|
|
![]() |
 |
| Using Ultrasound to Characterize Twin to Twin Transfusion Syndrome (TTTS) by Earline Finan |
|
Abstract
This paper introduces TTTS in monochorionic twins and discusses the imbalance of blood, nutrients and oxygen that affects both fetuses. Different fetal sizes and amniotic fluid volumes in the two sacs characterize TTTS. There is often a stuck twin found immobile at an edge of the placenta or uterine wall. Additional characteristics of this condition are a large bladder in the recipient fetus and an absent bladder in the donor fetus. Treatment options for TTTS include amnioreduction and laser ablation, each of which has associated risks. |
|
|
 |
| Figure 1. Equal and unequal placenta sharing. |
|
Keywords
Twin to twin transfusion syndrome, TTTS, monochorionic twins, stuck twin. |
|
|
 |
| Figure 2. Folding of inter-twin membrane. |
|
When there is more than one fetus in the uterus, each must share the resources provided by the mother.[1] Two main types of twin placentas exist.[1] In dichorionic twins each fetus is connected to a separate placenta, and each placenta occupies its own separate space.[1] Although there may be competition between fetuses there is no direct sharing of blood.[1] In monochorionic twins, however, two fetuses share one placenta.[1] Figure 1 shows an example of equal and unequal placenta sharing[2]. The top image shows equal sharing (50/50) with central cord insertion on the left and marginal cord insertion on the right[2]. The bottom image shows unequal sharing (70/30) with velamentous cord insertion on the left and central cord insertion on the right[2].
Monochorionic twins are always identical twins.[1] Although usually separated by a thin membrane, they have many direct connections in the placenta between blood vessels that radiate from each cord insertion.[1] These connections allow passage of blood between their circulatory systems.[1] If there is an equal exchange of blood in both directions then there is balanced sharing of nutrients and oxygen, and this is an uncomplicated monochorionic pregnancy.[1] However if the blood sharing is not balanced one twin may have an adequate supply of nutrients, fluid and oxygen while the other has an inadequate supply.[1] This condition is called selective intrauterine growth restriction.[1] This paper introduces TTTS in monochorionic twins and discusses the imbalance of blood, nutrients, and oxygen that affects both twins. |
|
|
 |
| Figure 3. Discrepancy in twin sizes. |
|
In twin-to-twin transfusion syndrome (TTTS), both fetuses are affected because there is unequal exchange of blood, nutrients and oxygen between the twins.[1] For one twin the amount of blood that leaves is not replaced by an equal amount coming back.[1] For the other, the amount of blood received in the veins coming back from the placenta is more than was sent in the arteries to the placenta.[1] This imbalance occurs because of blood vessel problems deep in the placenta.[1] Each twin has part of the placenta where his arteries put blood in and his veins drain blood out.[1] Between the fetuses there is an area where the blood is brought in by one fetus’ artery but drained away by the other’s vein.[1] This artery-vein mismatch produces the imbalance between circulatory systems, and affects each fetus differently.[1] |
|
|
 |
| Figure 4. Recipient showing clear profile in amniotic fluid. |
|
In about 30% of monochorionic twin pregnancies, imbalances in the net flow of blood across the placental vascular arterio-venous communications from one fetus, the donor, to the other, the recipient, results in twin-to-twin transfusion syndrome.[3] In about half of these the condition is severe.[3] Severe disease becomes apparent at 16-24 weeks of pregnancy.[3] Untreated TTTS that develops before 26 weeks has a perinatal mortality rate of 90%.[4] An early sign of disparity in amniotic fluid volume due to TTTS is inter-twin membrane folding.[3] In about 30% of monochorionic twin pregnancies at 15-17 weeks of gestation there is a membrane folding; half of these progress to severe TTTS while the other half progress to moderate TTTS.[3] In two-thirds of monochorionic twin pregnancies there is no membrane folding and these pregnancies are not at increased risk for miscarriage or perinatal death due to TTTS.[3] Figure 2 shows a folding of the inter-twin membrane toward the recipient sac (right), and increased echogenicity of the amniotic fluid in the donor sac (left)[3] (reprinted with permission of the Fetal Medicine Foundation). |
|
|
 |
| Figure 5. Donor tightly wrapped in membrane. |
|
The diagnosis of TTTS is made by ultrasound and is characterized by differences in fetal size and amniotic fluid volumes in the two sacs.[5] Figure 3 shows the discrepancy in twin sizes[5] (reprinted with permission of Elsevier). Although the diagnosis of TTTS has been reported as early as the first trimester, it is more commonly made during the second trimester.[5] Hallmarks of the diagnosis are a single placenta, same gender, greater than 20% growth difference between the fetuses, and different amniotic fluid volume between the sacs.[5] A discrepancy in the diameter of the umbilical cords and fetal hydrops may also be seen with the disorder.[5] When there is no measurable amniotic fluid pocket around one fetus it is called a ‘stuck’ twin where the dividing membrane is ‘shrink-wrapped’ against the fetus, making it difficult to visualize.[5] The stuck twin does not move freely within the amniotic cavity; with changes in maternal position it appears stuck in the same place.[5]
