Histological evidence

TTTS. Histological evidence

An interesting study on placental histology was performed in 1998.17 Tertiary villi were studied and the number of muscular arteries was counted in a subset of 9 consecutive TTTS diagnosed by ultrasound, with 20% growth discordance and oligo–polyhydramnios sequence. The placenta near the cord insertion of the donor and the recipient was studied blinded for the pathologist. They found significantly less muscular arteries in the donors (5.81 ± 0.55 vs 6.66 ± 0.24; p = 0.017). 

Concomittantly, the umbilical artery S/D Doppler index was higher in the donors than in the recipients. In a previous publication of the same cases, they found that 66% of the pairs had feto–fetal transfusion, and only 25% of them had anemia/ polycythemia. 

In-vivo studies

In-vivo studies

The description of placental vascular anastomoses from fetoscopies during laser photocoagulation of the chorionic plate may be used to study the placental architecture. This method is sensitive because of the endoscopic magnification of the image and is controlled because the placental surface remains with a white scar at the coagulation point. 

The problem is that technically difficult fetoscopies as anterior placentas or turbid amniotic fluid may give false negative for anastomoses detection. De Lia et al14 described ablating 8–10 communicating vessels per placenta in TTTS. 

Placenta

Placenta. Twin–Twin Transfusion Syndrome

The normal fetoplacental circulation consists of two arteries and one vein, which divide progressively in the chorionic plate (fetal surface of the placenta), and irrigate each cotyledon in a separate way. This means that normal vascularization of the placentas may be clearly recognized by the naked eye, until it reaches the corresponding cotyledon. 

All monochorionic placentas have vascular communications between the cords,5 which was demonstrated in 278 placentas from twin pregnancies. Monochorionic placentas always have vascular communications, in comparison with dichorionic fused placentas. Despite this finding, TTTS only occurs in 15% of monochorionic pregnancies. The pattern of the anastomoses is therefore determinant in the occurrence of TTTS. 

The case Definition problem

Twin–Twin Transfusion Syndrome.The case Definition problem

Before the use of ultrasound, TTTS was diagnosed by a 20% discordance in the weights and at least 5 g/dl difference in the hemoglobin concentration at birth of two twins of the same sex.3 These criteria were left aside because they were not always possible to demonstrate antenatally by ultrasound and because it is frequent in diamniotic twins as much as in monochorionic twins. Besides, these were used for surviving twins but not in cases of double- or single-fetal demise, which were more possibly affected by severe TTTS. 

With the development of ultrasound, new antenatal findings were correlated to adverse outcome. The polyhydramnios/oligohydramnios sequence has been found to be the condition with one of the highest mortalities in obstetrics, with 90% mortality without treatment. Recently, growth discordance with abnormal umbilical Doppler has been considered a new indication for laser photocoagulation of anastomoses on the chorionic plate. 

Evidence on physiopathology

Evidence on physiopathology

The twin–twin transfusion syndrome (TTTS) (Figure 4.1) occurs in 15% of monochorionic– diamniotic pregnancies,1 with a high perinatal mortality rate.2 These are morphologically normal fetuses, in which the vascular communications in the placenta are thought to be responsible for the development of the disease.

Besides the primary phenomenon, the disease may lead to disruptive lesions in both twins. This chapter will review the evidence for the development of the disease, its complications, and implications in therapeutical approaches. The chapter focuses at defining the clinical problem, the histopathology correlation of the clinical condition, and the ultrasonographic features that support or reject the pathophysiology of the disease. Some particular conditions that lead to TTTS are mentioned in order to understand the disease better.

Summary of the Pathophysiology

Summary of the Pathophysiology of Twin–Twin Transfusion Syndrome

TTTS presumably results from an unbalanced exchange of blood between two or more fetuses via placental vascular anastomoses. While this fundamental hypothesis has not been proven, circumstantial evidence suggests that this indeed may be the main operating pathophysiological mechanism. 

The etiology of the unbalanced blood exchange can be traced back to either an abnormal design of the placental vascular anastomoses (obligatory etiology), or, less often, to primary hemodynamic differences between the fetuses. Hypovolemia in the donor twin elicits the RAS and increased ADH, causing local vasoconstriction, oliguria, oligohydramnios, and renal tubular dysgenesis. Hypervolemia in the recipient twin results in increased ANF secretion, polyuria, polyhydramnios, and hypertension. 

Cause versus effect

Cause versus effect

The endocrine findings in TTTS are commonly viewed as the effect of the uneven blood exchange. Increased renin in the donor and increased ANF in the recipient, with their hemodynamic and renal consequences, follow the uneven exchange of blood. This view would be in accordance with the obligatory etiology as described above. 

Alternatively, increased renin, increased ADH, and a tendency to raise the blood pressure in the donor could result from placental insufficiency. The increased blood pressure in the donor would in turn force more blood through placental vascular anastomoses into the recipient twin, setting off the vicious cycle.