- Affects 1% of monochorionic twin pregnancies and is caused by abnormal vascular connections between a pump twin and an acardiac mass.
TRAP sequence occurs when a normal or ‘pump’ twin perfuses an acardiac mass via placental vascular connections that are found only in monochorionic placentae. The pump twin perfuses the acardiac mass with deoxygenated blood in a retrograde manner via a placental arterioarterial anastomosis and the acardiac mass returns further deoxygenated blood back to the pump twin through a direct venovenous anastomosis.
The pump twin can be compromised through at least three mechanisms:
- Congestive heart failure and polyhydramnios: Caused by a risen cardiac work due to the increased blood flow;
- Preterm premature rupture of membranes (pPROM), preterm labor and preterm delivery: Caused by uterine overdistension, since the acardiac twin is often bigger than pump twin and it can reach a considerable size;
- Hypoxia and intrauterine growth restriction: Caused by the deoxygenated blood that comes back to the pump twin through vascular anastomosis.
Based on the morphology of the acardiac fetus, four distinct types have been described:
- Acardius acephalus (M/C type, 60–75% cases): Good development of pelvis and legs and an absence of cephalic pole, thoracic organs and upper extremities.
- Acardius anceps (~10% cases): M/morphologically developed acardius, with a recognizable body shape and extremities and a rudimentary development of head and face.
- Acardius acormus (very rare, ~5% cases): Only presence of the cephalic pole, while the body (if present) is a shriveled mass. The head can be attached to the placenta either directly or through the umbilical cord.
- Acardius amorphus (~20% cases): Absence of recognizable structures; the fetus appears as a shapeless mass
- Gross differences in biometrical measurements of twins
- Absence of morphologically normal heart in one twin associated with several other malformations in head, trunk, upper and lower extremities
- Subcutaneous edema and fluid collections in anomalous twin
- Color doppler (CHARACTERISTIC): Paradoxical circulation in the acardiac twin, with arterial blood flowing towards, rather than away and in a caudal-to-cranial course in the abdominal aorta, which may be evident even during the first trimester.
The main goals in the management of the TRAP sequence are preserving the survival of the pump twin and reaching the term for delivery.
Surgical management:Technique of choice to interrupt the vascular supply to the acardiac twin:
- Ultrasound-guided laser coagulation
- Radiofrequency ablation of intrafetal vessels