Fetal and postnatal brain MRI in premature infants with twin-twin transfusion syndrome

Fetal neurosurgery

Among fetal surgical procedures, neurosurgery stands out due to the number of cases and the possibility of developing new procedures that can be performed in the fetal period. To perform fetal neurosurgical procedures, there is a need for specialized centers that have experts in the diagnosis of fetal pathologies and a highly complex obstetrics service with specialized maternal-fetal teams associated with a pediatric neurosurgery center with expertise in the diverse pathologies of the fetus and the central nervous system that offers multidisciplinary follow-up during postnatal life. Services that do not have these characteristics should refer their patients to these centers to obtain better treatment results. It is essential that the fetal neurosurgical procedure be performed by a pediatric neurosurgeon with extensive experience, as he will be responsible for monitoring these patients in the postnatal period and for several years. The objective of this manuscript is to demonstrate the diagnostic and treatment possibilities, in the fetal period, of some neurosurgical diseases such as hydrocephalus, tumors, occipital encephalocele, and myelomeningocele.

Incidence and outcome of prenatal brain abnormality in twin-to-twin transfusion syndrome: systematic review and meta-analysis

OBJECTIVES

To evaluate the incidence of antenatally diagnosed brain injury in twin pregnancy complicated by twin-to-twin transfusion syndrome (TTTS) and to quantify the perinatal mortality, morbidity and long-term neurodevelopmental outcome of these fetuses.

METHODS

MEDLINE, EMBASE, ClinicalTrials.gov and The Cochrane Library databases were searched. Inclusion criteria were studies reporting on brain abnormality diagnosed antenatally in twin pregnancies complicated by TTTS. The primary outcome was the incidence of prenatal brain abnormality. The secondary outcomes were intrauterine demise (IUD), neonatal death, termination of pregnancy (TOP) and long-term morbidity. Outcomes were explored in the population of fetuses with antenatal diagnosis of brain abnormality. Subgroup analysis according to the type of treatment, gestational age, Quintero stage at diagnosis and/or treatment, and cotwin death was planned. Meta-analysis of proportions was used to combine data and pooled proportions and their 95% CI were reported.

RESULTS

Thirteen studies including 1573 cases of TTTS and 88 fetuses with an antenatal diagnosis of brain abnormality were included in the systematic review. The meta-analysis included only studies reporting on brain abnormality in twin pregnancy complicated by TTTS cases and treated with laser surgery. Overall, brain injury occurred in 2.2% (52/2410) of fetuses (eight studies). Brain abnormality was reported in 1.03% and 0.82% of recipients and donors, respectively. The most common type of abnormality was ischemic lesions (30.4% (95% CI, 19.1-43.0%)), followed by destructive lesions (23.9% (95% CI, 13.7-35.9%)), ventriculomegaly (19.9% (95% CI, 10.6-31.3%)) and hemorrhagic lesions (15.3% (95% CI, 7.1-25.8%)). Spontaneous IUD occurred in 13.4% (95% CI, 5.1-24.8%) of fetuses, while TOP was chosen by parents in 53.5% (95% CI, 38.9-67.8%) cases. Neonatal death was reported by only three studies, with an incidence of 15.4% (95% CI, 2.8-35.4%). Finally, only two studies reported on composite morbidity, with an overall rate of the outcome of 20.4% (95% CI, 2.5-49.4%) and rates of 29.7% and 20.4% in the recipient and donor fetuses, respectively. Due to the small numbers, only composite morbidity was analyzed and no information on neonatal intensive care unit admission, respiratory distress syndrome or other long-term outcomes, such as neurodevelopmental delay and cerebral palsy, could be retrieved reliably.

CONCLUSIONS

The overall incidence of antenatally diagnosed fetal brain abnormality in twin pregnancy complicated by TTTS treated with laser surgery is around 2% and is mainly ischemic in nature (30.4%). TOP was chosen by parents in more than half of cases (53.5%). No information could be retrieved on morbidity outcomes, highlighting the urgent need for long-term follow-up studies of these children. © 2022 International Society of Ultrasound in Obstetrics and Gynecology.

