What is TTTS?
- The placenta. The developing fetus receives nutrition from the placenta through the umbilical cord where the umbilical cord meets the placenta it branches into many vessels that radiate out and penetrate the placenta like the roots of a tree. Arteries from the fetus enter the cord and bring blood to the placenta, and veins bring the blood, fresh with new oxygen and nutrients, back to the fetus.
- Twin placentas. When there is more than one fetus is in the uterus, they must share the resources provide by the mother. Two main types of twin placentas exist, and they are significantly different. In dichorionic twins each fetus is connected to a separate placenta. Each placenta occupies its own separate space, like two trees growing beside each other. Although there may be competition between fetuses, there is no direct sharing of blood between twins In monochorionic twins, two fetuses share one placenta, like two trees growing from the same trunk. Monochorionic twins are always identical twins, Although they are usually separated by a thin wall (membrane), they have many direct connections in the placenta. These are connections between the blood vessels that radiate from each cord insertion, called vascular anastomoses. They allow the passage of blood between their circulations. These anastomoses are found in all monochorionic pregnancies - all such twins exchange blood back and forth. In uncomplicated monochorionic pregnancies there is an equal exchange of blood in both directions keeping the balance between their circulations, and there is balanced placental sharing of nutrients and oxygen between twins.
Color Doppler images show blood vessel connections on the placenta.
- Complicated monochorionic twins. Although all monochorionic twins share blood through their placental anastomoses, sometimes this sharing is not balanced. In some case, one twin has adequate supply of nutrients, fluid and oxygen, while the other has an inadequate share of the placenta. This fetus has less placenta and therefore less support, and may be smaller, have less amniotic fluid, and be more at risk for fetal distress. This condition, where one identical twin is normal and the other is compromised, is known as selective intrauterine growth restriction (IUGR). This condition may require intrauterine prenatal treatment if the IUGR twin is in distress, because death of one twin will seriously affect the health of the other, due to their placental connections. Selective IUGR (one is normal, one is affected) is different from twin to twin transfusion syndrome, where both fetuses are affected.
Amniotic fluid volume alters imaging. The large baby on the left (recipient) has increased amniotic fluid, so a clear profile and clear imaging are obtained. The donor, on the right, is tightly wrapped in his membrane and imaging is very indistinct.
- Twin to twin transfusion syndrome (TTTS) may threaten both twins in a complicated monochorionic pregnancy. In this situation, there is unequal exchange of blood between twins - placental sharing of nutrients, fluid and oxygen is not balanced. For one twin, the amount of blood that leaves through the vascular anastomoses is not replaced by an equal amount coming back. For the other, the amount received in the veins coming back from the placenta is more than was sent in the arteries to the placenta. This imbalance occurs mainly because of blood vessel problems deep in the placenta. Each twin has part of the placenta where his arteries put the blood in, and his veins drain the blood out. 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. This artery-vein mismatch produces the imbalance between circulations. This imbalance affects each fetus, but in different ways.
What are the risks from TTTS?
- Untreated TTTS has serious consequences for each twin. The complications of TTTS arise due to the presence of placental anastomoses, unbalanced placental sharing, preterm labor, or a variable combination of all of these factors. If TTTS detected in the first 20 weeks of pregnancy is left untreated, there is an 70-100% chance that the twins will not survive.
- Donor twin. The donor is at risk for cardiovascular collapse, malformations due to compression, or malnutrition with severe growth restriction (IUGR). These effects may be made even worse if preterm labor starts, or the polyhydramnios becomes extreme, because both of these situations may worsen placental function. When IUGR is severe, the donor is very fragile and frequently cannot cope with the added stresses of prematurity.
- Recipient twin. The main risks for this twin are from cardiovascular volume overload. The first effects on the recipient's heart may to thicken the heart muscle, but eventually heart failure may develop. These effects on the heart may also last into newborn life, made worse if birth is premature (often the case if treatment is incomplete).
- Both twins. These circumstances are suboptimal for normal development of either twin and may account for the increased rate of developmental delay observed in monochorionic twins at 2 years of age. The placental anastomoses that are responsible for this degree of cardiovascular imbalance carry an additional danger in the event that one twin dies. When this happens, the surviving twin can lose a large amount of blood volume across the connecting vessels into the dead twin. This may cause a sudden drop in blood pressure in the surviving twin, which can result in a heart attack or a stroke. Thus the fate of one twin remains linked to the other through the placental anastomoses.
- The excessive urine production by the recipient leads to distention of the uterus tat may cause the mother discomfort and may put pressure on the cervix, the lower part of the uterus. With continuing pressure the cervix may open or the membranes may rupture resulting in miscarriage or preterm delivery. This often occurs at an early gestational age where the chances for survival are poor. Even if the babies might survive the dangers of prematurity, their complications from TTTS mean death or permanent injury.
- Maternal risks. Of course, loss of a wanted pregnancy is very serious for the family. There are physical risks to the mother from the overdistended uterus, from the attempted treatments, and even from the delivery, that all must be considered when making decisions about management of TTTS.
- Our Role. Because many factors need to be taken into account to assess the risks for individual patients a detailed and standardized examination of monochorionic pregnancies is essential. Each pregnancy complicated by TTTS presents unique features - no single approach will be best for all families. A critical role for our team is to provide the best possible assessment, so the family may make the best choice.
Ultrasound Assessment
There are several definitions for TTTS in use. Many of these are based on the pediatric literature - in other words, they are based on babies who have survived TTTS and are born alive. We now understand that the babies reported this way have the mildest form of TTTS, or with complications that developed near delivery. The lack of uniform diagnostic criteria made evaluation of patients and assessment of their risks difficult. In 1997 Dr Ruben Quintero proposed a staging system that has advanced the unified assessment of TTTS. The staging system means that there is general agreement on the condition of the twins before treatment has started. The objective comparison of modern management options has thus become possible. The severity of TTTS is graded by the characteristics of amniotic fluid volume status, bladder filling, cardiovascular function, in 5 Stages.
Stage 1 - Polyhydramnios. Vertical amniotic fluid pocket > 8cm in the recipient sac. AND Oligohydramnios . No vertical amniotic fluid pocket > 2cm in the donor sac
These images of the donor show reduction in amniotic fluid. On the left, the membrane is across the baby’s chest, with small pockets of fluid below it. On the right, this baby has no amniotic fluid left, and his legs are tightly pressed against his abdomen. The fluid in this picture belongs to his brother, the recipient, who has polyhydramnios.
Stage 2 - Absent bladder filling in the donor
The donor on the left has no visible bladder (the colored lines represent umbilical arteries, one on each side of the bladder, which is empty). The recipient on the right has a continuously over-filled bladder (the black circle in between the two arteries).
Stage 3 - Critically abnormal cardiovascular function in either twin
The recipient has excessive blood volume, which puts stress on the heart. These images show blood flowing backwards through the tricuspid valve due to borderline heart failure.
Stage 4 - Evidence of overt heart failure in either twin.
Each baby has circulatory problems as TTTS progresses. On the left, the donor has severe placental resistance and very low blood flow to the placenta. On the right, the recipient is struggling with heart failure, with blood running backward during the cardiac cycle.
Stage 5 - Intrauterine death of either twin
In addition to these minimum assessments necessary to carry out the “Quintero staging” there are several other assessments that help to individualize the risk assessment further.
Synonyms: TTTS, stuck twin, feto-fetal transfusion syndrome, interfetal transfusion syndrome, oligohydramnios polyhydramnios sequence, hydrops, acardiac twin.