TRAP Sequence - An Interesting Entity in Twins

INTRODUCTION

Twin reversed arterial perfusion (TRAP sequence), previously termed as ‘acardiac twin gestation’, represents a syndrome characterised by vascular anastamosis and partial or complete lack of cardiac development in one of the twins. It is a rare complication unique to monozygotic, multiple pregnancies. This condition occurs in 1 in 35000 pregnancies and in 0.3% of all monozygotic twin gestations.[12] Without prompt detection, follow-up, and treatment, mortality rates for pump twins have been noted to be as high as 50% to 70%.[12] Early sonographic diagnosis, appropriate follow-up, and intervention have been associated with substantial improvements in survival.[34] We report two cases that emphasize the value of gray-scale ultrasound and color Doppler techniques for both diagnosis of TRAP sequence and detection of prognostic features.

CASE REPORTS

Case 1

A 26-year-old female with G1P0L0A0 (Gravida 1/Para 0/Obortus 0) underwent ultrasound examination to detect the presence of congenital anamolies. Ultrasound scan revealed monochorionic, monoamniotic, twin pregnancies. The pump twin was normal. The recipient twin demonstrated well-developed lower limbs, partially developed lumbar and thoracic spine, absent cervical spine, under-developed abdominal and chest organs. Heart was absent, head and face were absent, and upper limbs were absent [Figure 1]. Cystic hygroma with diffuse soft tissue edema [Figure 2] and single umbilical artery [Figure 3] with reversal of flow [Figure 4] were detected. The weight of the recipient twin was less than 70% of the pump twin. Polyhydramnios was noted as Amniotic fluid Index (AFI) was 29 cm. Complications and options for treatment were discussed with the patient. Patient opted to continue the pregnancy without any treatment.

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Figure 1

Sonogram of Case 1 recipient twin demonstrates well-developed lower limbs, partially developed abdomen and thorax, absent head and upper limbs and diffuse soft tissue edema.

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Figure 2

Sonogram of Case 1 recipient twin demonstrates thickened soft tissue along the dorsal aspect suggestive of cystic hygroma.

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Figure 3

Color Doppler sonographic image of Case 1 demonstrates two vessel umbilical cords with single umbilical artery.

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Figure 4

Pulsed Doppler sonographic image of Case 1 demonstrates reversal of flow in the umbilical artery.

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Case 2

A 22-year-old G3 P1 L1 A1 female who had complaints of lower abdominal pain was referred to our department for sonography. Ultrasound findings showed monochorionic, monoamniotic, twin gestations with one normal twin (pump twin) and the other twin (recipient twin) demonstrating multiple anomalies. The recipient twin had well-developed lower limbs that were directed posteriorly [Figure 5], underdeveloped abdomen with absent anterior abdominal wall, underdeveloped chest with hypoplastic ribs, absent cardia, underdeveloped upper limbs with single forearm bone, oligodactyly and club hand [Figure 6], cystic hygroma with diffuse soft tissue edema [Figure 7], microcephaly with dilated lateral and third ventricles [Figure 7], bilateral cleft lip/palate, and single umbilical artery with reversal of flow in umbilical vessels. The weight of the recipient twin was less than 70% of the pump twin. Polyhydramnios was noted as AFI was 28 cm. Later the patient was subjected to TRAP circulation ablation using a 20-gauge needle and absolute alcohol (0.5 ml) under the guidance of color Doppler sonography. Color Doppler sonography on the next day confirmed the absence of blood flow in the recipient twin. Patient was monitored regularly with gray-scale and Doppler scans. These revealed no evidence of volume overload in the pump twin. At 35 weeks of gestation, emergency lower section caesarean section (LSCS) was carried out, as the patient went into labor. A healthy male twin (pump twin) weighing 2.03 kg and an anomalous twin (recipient) were delivered [Figure 8].

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Figure 5

Sonogram of Case 2 recipient twin demonstrates well-developed lower limbs with diffuse soft tissue edema.

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Figure 6

Sonogram of Case 2 recipient twin demonstrates club hand.

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Figure 7

Sonogram of Case 2 recipient twin demonstrates diffuse soft tissue edema and cystic hygroma (arrow).

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Figure 8

Specimen of Case 2 recipient twin demonstrates microcephaly, cystic hygroma, bilateral cleft lip, club hand, under developed chest and omphalocele.

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DISCUSSION

TRAP sequence is a rare anomaly that occurs in monozygotic twins. The etiology and patho-physiological mechanisms are not well understood. Many investigators have put forward the hypothesis that disruption of organogenesis occurs secondary to the presence of reversed flow through an abnormal artery-to-artery placental anastamosis resulting in cardiac and other malformations.[4] However, other researchers propose the hypothesis that initial abnormal cardiac formation during embryogenesis initiates the development of this anomaly.[5]

In TRAP sequence, the normal twin acts as the ‘pump’ twin which donates blood to the abnormal twin called ‘recipient’ twin through an abnormal artery-to-artery or venous-to-venous communication in the placenta. This results in reversal of flow in the umbilical cord vessels of the recipient twin, oxygenated blood enters the fetus through the umbilical artery and deoxygenated blood leaves the fetus through the umbilical vein, which is opposite to the normal blood flow pattern to a fetus. This has led to the term TRAP sequence for this anomaly.[6] As a result of an imbalance in the inter-fetal circulation, the caudal aspect of the perfused fetus receives blood with relatively more nutrients and oxygen than the upper torso, resulting in better development of pelvis and lower extremities. Fully de-saturated blood then flows in a retrograde fashion to the upper body and head, leading to faulty development of the heart and upper torso, which are either completely absent or severely deformed.[6]

Congenital anomalies are present in about 9% of pump twins. The overall perinatal mortality of pump twins is 50-55%. This is generally due to either polyhydraminos leading to premature delivery or secondary to congestive cardiac failure developing due to the increased cardiac output secondary to the abnormal inter-fetal circulation.[6] This high cardiac output also increases perfusion of the fetal kidneys, resulting in overproduction of fetal urine and polyhydramnios.

The sonographic findings that aid in diagnosis of TRAP sequence are summarized in Table 1.

Table 1 Summary of findings that aid in the diagnosis of TRAP sequence

CONCLUSION