Multimodal imaging in discriminating atrial septal defect combined with mobile echoic strands in fossa ovalis: a case report.

Background: Mobile echoic structures in the fossa ovalis region represent a common diagnostic pitfall in patients with atrial septal defect (ASD). Misdiagnosis as pathological masses may lead to unnecessary invasive interventions. This case demonstrates the value of a stepwise multimodal imaging strategy for differential diagnosis. Case summary: A 48-year-old female presented with intermittent chest discomfort and was initially diagnosed with suspected patent foramen ovale (PFO) at a referring hospital. After systematic evaluation at our centre, the diagnosis was revised to secundum ASD (fossa ovalis type) accompanied by a mobile linear hyperechoic structure at the defect margin. The patient underwent transoesophageal echocardiography (TEE), transthoracic contrast echocardiography, and contrast-enhanced cardiac computed tomography, with complete laboratory workup to exclude infectious and thrombotic aetiologies. Discussion: High-resolution TEE confirmed a true anatomical septal defect rather than a PFO flap channel. Multimodal imaging stepwise excluded pathological mimics, including infective vegetation, mural thrombus, and cystic haemangioma, and the lesion was finally identified as benign residual fragments of the primary septum. Accurate differentiation directly guided clinical decision-making and avoided unnecessary invasive intervention.