Monochorionic twins discordant for congenital heart disease: a referral center's experience and possible pathophysiologic mechanisms.

OBJECTIVE To describe the spectrum of cardiac defects in monochorionic (MC) twins discordant for congenital heart disease (CHD) in a referral center population. METHOD Retrospective study of all twin gestations undergoing echocardiography between 2000 and 2009 at our institution. RESULTS A total of 356 twin pairs were evaluated during the study period, 202 for suspected twin-twin transfusion syndrome (TTTS) and the remainder for other indications. Twenty-nine MC pairs were discordant for CHD: laterality defects, 24% [right (2) or left (3) isomerism (3), primitive heart (2)]; ventricular hypoplasia secondary to semilunar valve obstruction, 14% [hypoplastic left heart syndrome (2), severe pulmonary stenosis (PS) or atresia (2)]; valvar dysplasias in TTTS recipients, 27% [PS (4), mitral/tricuspid dysplasia (4)]; conjoining, 14% (4); and other developmental errors, 21% [conotruncal (1), tricuspid atresia (2), ventricular septal defect (2)]. CONCLUSIONS The spectrum of lesions in individuals assumed to be genetically identical and the disproportionate incidence of laterality and ventricular hypoplasia in this population leads us to propose potential mechanisms for the development of CHD in this population including local environmental or epigenetic factors influencing gene expression differentially, abnormal reciprocal laterality signaling between twinned embryos, or placental vascular factors affecting hemodynamics, either early in gestation or later in the setting of TTTS, leading to valvar lesions and ventricular hypoplasia.