Synonyms: Twin reversed arterial perfusion syndrome (TRAP), Acardius.

Prevalence: 0.3:10,000 pregnancies, 1% monozygotic twin pregnancies.

Definition: A complication of monozygotic twin pregnancies in which one fetus develops normally (pump twin) and the second twin (recipient twin) demonstrates cardiac maldevelopment ranging from complete absence of heart tissue to some formation of rudimentary myocardia.

Etiology: Probable reversal of circulation in the anomalous twin resulting from anastomosis of circulation between the twins.

 Pathogenesis: Malformation of the heart and heart dependent endodermal organs in the anomalous twin.

Associated anomalies: In the perfused twin: total or partial absence of cranial vault, holo­pros­en­ce­pha­ly, anencephaly, absent facial structures, anophthalmia, micro­phthalmia, cleft lip, cleft palate, absent or rudimentary limbs, dia­phragmatic defects, absent lungs and heart, esophageal atresia, ventral wall defects, ascites, absent liver and gallbladder, edema of the skin and single umbilical artery. In the pump twin: evi­dence of congestive heart failure (ascites, pleural effusions, poly­hydramnios, skin edema).

Differential diagnosis: Cystic hygromas, singleton pregnancies with intraamniotic tumors, pseudoacardiac twin.

Prognosis: Uniformly lethal for the recipient twin; perinatal mortality rates reported up to 50 percent in pump twin.

Recurrence risk: Not increased.

Management: Serial ultrasound for evaluation of growth and signs of congestive heart failure in the normal twin.

MESH Diseases‑in‑Twins‑diagnosis; Feto­fetal‑Transfusion‑diagnosis; Heart‑Defects,‑Congenital‑diagnosis; Pregnancy,‑Multiple; Monsters‑; Umbilical ‑Cord ‑blood‑supply ICD9 761.5 CDC 759.456(*)

* Address correspondence to: Luis Izquierdo, MD, University of New Mexico, School of Medicine, Dept. of Ob‑Gyn. 2211 Lomas Blvd., NE, Albuquerque, NM 87131‑5286 Ph: 505‑277‑8381 (voice), 277‑7621 (fax)

Introduction

Twin pregnancies occur in approximately 1.2% pregnancies in the United States. The management of twin pregnancies remains a challenge for perinatologists. Twin pregnancies, compared to singleton pregnancies, are complicated by a higher risk of preterm labor, preeclampsia, and gestational diabetes. However, besides these increased risks associated with twinning, congenital malformation sequences may occur which are unique to the twinning process. An example of this is the acephalus acardiac anomaly, also known as the Twin Reverse Arterial Perfusion sequence (TRAP). This malformation occurs in approximately 1% of monozygotic twin pregnancies, or about 0.3:10,000 pregnancies. The presence of an acardiac twin requires the normal twin to provide circulation for itself as well as the acardiac sibling by means of reversal of circulation through large artery‑to‑artery and vein‑to‑vein anastomoses ^{2, 3}. This condition is associated with profound alterations in twin fetal physiology and leads to increased perinatal mortality^3‑5. We present a case of the acephalus acardiac anomaly diagnosed and followed at our institution.

Case report

A 24 year old, G₂ P0 a1, female was referred to our antenatal testing unit at 19 weeks of gestational age. The patient had had an ultrasound performed by her primary physician, which revealed a twin pregnancy. One of the fetuses was reported as "deadâ€. At our institution, ultrasound evaluation was repeated and demonstrated a twin pregnancy with the following findings. Twin A (the pump twin) was found to be in a vertex presentation, with normal fetal anatomy consistent with 19 weeks. A thin complete membrane was present between the two sacs. Grossly abnormal anatomy was noted in Twin B (the recipient twin). There were multiple cystic cavities in the thorax with no identifiable cardiac structures present. There was no definition of the fetal skull above the thorax, and the spine ended abruptly in the cervical region (Figure 1).

Fig. 1: Sagittal view of the spine ending abruptly at the cervical spine. Note the cystic cavity at the cranial area.

This fetus was motionless. In the sac of the pump twin, there was evidence of poly­hydramnios but the pump twin had no evidence of ascites, pleural effusions, or skin edema. Doppler studies in the umbilical arteries of the pump twin failed to demonstrate any abnormalities in blood flow.

The ultrasound findings were consistent with the diagnosis of TRAP. After extensive counselling, the patient desired conservative management. She was advised of the prognosis and risk for preterm labor.

One week later, at 20 weeks of gestational age, the patient presented in advanced active labor and delivered. Figures 2 and 3 represent the products of this delivery, which confirmed the diagnosis of TRAP. In the anomalous twin, there was an anterior midline opening in the area of the upper thorax (Fig. 2). No cephalic structures were developed.

Fig. 3: Acardiac twin demonstrating an opening in the midline of the upper thorax.

