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

Abstract

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.

Keywords: Hyrtl's anastomosis; acardiac monochorionic twins; acardiac morphology; short umbilical cord syndrome; spontaneous aborted embryos.

FIGURE 1

Spontaneous aborted singleton embryos (fig. 1 of Poland et al. (1981), reproduced with permission of the publisher). DI, developmentally inconsistent; GD, growth disorganized. Poland gave 337 GD1, 286 GD2 (1–4 mm; <6 weeks gestation) (179 direct; 107 with body stalk up to 3 cm), 133 GD3 (up to 1 cm; <7 weeks), 31 GD4 (3–17 mm; <8.5 weeks; CRL, 50% <1 cm; <7 weeks), 78 DI (most CRL 1–2.1 cm; 7–9 weeks), 183 normal embryos. In Table 3 we used that 1.6 time more abnormalities occurred in monozygotic twin pregnancies (Myrianthopoulos, 1976), thus assumed also in monochorionic twins, than in singletons, hence we reduced the 183 aborted normal embryos to 183/1.6 ≈ 115. We also included 36 defects of specific systems (DSS) cases (Table 3). Thus, an assumed total of 1,016 spontaneous aborted monochorionic twin embryos with live cotwins

FIGURE 2

From fig. 502 of Gray (1918). Ductus venosus connecting to the inferior vena cava. Section 4c. Peculiarities in the vascular system in the fetus. We added the blood flows (black arrows) measured by Gembruch, Viski, Bagamery, Berg, and Germer (2001) in a just demised fetus at 25 + 3 weeks, during exsanguination of the other, live, twin. AA, VV are arterioarterial, venovenous; a is arterial, v is venous

FIGURE 3

Left upper: two umbilical arteries with a fenestrated Hyrtl's anastomosis, and the AA connected to one of the umbilical arteries. Right upper: two umbilical arteries with a fused Hyrtl's anastomosis, and the AA connected to the fused umbilical artery. The retrograde directed blood flows in the umbilical arteries are indicated with small black arrows. We have assumed that fused Hyrtl's anastomoses and 50% of the fenestrated anastomoses retain two umbilical arteries. The five smaller lower figures are other Hyrtl's anastomoses from Valsalan et al. (2018) and Ullberg et al. (2001). We have assumed that each of these five lower cases will develop into one umbilical artery

FIGURE 4

Pump/Acardiac umbilical venous diameter (UVD) ratios separating 17 pump twins with adverse outcome from 7 without adverse outcome (from fig. 4 of van Gemert et al., 2016). The separation line is drawn by eye and has no scientific foundation