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Twin-reversed arterial perfusion / TRAP sequence occurs only in the setting of a monochorionic gestation and complicates approximately 1% of monochorionic twin gestations, with an incidence of 1 in 35,000 births.
In the TRAP sequence, one twin is not viable, lacking a heart (acardiac) and defined anatomical features such as a head (acephalic). While in utero, the acardiac/acephalic twin receives all of its blood supply from the normal or "pump" twin.
It is important to exclude a chromosomal abnormality before offering a fetoscopic procedure in TRAP sequence, because the incidence of chromosomal abnormality in the pump twin may be as high as 9%. Fifty-one percent of TRAP sequence pregnancies are complicated by polyhydramnios, and 75% are complicated by preterm labor.
The difference in estimated fetal weight between the pump twin and the acardiac / acephalic twin is predictive of outcome. When the acardius-to-pump twin weight ratio exceeds 0.5, adverse pregnancy outcome is predicted in 64% of cases. If this weight ratio is greater than 0.7, the adverse pregnancy outcome for the pump twin is approximately 90%.
Techniques of sectio parva (selective removal of an anomalous twin) and ultrasound-guided embolization were used in an attempt to interrupt the vascular communication between the pump twin and the acardius. These procedures have been associated with substantial morbidity and unreliable outcomes, which led to the development of fetoscopic approaches to this problem.
McCurdy and associates were the first to report a case of fetoscopic cord ligation in TRAP sequence. The acardiac / acephalic twin's cord was successfully ligated, but only after the pump twin's cord was ligated and then released after the error was recognized. The pump twin developed persistent bradycardia and was noted to be dead on ultrasound examination on postoperative day one.
Quintero et al, reported the first successful umbilical cord ligation for TRAP sequence. The fetal surgery was performed at 19 weeks of gestation, using two percutaneous trocars and a 1.9-mm endoscope. The cord was successfully ligated, and except for some mild postoperative uterine irritability, the patient responded well.
Three weeks following the procedure the mother presented with leakage of amniotic fluid that subsequently resolved. The pregnancy continued until 36 weeks of gestation, when a healthy boy was delivered.
Several different techniques have been used to treat TRAP sequence by interrupting the connection between the acardiac/acephalic twin and the normal or pump twin to increase the chances that the pump twin will survive. These techniques include cord occlusion by embolization, ligation, laser photocoagulation, monopolar and bipolar diathermy. Intrafetal ablation has also been performed by alcohol injection, monopolar diathermy, interstitial laser, and radiofrequency ablation (RFA).In a review of the various techniques that have been reported to treat TRAP sequence, Tan et al concluded that intrafetal RFA ablation was superior to minimally invasive methods used to coagulate the umbilical cord. RFA was associated with lower rate of premature delivery, rupture of membranes before 32 weeks' gestation (23% vs 58% for other techniques), and a higher rate of clinical success than cord occlusion techniques.
A more recent study by Livingston et al found that primary treatment with RFA is a successful method for pregnancies complicated by TRAP sequence. The study included 17 women with pregnancies complicated by TRAP who were treated with RFA at the Cincinnati Fetal Center. All these women had one or more poor prognostic factors warranting intervention. Sixteen of the other 17 women (94%) treated with RFA delivered a pump twin survivor. The one pump twin who did not survive postoperatively had hydrops (in utero heart failure).
The average gestational age at time of treatment was 21 weeks (range 17-24 weeks), and the average gestational age at delivery was 37 weeks (range 26-39 weeks). The 94% success rate is identical to that reported by Tsao and colleagues who also used an identical RFA treatment method and compares favorably to the 80% survival in a study among 60% women treated with fetoscopic laser photocoagulation. We have performed 80 RFA procedures for TRAP sequence at the Fetal Center.
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