Idiopathic Hypereosinophilic Syndrome Presenting as Isolated Right Ventricular Endomyocardial Fibrosis With Intracavitary Thrombus: A Multimodality Imaging Case Report.

Idiopathic hypereosinophilic syndrome (HES) is an uncommon disorder characterized by persistent eosinophilia and eosinophil-mediated organ damage, with cardiac involvement representing its most severe complication. Isolated right ventricular endomyocardial fibrosis is exceptionally uncommon and may mimic other intracardiac pathologies, making diagnosis particularly challenging. We report the case of a 55-year-old man who presented with a one-month history of progressive bilateral lower-limb edema. Laboratory investigations revealed marked persistent hypereosinophilia associated with elevated cardiac biomarkers. Transthoracic echocardiography demonstrated an extensive right ventricular intracavitary mass with near-complete apical obliteration. Cardiac magnetic resonance confirmed diffuse right ventricular subendocardial late gadolinium enhancement consistent with endomyocardial fibrosis and identified an associated intracavitary thrombus. CT excluded pulmonary embolism and malignancy while confirming the intracardiac lesion. An extensive infectious, autoimmune, hematologic, molecular, and oncologic evaluation, including bone marrow examination and molecular testing for clonal eosinophilic disorders, was negative, leading to the diagnosis of idiopathic HES. Treatment with high-dose corticosteroids, vitamin K antagonist anticoagulation, and diuretic therapy was initiated. This case highlights the diagnostic challenge posed by isolated right ventricular eosinophilic endomyocardial fibrosis and underscores the complementary role of multimodality cardiovascular imaging in lesion characterization, exclusion of differential diagnoses, and guidance of appropriate management. Early recognition and a systematic etiological workup are essential to prevent irreversible cardiac damage and improve patient outcomes.