Malformations in acardiac twins are consistent with reversed blood flow: liver as a clue to their pathogenesis.

Because of the uncertainty concerning the origin of the malformations in the acardiac twin, its structure was studied to see if the malformations were random or with a pattern related to the twinning process. Included were 18 cases of dysmorphic twins in which an autopsy demonstrated a rudimentary or absent heart, and in which some polarity was evident. The organs and long bones with alterations were tabulated according to the embryonal-fetal circulation of blood from and to the placenta. Malformations were more often encountered in the superior limbs and organs; the number of altered organs decreased in a cranio-caudal direction. The liver was the exception being affected in 89% of the cases vs. an average of 54% for the other abdominal organs. As the liver lies first in the circulatory path from the placenta to the fetus, the pattern of the malformations was in accordance with the "twin reversed arterial perfusion sequence" (TRAP). The more frequent absence of distal bones (P = 0.0007) is compatible with reduced perfusion in each limb. The malformations found in the acardiac twins involved brain, esophagus and trachea, liver, other abdominal organs, diaphragm, vertebrae, limbs, anus, and omphalocele; vascular disruption may be the common pathogenesis for the acardiac twins as well as other dysmorphic infants with similar anomalies.