The coronary sinus is the persisting remnant of the left horn of the developing systemic venous sinus. During the fifth week of human development, the channel is incorporated, with its own walls, into the left atrioventricular groove. Sprouts from the newly formed sinus then provide the basis for formation of the great, middle, and small cardiac veins.
Postnatally, the sinus provides drainage of the greater part of the venous return from the heart. The left superior caval vein itself largely regresses, with only its terminal part persisting as the oblique vein of the left atrium. Union of the oblique vein and the great cardiac vein serves to mark the distal extent of the sinus, with its proximal boundary formed at its entrance to the right atrium, guarded by the Thebesian valve. The great cardiac vein itself forms the base of the left ventricular summit, with this area initially described as a triangle by Brocq and Mouchet. In the setting of congenital persistence of the left superior caval vein, the sinus, and its right atrial orifice, are usually dilated. An imperforate Thebesian valve produces atresia of the sinus, with the persisting left superior caval vein then providing a conduit for retrograde passage of cardiac venous return to the right atrium. Rarer malformations are so-called "unroofing" of the sinus, and aneurysmal dilation to form diverticulums in the inferior wall of the ventricular mass. The sinus can also be abnormal, or absent, in the setting of isomerism.
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