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Articles » Multiple gestations » Twin reversed arterial perfusion syndrome

1994-05-08-13 Twin, acardiac, amorphus  © Gross

Twin reversed arterial perfusion (acardiac), amorphus

Beth Gross, MD, Boris Petrikovsky, MD, PhD, Elizabeth Lane, MD

Address correspondence to Beth Gross, MD, Department of Radiology, North Shore University Hospital-Cornell University Medical College. 300 Community Drive, Manhasset, NY 11030-3876, Ph: 516-562-4799, Fax: 516-562-4794. Division of Maternal Fetal Medicine, Department of Obstetrics and Gynecology


Acardiac twinning, also referred to as the twin reversed arterial perfusion (TRAP) sequence1, likely represents an extreme manifestation of the twin transfusion syndrome and has an incidence of approximately 1% of monozygotic twins or 1 in 35,000 deliveries2,3. The acardiac twin is perfused by the normal co-twin by means of reversal of circulation through large vein-to-vein and artery-to-artery anastomosis4 and has no direct vascular communication with the placenta5,6. The acardiac twin presents with many bizarre anomalies thought to be due to low oxygen tension and dramatic alterations in fetal physiology7. Complications to the pump twin of an acardiac fetus include congestive heart failure, polyhydramnios, preterm labor and death in 50-70% of cases1,2. We present a case of acardiac amorphus which was initially mistaken for an amputated extremity in amniotic band syndrome in a third trimester fetus.

Case report

A 31-year-old G2P0 woman was referred from an outside institution at 31 weeks of gestation for evaluation of a possible upper limb amputation due to an amniotic band. The patient had a normal ultrasound examination at 10 weeks. Our initial examination demonstrated a 30-week size fetus in vertex presentation with normal lower and left upper extremities. The right upper extremity was thought to be visualized proximally, with swollen and heterogeneous soft tissues (fig.1).

Figure 1: Ultrasound of presumed abnormal fetal arm with internal bone (arrows). This was later identified as an acardiac twin.

An internal bone thought to be a humerus was consistent with 28 weeks. The right forearm and hand were not readily identified. Prolonged scanning allowed identification of a short, thin cord, connecting the distal portion of the apparent right arm to the region of the insertion of the normal 3 vessel umbilical cord onto the placental surface (fig. 2). Umbilical artery S/D ratio of 3 vessel cord was normal. Color and pulsed Doppler investigation of the thin cord (supposed band) demonstrated a minimal amount of flow (fig. 3). The possibility of acardiac twinning was considered. Although membranes could not be identified near the right upper extremity, an atypical amniotic band syndrome was also considered, given the lack of visualization of the distal right forearm and the swollen, distorted appearance of the proximal right arm.

Figure 2: A thin cord connects the acardiac twin (large arrow), with the placental surface in the region of insertion of normal 3 vessel cord (small arrow).

Figure 3: Pulsed Doppler of the acardiac twins cord demonstrates minimal amount of flow.

One week later, follow-up examination with a different fetal position revealed two entirely normal upper extremities. The previously identified swollen soft tissue mass thought to be the right arm was located superior to the fetal lower extremities and was clearly separate from the fetus. A diagnosis of cardiac amorphus was made. The patient was followed weekly until term, without change in the size or appearance of the acardiac mass; nor was there evidence of polyhydramnios or hydrops in the normal fetus. A normal female fetus was born at term via a spontaneous vaginal delivery. A fusiform mass, the acardiac twin, was also delivered.


A small portion of thin cord was still attached to the surface of the acardiac twin (fig. 4). The acardiac-amorphus had no recognizable organs, limbs or facial differentiation. One rudimentary bony structure was present. A monochorionic, monoamniotic placenta was identified, and had a normal 3 vessel cord which inserted marginally. A second, short umbilical cord containing one vessel inserted adjacent to the normal 3 vessel cord (fig. 5).

Figure 4: The amorphous acardiac twin covereed with skin and hair. The thin cord is still attached to one pole of the mass (arrows).

