Ultrasound assessment

Ultrasound assessment in twin–twin transfusion syndrome

The ultrasound assessment of patients with twin–twin transfusion syndrome (TTTS) can be particularly challenging. The presence of oligohydramnios in the sac of the donor twin frequently impairs adequate visualization of the fetal anatomy, fetal gender, and occasionally, adequate Doppler interrogation of the different vessels of this fetus. Polyhydramnios is typically associated with a frequently moving recipient twin, which makes the Doppler assessment of this fetus particularly difficult. 

Polyhydramnios may also result in significant maternal discomfort with back pain or light-headedness in the recumbent position, hindering further the completion of the ultrasound examination. Lastly, the examination is typically done under an atmosphere of psychological tension from maternal anxiety, given the grave prognosis in most cases. Despite these limitations, a thorough and complete ultrasound examination is necessary to correctly diagnose and assess the status of patients with TTTS. The ultrasound assessment of patients with TTTS can be systematized to be performed in several steps or levels, including diagnosis, staging, cervical length, and preoperative mapping. Each of these steps will be discussed below. 

STEP ONE: DIAGNOSIS OF TWIN–TWIN TRANSFUSION SYNDROME 

TTTS is a complication of monochorionic multiple pregnancies defined sonographically as the combined presence of polyhydramnios in one sac and oligohydramnios in the other sac in a monochorionic–diamniotic twin gestation. Polyhydramnios is defined as a maximum vertical pocket (MVP) ? 8 cm, and oligohydramnios as an MVP ?2 cm (poly8-oligo2). Monochorionicity is established by the presence of a single placenta, absence of a twin-peak sign, thin dividing membrane, and same gender. 

Variations in the definition 

Although the condition affects mostly twin pregnancies, TTTS can also occur in triplet or higher-order multiple gestations provided at least two fetuses are monochorionic. In monoamniotic twins, the lack of a dividing membrane precludes the presence of oligohydramnios, but the syndrome can be suspected by the presence of polyhydramnios and differences in bladder filling or Doppler studies of the two fetuses. In monochorionic triplet pregnancies, two or all three fetuses may be involved. 

Definitions no longer used 

Until a few years ago, TTTS was only diagnosed postnatally if an intertwin hemoglobin difference >5 g/dl1 and a birth weight difference >20%2 existed between the twins. However, in a classic study by Danskin and Neilson3 in 178 twin pairs, only four pairs had a hemoglobin difference >5 g/dl and a weight difference >20% but none of these pregnancies showed evidence of polyhydramnios or oligohydramnios. Similarly, percutaneous umbilical blood sampling in six TTTS patients failed to show hemoglobin differences >5 g/dl except in one pregnancy.4 A difference of only 1.7 g/dl was found in 4 patients by Saunders and collaborators.5 Because of these findings, the previous neonatal criteria based on birthweight discordance and hemoglobin differences are no longer used.