Twin reversed arterial perfusion (TRAP) sequence: In-utero treatment

Background: Twin reversed arterial perfusion (TRAP) sequence or acardiac twinning is a rare congenital anomaly unique to monozygotic multiple gestations. Existence of TRAP requires two conditions: (1) pump or forward flow failure in the acardiac twin and (2) a set of arterioarterial and venovenous placental anastomoses connecting the acardiac and “pump” twins’ circulatory systems. It is considered the most severe malformation of twinning seen in humans. Other historical though less commonly used terms include chorioangiopagus parasiticus and acardiac monster. Incidence is estimated to be 1:35000 pregnancies or 1% of all monochorionic pregnancies [1, 2], though it may be higher given that some cases of TRAP may go undiagnosed due to early pregnancy loss or reabsorption of the acardiac twin [3]. Pathophysiology: In TRAP, one twin, the acardiac, has a non-functioning (pseudoacardius) or absent (holoacardius) heart. This twin is hemodynamically dependent on the structurally normal donor or “pump” twin. Blood is shunted from the “pump” twin, bypassing the placenta, in a retrograde fashion through anomalous arterioarterial anastomoses and perfuses the acardiac twin [4]. The blood reaching the acardiac twin is poorly oxygenated and nutrient deplete. Once in the acardiac’s circulatory system, blood flows passively through the iliac arteries, first perfusing the lower body and extremities. The further deoxygenated blood then perfuses the caudal structures and subsequently flows via the acardiac’s umbilical vein back to the “pump” twin through direct venovenous anastomoses (see Figure 12.2.1). Thus, blood returning to the “pump” twin is a mix of oxygenated blood from the placenta and deoxygenated blood from the acardiac twin.