Obstetrica }i Ginecologia LXVI (2018) 129-133 Review article

UNDERSTANDING THE PATHOPHYSIOLOGY OF THE TWIN-TO- TWIN TRANSFUSION SYNDROME

N. Suciu1,2, A.Oncescu1, Andreea-Cătălina Fetecău1, L. Pop1, Oana Toader1,2

1Departament of Obstetrics and Gynecology, “Alessandrescu-Rusescu” National Institute for Mother and Child Care, Polizu Hospital, Bucharest. 2 Department of Obstetrics and Gynecology, ”Carol Davila” University of Medicine and Pharmacy, Bucharest

Abstract

Twin-to-twin transfusion syndrome (TTTS) also known as twin - sequence (TOPS) is considered to be one of the most frequent complications of monochorionic diamniotic . The lies at the core of the TTTS pathophysiology and during the past two decades, numerous studies have been carried out in order to help understand the transfusional mechanisms. The placental anastomoses, which are the primary mediators of transfusional imbalance are incapable of fully explaining the disease and also the negative impact it has on both the renal and cardiovascular systems. The current paper is aimed at providing an insight into the placental and fetal pathophysiology of the syndrome after reviewing the current articles published in the literature.

Rezumat

Sindromul transfuzor-transfuzat (TTTS), cunoscut si sub denumirea de secventa oligohidramnios- polihidramnios (TOPS) este considerat a fi una dintre cele mai frevvente si severe complicatii ce apar la sarcinile gemelare monocoriale, biamniotice. Placentatia jocă un rol substantial în fiziopatologia sindromului transfuzor- transfuzat si, în ultimii 20 de ani, numaroase studii au fost efectuate pentru a ajuta la întelegerea mecanismelor transfuzionale ce stau la baza acestui tip de sindrom. Anastomozele placentare, depistate la sarcinile gemelare cu TTTS si principalii mediatori răspunzători de dezechilibrul transfuzional sunt insuficienti pentru a putea explica aparitia sindromului precum si impactul negativ al acestuia asupra sistemelor renal si cardiovascular fetale. Lucrarea de fată îsi propune să ofere o mai bună cunoastere a fiziopatologiei placentare si fetale ale sindromului după evaluarea articolelor publicate in literatura de specialitate.

Introduction and epidemiology

Twin pregnancies account for 1.2 % of become the gold standard for all stages of TTTS spontaneously conceived pregnancies. Twin-to-twin diagnosed before 26 weeks of gestation. [1] transfusion syndrome (TTTS) is a rare and potentially Out of the naturally conceived twin dangerous complication which occurs in one in five pregnancies one third are monozygotic and of those, monochorionic diamniotic pregnancies. This condition two-thirds are monochorionic with a prevalence of is associated with a high mortality rate and poses 2-3/1000 births. Approximately 10-20% of significant management difficulties despite the recent monochorionic pregnancies will develop TTTS with introduction of the fetoscopic laser therapy which has an overall prevalence of 1-3:10000. In what regards CORESPONDENCE:Nicolae Suciu, mail: [email protected] KEY WORDS: TTTS, placenta, Obstetrica }i Ginecologia 129 Understanding the pathophysiology of the twin-to-twin transfusion syndrome artificially obtained pregnancies, the majority of IVF cases, AV anastomosis in one direction can be twin pregnancies are dizygotic, but monozygosity balanced by those in the other direction which can occurs in 1.5% of pregnancies after ovulation have a compensating effect thus reducing the risk of induction with gonadotropins. The occurrence of developing TTTS. [14] monozygotic pregnancies depends on the day of Arterioarterial anastomoses are superficial embryonic splitting. If the embryo splits before day 3 and can be found on the chorionic plate, similar to after fertilization, two separate with separate VV anastomoses. Through these anastomoses will result. Monochorionic diamnotic flows bidirectionally, the exact amount depending on pregnancies appear whenever the splitting occurs the pressure gradient supplied from each fetal after day 4, whereas division after day 8 results in a circulation. AA anastomoses consist of one artery monochorionic monoamniotic pregnancy. from one and one artery from the other with In monochorionic diamniotic pregnancies, the smaller arteries branching from the anastomoses existence of a single placenta leads to the formation towards cotyledons. VV anastomoses, which are the of vascular anastomoses which stand at the core of least common of all three types of anastomoses, the TTTS. These lead to an unbalanced hemodynamic connect chorionic veins of both fetuses. [12] exchange between fetuses [2]. Anastomoses have been reported in 80% of monochorionic placentations in ex vivo injection Placental Pathophysiology studies, with a median of one anastomosis per placenta. This type of anastomoses can be diagnosed The first description of TTTS was published using Doppler ultrasound which shows bidirectional by Friedrich Schatz in 1875 and later on in 1882. He interference flow pattern. Resistance to flow in these described that the pathophysiological substrate of the superficial true anastomoses is 20 times lower than TTTS is a placental disease which affects both of in the deep transcapillary AV anastomoses which the fetuses. He then proceeded to injecting the fetal suggests that AA anastomoses can compensate placental blood vessels of which unidirectional flow from AV anastomoses in order to showed the intertwin anastomoses, most frequently prevent the chronic blood transfer that results in artery-to-artery (AA), but also artery-to-vein (AV) TTTS. In the presence of deep anastomoses, the risk and vein-to-vein (VV). Due to the fact that the deep of developing TTTS is reduced by superficial anastomoses could not be seen on the placental anastomoses even though it is not fully eliminated. surface, Schatz named them the “third circulation” [8,15] which is responsible for chronic TTTS. The VV anastomoses are the least common type pathophysiological substrate of acute TTTS was of anastomoses. Their incidence is higher in cases acknowledged to be direct superficial anastomoses. complicated with TTTS with a diameter of 2.6 mm. This type of TTTS occurs less frequently but can The presence of these anastomoses is associated with cause acute of both fetuses, especially an overall poorer perinatal survival rate regardless during delivery. [3] of the presence of TTTS. [4] Arteriovenous anastomoses are represented The “deep” anastomoses described by by shared cotyledons and situated deep in the placenta Schatz alter the blood flow between the two fetuses, which allow unidirectional blood flow and can be thus leading to an increase in the net flow from the found in 95% of monochorionic placentation. The donor towards the recipient, leading to hypo- and artery and vein are located close to each other on hypervolemia in the respective twins. Hypervolemia the chorionic plate but the anastomosis is situated in the recipient causes an increase in the renal blood beneath and cannot be seen. A typical AV flow thus leading to polyuria and polyhydramnios anastomosis is formed by the artery of one fetus and which represent the primary changes in the recipient the vein of the other one connected through a capillary twin who is also at risk of developing progressive network which becomes a shared cotyledon. In many cardiomyopathy manifested through cardiac