Discussion

While the antenatal diagnosis of TRAP has been reported by several authors^5‑9, the pathogenesis of the TRAP sequence remains controversial. Evidence suggests that intrauterine growth is achieved by perfusion from the normal twin via a large blood vessel anastomosis in the placenta. Perfusion of the anomalous twin occurs by reversal of flow through the umbilical vessels of that twin². The arterial blood that enters the body of the affected twin is presumably under reduced oxygen tension, which may cause disruption of organ morphogenesis^{2, 4}. It has also been suggested that the primary defect is a failure of embryologically paired structures, including the heart, to fuse in the midline, with subsequent maldevelopment².

Types of acardiac twins

Types of acardiac twins have been categorized based on the degree of cephalic and truncal maldevelopment (Table 1).

The acardiac acephalus fetus has no cephalic development at all. The acardius anceps fetus has cranial structures and/or neural tissue development. The acardius acormus fetus demonstrates cephalic structures but has limited or no truncal development. The fourth type, and most seriously maldeveloped, is the acardius amorphus fetus, which retains virtually no cephalic or truncal organization at all⁵.

Diagnosis

Whenever a twin pregnancy is detected by ultrasonography, and the twins have a discordant or a grotesque malformation; acardia should be strongly suspected. Serial ultrasounds are indicated to assess these complicated pregnancies.

Doppler velocimetry has been used to investigate the TRAP syndrome. Sherer et al⁷ reported the use of the Doppler velocimetry of the umbilical cord in the TRAP syndrome, reporting a markedly abnormal peak systolic to end diastolic velocity (S/D) ratio. Our case, probably due to the early gestational age, failed to demonstrate any Doppler changes.

Management

The major perinatal problems associated with acardiac twinning include congestive heart failure and hydrops in the normal twin, hydramnios, and preterm delivery. Increasing perinatal morbidity and mortality has been associated with the relative size of the affected twin. Evidence for this is suggested by Moore et al³. In a review of 49 cases of acardiac twinning, they found an overall perinatal mortality of 55 percent. To assess the effect of the anomalous twin"s size on perinatal outcome, the weights of the twins were expressed as a ratio called the Twin Weight Ratio (TWR). The TWR is the wet weight of the acardiac twin divided by the weight of the normal twin. Preterm delivery, hydramnios and pump twin congestive heart failure in the normal twin were seen more commonly if the TWR was above 70%. This implies that the increased perfusion demands of the relatively large acardiac twin, compared to the normal twin, is related to prognosis. This data may be useful the counseling and management of these complicated pregnancies.

Interventional procedures

Attempts to improve perinatal outcome by interruption of the circulation to the anomalous twin have been proposed. Platt et al⁹ speculated that termination of the circulation to the abnormal fetus may reduce the amount of hydramnios and subsequently prolong pregnancy. Seeds et al⁵ have suggested selective feticide in these pregnancies, but that the injection of a lethal substance would also endanger the normal twin. Recently, Fusi et al.¹⁰reported that the pump twin remains at increased risk of sudden death even without ultrasound evidence of cardiac failure, possibly due to acute disseminated intravascular coagulation. Hamade et al.¹¹ reported a steel coil placement in the abdominal wall of a acardiac fetus under ultrasound guidance. The patient delivered at term a 2237g normal baby and an acardiac twin weighing 110g.

Management of TRAP syndrome should include serial ultrasound to assess the growth rate and cardiovascular status of the normal twin.

References

1. Deacon JS, Machin GA, Martin JME, Nicholson S, Nwankwo DC, Wintemate R. Investigation of Acephalus. Am J Med Gen 5:85‑99, 1980

2. Benirschke K, Harper VDS. The acardiac anomaly. Teratology 15:311‑316, 1977

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

4. Romero R, Pilu G, Jeanty P, Ghidini A, Hobbins TC. Prenatal Diagnosis of Congenital Anomalies. Appleton and Lange 409‑411, 1988

5. Seeds JW, Herbert WNP, Richards DS. Prenatal Sonographic Diagnosis and management of a twin pregnancy with placenta previa and hemicardia. Am J Perinat 4:313‑316, 1987

6. Van Groeninghen JC, Franssen AMHW, Willemsen WNP, Nijhuis JG, Puts JJG. Europ J Obstet Gynecol Reprod Biol 19:317‑325, 1985

7. Sherer DM, Armstrong B, Shah YG, Metlay LA, Woods JR. Prenatal sonographic diagnosis Doppler velocimetric umbilical cord studies, and subsequent management of an acardiac twin pregnancy. Obstet Gynecol; 74:472‑475, 1989

8. Platt LD, De Vore GR, Bieniarz A, Benner P, Rao R. Antenatal diagnosis of acephalus acardia: a proposed management scheme. Am J Obstet Gynecol 143:857‑859, 1983

9. Stiller RJ, Romero R, Pace S, Hobbins JC. Prenatal identification of twin reversed arteral perfusion syndrome in the first trimester. Am J Obstet Gynecol 160:1194‑1196, 1989

10. Fusi L, Fisk N, Talbert D et al.: When does death occur in an acardiac twin? Ultrasound diagnostic difficulties. J Perinat Med. 18: 223‑7, 1990

11. Hamada H, Okane M, Koresawa M et al.: Fetal therapy in utero by blockage of the umbilical blood flow of acardiac monster in twin pregnancy. Nippon Sanka Fujinka Gakkai Zasshi 41: 1803‑9, 1989