Laterality defects in conjoined twins: Implications for normal asymmetry in human embryogenesis

Abstract

We evaluated six pairs of conjoined twins: four pairs were dicephalus, and two were of the ischiopagus type. In three of the four dicephalus pairs, the right twin had an abnormality of laterality that included a right aortic arch, reversed great vessel orientation, bilateral right-sided isomerism of the lungs, asplenia, and situs inversus of the viscera. The left twin had normal great vessel orientation and situs solitus in each case. The finding that was unique in these three dicephalus twin pairs was their fused hearts, which were similar in orientation and configuration. The fourth dicephalus twin pair had one normally rotated heart, which was located in the midline and had normally placed chambers and great vessels. Each twin of this pair had normal visceral situs. In the two pairs of ischiopagus twins, each pair had two separate hearts, with normal cardiac structure and great vessel relationships. The viscera expressed normal laterality. Documentation of a defect in laterality in the right twin in three conjoined twin pairs with fusion of the hearts, combined with the presence of normal laterality in three pairs without cardiac fusion, has implications regarding the mechanisms leading to laterality of the human embryo. We suggest that rotation of the heart initiates the embryo's process of lateralization and that the laterality defects of the viscera seen in the right twin are a result of their abnormal cardiac rotation.

Vaginal Delivery of Dicephalic Parapagus Conjoined Twins: Case Report and Literature Review

“…Past studies describe fused hearts with complex anatomy, including right aortic arch and reversal or transposition of great vessels (Cunniff et al 1988;Hammond et al 1991;Spencer 1992;Itoh et al 1993;Gilbert-Barness et al 2003;Tansel and Yazicioglu 2004). When two hearts are present, the right twin's heart is usually more severely malformed (Spencer 1992).…”

“…Defects of laterality, with absent spleen or situs inversus of the right twin's abdominal organs, often accompany heart abnormalities (Cunniff et al 1988). If the hearts are not fused, abdominal viscera are situated normally (Cunniff et al 1988). …”

“…There are generally two sets of lungs (Spencer 1992), which may be underdeveloped or anomalous, especially in the right twin (Cunniff et al 1988;Hammond et al 1991;Yang et al 1994;Gilbert-Barness et al 2003). The liver, pancreas, gallbladder, genitourinary tracts, and rectum may be shared (Cunniff et al 1988;Spencer 1992;Yang et al 1994;Groner et al 1996;GilbertBarness et al 2003).…”

“…The liver, pancreas, gallbladder, genitourinary tracts, and rectum may be shared (Cunniff et al 1988;Spencer 1992;Yang et al 1994;Groner et al 1996;GilbertBarness et al 2003). Neural tube defects, cystic hygroma, clubfoot, and imperforate anus have occurred in parapagus twins (Itoh et al 1993;Machin 1993;Bondeson 2001;Mackenzie et al 2002).…”

“…Complex anomalies of the heart and abdominal laterality are seen more often in dicephalus and thoracopagus than in other types of CTs, and might result from disturbed cross-signaling between tissues in adjacent primitive streaks, as suggested in animal models (Gilbert-Barness et al 2003). Cunniff et al (1988) hypothesized that rotation of the heart at the atrial level initiates lateralization in the embryo, and that interference with heart rotation causes abnormalities in some right dicephalus twins.…”