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.

Associated anomalies

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.

Differential diagnosis

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.


1. Grab D, Schneider V, Keckstein J, Terinde R: Twin, acardiac, outcome. Fetus 2:11-13,1992.

2. Al-Malt A, Ashmead G, Judge N, et al: Color-flow and Doppler velocimetry in prenatal diagnosis of acardiac triplet. J Ultrasound Med 10:341-345,1991.

3. Sherer DM, Armstrong B, Shah Y, et al: Prenatal sonographic diagnosis, Doppler velocimetric umbilical cord studies, and subsequent management of an acardiac twin pregnancy. Obstet Gynecol 74:472-475,1989.

4. Benson CB, Bieber FR, Genest D, et al: Doppler demonstration of reversed umbilical blood flow in an acardiac twin. JCU 17:291-295,1989.

5. Nyberg DA, Mahony BS, Pretorius DH: Diagnostic ultrasound of fetal anomalies: text and atlas. St. Louis, MO: Mosby-Year Book, 1990, pp. 608-609.

6. Izquierdo L, Smith J, Gilson G, et al: Twin, acardiac, acephalus. Fetus 1:1-3,1991.

7. Nevils BG, Maciulla JE, Izquierdo LA, et al: Twin, acardiac, anceps. Fetus 3:15-17,1993

8. Gibson JY, D"Cruz CA, Patel RB, Palmer SM: Acardiac anomaly: review of the subject with case report and emphasis on practical sonography. JCU 14:541-545,1986.

9. Seeds JW, Cefalo RC, Herbert WNP: Amniotic band syndrome. Am J Obstet Gynecol 144:243-248,1982.

10. Pretorius DH, Leopold GR, Moore TR, et al: Acardiac twin, report of Doppler sonography. J Ultrasound Med 7:413-416,1988.

11. Langlotz H, Sauerbrei E, Murray S: Transvaginal Doppler sonographic diagnosis of an acardiac twin at 12 weeks gestation. J Ultrasound Med 10:175-179,1991.

12. Nerlich A, Wisser J, Draeger A, et al: Human acardiac anomaly: a report of three cases. Eur J Obstet Gynecol Reprod Biol 38:79-85, 1990.

13. Bhatnagar KP, Sharma SC, Bisker J: The holocardius: a correlative computerized tomographic, radiologic and ultrasonographic investigation of a new case with review of literature. Acta Genet Med Gemellol 35:77-89,1986.

14. Moore CA, Buehler BA, McManus BM, et al: Brief clinical report: acephalus-acardia in twins with aneuploidy. Am J Med Genet (Suppl) 3: 139-143,1987.

15. Moore TR, Gale S, Benirschke K: Perinatal outcome of forty-nine pregnancies complicated by acardiac twinning. Am J Obstet Gynecol 163:907-912,1990.

16. Lebel RR, Mock C, Israel J: Acardia/acephaly presenting at full term. Fetus 4:3 7615-23, 1994.

17. McCurdy CM, Childers JM, Seeds JW: Ligation of the umbilical cord of an acardiac-acephalus twin with an endoscopic intrauterine technique. Obstet Gynecol 82:708-711,1993.

18. Quintero RA, Reich H, Puder KS, et al: Brief report: umbilical-cord ligation of an acardiac twin by fetoscopy at 19 weeks of gestation. N Engl J Med 330:469-471,1994.

19. Robie GF, Payne GG, Morgan MA: Selective delivery of an acardiac, acephalic twin. N Engl J Med 320:512-513,1989.

20. Ginsberg NA, Applebaum M, Rabin SA, et al: Term birth after midtrimester hysterotomy and selective delivery of an acardiac twin. Am J Obstet Gynecol 167:33-37,1992.

21. Simpson PC, Trudinger BJ, Walker A, Baird PJ: The intrauterine treatment of fetal cardiac failure in a twin pregnancy with an acardiac acephalic monster. Am J Obstet Gynecol 147:842-844,1983.

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