Congenitally corrected transposition of the great arteries (ccTGA) associates atrioventricular discordance and ventriculo-arterial discordance. The anatomy of the associated ventricular septal defect (VSD) remains controversial.

We analyzed 102 human heart specimens: 31 ccTGA, 36 TGA, 35 normal hearts (NH), to compare the right ventricular septal anatomy between the 3 groups and to determine the anatomy of the VSD in ccTGA. VSD were classified as outlet if above the septal insertions of the tricuspid valve, inlet if underneath. We measured the lengths of the anterior (AL) and posterior (PL) limbs of the septal band, the angle between the two limbs and, in order to assess the orientation of the septal band, the angle between AL and the arterial valve above (AL-AV).

VSD was present in 26 ccTGA (83.9%) and was outlet in 16 cases (62%). Mean AL-PL angle was 76.4° (ccTGA) compared to 90.6° (TGA, P=0.011) and 76.1° (NH, P=ns). Mean AL-AV was 70.6° (ccTGA) compared to 90.6° (TGA, P=0.0004) and 69.1° (NH,P=ns). PL was significantly shorter in ccTGA: AL/PL length ratio 21.4 (ccTGA), 2.2 (TGA), 1.5 (NH), P<0.0003.

The typical VSD in ccTGA is an outlet VSD. Its frequent misdiagnosis as an inlet VSD is due to the short PL, which creates the illusion of a posterior VSD. Surprisingly, the orientation of the septal band is similar in ccTGA and NH, despite the atrioventricular discordance, and different in ccTGA and TGA, despite the ventriculo-arterial discordance. ccTGA is not a TGA and the term “double discordance” might be more appropriate.