Twin reversed arterial perfusion (TRAP) sequence is considered a severe variant of the twin to twin transfusion syndrome (TTTS). The hallmark of this condition which only happens in monochorionic pregnancies is the lack of placental perfusion of one of the twins (so-called acardiac twin), with all perfusion supplied by the donor twin via an arterial to arterial anomalous anastomosis on the surface of the placenta. This can be shown on Doppler ultrasound by reversal of the acardiac twin umbilical arterial blood (i.e. flow towards the fetus). It can also rarely occur with other multifetal pregnancies 3.
It is thought to occur in ~0.5% (range 0.3-1 % 1,4) of monozygotic pregnancies with an estimated incidence of 1:35,000 births overall 1,10.
TRAP is considered to primarily result from an abnormal placental arterial-to-arterial anastomosis. Occasional venovenous anastomoses have also been described 11.
Classically this results in one normal and one abnormal twin:
- normal twin (pump twin): smaller, viable
- abnormal twin (recipient/acardiac twin): larger, amorphous, non-viable
The acardiac twin is haemodynamically disadvantaged receiving deoxygenated blood from donor twin which then supplies iliac arteries first. This will result in more perfusion compromise in the upper part of the body so atrophy of the heart and dependent organs (brain) are more pronounced. Eventually, a characteristic set of anomalies including acardia and acephalus develop.
- shows one morphologically normal and one abnormal twin, with the latter being seen morphologically abnormal without a heartbeat
- on Doppler interrogation, flow in the acardiac twin umbilical artery is seen entering the fetus and blood leaving via the umbilical vein 13
- development of hydrops fetalis in pump (donor) twin from cardiac failure
Treatment and prognosis
As the acardiac twin is non-viable, the majority of efforts in management are focused or maintaining the viability of the other donor (pump) twin. The perinatal mortality for the pump twin can be high as 50% 10.
Treatment is around the surgical destruction of the inter-twin anastomosis and includes:
- endoscopic laser coagulation/radio-frequency ablation 5
- surgical (fetoscopic) ligation of acardiac twin umbilical cord
- selective delivery of acardiac twin 7
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