Spectrum of Specific Complications
The monochorionic type of placentation causes a number of pathological conditions not found in other types of pregnancies. These include:
- Twin-to-twin transfusion syndrome (TTTS).
- Twin reversed arterial perfusion sequence.
- Intrauterine fetal demise of one of the twins.
- Congenital malformations or chromosomal abnormalities in one of the twins.
- Formation of conjoined (monsters) twins.
A special high-risk group is represented by monochorionic monoamniotic twins. With this form, the danger of umbilical cord entanglement increases critically both during pregnancy and in labor. The optimal delivery method in this case is recognized as planned cesarean section at 33–34 weeks.
In addition, during labor there is a high risk of acute intrapartum transfusion. This condition can be fatal for the second twin, as it causes acute hypovolemia, leading to anemia, brain structure damage, or intrapartum death.
Pathogenesis and Clinical Presentation of TTTS
The triggering factor for the development of TTTS is the presence of arteriovenous anastomoses within the thickness of the monochorionic placenta, which pass through the capillary bed of the cotyledons. The severity of the condition depends on the number, size, and direction of these vessels through which the pathological redistribution of blood occurs.
As a result, a vicious cycle is formed:
- The donor twin suffers from blood loss. Hypovolemia and hypoxia develop. Due to increased peripheral resistance in the placenta, blood begins to shunt to the second twin.
- The recipient twin receives an excess volume of circulating blood. The body tries to compensate for hypervolemia through pronounced polyuria. Subsequently, due to volume overload, the recipient develops heart failure.
A pathognomonic echographic marker of severe disease is a contrast picture: marked oligohydramnios in the donor against the background of marked polyhydramnios in the recipient.
TTTS Staging
To determine obstetric tactics and indications for intrauterine surgery, the Quintero staging classification is used, based on ultrasound data:
- Stage I — the bladder of the donor twin is visualized on ultrasound.
- Stage II — the donor's bladder is not detected, but blood flow parameters (in the ductus venosus and/or umbilical artery) are not critical.
- Stage III — a critical state of blood flow in the donor and/or recipient is recorded.
- Stage IV — signs of hydrops appear in the recipient twin.
- Stage V — antenatal death of one or both twins.
Surgical Correction and Alternative Tactics
Main treatment method The gold standard of TTTS therapy is fetoscopic laser ablation of vascular anastomoses. The surgery is performed under combined echographic and visual control:
- The fetoscope is introduced transabdominally into the recipient's amniotic cavity.
- The surgeon examines the chorionic plate along the intertwin membrane.
- Identified anastomoses are coagulated with a laser. This process is called "placental dichorionization", as it functionally separates the blood flows of the fetuses.
- In conclusion, excess amniotic fluid is drained in the recipient.
The intervention makes it possible to prolong the pregnancy by an average of 14 weeks and reliably reduces the risk of intrauterine death.
Alternative tactics If laser ablation is impossible, amniodrainage is used — the removal of excess fluid from the recipient's cavity under ultrasound control. This is a palliative measure: it does not eliminate the cause of the syndrome, but reduces intraamniotic pressure. As a result, the compression of the umbilical cord (which in multiple pregnancies often has a velamentous insertion) and superficial placental vessels decreases, which temporarily improves the condition of both fetuses. The procedure is used repeatedly in dynamics.
Briefly: answers to topical questions
What is the pathogenesis of twin reversed arterial perfusion (TRAP sequence) in monochorionic twins?
The pathogenesis of the twin reversed arterial perfusion sequence is due to the presence of umbilical arterio-arterial anastomoses, due to which blood circulation is distributed so that one fetus develops at the expense of the other.
- The donor twin acts as a "pump" and provides hemodynamics for both fetuses, while structural anomalies in it are, as a rule, absent.
- The recipient twin is "parasitic", develops exclusively at the expense of the donor twin, and always has multiple anomalies incompatible with life.
This condition is the most pronounced manifestation of twin-to-twin transfusion syndrome and is specific only to monochorionic twins.
What are the ultrasound criteria for twin anemia-polycythemia sequence (TAPS) in monochorionic twins?
The ultrasound criteria for twin anemia-polycythemia sequence are based on the assessment of peak systolic velocity in the middle cerebral artery (PSV MCA) and the absence of differences in amniotic fluid volume.
- The donor twin has signs of anemia, in which PSV MCA > 1.5 MoM (with progression of blood flow velocity discordance — greater than 1.7 MoM).
- The recipient twin has signs of polycythemia, in which PSV MCA < 0.8 MoM (antenatal criteria also include a value of 1.0 MoM).
An important diagnostic sign is the absence of oligohydramnios or polyhydramnios, which distinguishes this pathology from twin-to-twin transfusion syndrome.
What types of vascular anastomoses in the placenta participate in hemodynamics in monochorionic twins?
Arteriovenous and arterio-arterial vascular anastomoses participate in hemodynamics in monochorionic twins.
- Arteriovenous anastomoses are located in the thickness of the placenta and almost always pass through the capillary bed of the cotyledon. The degree of blood redistribution through them determines the severity of twin-to-twin transfusion syndrome.
- Arterio-arterial anastomoses are umbilical vascular communications, the presence of which leads to the development of the twin reversed arterial perfusion sequence, where one fetus acts as a "pump" and the second develops at its expense.
To remember the condition of the fetuses in TTTS: Donor — "dries up" (hypovolemia, oligohydramnios, empty bladder), Recipient — "swells up" (hypervolemia, polyhydramnios, polyuria, hydrops).
Class and exam questions
Why is there a high risk of demise of the second twin during labor in monochorionic pregnancies?
Due to the risk of acute intrapartum twin-to-twin transfusion. It leads to severe hypovolemia in the second twin, which is fraught with brain damage, severe anemia, and death.
Which ultrasound sign is pathognomonic for severe TTTS?
Combination of pronounced polyhydramnios in the recipient twin and pronounced oligohydramnios in the donor twin.
What is the essence of surgery for TTTS?
Under ultrasound and fetoscopic control, laser ablation of anastomoses in the placenta is performed. This separates the blood flows of the fetuses (dichorionization) and eliminates the cause of the syndrome itself.
What else to study on this topic
- Features of vaginal delivery management in twins
- Fetal collision: biomechanism of interlocking and emergency tactics
- Indications for planned cesarean section in multiple pregnancy
- Intrapartum placental abruption after the birth of the first twin
- Cervicometry: differential diagnosis of the state of the cervix
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