Loeffler endocarditis in hypereosinophilic syndrome, the paradox of absent peripheral eosinophilia: a case report.

Background: Loeffler endocarditis is a cardiac disease characterized by infiltration of eosinophils in hypereosinophilic syndrome (HES), progressing through acute necrotic, thrombotic, and fibrotic stages. It can ultimately lead to heart failure and restrictive cardiomyopathy. Early recognition, multidisciplinary approach, and accurate treatment are crucial to prevent these complications. Case summary: A 51-year-old man presented with dyspnoea and chest discomfort. Imaging revealed a large left ventricular (LV) thrombus, a reduced LV ejection fraction (LVEF 28%), and a typical 'double-V pattern' of endomyocardial fibrosis. Despite characteristic imaging findings of Loeffler endocarditis, the absence of peripheral eosinophilia reduced clinical suspicion. Coronary disease was excluded by CT coronary angiography. Extended bloodwork for infectious, autoimmune, oncologic, and HES-related diseases remained negative. Myeloperoxidase tested positive; however, there were no clinical findings suggestive of eosinophilic granulomatosis with polyangiitis. Subsequently, a bone marrow biopsy confirmed eosinophilia, establishing the diagnosis of Loeffler endocarditis. The patient was treated with anticoagulation, heart failure therapy, and prednisone. At 6-month follow-up, LVEF improved slightly to 35% but the typical V-shaped LV thrombus persisted, and heart failure symptoms (NYHA III) remained. Discussion: Loeffler endocarditis is an eosinophil-mediated disease but may present without peripheral eosinophilia, particularly in longstanding cases of HES. This underscores the importance of cardiac imaging, bone marrow evaluation, and multidisciplinary collaboration in suspected cases to ensure timely diagnosis and treatment, thereby preventing disease progression.