TRAP sequence

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Twin reversed arterial perfusion (TRAP) sequence is a rare complication of monochorionic pregnancies. It develops when the following conditions are present:

1. lack of a well-formed heart in one of the twins (so-called acardiac twin), and

2. a superficial artery to artery placental anastomosis providing perfusion of the acardiac twin by the donor (pump) twin. This can be shown on Doppler ultrasound by reversal of the acardiac twin umbilical arterial blood (i.e. flow towards the fetus).

Epidemiology

It is thought to occur in ~0.5% (range 0.3-1 % )of monozygotic pregnancies with an estimated incidence of 1:35,000 births overall .

It can rarely occur with higher-order multifetal pregnancies .

Pathology

TRAP is considered to primarily result from an abnormal placental arterial-to-arterial anastomosis. Venovenous anastomoses may also be present .

Classically this results in one normal and one abnormal twin:

viable twin (pump twin)
- usually normal, but 9% risk of abnormal karyotype
non-viable twin (recipient/acardiac twin)
- this twin can have four morphological types

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.

Associations

single umbilical artery in acardiac twin ~66%
chromosomal anomaly in acardiac twin ~33%

Radiographic features

Antenatal ultrasound

Acardiac twin

severely abnormal, with variable degrees of morphologic development (see morphologic types above).
the heart is absent or structurally abnormal, with <20% of fetuses having identifiable cardiac tissue
most commonly the head and upper extremities are absent or severely underdeveloped, while the lower body is better developed
may be amorphous with no distinguishable anatomic structures
severe subcutaneous edema and cystic hygromas common
single umbilical artery common

Pump twin

often, but not invariably is structurally normal.
multiple anomalies have been described, including
at risk high output cardiac failure

Anastomosis

on Doppler interrogation, flow in the acardiac twin umbilical arteries seen entering the fetus and blood leaving via the umbilical vein

Fetal MRI may be considered as an adjunct to ultrasound, to evaluate non-cardiac related abnormalities in the pump twin .

Complications

TRAP sequence pump twin is at risk of:

cardiac failure
cerebral ischemic sequelae
preterm birth
fetal demise in utero

The risk of adverse outcomes in the pump twin is closely related to its size ratio to the acardiac twin.

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 as high as 50% .

Treatment is around the surgical destruction of the inter-twin anastomosis and includes:

endoscopic laser coagulation/radio-frequency ablation
surgical (fetoscopic) ligation of acardiac twin umbilical cord
selective delivery of acardiac twin