Figure 5: Chorionic surface of the placenta.The thin cord belongs to the acardiac twin and inserts marginally to the normal cord.
Prenatal diagnosis of the twin reversed arterial perfusion sequences has been previously reported. The pump twin typically develops normally, while the recipient, acardiac twin demonstrates total or partial absence of the heart and reduction anomalies affecting all tissues. Four types of acardiac twins have been categorized based on the degree of cephalic and truncal maldevelopment:
g Acardius anceps: Head and face are partially developed.
g Acardius acephalus: No cephalic structures; this is the most common type, seen in 60-75% of the reported cases.
g Acardius acormus: There is a head, but no body; this is the rarest type.
g Acardius amorphus: A formless blob is present, containing all tissue types, but no recognizable organs. This differs from a teratoma only by its attachment to an umbilical cord8.
This report illustrates an acardiac twin of the amorphus variety, which was initially confused with an amniotic band syndrome. The amniotic band syndrome is a more common entity than the TRAP sequence, with an incidence of 0.7-8.3:10,000 live births, often presenting as an extremity amputation, sometimes with an associated fibrous band at constriction points9. As the management of the TRAP sequence may be complicated by the development of polyhydramnios and congestive heart failure in the pump twin, differentiating between TRAP and amniotic band syndrome clearly carried great importance.
It has been suggested that the anomaly develops as a result of extensive artery-to-artery and vein-to-vein anastomoses in the twin placenta leading to perfusion of the abnormal, acardiac twin by the pump co-twin. The reversed circulation in the anomalous twin may alter the hemodynamic forces needed for normal cardiac morphogenesis and results in acardia4.
Doppler sonography of the acardiac twin umbilical vessels has demonstrated reversal of normal flow3,4,10. The umbilical artery S/D ratio of the donor twin reflects the vascular resistance to blood flow in the placenta and connecting vessels to the acardiac twin, and has been observed to be normal. The S/D ratio of the reversed flow in the acardiac twin umbilical artery has been reported to be elevated and provides a measure of the systemic vascular resistance of the acardiac fetus itself2,3. The recipient twin cord contains a single umbilical artery in 50% of cases4,8.
It has been postulated that preferential perfusion of the parasitic twin lower body with low pressure, hypoxemic blood results in acardia, as well as varying degrees of upper body regression11. Associated anomalies include: total or partial absence of cranial vault, holoprosencephaly, anencephaly, anophthalmia, absent facial structures, microphthalmia, cleft lip, cleft palate, diaphragmatic defects, absent or rudimentary limbs, absent heart and lungs, esophageal atresia, ventral wall defects, ascites, absent liver and gallbladder, skin edema, and cystic hygroma6,7. As there have been reports of anomalies in the pump twin, careful search for malformations in the typically normal twin is recommended1.
Nerlich reported an acardiac twin which was associated with a non-acardiac twin showing features typical of amniotic band syndrome12. Half of acardiac twins have various chromosomal abnormalities11. There have been scattered reports of chromosomal anomalies in the donor fetus, including autosomal trisomies2,13 and the Klinefelter syndrome14.
The acardiac twin may be mistaken for a dead anomalous twin, especially when cranial structures are present2,7. At the other extreme, when the acardiac twin has minimal resemblance to a human form, as in our case, confusion with the amniotic band syndrome can occur.
Moore studied a series of 49 acardiac twin pregnancies and detected an overall perinatal mortality of 55% for the donor twin. The prognosis was directly related to the respective weights of the acardiac fetus and pump twin, with a higher weight of the recipient twin resulting in a higher likelihood of the development of polyhydramnios, cardiac insufficiency in the donor twin, and premature delivery. Moore compared the weights of the twins via a twin weight ratio (TWR) which was computed as the weight of the acardiac twin expressed as a percentage of the pump twin weight. A TWR greater than 70% resulted in significantly higher incidence of the above complications15. Lebel et al. reported a remarkable exception to the Moore generalization16.
TRAP sequence management includes serial ultrasound to follow growth patterns of the normal twin and look for evidence of congestive heart failure in the donor. Once there is evidence of the latter, several authors have suggested intervention in an attempt to improve perinatal outcome by interrupting the circulation to the acardia twin1,17-21. McCurdy et al. recently reported successful ligation of the acardiac twin"s umbilical cord with endoscopic technique; however, both twins ultimately died17. On the other hand, Quintero et al. were more successful with this method18. Grab attempted fibrin occlusion of the umbilical vein of an acardiac fetus, which also resulted in death of both twins1. Robie reported a case of sectio parva, with selective delivery of an acardiac twin at 22.5 weeks and ultimate prolongation of the pregnancy to 33 weeks19. Ginsberg et al. achieved term delivery of a healthy pump twin after section delivery of the acardiac fetus in the second trimester20. Simpson successfully treated an acardiac pregnancy with digoxin after there were sonographic signs of cardiac insufficiency of the pump twin21.
Our case of acardiac amorphus, which was initially confused with an amniotic band syndrome, resulted in an uncomplicated course without morbidity to the donor twin, likely due to the small size of the acardiac twin relative to the donor.
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