sIBM is a progressively worsening inflammatory muscle disorder marked by muscle weakness and atrophy, particularly in the quadriceps and finger flexors. While the exact cause remains unclear, it is believed to be a complex condition with various contributing factors, including immune-mediated muscle damage and degenerative changes [13]. Magnetic resonance imaging of the thigh in patients with sIBM typically shows edema, atrophy and fatty replacement of the anterior compartment with relative sparing of the medial and posterior compartments [14, 15]. On histopathology, the condition commonly exhibits inflammatory infiltrates in muscle tissue, primarily composed of cytotoxic T cells and macrophages. Autoantibodies such as anti-cN1A have been detected in some sIBM patients, suggesting a possible autoimmune aspect [16, 17]. Additionally, histopathological changes detectable by light microscopy often reveal rimmed vacuoles, cytoplasmic protein aggregates, and mitochondrial abnormalities, indicating a degenerative component that may be related to protein misfolding and impaired autophagy [13, 16, 18].

Compared to other idiopathic inflammatory myopathies, such as dermatomyositis and polymyositis with a cardiac involvement of between 6 and 75% with a variability depending on a definition of heart involvement and the detection method used, cardiac involvement in sIBM is considered rare, although the exact frequency also remaining unclear [3, 19]. Cardiac involvement has been rarely reported with, mainly, dilated cardiomyopathy and complex ventricular arrhythmias [20, 21]. There was no description of focal fatty lesions in any of these cases. However, CMR assessment was not done in all cases. In 2010, a cohort study included 51 sIBM patients and looked for heart involvement with electrocardiographic and echocardiographic data [22]. No evidence of cardiac involvement was found, leading the authors to conclude that routine comprehensive cardiac evaluation in sIBM patients without cardiac symptoms should not be recommended. However, CMR was not performed in that study.

Another recent study on 20 sIBM patients without cardiovascular symptoms showed LGE patterns consistent with myocardial fibrosis in 7 (35%) patients [23]. Although the authors concluded that it was a nonspecific finding, one could hypothesize that without conducting a multiparametric CMR study, subclinical cardiac involvement might not have been detected. Indeed, in sIBM patients, patchy intramural LGE was located in the inferior and inferolateral segments.

Our case report is the first, to our knowledge, that found fatty intramyocardial infiltrations detected by specific multiparametric analysis in the absence of scar or fibrosis in sIBM. Fatty infiltration of the heart observed on CMR can indicate various conditions that should be considered in the differential diagnosis, including arrhythmogenic right ventricular cardiomyopathy (ARVC), cardiac sarcoidosis, and Neurofibromatosis type 1 (Recklinghausen Disease). Specifically regarding ARVC, recent studies suggest that a “bite-like” fibrofatty replacement pattern in the left ventricle is highly specific for ARVC with left ventricular involvement when assessed within the appropriate clinical context [24]. In the absence of an alternative explanation, such as sIBM, the presence of this feature in a patient with suspected ARVC should strongly prompt suspicion of the disease and warrant a comprehensive diagnostic evaluation, including resting ECG, Holter monitoring, and genetic testing. These findings must always be correlated with both clinical presentation and CMR results for accurate diagnosis.

Early identification of morphologic and functional alterations in the heart can probably be of prognostic value. Though the exact percentage can vary slightly depending on the study and cohort characteristics, cardiovascular causes are responsible for approximately 20–30% of deaths in patients with sIBM due to a combination of factors including chronic muscle inflammation, reduced physical activity and associated comorbidities such as hypertension and diabetes. Therefore, monitoring disease and in particular cardiac involvement may guide the initiation and monitoring of various interventions. In this context, CMR appears to be the imaging method of choice. Additionally, this disease is slowly progressive, and current immunosuppressive and immunomodulatory treatments have shown limited effectiveness [10, 25]. Therefore, scientific societies recommend the implementation of pharmacological treatment in cases of severe inflammation disproportionate to fibrosis on muscle biopsy and/or magnetic resonance imaging or in those patients with other autoimmune diseases, which are more commonly associated with cardiac involvement. Hence, the discovery of myocardial lesions in sIBM should prompt closer cardiological follow-up. In order to better understand if these fatty lesions could be frequently associated with sIBM, large cohort studies should be performed, which may secondarily drive the development of new therapeutic modalities.