Twin-to-twin transfusion syndrome complicated with in utero limb ischemia of the donor twin - a case report

Background

In utero limb ischemia is a rare complication of the monochorionic twin pregnancies complicated with twin to twin transfusion syndrome (TTTS). The condition is more often seen in recipient twins. There are few theories of the pathogenesis including in utero venous thromboembolism, but the cause remains unclear. However, limb ischemia is thought to be unrelated with any prenatal intervention.

Case Presentation

We present a case of a monochorionic twin pregnancy complicated with TTTS admitted to the Clinic for selective fetoscopic laser photocoagulation. The invasive procedure failed due to poor visibility. In the following weeks of pregnancy, amnioreduction procedures were performed. At 28 weeks of gestation due to twin anemia-polycythemia sequence diagnosis the patient was qualified for cesarean section. Postnatally, the donor twin was diagnosed with lower right limb ischemic necrosis. The extremity was amputated 2 days later with an uncomplicated recovery. After speculations of the potential pathogeneses it was suggested that the ischemic limb occurred as a complication of the main condition – TTTS.

Conclusions

In literature, there have been no cases reported of TTTS stage I complicated with donor twin limb ischemia. The actual cause of the in utero limb ischemic necrosis in monochorionic twins remains unknown. Nevertheless, increased attention to the potential complication after failed invasive procedures or conservative treatment should be required.

Twin-to-Twin Transfusion Syndrome: Diagnostic Imaging and Its Role in Staving Off Malpractice Charges and Litigation

The study aims to expound upon the imaging-based diagnostic methodologies aimed at identifying twin-to-twin transfusion syndrome (TTTS), a serious, somewhat rare prenatal condition that takes place in pregnancies where identical twins, or other multiples, share a placenta (monochorionic placenta), highlighting how medico-legal outcomes can be affected by provable compliance with consolidated diagnostic guidelines or best practices. It is of utmost importance to produce a prompt identification of TTTS instances; an early diagnosis is in fact critical in order to effectively treat and manage TTTS. By virtue of TTTS being a highly progressive condition, a delay in diagnosis can result in disastrous outcomes; just a few weeks delay in the diagnosis of TTTS can turn out fatal for one or both twins. Hence, most TTTS malpractice claims involve allegations of medical negligence, namely the failure to recognize the condition in a timely fashion, or to proceed with adequate diagnostic and therapeutic pathways. In that regard, case law databases have been pored over (Justia, Lexis, Leagle), and five significant court cases have been examined and discussed in an attempt to identify objective medico-legal standards and bring to the forefront relevant forensic dynamics. In fact, when health professionals are capable of proving adherence to guidelines or best practices, this can shield them from malpractice allegations and ensuing litigation.

Antenatal diagnosis of fetal intraventricular hemorrhage: systematic review and meta-analysis

AIM

To determine how the severity of antenatally diagnosed germinal matrix-intraventricular hemorrhage (GMH-IVH) relates to morbidity and mortality, and to explore potential risk factors.

METHOD

We conducted a systematic review and individual patient data meta-analysis of antenatally diagnosed fetal GMH-IVH. The primary outcomes were mortality and morbidity. Potential associations with clinical factors during pregnancy were explored. Analysis employed Fisher's exact test and logistic regression.

RESULTS

We included 240 cases from 80 studies. Presence of venous infarction was associated with mortality (odds ratio [OR] 4.3, 95% confidence interval [CI] 1.4-13.25), motor impairment (OR 103.2, 95% CI 8.6-1238), epilepsy (OR 6.46, 95% CI 2.64-16.06), and developmental delay (OR 8.55, 95% CI 2.12-48.79). Shunt placement was associated with gestational age at GMH-IVH diagnosis and in utero progression. Many cases had uncomplicated pregnancies but possible co-occurring conditions included twin gestation, small for gestational age, and congenital anomalies.

INTERPRETATION

Severity of fetal GMH-IVH, specifically venous infarction, is associated with overall mortality and morbidity. Risk factors for fetal GMH-IVH are poorly understood and controlled studies are required.

WHAT THIS PAPER ADDS

Preterm germinal matrix-intraventricular hemorrhage (GMH-IVH) grading can be applied to fetuses. Many fetal germinal matrix hemorrhages occur in otherwise typical pregnancies. Half of fetuses with post-hemorrhagic ventricular dilatation receive a shunt after delivery. Fetuses with grade I or II GMH-IVH have few sequelae. Fetuses with periventricular hemorrhagic infarction have a high burden of motor impairment.

