Atrioventricular canal defect and hypoplastic left heart syndrome as discordant congenital heart defects in twins.

We report on a twin pair presenting with atrioventricular canal defect (AVCD) with right ventricular dominance in one twin, and classic hypoplastic left heart syndrome (HLHS) in the other. According to the developmental-mechanistic approach, AVCDs belong to the group of extracellular matrix abnormalities, whereas classic HLHS is included among flow lesions. Twin pairs with congenital heart defect (CHD) generally have concordant defects by mechanistic group. The occurrence of AVCD and classic HLHS in twins or siblings has never been reported. Interestingly, hypoplasia of the left ventricle is the anatomic characteristic which unifies the discordant CHDs observed in our twins. The occurrence of CHD in both members of the twin pair implies a strong influence of genetic factors. At present, the genetic basis determining the different cardiac phenotypes observed in our twins is unknown. The report of these peculiar associations may be useful to stimulate further studies and shed light on the etiology of CHDs.