Effect of unbalanced blood exchange in the recipient twin

Effect of unbalanced blood exchange in the recipient twin

The excessive blood volume in the recipient twin presumably results in an increased production of atrial natriuretic factor, or ANF.9 Wieacker et al studied three cases of severe twin transfusion syndrome and demonstrated that the concentration of ANF in the cord blood of recipient twins is significantly elevated compared to that of donor twins. Increased ANF results in increased fetal urine production, bladder distention, and polyhydramnios. 

The discrepancy between recipient and donor concentration correlated with the volume of transfusion. They proposed the following pathophysiological mechanism to explain the development of polyhydramnios in recipient twins: chronic blood volume overload in twins causes enhanced release of ANF from the fetal heart; increased fetal urine production leads to polyhydramnios, which is additionally enhanced by inhibition of antidiuretic hormone (ADH) release. 

Consequences in the donor twin

Consequences in the donor twin

Net blood loss in the donor twin presumably actives the renin–angiotensin system (RAS). Mahieu-Caputo et al have performed immunohistochemical studies with renin antiserum and with in-situ hybridization using riboprobes complementary to renin mRNA, and reninsecreting cells (RSCs) to assess the renin production in twin pairs affected by transfusion syndrome.8 The overall maturation of the renal cortex, as determined by the percentage of immature glomeruli, was simultaneously assessed. Although donor twin kidneys were smaller than those of recipients, the maturation of the renal cortex was not significantly different (28.2% immature glomeruli in the donor and 24.4% in the recipient kidney). 

Facultative etiology

Facultative etiology of Twin–Twin Transfusion Syndrome

Monochorionic twins may be viewed hemodynamically as two pumps connected in parallel via placental vascular anastomoses. The hemodynamic equilibrium is maintained as long as both fetuses pump the same amount of blood in opposite directions. 

Selective pump failure may occur in one of the twins from cardiac dysfunction secondary congenital heart disease, myocarditis or obstruction at the level of the umbilical cord. The affected fetus may not be able to tolerate the additional amount of blood coming from the other twin. As a result, the affected fetus becomes the recipient and the other twin the donor. 

Obligatory etiology

Obligatory etiology. Twin–twin transfusion syndrome

In obligatory TTTS, the placental vascular design is deterministic or fatalistic. The faulty placental vascular design consists of the presence of either more numerous or larger arteriovenous (AV) anastomoses from donor to recipient. As a result, more blood flows from donor to recipient. To investigate this hypothesis, we assessed the size, number, and direction of arteriovenous anastomoses. Demonstration of faulty placental vascular design: The DR score 3 Detailed endoscopic analysis of the size, number, and direction of AV anastomoses in TTTS placentas indeed suggests larger or more numerous AV anastomoses from donor to recipient.

In a study we conducted on 20 TTTS patients with pure AV anastomoses identified endoscopically during laser surgery, the number of vascular communications was established in two ways:
• as a single communication, if only an artery and a vein participated
• as multiple communications, if prolific branching precluded an accurate estimate of the number of vessels.

Pathophysiology

Pathophysiology. Twin–twin transfusion syndrome (TTTS)

Twin–twin transfusion syndrome (TTTS) is thought to result from an unbalanced sharing of blood between two fetuses via placental vascular anastomoses. The negative corollary is evident from the fact that the disease does not occur in dichorionic pregnancies, as vascular anastomoses do not develop in such placentas. 

As a result of the unbalanced blood exchange, one fetus receives too much blood (the recipient twin, or recipient), and one fetus loses too much blood (the donor twin, or donor). The unbalanced blood sharing triggers a series of pathophysiological changes that characterize the natural history and outcome of the disease. Most monochorionic placentas have vascular anastomoses but only 5–10% of monochorionic pregnancies develop TTTS.

What determines then the development of the disease in these patients? Two theories have evolved over the years in our laboratory regarding the etiology of TTTS and, analogous to the microbiology terminology, we have termed them ‘obligatory’ and ‘facultative.’

Summary

Summary of Twin–Twin Transfusion Syndrome

Evaluation of the incidence of TTTS is limited by the continuum of mild to severe disease and by the assessment in select populations, including referral centers and population-based studies. By current diagnostic criteria, less than 2% of twins or higher multiple births born >20 weeks’ gestation may develop TTTS. 

This incidence is likely to change with increasing rates of multiple births worldwide. Predicting TTTS in early pregnancy may be more valuable than the diagnosis of clinically overt disease. In addition to its prognostic value, 

Doppler ultrasound studies may play a bigger role in diagnosing TTTS in the future. The true incidence of TTTS may fall somewhere in between the percent of histopathologically abnormal monochorionic placentas and the yet immeasurable subtle manifestations of the syndrome.

Prognostic Factors for adverse outcome

Prognostic Factors for adverse outcome

Absent or reversed end-diastolic flow in the donor umbilical artery, abnormal pulsatility of the venous system in the recipient, and absence of an arterioarterial anastomosis are predictive of fetal mortality. Preoperative absent or reversed end-diastolic velocity in the umbilical Doppler, but not other Doppler studies, may predict demise in the donor twin following laser therapy. 

Short cervical length, parity, and intrauterine demise of one twin have been shown to be predictors of preterm delivery in pregnancies identified with TTTS following laser coagulation of vascular anastomoses.