Thrombus straddling the patent foramen ovale: a case report.

Background: A transseptal thrombus straddling the patent foramen ovale (TSFO) is a rare but potentially life-threatening imaging finding that often serves as direct evidence of paradoxical embolism. Given its dynamic and transient nature, timely recognition is crucial to prevent systemic embolism. Case summary: We report an 81-year-old woman admitted for acute dyspnea. Transthoracic echocardiography (TTE) on admission revealed no intracardiac thrombus. However, contrast-enhanced cardiac computed tomography (CT) performed the following day unexpectedly demonstrated a linear hypodense lesion spanning the interatrial septum at the fossa ovalis, exhibiting the "straddling sign" and confirming the diagnosis of TSFO. Following shared decision-making with the patient, anticoagulation alone was deemed the optimal management. Telephone follow-ups at 1 and 3 months, and up to 2 years post-discharge, demonstrated a favorable clinical recovery. Discussion: This case highlights the diagnostic value of contrast-enhanced CT angiography in identifying TSFO in the setting of acute pulmonary embolism. Although TSFO is rare, it represents a "window period" manifestation of paradoxical embolism, necessitating heightened vigilance from both radiologists and clinicians. Early identification and initiation of anticoagulant therapy can effectively prevent systemic thromboembolic events and improve prognosis. Due to its dynamic and transient nature, imaging capture particularly on CT angiography is exceedingly rare. This case demonstrates a clear image of a transseptal thrombus, holding significant clinical and educational value.