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.

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The amniotic band disruption complex (ABDC) has been attributed to vascular disruption by some authors, not by others. Acardiac twins (ATs), however, have been generally accepted as a prime example of vascular disruption. In this study a comparison was made of these two entities to determine if they were similar or not, and thus we attempted to resolve the controversy of the mechanisms in the ABDC. A female tendency (2:1) was found in the ABDC in contrast to the "normal" sex distribution (0.88:1) in the ATs (p < 0.001). Most types of malformations (66%) were mutually exclusive, notably those of the cranium/brain, abdominal wall, and most internal organs; 83% were more significantly related to one or other of the entities. The ABDC malformations tended to occur unilaterally, but in the ATs they occurred bilaterally (p < 0.0001); the former tended to involve external organs and the latter internal organs (p < 0.0001). With so many differences, the two entities are unlikely due to the same mechanism: the ABDC is more likely to be due to external disruption.

Localized lesions, whether involving the cranium or lumbar spine, are formed by a mechanism that favors the female gender, whereas upper spina bifida, which is usually accompanied by cranial involvement, results from another mechanism (possibly vascular disruption) and is frequently accompanied by other malformations.

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