The superior aspect of coronary sinus and a portion of the left atrium share a common wall .Embryological defects in this area result in a communication between left atrium and coronary sinus. This defect descriptively called as unroofed coronary sinus .This entity is most commonly associated with persistent left SVC. Extreme form of unroofing is some times termed as absent coronary sinus.
How to diagnose it ?
A high degree of anticipation is necessary in all patients with ASD or LSVC. A dilated coronary sinus in routine echocardiography warrants full investigation. A contrast echocardiography with agitated saline injection in left cubital vein will clinch the diagnosis as contrast enters LA after opacifying the dilated coronary sinus. During right heart catheterisation catheter course entering coronary sinus and advancing into LA through the fenestrations (Unroofing) will confirm the defect.
How do you classify unroofed coronary sinus ?
The morphologic type of URCS was classified as Kirklin and Barratt-Boyes
Type I, completely unroofed with LSVC;
Type II, completely unroofed without LSVC;
Type III, partially unroofed midportion;
Type IV, partially unroofed terminal portion
What is the clinical relevance of this entity ?
This entity should be suspected in every patient with persistent LSVC, ( and LSVC should be suspected in every patient with ASD). The hemodynamics is that of an ASD but if sufficient mixing of LSVC blood and LA blood takes place the child will have mild cyanosis.Some times when the coronary sinus is totally absent it will present as a typical dusky ASD picture which can closely mimic a TAPVC clinically.
Surgeons have a greater role in recognising and treating this entity. A typical repair will be done like this
Links to some of interesting articles on this topic
Raghib circulation 1965
Dr.S.Venkatesan MD 
We just did an echo on a pediatric patient and my clinical instructor was telling me why we take a detailed look at the coronary sinus. This is why. Very neat article, thanks!