Monochorionic multiple gestations
MC twins are at substantially increased risk of adverse outcomes compared to DC twins,44 secondary to typical angioarchitecture of a monochorionic placenta.45Vascular anastomoses maintain a dynamic bidirectional flow between the co-twins. Unique complications such as twin-to-twin transfusion syndrome, twin reversed arterial perfusion sequence, selective intrauterine growth restriction or discordant malformations are encountered in monochorionic gestations due to altered vascular connections. When one of the twins suffers major structural anomaly or shows doppler patterns suggestive of impending demise, ST of the specific fetus is the only therapeutic option available to optimize the survival of co-twin. In case of intrauterine demise of one fetus, co-twin may acutely exsanguinate across vascular anastomoses and die or suffer neurological damage in 30-50% cases.46
ST using various vaso-occlusive techniques that completely occlude flow through the targeted fetus’s umbilical cord have been developed.47 Initial attempts were made with cord ligation or cord embolization with thrombogenic coils or sclerosants. These were often complicated by method failure and high pregnancy loss-rates. Presently techniques used are namely bipolar cord coagulation (BCC), laser cord coagulation, radiofrequency ablation (RFA), microwave ablation and High Intensity Focussed Ultrasound to achieve selective termination. The main complications associated with these interventions are preterm rupture of membranes (PPROM), hemorrhage and PTB.
Bipolar cord coagulation is the standard procedure for ST in monochorionic pregnancies. Under ultrasound guidance, a 3mm port is inserted into the sac of the affected fetus and a bipolar coagulation forceps is passed and used to grasp the cord of the affected fetus which is coagulated using bipolar energy between 20 and 50W. This technique simultaneously obliterates the umbilical arteries and vein, causing immediate cessation of flow and thus preventing any interfetal hemorrhage. Overall survival rates for co-twin are between 76% and 88% with low risk of neurological damage (2.6%).48-50
Radiofrequency ablation uses a 17G radiofrequency electrode to generate high-frequency sinusoidal current (400–500 kHz) which induces local tissue ionic agitation resulting in frictional heat to cause thermal coagulation. Under ultrasound guidance, using local anesthesia RFA needle is inserted into the fetal abdomen at the level of cord insertion, guided by color Doppler mapping. Radiofrequency energy is applied until there is ‘roll-off’. Tissue necrosis results in increased tissue impedance that prohibits the passage of electrical current, hence there is a drop-in power output indicated as roll-off. Coagulative damage is usually restricted to 2cm area around the needle tip. Commonly used devices include LeVeen RFA probe (Boston Scientific, Natick, MA), Starburst SDE_ radiofrequency needle (Angiodynamics, Inc., Queensbury, NY, USA) and Covidien Cool-tip™ RF ablation system (Medtronics) Each function slightly differently, but the net effect achieved is the same. Occlusion of blood flow occurs slowly only once adequate power has been applied to achieve tissue desiccation. This may result in an increased risk of thermal damage to membranes and co-twin demise due to acute exsanguination.
Fetoscopically guided Nd:YAG laser coagulation of the cord or the vessels at the route of the cord has been used for ST in MC gestations. A 400–600-micron laser fiber positioned under ultrasound guidance in the fetoscope and is advanced a few millimeters beyond the tip of the needle into the target tissue. Power is applied in short bursts until the blood flow stops. Coagulation occurs slowly only once adequate thermal energy is applied often requiring multiple applications of laser. Slower occlusion again increases the risks for co-twin.
Newer techniques employed are microwave ablation or High frequency focused ultrasound (HIFU). A thin microwave antenna is placed directly into the fetus using ultrasound guidance. This antenna emits an electromagnetic wave through its exposed, non-insulated tip which agitates water molecules in the surrounding tissue, producing friction and heat, thus, inducing cellular death via coagulation necrosis.51 The antennae are internally cooled with either room-temperature fluid or carbon dioxide to reduce conductive heating and to prevent possible skin damage.
Modality of HIFU has been used to manage TRAP sequence cases.52 This technique involves the transmission of a beam of ultrasound through solid tissue which is focused within the fetus, enabling thermal coagulation necrosis. The chief advantage lies in being truly non-invasive. However only a few reports are published and in one of them the co-twin was delivered with pseudoarthosis52, and the role of ultrasound beam is not clear. The procedure required several attempts to achieve complete occlusion, thus leaving the potential for transfusion-related complications.
Rossi et al50 presented a systematic review of 345 complicated MC twins who underwent ST. Overall survival rate is almost similar for BCC (82%) or RFA (86%). Nearly 22% had PPROM and in one-sixth of cohort, co-twin demise was reported. Roman et al49 also showed no clear superiority of any one technique in their comparative series.
Bebbington53 reported that BCC was associated with a higher rate of overall survival (85.2% v 70.7%) compared to those who had RFA for ST. The difference was attributed to an increased survival rate of the co-twin below 28 weeks (BCC 31.6% Vs RFA 10.5 %) probably as there was a complete cessation of blood flow in BCC cases whereas, in RFA, there is slow cessation.
O’Donoghue54 reported outcomes for the use of interstitial laser in ST of thirty MC pregnancies. Overall perinatal survival was reported in 68% cases. Two of 26 neonates (8%) were diagnosed with aplasia cutis congenita. They postulated that the procedure-related losses may be related to difficulty in maintaining the correct position of the laser fibre during repeated laser applications. However, Jennifer55 demonstrated laser cord occlusion is a reasonable surgical modality for ST in their retrospective review with high PPROM (32.6%) and co-twin survival rates (95.3%).
Meng et al56 described their preliminary novel experience in the application of microwave ablation for ST and reported it to be safe and effective. None of the surviving co-twins had evidence of thermal injury or neurological abnormalities and the overall survival rate was 73.3% (n=33/45).
All techniques for ST are associated with some risk of co-twin demise and complications such as miscarriage, PPROM, preterm birth, etc. (Table 1). Each of ST technique has its own merits, no single perfect technique exists to manage complicated MC gestations. No randomized control studies are available to assess the superiority of one method over others, the evidence is though available through various systematic reviews and metanalyses. The balance between benefits and risks with these procedures is not always completely clear and the decision is usually based on technical considerations, local experience, available instrumentation and above all parental wishes.
Conclusion: Management of multifetal pregnancies is challenging due to the associated high rates of complications. MFPR aims to increase the likelihood of a successful pregnancy and the best current means of evading undesired sequelae of extreme prematurity associated with high-order multiples. Selective termination for anomalies in multifetal pregnancies has significantly advanced with acceptable low procedure-related loss rates. Couples should be provided balanced counseling about benefits and risks associated with these fetal interventions so that they can make informed choices for their pregnancy.
Conflict of Interest: All authors declare that there are no conflicts of interest.
Contribution to authorship: AB did the literature search and wrote the article. YSH gave expert opinion and contributed to the conclusion. All authors approved the final version.
Funding Information: No funding was obtained for this study.