Cyclopia, aprosencephaly, and acardiac twinning: Is hypoxia-ischemia a unifying mechanism?

In a recent case of monochorionic diamniotic twinning we observed one twin with acardia, cyclopia, and aprosencephaly, an association not reported previously. In most cases of acardia, the development of tissues in superior regions of the body is disrupted severely, while inferior structures develop more normally. A common explanation for this disruption is hypoxia-ischemia due to twin reversed arterial perfusion (TRAP). In this condition, arterial-arterial and venous-venous anastomoses in the placenta permit twin-twin transfusion and reversal of blood flow in the umbilical vessels and aorta of the recipient twin. The heart is absent or severely deficient, either by secondary atrophy or possibly a more primary, though currently unknown, mechanism. As a result, cranial tissues are less likely to be perfused with oxygenated blood than caudal tissues. A host of craniocerebral anomalies are observed in acardia, including total absence of the head and brain, rudimentary brain, anencephaly, holoprosencephaly, neuronal migration defects, and near-normal brain. Hypoxia-ischemia could be an important factor in the disruption of tissues in the present case, although a more generalized process affecting heart, head, and brain cannot be excluded. The findings suggest that hypoxia-ischemia may play a role in the pathogenesis of some cases of holoprosencephaly and aprosencephaly. This mechanism has been underreported and requires additional investigation.