Perinatal outcomes of twin pregnancies complicated by late twin-twin transfusion syndrome: A systematic review and meta-analysis

INTRODUCTION

Untreated twin-to-twin transfusion syndrome (TTTS) is associated with a high risk of perinatal mortality and morbidity. Laser surgery is recommended before 26 weeks of gestation. However, the optimal management in case of late TTTS (occurring after 26 weeks of gestation) is yet to be established.

MATERIAL AND METHODS

We conducted a systematic review and meta-analysis to evaluate the outcomes of monochorionic-diamniotic twin pregnancies complicated by late TTTS according to different management options (expectant, laser therapy, amnioreduction, or delivery). The primary outcome was mortality, including single and double intrauterine, neonatal, and perinatal death. Secondary outcomes were composite morbidity, neuromorbidity, respiratory distress syndrome, admission to neonatal intensive care unit, intact survival (ie, free from neurological complications), and preterm birth before <32 weeks of gestation. Outcomes were reviewed according to the management and reported for the overall population of twins and disease status (ie, donor and recipient separately). Random-effect meta-analyses of proportions were used to analyze the data.

RESULTS

Nine studies including 796 twin pregnancies affected by TTTS were included. No randomized controlled trials were available for inclusion. TTTS occurred at ≥26 weeks of gestation in 8.7% (95% CI 6.9%-10.9%; 67/769) of cases reporting TTTS at all gestations. Intrauterine death occurred in 17.7% (95% CI 4.9%-36.2%) of pregnancies managed expectantly, 5.3% (95% CI 0.9%-12.9%) of pregnancies treated with laser, and 0% (95% CI 0%-9%) after amnioreduction. Neonatal death occurred in 42.5% (95% CI 17.5%-69.7%) of pregnancies managed expectantly, in 2.8% (95% CI 0.3%-7.7%) of cases treated with laser, and in 20.2% (95% CI 6%-40%) after amnioreduction. Only one study (10 cases) reported data on immediate delivery after diagnosis with no perinatal deaths. Perinatal death incidence was 55.7% (95% CI 31.4%-78.6%) in twin pregnancies managed expectantly, 5.6% (95% CI 0.5%-15.3%) in those treated with laser, and 20.2% (95% CI 6%-40%) in those after amnioreduction. Intact survival was reported in 44.4%, 96.4%, and 78% of fetuses managed expectantly, with laser or amnioreduction, respectively.

CONCLUSIONS

Evidence regarding perinatal mortality and morbidity in twin pregnancies complicated by late TTTS according to the different managements was of very low quality. Therefore further high-quality research in this field is needed to elucidate the optimal management of these pregnancies.

Magnetic resonance imaging of the fetal brain in monochorionic diamniotic twin gestation: correlation of cerebral injury with ultrasound staging and survival outcomes

BACKGROUND

The current staging system of twin-twin transfusion syndrome (TTTS) is based on the ultrasound criteria and does not consider the cerebral injury.

OBJECTIVE

To assess the incidence of cerebral damage on fetal MRI and correlate abnormal cerebral diffusion-weighted imaging (DWI) findings with survival outcome and the ultrasound staging of TTTS.

MATERIALS AND METHODS

We conducted a retrospective review of MRI/DWI of fetal brains and the electronic medical records in monochorionic diamniotic twin gestations. Axial DWI of each fetal brain was performed on 1.5-tesla (T) clinical magnet with b values of 0 s/mm² and 700 s/mm². We correlated MRI/DWI abnormalities with the Quintero staging system and survival outcomes of the fetuses.

RESULTS

Thirty-four pregnancies (68 fetuses) with 42 fetal cerebral abnormalities were identified by MRI/DWI alone. Of these 42 fetal cerebral abnormalities, 33 fetal brain lesions were visible only on DWI (n=25 donor, n=8 recipients; n=30 unilateral, n=3 bilateral; n=26 diffuse, n=7 focal). Quintero staging in these 34 pregnancies was as follows: 9 Stage I, 7 Stage II, 13 Stage III, 4 Stage IV, 1 Stage V. There was no significant correlation between the presence of cerebral infarction or hemorrhage using MRI/DWI and ultrasound staging (P=0.138). The overall survival rate was 63.2% (43/68). There was a significant correlation between the presence of cerebral infarction or hemorrhage on MRI/DWI and delivery status (P=0.009).