130 Obstetrica }i Ginecologia Nicolae Suciu dysfunction, atrioventricular valve regurgitation, weeks of gestation. Another parameter which can pulmonary outflow obstruction, abnormal blood flow be evaluated in the second trimester is represented in the ductus venosus and, in the end, . by the thickness of the intertwin membrane. This, In contrast, the donor is characterized by however is highly unreliable as it is associated with hypovolemia and oligo-/anhydramnios, which is significant interobserver variability. In MC, DA associated with an empty bladder, intrauterine growth pregnancies, the intertwin membranes thickness does restriction (IUGR) and abnormal Doppler flow in the not usually succeed 1.5 mm. However, intertwin umbilical artery. [2,16] membranes of more than 1.5 mm are not diagnostic for DC pregnancies in all cases. [13] Diagnosis In the first stage of the Quintero classification, the mildest form of TTTS, the sole In the vast majority of cases, the clinical modification is represented by abnormalities in manifestations of TTTS appear during 16 and 24 production with polyhydramnios in the weeks of gestation and can be classified using the recipient and oligohydramnios in the donor twin. The Quintero classification which is used worldwide and bladder of the donor is still visible. In the recipient, defines prognostic subgroups according to the severity hypervolemia leads to atrial distension and of the disease. consequently to the release of atrial (ANP) and brain Once a twin gestation has been diagnosed, natriuretic peptide (BNP). Despite the fact that the the following step is to establish chorionicity. The renin-angiotensin system (RAS) is suppressed in the difference between momochorionic (MC) and recipient twin, the blood levels of renin and dichorionic (DC) pregnancies can be made before angiotensin are elevated due to the transplacental the 14th week of pregnancy and in order to evaluate transfer from the donor and also increased production chorionicity the intertwin membrane should be by the placenta. Both ANP and BNP are diuretics visualized ai its insertion into the placental mass. If and vasodilators which lead to polyuria and thus to a the extension of the chorionic tissue into the base of distended bladder, failure to complete bladder the intertwin membrane can be visualized, then this emptying and polyhydramnios (greater than 8 cm is highly suggestive for a DC placentation, which is maximal vertical pocket). In the donor twin the renal called the lambda or twin peak sign. If, instead, a T- RAS is upregulated which promotes vasoconstriction shaped junction (T sign) is detected at the fusion of and oliguria. The clinical signs of oliguria in the donor the two amniotic membranes, and , twin are represented by decreased bladder distension, without any intervening chorionic tissue, the diagnosis decreased frequency of bladder emptying. These is represented by monochorionic diamniotic findings are associated with decreased urine pregnancy. [7] production and also a decrease in the amniotic fluid Even though two separate placentas can be volume which leads to oligohydramnios (less than 2 visualized this criterion alone is not enough to diagnose cm maximal vertical pocket). DC placentation as in approximately 3% of In Quintero stage II, the donor twin becomes monochorionic pregnancies two separate placentas “stuck” as it has little room to move in the can be observed. Similarly, a single placental mass anhydramniotic sac, without bladder filling. does not prove MC placentation due to the fact that In stage III, fetal Doppler flow is abnormal. in 50% of DC cases the two placental masses appear In the donor twin, there is absent or reversed end- fused and resemble a single placental mass on diastolic flow in the umbilical artery, and in the ultrasound. [7] recipient there is absent or reversed flow in the ductus In the second trimester of pregnancy, after venosus. 14 weeks of gestation the ability to determine Stage IV is characterized by fetal hydrops chorionicity is significantly reduced. The lambda sign and stage V by fetal death. Wee and Fisk suggested can no longer be visualized in 7% of DC after 20 that a rapid progression to a higher stage is the best

