What is Vanishing Twin Syndrome?

Twin blood transfusion syndromes (TTTs) are a serious complication in twin pregnancy, and the perinatal mortality rate is extremely high, with untreated mortality ranging from 70% to 100%. At present, the treatment of TTTs by fetal microscopy with placental traffic vascular laser coagulation has become the preferred treatment method for many fetal medical centers in the world, which can make the survival rate of at least one of the fetuses reach 75% to 80%.

Basic Information

English name: twin to twin transfusion syndrome
Visiting department: Obstetrics and Gynecology
Multiple groups: Pregnant woman

Common causes: Imbalance of blood circulation caused by deep placental arteriovenous anastomosis
Common symptoms: Polyhydramnios, twin fetal vein anastomosis, paper fetus, fetal edema

Causes of Twins Transfusion Syndrome

Most TTTs occur in double amniotic sac single chorionic twins (MCT). There are superficial and deep vascular anastomosis in MCT placenta. There are four types of vascular connection: superficial anastomosis of capillaries; arterial anastomosis between large vessels; venous anastomosis between large vessels; villous capillary anastomosis. The first three are superficial vascular anastomosis. Superficial anastomosis refers to the anastomosis of larger blood vessels on the surface of the fetal surface of the placenta. Most of them are arterial-arterial direct anastomosis, and few are direct-venous anastomosis. The imbalance of blood circulation caused by deep placental arteriovenous anastomosis is the cause of TTTs. Under normal circumstances, the blood flow exchange between the two placens is balanced, and the superficial blood vessels of the placenta are anastomosed into two-way blood flow, so the hemodynamic balance of the two places is maintained. The deep anastomosis is in one or more placental lobes adjacent to the placenta where the two fetuses belong. In these placental lobes, the placental arterial and venous anastomosis, the blood flow distribution is equal, and the result is unit time The blood flow from fetus A to fetus B is equivalent to that from fetus B to fetus A, so the fetal development speed is similar. Deep arteriovenous anastomosis tends to show unidirectional blood flow. When a superficial arterial-arterial, venous-venous anastomosis is lacking, an unidirectional blood supply can be caused, resulting in hemodynamic imbalance. The blood flow from fetus A to fetus B is greater than that from fetus B to fetus A within a unit of time. Fetus A becomes the donor and fetus B receives the blood. Imbalance in blood flow leads to a series of pathological changes. Because vascular traffic occurs in 85% to 100% of MCTs, it is rare in twin chorionic twins. Therefore, almost all TTTs occur in single chorionic twins. The blood-supplying fetus is in a state of low blood volume and anemia due to continuous blood transfusion to the fetus. Blood fetuses have high blood volume, increased urine volume causes increased amniotic fluid, larger fetuses, increased heart, liver, kidney and other organs, increased red blood cells, increased red blood cells, and fetal edema.

Clinical manifestations of twin transfusion syndrome

Excessive amniotic fluid

The amount of amniotic fluid during normal pregnancy increases with the increase of gestational weeks, and gradually decreases in the last 2 to 4 weeks. The amount of amniotic fluid at full-term pregnancy is 1000ml (800 1200ml). Those who have more than 2000ml amniotic fluid during any period of pregnancy are called polyhydramnios. Up to 20000ml. Most pregnant women increase slowly and develop over a long period of time, called chronic polyhydramnios; a few pregnant women have a sharp increase in amniotic fluid within a few days, called acute polyhydramnios.

2. Twin fetal vein anastomosis

Twins are divided into monozygotic twins and twins. Single egg twins are divided into double amniotic sac double chorionic twins, double amniotic sac single chorionic twins and single amniotic sac single chorionic twins. The blood circulation is interlinked, including arterial-arterial, venous-venous, and arterial-venous anastomosis. Vascular anastomosis can be divided into superficial and deep. Superficial anastomosis refers to the anastomosis of larger blood vessels on the surface of the fetal surface of the placenta. Most of them are arterial-arterial direct anastomosis, and a few are direct-venous anastomosis. On the fetal surface of a few single chorionic twin placenta, both anastomoses are present. The deep anastomosis is located in one or more placental lobes adjacent to the placenta where the two fetuses belong. Although there are multiple anastomoses through the capillaries, there is no direct arterial or venous anastomosis. But its blood flows from one fetus to another, which Schaty (1900) calls "the third cycle." Twin venous anastomosis is a manifestation of twin transfusion syndrome. Twin blood transfusion syndrome is caused by a single zygote, single chorion, double amniotic sac twin, and a fetus (blood donor) delivers blood to another fetus (blooded child) through the unbalanced placental anastomosis network in the official cavity A series of pathophysiological changes and clinical symptoms are serious complications of twin or multiple pregnancy. The disease can be divided into two types, acute and chronic, which are usually referred to as chronic. The incidence of this disease in single chorionic twins is 10% to 15%, and the prognosis is poor.

