In twin reversed arterial perfusion (TRAP) sequence, blood flows via an arterioarterial anastomosis from an umbilical artery of a pump twin into the umbilical artery of an acardiac twin, and usually returns through a venovenous anastomosis to the pump twin. The direction of blood flow in the umbilical artery and vein of the acardiac twin is therefore reversed in comparison to that in normal fetal circulation. The acardiac twin in a monochorionic twin pair presents with abnormal development, which may result from an early intrauterine demise with further cell growth and differentiation supported by the existing anastomoses[1-3].
We report a case of a dichorionic triamniotic triplet pregnancy with an acardiac twin within the monochorionic pair presenting as an intrauterine cyst. The pregnancy was conceived after ovulation induction. One of the fetuses, a female, presented normal development. The second fetus, a male, presented with a complex cardiac malformation (hypoplastic right heart, ventricular septal defect, double outlet right ventricle and transposition of the great arteries with pulmonary stenosis). Amniocentesis was performed for the second fetus and findings were normal on molecular karyotyping. This fetus had a connection through an arterioarterial anastomosis with an unusual acardiac twin, which presented as an intrauterine cyst (50 mm in diameter) containing an amorphous hyperechoic mass with a vascular connection to the placenta (Figure 1). The vascular supply to the acardiac twin was minimal and remained stable throughout pregnancy. The second fetus developed polyhydramnios and, due to preterm labor, the infants were delivered at 32 weeks, the girl weighing 2040 g and the boy (with cardiopathy) weighing 1640 g. At the time of writing, both infants were doing well at 2 months of age. Pathological examination confirmed the presence of an acardiac twin in the monochorionic twin pair.
TRAP sequence is a rare condition, occurring in about 1% of monochorionic twin pregnancies. It is associated with a high risk of perinatal death, caused by a combination of high-output cardiac failure and polyhydramnios-related preterm birth. The outcome may be improved by intervention to arrest the circulation of the acardiac twin. The technique used depends on the clinical presentation, with available methods including coagulation of the umbilical cord and/or placental anastomoses by intrafetal laser or radiofrequency ablation[4-7].
We suspect that in this pregnancy, a small vascular connection resulted in the continuation of some cell growth following the early fetal demise of one of a monochorionic twin pair, but that the minimal blood flow involved did not put the surviving fetus at risk for decompensation.