal areas on T2-weighted images, suggesting non-ischemic and inflammatory heart disease. Although the peripheral eosinophil count was not elevated on admission, it gradually increased during hospitalization. These findings encouraged us to perform endomyocardial biopsy, which confirmed myocardial eosinophilic infiltration with mild fibrosis and necrosis, leading to the diagnosis of EM. Immunosuppressive treatment with oral corticosteroids improved LV dysfunction and completely resolved severe secondary MR. The current case highlighted that comprehensive assessment of laboratory, imaging, and pathological examinations including CMR is crucial to develop the appropriate therapeutic strategy for refractory heart failure. Immunosuppressive treatment should be considered as the first therapeutic option even in EM cases with severe secondary MR, possibly requiring mitral valve repair. <Learning objective:The gradual increase in peripheral eosinophils during hospitalization, without significant peripheral eosinophilia on admission, is crucial for the diagnosis of eosinophilic myocarditis. Comprehensive assessment of laboratory, imaging, and pathological examinations including cardiac magnetic resonance imaging is mandatory when building an appropriate therapeutic strategy for refractory heart failure. Severe secondary mitral regurgitation with mitral valve disruption can be completely resolved via immunosuppressive treatment in cases of eosinophilic myocarditis.>