Mathematical Modeling

TTTS. Mathematical Modeling

Animal models of monochorionic multiple pregnancies are lacking, so mathematical modeling has become an interesting way of studying the pathophysiology of TTTS. The first model36 was based on two circulations in which a communication was produced at 28 weeks of pregnancy, reproducing the oligo/polyhydramnios sequence. 

An important pathophysiological consideration is the timing on the development of TTTS relative to existence of the vascular anastomoses. Since TTTS develops in the midtrimester, despite the existence of vascular anastomoses since the beginning of the pregnancy. Later mathematical models suggested that the growth of the vessels diminish the vascular resistance faster than the growth of the fetus, which in turn the existent anastomoses into functional low resistance anastomoses. 37 The models are explained in detail in Chapter 6. 

CASE REPORTS 

Some pathophysiological arguments come from particular cases. Acute TTTS developed at 32 weeks with previously normal amniotic fluid and non-discordant monochorionic twins.38 After delivery, a recent thrombosis of a superficial arterioarterial anastomosis was found. In addition, the placenta had 5 donor-to-recipient deep AV anastomoses, and 2 recipient-to-donor deep AV anastomoses. 

CONCLUSIONS 

The pathophysiology of TTTS is still poorly understood. It is generally accepted that neonatal criteria are inadequate for the diagnosis, and have been replaced by ultrasonographic criteria of amniotic fluid discordance. Most authors agree that the placental vascular connections are the main basis for the development of the condition. The TTTS stages may represent natural history or different presentations of TTTS, rather than progressive stages. 

Data from cordocentesis has concluded that the ultrasonographic diagnosis of TTTS (oligo/polyhydramnios) has rarely anemia/polycythemia, with similar iron/ ferritin profile in both fetuses. The renin– angiotensin system is unbalanced, with overfuncion in the donor and down-regulation in the recipient, but the effect of the oversecretion in the recipient may be part of the passage of blood from donor to recipient, aggravating the condition. Arterial Doppler measurements are not different between twins in general, even in middle cerebral artery territory. Venous flow measurement supports the theory that the main disturbance is volume overload in one fetus and depletion in the other.