Obstetrica }i Ginecologia 131 Understanding the pathophysiology of the twin-to-twin transfusion syndrome predictor of poor outcome. Kagan et al. showed that converting enzyme (ACE) produced by the lungs. discordance in the nuchal translucency thickness of Therefore, AT2 is the active hormone which interacts the two twins at 11 to 14 weeks of gestation can be with the angiotensin receptors AR1 and 2. The RAS used to predict TTTS. A discordance of more than system is activated in the late first and second 20% increases the risk of severe TTTS, necessitating trimester of pregnancy. Decreased renal perfusion intrauterine therapy to 30%, whereas the risk is less in the donor twin leads to an increase in the renin than 10% if the discordance is less than 20%. Other level which is virtually absent in the recipient twin. risk factors for the development of severe TTTS are Despite these considerations, blood levels of renin membrane folding at 15–17 weeks, the absence of are shown to be similar in both donors and recipients arterio-arterial anastomoses and velamentous cord which can be explained by the transfer of RAS insertion. [11] components from donor to recipient. [5] [20] In what concerns the cardiovascular system, Fetal Pathophysiology the cardiac changes can be seen exclusively in the recipient in Stage IV, even though some abnormalities The complex mechanism through which the can be visualized using Doppler ultrasound in the modified hemodynamics induced by the TTTS donor as well. mediates the pathological changes in the twin fetuses One marker of cardiovascular dysfunction is yet to be thoroughly understood. All major organ is represented by the left-sided myocardial systems are concerned. performance index (MPI) which evaluates the systolic The oligo-polyhydramnios sequence, highly and diastolic myocardial function and is significantly representative for Quintero stage 1, is caused by the higher with a lower ejection fraction (EF). Another polyuric, volume-overloaded recipient and an oliguric, marker consists of atrioventricular valve regurgitation volume-depleted donor. If the hypoperfusion is represented mainly by mitral valve regurgitation which prolonged, this leads to an underdeveloped renal occurs in 13% to 40% of cases. However, tricuspid system characterized by renal tubular dysgenesis valve regurgitation is more common, suggesting also, which is a common feature. [23] that right-sided dysfunction is more frequent this being In both fetuses, major renovascular hormonal attributed to volume overload of the heart. [19] changes can be identified which are involved in coping The risk for congenital heart disease is with the hemodynamic insults induced by the TTTS. increased 13-14 fold in MC pregnancies complicated ANP produced in the cardiac atria is involved in the with TTTS. This is associated with cardiomegaly, production of amniotic fluid in the recipient and is pulmonary stenosis and atrial septal defects, the first consistent with polyhydramnios. Vasopressin (AVP) two being attributed to the modified intracardiac is an antidiuretic hormone releases in response to hemodynamics in the recipient twin. The hypovolemic or hypoosmotic conditions. Fetal AVP cardiothoracic ratio is larger in recipients compared contributes to water retention, increases urine to donors with a reported frequency from 17% to osmolarity and induces peripheral vasoconstriction. 100% compared to non-TTTS twins. [5] The AVP levels are significantly higher in the donor and lower in the recipient. In cases with hydrops, Conclusions AVP levels are high in both the recipient and the donor. [6,22] TTTS is associated with an increased risk The Renin Angiotensin System (RAS) plays of neonatal mortality and morbidity and represents a key role in the pathophysiology of TTTS. one of the main reasons why MC DA pregnancies Angiotensinogen is produced by the liver and need to be closely monitored. The high rate of converted to angiotensin 1 (AT1) under the action of complications in MC twins is frequently linked to renin produced by the kidney which in turn is which has not only a social but also converted to angiotensin 2 (AT2) by angiotensin economic toll. The Quintero Staging System plays a

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