CONCLUSION

Abnormal cerebral imaging findings on MRI/DWI from hypoxic-ischemic injury or hemorrhage can be seen at the beginning of the second trimester and do not correlate with the current ultrasound staging system; however, they do correlate with decreased survival. Fetal cerebral abnormalities could be incorporated into the TTTS staging system as an independent risk factor.

Neurodevelopmental impairment at 3 years of age after fetoscopic laser surgery for twin-to-twin transfusion syndrome

BACKGROUND

Data on neurodevelopmental outcomes of children surviving after fetoscopic laser surgery (FLS) for twin-to-twin transfusion syndrome (TTTS) are scarce.

METHODS

We retrospectively investigated children surviving after FLS for TTTS at 16 to 26 weeks' gestation between 2003 and 2014. Children were evaluated by standardized neurologic examinations using the Kyoto Scale 2001 at a corrected age of 3 years ± 6 months. Neurodevelopmental impairment (NDI) was defined as cerebral palsy (CP), bilateral blindness, bilateral deafness or a developmental quotient (DQ) < 70 points. Brain magnetic resonance imaging (MRI) was performed at term-equivalent age.

RESULTS

A total of 188 children from 110 twin pregnancies were evaluated. NDI was detected in 16/188 (8.5%) children, including six cases of CP (3.2%). No children had bilateral blindness or deafness. An earlier gestational age at delivery was associated with a higher incidence of NDI (P < .001). Abnormal brain MRI findings were detected in 9/16 (56%) of children with NDI, including 6/6 (100%) with CP.

CONCLUSION

The incidence of NDI in children following FLS at 3 years old was 8.5%. Prematurity is a strong risk factor for NDI. Brain MRI may predict the development of CP.

Twin-twin transfusion syndrome in the era of fetoscopic laser surgery: antenatal management, neonatal outcome and beyond

Introduction: Twin-twin transfusion syndrome (TTTS) is a devastating complication of monochorionic twin pregnancy and remains a major challenge for worldwide fetal medicine specialists. In TTTS, intertwin transfusion through vascular anastomoses in the shared placenta leads to severe hemodynamic imbalance. This review summarizes the current knowledge of TTTS.Areas covered: The most recent insights concerning the management of TTTS, as well as fetal and neonatal complications are described. Relevant articles were selected based on a Pubmed search using the keywords below. Understanding of the underlying pathophysiology has improved greatly as a result of placental injection studies. Advancements in antenatal management have led to increased perinatal survival and a decreased incidence of neonatal complications, including brain injury and neurodevelopmental impairment.Expert opinion: Further opportunities for improvement comprise technological innovations in laser procedures and the prevention of preterm rupture of membranes with subsequent prematurity. A noninvasive treatment such as high-intensity focused ultrasound (HIFU) seems to hold promise for the future treatment of TTTS. Fetal MRI studies are important to improve our understanding of fetal brain injury and should relate their findings to long-term neurodevelopment. International collaboration and centralization of care are of paramount importance to ensure the best care for our patients.

Early postnatal cardiac manifestations are associated with perinatal brain injury in preterm infants with twin to twin transfusion syndrome

Altered hemodynamics associated with twin to twin transfusion syndrome (TTTS) can be manifested in the fetal and neonatal heart. This study evaluated the association between cardiac manifestations immediately after birth and brain injury in preterm infants with TTTS. Medical records of preterm infants who were born at <35 weeks of gestation with TTTS and admitted to the neonatal intensive care unit at Seoul National University Children's Hospital between January 2011 and January 2018 were reviewed. TTTS was prenatally diagnosed and staged according to the Quintero criteria. Echocardiographic findings, brain ultrasound and MRI imaging findings were analyzed. Fifty-three infants were enrolled in this study. Thirty-two infants (60.3%) were treated by fetoscopic laser coagulation. Brain injury developed in 15 infants (28.3%). Hypotension within the first week and immediate postnatal cardiac manifestations were more prevalent in the brain injury group. In the multivariate analysis, acute kidney injury and cardiac manifestations, such as ventricular dysfunction and tricuspid regurgitation, were statistically associated with brain injury in the study population. Immediate postnatal cardiac manifestations, such as ventricular dysfunction and tricuspid regurgitation, can serve as surrogate markers for perinatal hemodynamic disturbance, which are associated with early neonatal brain injury in preterm infants with TTTS.

