Introduction: Unroofed coronary sinus-atrial septal defect (CS-ASD) is a very rare anomaly that may be easily missed by transthoracic echocardiography. Case: A 59 year old woman presented with chronic and progressively worsening dyspnea. TTE demonstrated preserved ejection fraction, right ventricular and atrial dilation, and elevated pulmonary artery pressures. Color doppler demonstrated a persistent diastolic flow at the interatrial septum immediately above the plane of the tricuspid valve, suspicious for atrial septal defect (Fig. 1). Such aberrant flow was not observed on multiple prior TTEs. Decision-Making: Given the uncertain nature of the abnormal color flow seen, further imaging was pursued with a gated Cardiac CT with contrast. CT imaging demonstrated a dilated coronary sinus with a communication between the roof of the terminal coronary sinus and the left atrium, consistent with an unroofed coronary sinus-atrial septal defect (CS-ASD) (Fig. 2). There was no evidence of a persistent left superior vena cava. Cardiac MRI demonstrated a significant right to left shunt with a Qp/Qs of 1.53 using phase contrast imaging. Discussion: CS-ASD is a very rare cardiac anomaly and is the most uncommon type of ASD (<1%), often associated with a persistent left superior vena cava. Given the posterior nature of the defect, it is often missed on TTE. In our case, suspicion was raised by a small abnormal color jet of flow coming from the interatrial septum. Cardiac CT is an excellent diagnostic modality for CS-ASD given its excellent visuospatial capabilities and ability to evaluate pulmonary veins. Cardiac MRI may serve as a helpful adjunct in quantification of shunt flow.

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