Complete atrioventricular septal defect (CAVSD) is associated with Down syndrome (DS) in almost half of cases, and is due in this setting to an abnormal development of the vestibular spine, derived from the posterior second heart field (SHF). Clinical studies have shown that Tetralogy of Fallot (TF), an outflow tract defect involving the anterior SHF, is more frequent in CAVSD patients with DS. We therefore decided to look for an outlet extension of the VSD in DS and non-DS.

We reviewed retrospectively all 208 consecutive patients hospitalized in our unit with the diagnosis of CAVSD between 01/01/2017 and 09/12/2019. All echocardiographic examinations were screened by 2 pediatric cardiologists (ZB/LH). We excluded 46 patients whose VSD anatomical type could not be determined. We used the classification as agreed upon by ICD-11 to determine the VSD anatomical type in the 162 patients included.

Among the cohort, 101/162 had DS (62.7%). An outlet extension of the VSD, with anterior malalignment of the outlet septum, was found in 88/101 (87.1%) DS vs. 4/61 (6.5%) non-DS (P<0.0001). Associated TF was found in 12 DS vs. 1 in non-DS (P=ns). Ventricles were balanced in 126 patients, 23 had a hypoplastic left ventricle (LV), 13 had a hypoplastic right ventricle (RV). There was no significant difference regarding balanced or not ventricles between DS and non-DS. However, the hypoplastic ventricle was always the LV in non-DS, and was the RV in 62% of DS (P=0.006).

This echocardiographic study describes for the first time that outlet extension of the VSD is the rule in CAVSD with DS and that hypoplastic RV occurs only in DS. This seems to confirm the involvement of the two parts of the second heart field in the morphogenesis of CAVSD in DS patients. The presence of a malaligned outlet septum in a fetus with CAVSD should raise awareness of the high risk of associated DS.