Twin reversed arterial perfusion syndrome (TRAP or acardiac twin)

Twin reversed arterial perfusion syndrome (TRAP or acardiac twin)


Twin-reversed arterial perfusion (TRAP) sequence is a rare condition (0.3:10,000 births) that occurs in monochorionic twin pregnancies, resulting in coexistence of a normal ‘pump’ twin and an acardiac ‘recipient’ twin. The proposed pathogenesis is the association of paired artery-to-artery and vein-to-vein anastomoses within the placenta combined with delayed cardiac function of one of the twins early in pregnancy. This situation allows blood pumped from the healthy twin to perfuse retrogradely the heart of the other twin. This retrograde perfusion interferes with normal cardiac development, which rarely progresses beyond the formation of a tubular heart. The upper half of the recipient is poorly developed. Head, cervical spine and upper limbs are usually absent. Edema and sonolucent areas in the upper body, consistent with cystic hygroma, are common. The lower half of the body, although malformed, is better developed. This pattern may be explained by the mechanism of perfusion of the acardiac twin. Blood that enters the abdomen of the fetus is deoxygenated blood that left the normal twin. Occasionally the umbilical artery of the acardiac twin connects to the superior mesenteric artery (instead of the iliac artery). This represents a persistence of the vitelline supply. Most of the oxygen available is extracted when the blood enters the acardiac twin, allowing for some development of the lower body and extremities. Once blood reaches the upper half of the body, oxygen saturation is extremely low, halting development in this area. The degree of caudal and rostral development determines the classification of the recipient twin: · Acephalus – no cephalic structure present · Anceps – some cranial structure and/or neural tissue present · Acormus – cephalic structure but no truncal structures · Amorphus – no distinguishable rostral or caudal structure The recipient twin is essentially a parasite. It requires blood pumped from the normal twin to keep developing, putting the pump fetus at risk of high output cardiac failure. The risk is directly dependent on the size of the recipient twin: the higher its weight, the higher the risk of cardiac failure and death for the normal twin. Overall only 50% of pump twins survive. Management has consisted in termination of pregnancy, or interruption of flow to the recipient fetus by open or fetoscopic umbilical cord ligation, embolization, or laser vaporization. Conservative management aimed at supporting the cardiac function of the pump twin with digoxin has also been attempted. Large numbers are not available to compare the various techniques.


The pathognomonic finding is of reversed arterial perfusion. On imaging the umbilical cord with color and pulsed wave Doppler, arterial waveforms are observed from the placenta towards the acardiac twin. Venous waveforms are noted in the opposite direction. The recipient twin will demonstrate absent or impaired development of the cephalic pole, heart, upper limbs and viscera; there will be relative preservation of the lower limbs. Talipes is common. There may be inconsistent or incomplete membrane development between the twins.

Differential Diagnosis

Twin-to-twin transfusion syndrome may occasionally resemble a TRAP, but may be excluded by the definition of a dividing membrane and of cardiac activity in the smaller fetus. Fetal demise in a twin pregnancy may also resemble an acardiac, but there should be no Doppler signal within the dead fetus.

Sonographic Features

Reversed arterial perfusion to one twin (recipient)

Impaired or absent development in recipient of: cephalic pole heart upper limbs

Incomplete dividing membrane

Associated Syndromes