The Deceptive Shadow: Inverted Left Atrial Appendage Mimicking Thrombus After Infant Open Heart Surgery

Inversion of the left atrial appendage (LAA) is a rare postoperative complication following cardiac surgery, often misdiagnosed as thrombus or vegetation on imaging, potentially leading to inappropriate management. We report a case of a five-month-old female infant who underwent elective surgical closure of a perimembranous ventricular septal defect via a right atrial approach under cardiopulmonary bypass. On postoperative day 2, transthoracic echocardiography (apical four-chamber view) revealed a 1 × 0.9 cm echogenic mass attached to the anterior mitral leaflet in the left atrium. The mass caused progressive mitral inflow obstruction, and by postoperative day 5, the patient developed acute pulmonary edema. Although initially suspected to be a thrombus, the diagnosis of LAA inversion was confirmed intraoperatively. Given the elongated nature and prolapse of the appendage into the mitral orifice, external ligation and clipping were chosen to ensure complete exclusion and prevent recurrence. Postoperative echocardiography demonstrated complete resolution of the mass and unobstructed mitral inflow, with sustained clinical recovery. This case highlights the importance of considering LAA inversion in the differential diagnosis of left atrial masses following congenital heart surgery. Early recognition and appropriate surgical management can prevent life-threatening complications.