Abstract Body (Do not enter title and authors here): Background: Lipomatous hypertrophy of the interatrial septum (LHIS) is a rare, benign cardiac anomaly involving non-encapsulated fat infiltration of the septum, typically sparing the fossa ovalis. Though often asymptomatic, LHIS may be associated with arrhythmias and, very rarely, embolic events. It can mimic myxomas on imaging and is seldom considered a direct cause of ischemic stroke.

Research questions: Can LHIS serve as a source of paradoxical embolism leading to ischemic stroke? Could structural changes from LHIS precipitate atrial fibrillation?

Approach: An 82-year-old woman with hypertension, diabetes, and dementia presented with acute right gaze deviation, left hemineglect, and generalized weakness. CT perfusion confirmed a right middle cerebral artery (MCA) infarct, and she underwent mechanical thrombectomy. Workup included echocardiography, which revealed normal ejection fraction but a right atrial mass and a positive bubble study, indicating a patent foramen ovale (PFO). Transesophageal echocardiogram and cardiac CT angiography identified a 1×1 cm mass in the interatrial septum. Initially presumed to be a myxoma, the patient underwent mass excision, PFO closure, MAZE procedure, and left atrial appendage ligation following development of new-onset atrial fibrillation.

Results: Histopathology confirmed lipomatous hypertrophy of the interatrial septum, not myxoma. The stroke was attributed to embolization of LHIS fragments through the PFO into systemic circulation, resulting in a right MCA infarction. Atrial fibrillation was likely precipitated by atrial distortion from the septal mass. The patient was discharged on metoprolol, diltiazem, and amiodarone in stable condition.

Conclusion: This case highlights a rare cause of embolic stroke—LHIS—as both a source of paradoxical embolism and a trigger for atrial fibrillation. Accurate diagnosis requires histopathological confirmation, as imaging may misidentify LHIS as a myxoma. Clinicians should consider LHIS in cryptogenic stroke, especially in the presence of a PFO and interatrial masses.