Unroofed coronary sinus

Imaging description Unroofed coronary sinus is a communication between the coronary sinus and left atrium. The result is a left-to-right shunt that allows flow of oxygenated blood from the left atrium into the coronary sinus. In the normal situation, the coronary sinus courses inferior to the undersurface of the left atrium in the left atrioventricular groove, emptying into the right atrium. In unroofed coronary sinus there is a variably sized communication between the two structures (Figure 24.1). In partial unroofing, a single or several small orifices are seen. In complete unroofing, there is total absence of the tissue separating the left atrium and coronary sinus. At crosssectional imaging, defects are optimally visualized in a shortaxis plane parallel to the atrioventricular groove (Figure 24.2). Associated signs include enlargement of the right atrium and right ventricle due to shunting, which can be quantified using phase-contrast MRI. In the case of small restrictive defects, turbulent flow jets may be visualized in the left atrium on cardiac MRI. Importance Unroofed coronary sinus is a rare cause for left-to-right shunt. It may be challenging to make the diagnosis on transthoracic echocardiography due to limited imaging windows, resulting in referral of patients to cardiac CT or MRI to evaluate for occult shunt. Diagnosis is important due to potential for transient right-to-left shunting that can result in systemic emboli or brain abscess. Typical clinical scenario Unroofed coronary sinus is a rare disorder that is the least common type of atrial septal defect, representing less than 1% of these anomalies. There is a frequent association with left-sided superior vena cava, which is seen in approximately 63–75% of cases. Many patients will have additional congenital cardiac defects. Differential diagnosis Unroofed coronary sinus should be distinguished from other types of potentially difficult to diagnose left-to-right shunts, such as sinus venosus atrial septal defects and partial anomalous pulmonary venous return.