Cortical malformations and COL4A1 mutation: Three new cases

AIM

The COL4A1 gene (13q34) encodes the α1 chain of type IV collagen, a crucial component of the basal membrane. COL4A1 mutations have been identified as a cause of a multisystem disease. Brain MRI in COL4A1-mutated patients typically shows vascular abnormalities and white matter lesions. Cortical malformations (specifically schizencephaly) have also recently been described in these patients, suggesting that these, too, could be part of the phenotypic spectrum of COL4A1 mutations. The aim of our work was to retrospectively evaluate COL4A1-mutated subjects diagnosed at our centers in order to assess the frequency and define the type of cortical malformations encountered in these individuals.

METHOD

We retrospectively reviewed MRI data of 18 carriers of COL4A1 mutations diagnosed in our centers between 2010 and 2016.

RESULTS

We identified polymicrogyria in two patients, and schizencephaly in the mother of a further patient.

INTERPRETATION

Our findings confirm that cortical malformations should be considered to fall within the phenotypic spectrum of COL4A1 mutations and show that not only schizencephaly but also polymicrogyria can also be found in mutated individuals. Although further studies are needed to clarify the underlying pathogenetic mechanism, independently of this, the timing of the brain damage could be the crucial factor determining the type of lesion.

Outcomes of extremely premature infants with twin-twin transfusion syndrome treated by laser therapy

OBJECTIVE

To compare short-term and long-term outcomes of preterm infants born at <29 weeks of gestational age (GA) with twin-twin transfusion syndrome (TTTS) treated with laser therapy to preterm twin infants without TTTS.

DESIGN

Retrospective case-control study comparing 33 preterm TTTS twins to 101 preterm diamniotic-dichorionic (di-di) twins born at our institution between 2006 and 2015.

RESULTS

GA at birth were 26.4 ± 1.4 weeks (TTTS) and 26.9 ± 1.6 weeks (di-di) (p = 0.07). TTTS premature newborns were less exposed to antenatal steroids (p = 0.01), more frequently born by C-section (p = 0.005), received more surfactant therapy (p = 0.004, and were smaller for GA (p < 0.001). When adjusted for antenatal steroids and birth weight, TTTS status was not associated with increased mortality (HR 1.66, 95% CI 0.77-3.56, p = 0.20). No differences were found on neurodevelopmental outcomes at 18 months of corrected GA.

CONCLUSION

Premature TTTS newborns treated with fetal laser therapy had similar survival and neurodevelopmental outcomes compared to preterm di-di twins without TTTS.

Fetal stroke and cerebrovascular disease: Advances in understanding from lenticulostriate and venous imaging, alloimmune thrombocytopaenia and monochorionic twins

Fetal stroke is an important cause of cerebral palsy but is difficult to diagnose unless imaging is undertaken in pregnancies at risk because of known maternal or fetal disorders. Fetal ultrasound or magnetic resonance imaging may show haemorrhage or ischaemic lesions including multicystic encephalomalacia and focal porencephaly. Serial imaging has shown the development of malformations including schizencephaly and polymicrogyra after ischaemic and haemorrhagic stroke. Recognised causes of haemorrhagic fetal stroke include alloimmune and autoimmune thrombocytopaenia, maternal and fetal clotting disorders and trauma but these are relatively rare. It is likely that a significant proportion of periventricular and intraventricular haemorrhages are of venous origin. Recent evidence highlights the importance of arterial endothelial dysfunction, rather than thrombocytopaenia, in the intraparenchymal haemorrhage of alloimmune thrombocytopaenia. In the context of placental anastomoses, monochorionic diamniotic twins are at risk of twin twin transfusion syndrome (TTTS), or partial forms including Twin Oligohydramnios Polyhydramnios Sequence (TOPS), differences in estimated weight (selective Intrauterine growth Retardation; sIUGR), or in fetal haemoglobin (Twin Anaemia Polycythaemia Sequence; TAPS). There is a very wide range of ischaemic and haemorrhagic injury in a focal as well as a global distribution. Acute twin twin transfusion may account for intraventricular haemorrhage in recipients and periventricular leukomalacia in donors but there are additional risk factors for focal embolism and cerebrovascular disease. The recipient has circulatory overload, with effects on systemic and pulmonary circulations which probably lead to systemic and pulmonary hypertension and even right ventricular outflow tract obstruction as well as the polycythaemia which is a risk factor for thrombosis and vasculopathy. The donor is hypovolaemic and has a reticulocytosis in response to the anaemia while maternal hypertension and diabetes may influence stroke risk. Understanding of the mechanisms, including the role of vasculopathy, in well studied conditions such as alloimmune thrombocytopaenia and monochorionic diamniotic twinning may lead to reduction of the burden of antenatally sustained cerebral palsy.

