Relationships of the tricuspid valve to the membranous ventricular septum in Down's syndrome without endocardial cushion defect: study of 28 specimens, 14 with a ventricular septal defect.

The commissure between the anterior and medial leaflets of the tricuspid valve is commonly absent in Down's syndrome without endocardial cushion defect (19 of 28 specimens). As a result, aneurysm of the membranous ventricular septum may develop (eight of 14 specimens with ventricular septal defect limited to the membranous ventricular septum) and the potential for left ventricle-to-right atrial communication is increased.