Figure 4 shows the recipient with increased amniotic fluid, and figure 5 shows the stuck donor tightly wrapped in the membrane (reprinted with permission of the Center for Advanced Fetal Care). The features of severe TTTS by ultrasonographic examination are the presence of a large bladder in the polyuric recipient fetus in the polyhydramniotic sac.[3] The anuric donor fetus has an ‘absent’ bladder and is found stuck and immobile at an edge of the placenta or the uterine wall where it is held fixed by the collapsed membranes of the anhydramniotic sac.[3] Ultrasonographic features of the underlying changes in severe TTTS may be present as early as 10-13 weeks of gestation and manifest as increased nuchal translucency thickness in one or both of the fetuses.[3] Another early ultrasound marker of severe TTTS may be abnormal Doppler flow velocity waveforms in the ductus of the recipient twin.[3]
Figure 6 shows twin A (right) freely moving in the amniotic cavity, while twin B (left) is stuck[4](reprinted with permission of Elsevier). Figure 7 shows a stuck twin against the posterior wall of the uterus, despite maternal position changes[4] (reprinted with permission of Elsevier). Ultrasound is very useful in determining placentation, which is important in predicting the prognosis for twin pregnancy complications.[4] Determining placentation includes characterizing the dividing membrane.[4] A thin wispy membrane, which is often difficult to visualize, may be indicative of monochorionic placentation.[4] A thicker, more apparent wavy membrane is more typical in dichorionic placentation.[4] |
|
|
 |
| Figure 6. Twin A (right) freely moves while twin B (left) is stuck. |
|
Amnioreduction and septostomy (intentional perforation of the intervening membrane) both result in the survival of at least one fetus in approximately 80% of cases.[6] However significant long-term neurologic problems have been reported in cases of TTTS managed with amnioreduction.[6] In one series of 25 cases treated with amnioreduction, evaluation after 18 months of age revealed a rate of cerebral palsy of 5.8%, with four additional infants exhibiting severe intellectual disability.[6] Laser ablation of the intraplacental anastomoses is the most effective treatment option for severe TTTS to date.[6] This treatment has been available since 1990.[6] The survival of at least one fetus is 76% with laser ablation, compared to 56% using amnioreduction.[6] However a comparison of the neonatal cranial ultrasound findings in 48 cases of TTTS that underwent laser therapy and 60 cases of monochorionic twin gestations without TTTS showed severe cerebral lesions in 10% of the TTTS group vs. 2% of the control group.[6] So laser ablation is not without risks.[6] Functional neurologic assessments are recommended for infants that survive in utero laser treatment of TTTS.[6] |
|
|
 |
| Figure 7. Stuck twin against posterior uterus wall. |
|
|
|
|
|
This paper introduced TTTS in monochorionic twins and discussed the imbalance of blood, nutrients and oxygen that affects both fetuses.[1] The risk of severe TTTS is about 15% of monochorionic twins, and the mortality rate is 90% if left untreated.[3][5] Different fetal sizes and amniotic fluid volumes in the two sacs characterize TTTS.[4] There is often a stuck twin found immobile at an edge of the placenta or uterine wall.[4] The stuck twin does not move freely within the amniotic cavity.[4] Additional characteristics of this condition are a large bladder in the recipient fetus and an absent bladder in the donor fetus.[3] Treatment options for TTTS include amnioreduction and laser ablation, each of which has associated risks.[6] |
|
|
1.Twin to Twin Transfusion Syndrome, University of Maryland Medical Center. Available from http://images.google.com/imgres?imgurl=http://www.obgyn.umm.edu/ttts/images/06%2520becoming%2520a%2520stuck%2520twin.jpg&imgrefurl=http://www.obgyn.umm.edu/ttts/what.html&h=271&w=364&sz=48&hl=en&start=6&tbnid=NSGzUnJU3k0paM:&tbnh=90&tbnw=121&prev=/images%3Fq%3Dttts%2Bultrasound%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DG.
2. Reprinted from American Journal of Obstetrics and Gynecology: 195(1): p. 179-180, Fick, A, et. al.: “Unequal Placental Sharing and Birth Weight”, © 2006 Mosby, with permission from Elsevier.
3. K.H. Nicolaides, N.J. Sebire, R.J.M Snijders, A.P. Souka. Chorionicity and Pregnancy Complications. Available from http://www.fetalmedicine.com/11-14book/chap4/chap04-04.htm.
4. J.F.X. Egan, A.F. Borgida. Multiple gestations: the importance of ultrasound. Obstetrics and Gynecology Clinics, vol. 31, no. 1. March 2004.
5. A. Monteagudo, A.S. Roman. Ultrasound in Multiple Gestations: Twins and Other Multifetal Pregnancies. Clinics in Perinatology, vol. 32 no. 2. June 2005.
6. K.J. Moise. Neurodevelopmental outcome after laser therapy for twin-twin transfusion syndrome. American Journal of Obstetrics and Gynecology, vol. 194, no. 5. May 2006.
|
|
|
Grading Form
Click here to download grading form in Word
Please email results to lpenrose@coosavalleytech.edu |
|
|
|
|
Copyright CVTC 2003-2006. All Rights Reserved.