Discordant Manifestation of Congenital Heart Block in a Twin Pregnancy: A Case Report

Background: Congenital heart block (CHB) is a severe disorder, with an estimated incidence of 1 in 20,000 live births. In most cases, the underlying pathophysiology involves transplacental transfer of maternal anti-Ro/SSA and/or anti-La/SSB antibodies, which are thought to bind to L-type calcium channels on fetal cardiac conduction cells, triggering apoptosis and initiating an inflammatory response that ultimately leads to destruction of the conduction system. Case presentation: We report a case of a dichorionic diamniotic twin pregnancy in which one fetus exhibited complete heart block in the 21st week of gestation, whereas the co-twin showed no abnormalities. The maternal history was notable for multiple autoimmune disorders among first- and second-degree relatives. Serological testing revealed markedly elevated titers of anti-SSA autoantibodies. Following parental decision, the pregnancy was terminated. Macroscopic evaluation of the affected fetal heart showed ventricular dilation, while histopathological examination demonstrated fibrotic and degenerative changes as well as focal calcification in the atrioventricular junctional region. In contrast, the cardiac conduction system of the unaffected fetus was preserved. Discussion: Fetuses of seropositive mothers have a 2-5% first-event risk of developing CHB during gestation. The available literature suggests that the underlying pathophysiology in discordant twin pregnancies likely reflects unequal placental transfer of antibodies in combination with subtle differences in fetal susceptibility and local immune responses. Conclusions: This case illustrates the unpredictable course of anti-SSA-associated CHB in twin pregnancies and highlights the importance of careful fetal cardiac surveillance in seropositive women, even in the absence of a clinically apparent autoimmune disease. Furthermore, the comprehensive histopathological documentation provides additional insight into the pathological basis of CHB.