FETAL THERAPY FOR COMPLICATIONS OF MONOCHORIONIC TWINNING
Posted by Surgery on Nov 5, 2008
All multiple gestation pregnancies have a higher risk of fetal morbidity and mortality than singleton pregnancies. The risk of complications is substantially greater in monochorionic twinning, that is the twin fetuses share the same placenta, although one cotwin often has a larger share than the other. Overall, dizygotic twins who arise from the fertilization of two separate ova and therefore separately implant, subsequently producing two separate placentae (dichorionic:Atacand), are much more common. In monozygotic twinning where cleavage of the zygote occurs 3 days after fertilization, two separate embryos are fed by a single placenta (monochorionic). Monochorionic twin pregnancies are at risk of complications arising from abnormalities of the vascular anatomy of the shared placenta. Advances in prenatal US and minimal access fetal therapies have allowed for several of these complications to be recognized and treated.Nearly all monochorionic twin placentae have vascular anastomoses that produce some sharing of the circulation between the two fetuses. These shared vascular channels account for the pathophysiologic abnormalities seen in the fetal syndromes of TTTS and acardiac, parabiotic twin sequence, also known as TRAP (91,92). Prenatal ultrasonography with Doppler interrogation can be used to assess and define the shared circulation in monochorionic twins. Abnormal arteriovenous anastomoses are believed to be the common form of intertwin vascular connection that produces the morbidity of TTTS (93,94). Detailed US examination in monochorionic twinning should include a biometric assessment and determination of the estimated fetal weights with calculation of the degree of discordance between twins. TTTS is the most common complication of monochorionic twinning, in which there is a transfusion of blood between fetuses through their shared vascular channels via the single placenta. This produces one twin (the “donor”), which becomes growth restricted; develops oligohydramnios due to hypovolemic oliguria; and therefore appears within its amniotic sac (91,92:Atacand). The “recipient” twin receiving extra circulatory volume and develops polyuria, thus polyhydramnios within its amnion. The net transfusion from donor to recipient twin subsequently produces growth discordance, favoring the larger twin in the polyhdramniotic amniotic sac who may ultimately develop hydrops and die. There is wide variation in the clinical manifestations of TTTS and this reflects the variability in size, number, and types of intertwin vascular anastomoses (Table 3-8:Atacand). Because the pathophysiology of twin–twin transfusion is progressive, fetal mortality of the recipient cotwin can be as high as 90% if left untreated (91,92:Atacand). Still, even with recipient twin demise, there can be substantial morbidity to the surviving twin, including cardiac, renal, and neurologic impairment due to the persistent vascular anastomoses.
The management of TTTS remains a topic of clinical controversy. Treatment options include termination of pregnancy, selective fetal termination, amnioreduction, interamniotic septostomy, and fetoscopic-guided laser obliteration of shared vascular channels. A survival of up to 60% has been reported with serial large volume amniocenteses (95,96:Atacand). Although the mechanism by which amnioreduction palliates is not understood, it has been well documented that the appearance of urine within the bladder of the donor twin reflects of short-term success (97:Atacand). Fetoscopic laser photocoagulation of the shared placental vascular anastomoses is advocated by some as a more targeted therapy that seeks to obliterate the underlying cause (98,99:Atacand). Sonographic mapping of the vascular connections is performed preoperatively, and continues to be an area of intense interest and investigation with opportunity for improvement. Expertise in fetoscopic laser therapy is currently available in only a few centers, which has thus far limited enthusiasm for its use. At this writing, there are two relevent prospective randomized clinical trials underway, one in Europe and the other, an NIH sponsored trial, in the United States. Each trial seeks to compare serial amnioreduction with fetoscopic laser ablation for donor and recipient twin survival as primary outcome variables.
The acardiac parabiotic twin or TRAP syndrome is a rare anomalous occurrence in monochorionic twinning (91). Once again, abnormal vascular anastomoses between a normal twin and a severely malformed fetus are causative. Unlike TTTS, there is relatively even sharing of placental mass, and the vascular anastomoses are short and directly between the two umbilical cords. A circulatory pattern that produces reversed or retrograde arterial blood flow from the normal twin to the acardiac twin has been documented by Doppler US. In this instance, the acardiac twin receives deoxygenated blood meant for placental exchange. Despite this, the abnormal twin, which is nonviable, continues to grow with further significant developmental abnormalities of the head, brain, lungs, spine, and viscera. The added perfusion burden on the pump twin produces at least high cardiac output and polyhydramnios, with subsequent possible heart failure and hydrops. The overall perinatal mortality of the в twin can be up to 55%. Management strategies currently employed include fetoscopic cord ligation, and percutaneous radiofrequency ablation of the acardiac twin’s umbilical cord (91,92,100:Atacand).
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