Twin reversed arterial perfusion

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)

2. a superficial artery-to-artery placental anastomosis providing perfusion of the acardiac twin by the donor (pump) twin (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% ^1,4) of monozygotic pregnancies with an estimated incidence of 1:35,000 births overall ^1,10. 

It can rarely occur with higher-order multifetal pregnancies ³.

Associations

• single umbilical artery in acardiac twin ~66% ⁷

• chromosomal anomaly in acardiac twin ~33% ⁷

Pathology

Twin reversed arterial perfusion is considered to primarily result from an abnormal placental arterial-to-arterial anastomosis. Venovenous anastomoses may also be present ¹¹.

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

    • acardius acephalus

    • acardius anceps

    • acardius amorphus

    • acardius acormus

The acardiac twin is haemodynamically disadvantaged, receiving deoxygenated blood from the 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) is more pronounced. Eventually, a characteristic set of anomalies including acardia and acephalus develop. 

Radiographic features

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 ¹⁵

• 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 risk of high-output cardiac failure

Anastomosis

• on Doppler interrogation, flow in the acardiac twin umbilical arteries is seen entering the fetus, with blood leaving via the umbilical vein ¹³

MRI

Fetal MRI may be considered as an adjunct to ultrasound, to evaluate non-cardiac related abnormalities in the pump twin ¹⁵. 

Treatment and prognosis

As the acardiac twin is non-viable, the majority of efforts in management are focused on maintaining the viability of the other donor (pump) twin. The perinatal mortality for the pump twin can be as high as 50% ¹⁰.

Treatment is centered on the surgical destruction of the intertwin anastomosis and includes:

• endoscopic laser coagulation/radiofrequency ablation ⁵

• surgical (fetoscopic) ligation of acardiac twin umbilical cord

• selective delivery of acardiac twin ⁷

Complications

A twin reversed arterial perfusion 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.

See also

• twin to twin transfusion syndrome