3. Paper-like fetus

Paper-like fetus refers to a fetus with twin or multiple pregnancies. Due to the restricted fetal growth, early death is caused by other fetuses to be thinly sliced. This paper-like fetus mostly occurs in twin blood transfusion syndrome.

4. Fetal Edema

The incidence of edema fetus is 1 / (1400 4000) pregnancy, there are two types, namely immune and non-immune. Maternal and child blood type incompatibility is the main cause of immune fetal edema, which has been rare in recent years. The causes of non-immune edema are complicated, including severe anemia (such as Bart's edema), cardiovascular abnormalities, chromosomal abnormalities, and intrauterine infections. Thalassemia (thalassemia) is a single-gene inheritance common in Southeast Asia.

Diagnosis of twin blood transfusion syndrome

In the past 20 years, B-ultrasound can diagnose TTTs before giving birth.

Prenatal diagnosis

(1) Determination of monozygotic twins TTTs are generally single chorionic twins, so it is important to diagnose them as single chorionic twins by ultrasound.

(2) Differences in fetal weight and fetal performance At present, among the parameters for estimating the weight of the fetus by B-ultrasound, if the single item is used, the abdominal circumference is the most accurate. Many scholars believe that if the difference in abdominal circumference is 20mm, the difference in weight is about 20%.

(3) How much is the difference in amniotic fluid? Excessive amniotic fluid and oligohydramnios are one of the important diagnostic conditions for TTTs.

(4) The difference between the umbilical cord and the placenta B-ultrasound shows that the umbilical cord of the recipient is thicker than the donor, and sometimes the recipient's umbilical cord is accompanied by a single umbilical artery. Observation of the placenta by color Doppler ultrasound may help to determine the branch of placental blood vessels in TTTs.

(5) Differences between the internal organs of the two fetuses.

(6) Umbilical puncture Some people think that the puncture of umbilical blood vessels under the guidance of B-ultrasound can greatly help diagnose TTTs. First, blood samples can be used to confirm that they are monozygotic twins, second, the hemoglobin level between the two fetuses can be known, and third, the donor's anemia status can be known.

(7) Whether there is edema in the fetus Any serious fetus may have edema or even stillbirth, or one of them is an adhesion child.

2. Postnatal diagnosis

(1) Placenta The placenta of the blood-supplying child is pale, edema, and atrophic. The villi have edema and vasoconstriction. There is amnion nodules on the amniotic membrane due to lack of amniotic fluid. The placenta of the affected child is red and congested.

(2) Hemoglobin level Generally, the difference between the hemoglobin levels of the blood receiving children and the donors of TTTs is more than 5g / dl, so the difference is currently 5g / dl as the diagnostic standard. However, there are reports that the difference is less than 5g / dl, especially in the second trimester.

(3) Weight difference The standard for weight difference between two babies is generally set at 20%.

Twins Transfusion Syndrome Treatment

Studies have shown that the increase in neonatal morbidity and mortality is due to an increase in preterm births, so the core of treatment should be the extension of gestational weeks.

1. Fetal microscopy selective laser electrocoagulation

Under fetal microscopy, the anastomotic branch of the placenta is burned with laser to block the blood flow. Theoretically, this disease can be fundamentally treated. This is an effective method for each stage, but requires professional technology and sufficient equipment. So far, for TTTS before 26 weeks, selective anastomosis of the placenta under fetal microscopy is the first choice. Compared with continuous amniotic fluid reduction, laser electrocoagulation can improve the perinatal survival rate and reduce the incidence of the nervous system.

2. Loss of amniotic fluid

Is one of the main treatment methods at present, in the acute TTTS after 26 weeks, amniotic fluid reduction is the first choice for treatment. Multiple amniocentesis extractions of amniotic fluid, although the blood transfusion between the twins cannot be interrupted, but the volume and volume of the venous bed of the placental blood vessel can be reduced after the reduction, which improves the blood flow in the umbilical cord and uterus. After the amniotic fluid is reduced, the echocardiogram can be used to monitor the waveform of the inferior vena cava of the donor to accurately predict the change in cardiac function in the recipient. Compared with electrocoagulation treatment, the operation and equipment required by this method are simple and easy to popularize, but multiple operations will increase the chance of infection.

3. Amniotic stoma

A stoma is made on the separation membrane to balance the amniotic fluid flow in the two amniotic sacs, thereby improving the placental circulation. There is no significant difference between amniotic ostomy and amniotic fluid reduction in terms of improving perinatal survival and reducing the incidence of neurological diseases. But compared to amniotic fluid reduction surgery, it only needs one operation, so it is easier to accept.

Prognosis of twin transfusion syndrome

The prognosis of untreated TTTs is not good. The earlier TTTs appear, the worse the prognosis. Those who appear earlier, if not treated, have a perinatal mortality rate of almost 100%. In general, the diagnosis and treatment before 28 weeks of pregnancy, the perinatal mortality rate is still 20% to 45%, which is significantly higher than double amniotic sac twin chorionic twins.