The term transitional AV canal most commonly refers to the combination of an atrial septal defect (primum ASD), a mitral valve cleft and a small inlet ventricular septal defect, which is often mostly or completely occluded by accessory AV valve tissue. A single valvar annulus is usually present where the anterior and posterior bridging leaflets fuse overlying the ventricular septum. Because of the leaflets' fusion, two distinct valvar orifaces are observed.