Fetal neuroimaging: an update on technical advances and clinical findings

This paper is based on a literature review from 2011 to 2016. The paper is divided into two main sections. The first section relates to technical advances in fetal imaging techniques, including fetal motion compensation, imaging at 3.0 T, 3-D T2-weighted MRI, susceptibility-weighted imaging, computed tomography, morphometric analysis, diffusion tensor imaging, spectroscopy and fetal behavioral assessment. The second section relates to clinical updates, including cerebral lamination, migrational anomalies, midline anomalies, neural tube defects, posterior fossa anomalies, sulcation/gyration and hypoxic-ischemic insults.

Fetal Brain Injury in Survivors of Twin Pregnancies Complicated by Demise of One Twin: A Review

Perinatal mortality is increased considerably in multiple pregnancies compared to singleton pregnancies, with single intrauterine fetal demise (sIUFD) presenting a rare but unique perinatal problem. Monochorionic pregnancies are at particular risk of sIUFD due to bidirectional inter-twin placental vascular anastomoses. The resulting inter-twin blood flow can become unbalanced, causing acute and chronic inter-twin transfusion and profound anemia secondary to fetal exsanguination into the low-pressure circulation of the dead fetus. If the sIUFD occurs after 14 weeks’ gestation it is believed to have the most significant effect on the continuing pregnancy as the co-twin is at increased risk of preterm delivery, long-term neurological complications, and death. This article will focus on fetal brain injury in the surviving co-twin in the case of sIUFD, as it is the most common kind of injury in sIUFD, and one which concerns parents and may be the basis for terminating the pregnancy. We will outline how these brain injuries are thought to occur and describe potential pathophysiological mechanisms. We will discuss risk factors for brain injury in cases of sIUFD, including: chorionicity, cause of the sIUFD (spontaneous or secondary to an underlying pathological process such as twin-to-twin transfusion syndrome), gestation of delivery and how to prevent brain injury in the co-twin. We also review modes of imaging, discuss the difficulties in predicting the long-term outcome for co-twin survivors, and highlight the dearth of research in this area.

Early detection by diffusion-weighted sequence magnetic resonance imaging of severe brain lesions after fetoscopic laser coagulation for twin-twin transfusion syndrome

OBJECTIVE

Monochorionic twins treated by fetoscopic laser coagulation (FLC) for twin-twin transfusion syndrome (TTTS) are at increased risk of neurodevelopmental impairment. Our aim was to evaluate the additional value of diffusion-weighted imaging (DWI) over fetal sonography and T2 magnetic resonance imaging (MRI) in the detection of acute ischemic cerebral lesions shortly following FLC.

METHODS

This was a prospective cohort study of fetuses with TTTS treated by FLC. All fetuses underwent brain DWI and T2-MRI within 24-96 h after the procedure and a follow-up MRI at 30-32 weeks' gestation. All fetuses also had frequent ultrasound evaluation until delivery.

RESULTS

Thirty pregnancies with TTTS were included in the study. Eight resulted in survival of only one twin after the procedure (Group A), while the other 22 pregnancies resulted in the survival of both twins (Group B), as assessed at the time of initial MRI. Two fetuses in Group A and four in Group B had evidence of transient bleeding in the germinal matrix. Three fetuses in Group B and none in Group A had diffuse ischemic changes in the white matter. One fetus died in utero 24 h after MRI. The other two fetuses in Group B demonstrated an acute infarct 24-48 h after the procedure. Subsequent scans at 28-29 and 32 weeks showed brain atrophy compatible with an old infarct.

CONCLUSIONS

DWI can demonstrate cerebral damage immediately after FLC. Larger cohorts are needed to determine the role of fetal MRI in the prenatal assessment and follow-up of patients with TTTS.