Figure 9: Macroscopic appearance of the two twins.
The autopsy and dysmorphologic evaluation revealed no major or minor malformations, signs of congestive heart failure, emboli, or disseminated intravascular coagulation in the pump twin, and radiographs were normal. The arterial anastomosis was noted just within the placenta at the base of the umbilical cords. The acardiac acephalic twin was affected with the anomalies defined by ultrasound evaluation, with the following two exceptions: the amniotic band thought to exist between the lower extremities of the anomalous twin was actually one of several sheets of amnionic membrane attached to the umbilical cords near the placental insertion, which we interpret as incomplete development of two amnionic cavities. The anomalous twin also had complete syndactyly of the left 4th and 5th toes. The absence of lungs, heart, liver and spleen were confirmed.
Three platinum coils were adjacent to the umbilical artery, and one was partially outside of the vessel. No evidence of significant hemorrhage was present. The abdominal aorta was absent. The anomalous umbilical artery branched into many vessels after turning cephalad. A urachus with protrusion at the umbilical ring was present. Karyotype of cord blood was normal, Cell culture of fluid aspirated from the cystic hygroma at the time of embolization and from fascia postmortem from the anomalous twin was unsuccessful.
Two distinct etiologic mechanisms have been proposed since the 19th century: 1) both twins may initially be normal, and the anastomosis enables the heart of the twin which is more advanced in development to disrupt normal cardiac development of the recipient twin; 2) the anomalous twin initially has abnormal cardiac development, either from genetic or non-genetic etiology, which is further compromised and finally overcome by abnormal pressures from retrograde flow from the pump twin via the artery-to-artery anastomosis.
Because both karyotypically normal twin pairs as well as discordant twin pairs (the acardiac twin always being cytogenetically abnormal) have been documented, it is possible that the condition is, in fact, etiologically heterogeneous and both theories are correct.
Cardiac anomalies are common in cytogenetically anomalous fetuses as well as in cytogenetically normal monozygous twins. Anomalous development of the cardiovascular systems in some of these twinsâ€”in the present case the umbilical vein descended caudad from the umbilical insertion and the abdominal aorta was absentâ€”suggests that cardiovascular anomalies are not solely the result of reversed perfusion. However, the pattern of malformation of these twins is clearly heavily influenced by the reversed perfusion. The anomalous twin receives blood from an artery-to-artery anastomosis and hence receives oxygen- and nutrient-poor blood which has already been circulated through the pump twin. Since the richest blood arrives through the iliac arteries, there is a relative sparing of the lower extremities and anomalies related to anoxia, and poor nutrient supply increases with increased distance from the entry of the vessels. Blood returns from the anomalous twin via its umbilical vein and anastomoses either with the umbilical vein of the normal twin or with venous vessels in the placenta.
Claudius pointed out in 1859 that the allantoic stalk of twin gestations reaches the chorion at different times, and anastomosis between allantoic arteries could occur then. Early fusion could result in complete disruption of the normal cardiovascular system prior to development of the cardiac flexure, and later fusion could disrupt an already-present primitive heart. Cytogenetically anomalous twins could grow at a slower rate, and hence would be succeptible to "takeover" via the arterial anastomosis, accounting for the significant proportion of karyotypically anomalous twins. In cytogenetically and phenotypically normal twin embryos, whichever twin"s heart begins effective circulation first, with greater pressure, would result in retrograde flow to the cardiovascularly more immature twin. Similarly, lower blood pressure could result from a unilateral cardiac defect, with onset of retrograde flow from the normal twin.
Essentially every single organ system is affected, but the following is a list of the most characteristic anomalies. The heart may be completely absent, reduced to a tubular or even a 2-chamber heart. A single umbilical artery is the rule. The lower extremities of the abnormal twin are usually spared the most, presumably because the femoral artery contains relatively the most oxygen and nutrients. Malformations of the lower extremities include ray deficiencies and equinovarus malformation. The abdominal visceral anomalies include absence of any discernable abdominal organs, persistence of embryonic structures like the urachus, and aberrant vasculature. The external genitalia may be normal, deficient or absent. The thoracic structures, including ribs and vertebrae, may be present, rudimentary, severely dysplastic or absent. Marked edema and cystic hygromata which may attain massive proportions are typical. Acardiac fetuses typically are acephalic with absent upper extremities. When the head is present, many malformations have been described, including anencephaly, holoprosencephaly, and facial clefting.
The normal (pump) twin may develop cardiac failure with polyhydramnios, cardiomegaly, pericardial, pleural, and peritoneal effusion, engorged umbilical vein, and subcutaneous edema. Heart failure may occur in the absence of polyhydramnios, however. The pump twin is also at risk of those malformations associated with monozygous twinning4, and of disseminated arterial calcification (reported once)5.
Affected pregnancies are at risk of premature delivery secondary to polyhydramnios and of sudden demise of the apparently normal pump twin, which may occur even in the absence of signs of cardiac decompensation.
A significant percentage of anomalous twins in TRAP sequence are chromosomally anomalous and may not be genotypically identical; the embryologic origin remains controversial. The first cytogenetic discordant twins were reported in 196619.
One remarkable case provides firm evidence that polar-body fertilization occurs in man, by documenting a triploid acardiac twin, the result of fertilization of a single ovum by two genetically dissimilar sperm, one uniting with the first mitotic polar body20.
At the time of the procedure, we were aware of one other embolization procedure for this indication which had been done two months prior, with growth of the anomalous twin ceasing and continued normal growth of the normal twin post-procedure. Subsequently, this twin successfully went to term, as occurred in another case report from the Japanese literature16.
It is clear that demise of the pump twin can occur post-procedure even in the absence of any signs of cardiac failure21; in our case, the normal fetus had no ultrasonic signs of failureâ€”no effusion, cardiomegaly, or engorgement of the umbilical vein or inferior vena cava, and no signs whatsoever of emboli or disseminated intravascular coagulation at autopsy. We theorized that the sudden elimination of the parasitic circulation after embolization would relieve strain on the normal heart, and given that the patient lost 10 lbs in the ensuing two days suggests resolving polyhydramnios, hence relief of cardiovascular strainâ€”yet the normal twin died.
Retrospectively, it might have been prudent to digitalize prior to this procedure in hopes of reducing any unforseen potential shock to the cardiovascular system of the normal twin. However, unexplained demise of a monozygous twin occurs more commonly than in singleton pregnancies (as may be the case in instances of fetus papyraceous), and it is possible that the demise was not related to the procedure.
Because of their rarity and the absence of a registry on prenatal management of acardiac twins, several management questions are difficult to answer. What is the relative risk of demise or premature delivery when polyhydramnios first occursâ€”is it markedly increased in 2nd trimester pregnancies versus 3rd trimester pregnancies? Are the benefits of an embolization procedure identical to those derived from hysterotomy? Are the risks of fetal demise post-embolization the same as post-hysterotomy? Is hysterotomy preferable, removing a significant possibility of transfer of toxic products or emboli to the normal twin, or does embolization prevent any potential circulation of toxic products or emboli as efficiently as removal with hysterotomy? Because twin embolization phenomena have not been reported in acardiac twinning, such risk is not similar to that in twin to twin transfusion syndrome and is undefined post-hysterotomy or post-embolization procedure. The risk of emboli thrown post-embolization procedure or post-hysterotomy/removal may be very low, and can only be defined with a registry of these procedures. We would be pleased to act as a registry for therapy in acardiac twin pregnancy.
Acknowledgements: We are grateful to Dr. Karim Iskander for referral of his patient and to Dr. Richard Porreco for sharing information on his embolization procedure.
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