Neuropathological features of the brain in acardius acormus

Hypothesized pathogenesis of acardius acephalus, acormus, amorphus, anceps, acardiac edema, single umbilical artery, and pump twin risk prediction

Background

Acardiac twinning complicates monochorionic twin pregnancies in ≈2.6%, in which arterioarterial (AA) and venovenous placental anastomoses cause a reverse circulation between prepump and preacardiac embryos and cessation of cardiac function in the preacardiac. Literature suggested four acardiac body morphologies in which select (groups of) organs fail to develop, deteriorate, or become abnormal: acephalus (≈64%, [almost] no head, part of body, legs), amorphus (≈22%, amorphous tissue lump), anceps (≈10%, cranial bones, well‐developed), and acormus (≈4%, head only). We sought to develop hypotheses that could explain acardiac pathogenesis, its progression, and develop methods for clinical testing.

Methods

We used qualitatively described pathophysiology during development, including twin‐specific AA and Hyrtl's anastomoses, the short umbilical cord syndrome, high capillary permeability, properties of spontaneous aborted embryos, and Pump/Acardiac umbilical venous diameter (UVD) ratios.

Results

We propose that each body morphology has a specific pathophysiologic pathway. An acephalus acardius may be larger than an anceps, verifiable from UVD ratio measurements. A single umbilical artery develops when one artery, unconnected to the AA, vanishes due to flow reduction by Hyrtl's anastomotic resistance. Acardiac edema may result from acardiac body hypoxemia combined with physiological high fetal capillary permeability, high interstitial compliance and low albumin synthesis. Morphological changes may occur after acardiac onset. Pump twin risk follows from UVD ratios.

Conclusion

Our suggested outcomes agree reasonably well with reported onset, incidence, and progression of acardiac morphologies. Guidance for clinical prediction and testing requires ultrasound anatomy/circulation study, from the first trimester onward.

An acardiac twin with advanced brain development and a minor form of holoprosencephaly and intracerebral retina-like pigmented tissue: a case report and review of the literature

The development of an acardiac twin in a monochorionic multiple pregnancy is a rare and severe complication of abnormal placental vascular anastomoses. These malformed fetuses present with a very bizarre morphology and a plethora of different malformations. However, all acardiac twins show either a complete absence or an anlage of the heart. Cerebral development is usually poor. We report, according to our review of the literature, for the first time, a very unusual case of acardius with features of acardius amorphus and acormus (fused head and malformed axial skeleton without macroscopically detectable internal organs) with lobar holoprosencephaly and intracerebral pigmented retina-like tissue.

Modeling Acardiac Twin Pregnancies

Acardiac twin pregnancies are a rare but severe complication of monochorionic twinning, where the acardiac twin lacks cardiac function but nevertheless grows during pregnancy because it is perfused by the pump twin through a set of placental arterioarterial and venovenous anastomoses. Because the acardiac twin's body is only perfused by the pump twin's arterial blood, acardiac twins have reduced blood oxygen saturation (SO(2)) levels. Furthermore, the pump twin has reduced blood oxygen saturation because the anastomoses merge the blood of the two twins. We suggested that angiogenesis from hypoxia mediated neovascularization increases the capillary density in the acardiac twin and causes a continuously decreasing vascular resistance of the acardiac body. The pump twin therefore has a continuously increasing cardiac output and decreasing oxygen saturation, which may cause a vicious circle of increasing levels of pump twin complications, often leading to intrauterine fetal death. Our aim in this article was twofold. First, to summarize our previous modeling work of acardiac twin pregnancies, and add an estimate of the capillary density in mammals at different levels of SO(2) from a literature search on angiogenesis related to SO(2), Hypoxia Inducible Factor, and Vascular Endothelial Growth Factor. Second, to speculate that combination of these efforts and our most recent model of monochorionic twin pregnancies and twin-twin transfusion syndrome may result in an improved computational model for acardiac twin pregnancies, aimed at identifying early clinical prognostic markers for pump twin complications.