Abstract Body (Do not enter title and authors here): Background: Loeffler endocarditis is a rare and clinically challenging presentation of hypereosinophilic syndrome (HES). Early detection and treatment are critical for this disease that otherwise carries a high risk of morbidity and mortality. We present a complex case of a patient with HES and sequelae.

Case: A 41-year-old male with history of asthma presented with cough, dyspnea on exertion, orthopnea, and right arm pain. He noted a 15-pound weight loss in the past few months. In addition to right upper superficial vein thrombus, he was initially diagnosed with pneumonia, but labs were concerning for significant leukocytosis (WBC 54.23 10³/uL) and hypereosinophilia (45%), as well as anemia and thrombocytopenia. Cardiology was consulted for evidence of cardiac involvement with high sensitivity troponin 480 ng/L and BNP 399 pg/mL. Transthoracic echocardiography and cardiac MRI showed mild apical biventricular dilatation with moderate apical hypokinesis. There was nonenhancing T2 hypointense eccentric signal in both apices, suggestive of organized thrombus. Circumferential mid-cavity subendocardial delayed enhancement in the left ventricle and right ventricular apex was noted. Systolic function was preserved, but there were signs of pulmonary arterial hypertension. These findings were consistent with Loeffler endocarditis. He underwent bone marrow biopsy, which confirmed diagnosis of FIP1L1-PDGFRA+ HES.

The presence of the FIP1L1-PDGFRA mutation is correlated with an increased risk of cardiac involvement in patients with HES and is also associated with worse prognosis. Following an interdisciplinary discussion between the hematology, rheumatology, and cardiology teams, he was started on imatinib, methylprednisolone followed by prednisone taper, and apixaban. His symptoms and blood counts have since drastically improved, and he is being monitored closely for resolution of the thrombi and for any signs of heart failure.

Conclusion: HES complicated by Loeffler endocarditis is a unique presentation of infiltrative disease causing inflammatory, thrombotic, and fibrotic cardiac sequelae. It can portend deadly complications, including valvular involvement, thromboembolic events, and heart failure. A high clinical suspicion is required due to the multitude of possible symptom presentations. Multimodality imaging and interdisciplinary management with early accurate diagnosis and treatment are necessary to slow disease progression and reduce morbidity risk.