Twin Reversed Arterial Perfusion Sequence

Twin reversed arterial perfusion (TRAP) sequence (acardiac twinning) is a rare complication of monochorionic twin pregnancies (1 in 35,000 pregnancies, 1% of monochorionic twins).

One twin (the "pump" twin) has a normal heart; the other (acardiac twin, absent or rudimentary heart) is perfused by retrograde flow through an artery-to-artery and a vein-to-vein anastomosis, with deoxygenated blood from the pump twin, causing disruption of development of the head and upper body.

The pump twin is at risk of high output cardiac failure, polyhydramnios, hydrops, preterm birth and death (mortality about 50 to 55% if untreated; risk is higher if the acardiac twin is more than 50% of the pump twin's weight).

• Usually asymptomatic early; polyhydramnios (common), preterm labour, and maternal discomfort later
• Acardiac twin: severely malformed (acephalus, acormus, amorphous mass) and anechoic subcutaneous oedema
• Pump twin: normal anatomy but with signs of cardiac strain (cardiomegaly, pericardial effusion, tricuspid regurgitation, polyhydramnios, hydrops, abnormal ductus venosus waveform)

• Ultrasound at 11 to 14 weeks: monochorionic twin with one fetus lacking cardiac activity and reversed umbilical artery flow direction (flow towards the acardiac fetus in the umbilical artery and away in the vein) on colour Doppler; the acardiac mass may continue to grow
• Prognostic measures: ratio of the acardiac twin's estimated weight to the pump twin's weight (above 70% poor), size of the acardiac twin, presence of polyhydramnios, hydrops, pump twin cardiac function, and acardiac twin movement
• Fetal echocardiography of the pump twin
• Karyotype or microarray of the pump twin (acardiac twin is often aneuploid)

• Expectant management is possible for small acardiac twins (ratio below 50%) with normal pump twin, with close ultrasound surveillance every 2 weeks (growth, fluid, Doppler, fetal echocardiography); about 30 to 40% eventually need treatment.
• Intervention (typically at 12 to 16 weeks, once the fetuses are sufficiently developed): selective occlusion of blood flow to the acardiac twin by radiofrequency ablation (preferred, using a 14 to 17 gauge needle inserted into the intra abdominal cord insertion of the acardiac twin), bipolar or laser cord coagulation, or interstitial laser; the pump twin survival is about 80 to 90%. Early intervention at 12 to 14 weeks provides similar survival with fewer complications.
• Complications: PPROM (about 10 to 20%), preterm delivery (average about 35 weeks), pump twin demise (about 10 to 15%), and fetal loss.
• Post-procedure follow up: ultrasound weekly for 2 weeks and then every 2 to 4 weeks, with antenatal steroids at 24 to 34 weeks if preterm delivery threatened; delivery at 36 to 38 weeks.
• Neonatal: paediatric review, genetic evaluation.