Influence of vasopressin in the pathogenesis of oligohydramnios-polyhydramnios in monochorionic twins

The pathophysiology of oligohydramnios-polyhydramnios in monochorionic (MC) twins complicated by chronic twin-twin transfusion syndrome (TTTS) is poorly understood. We hypothesise that oliguria and oligohydramnios in the donor twin of chronic TTTS, occurs due to antidiuretic and vasoconstrictive activity of vasopressin (AVP).

We measured AVP levels in maternal, fetal and amniotic fluid samples obtained in utero and at birth from 44 MC twins with (n=27) or without chronic TTTS (n=17). Concentrations of AVP in pg/ml were determined by immuno-radiometry assay.

In donor fetuses, plasma and amniotic fluid AVP levels were higher than those of the recipient twins in utero (P<0.001) and at birth (P<0.001). No such differences were found between the non-TTTS twins. The plasma AVP concentrations were higher in the recipient fetuses with severe hydrops than those without hydrops (2.8+/-0.7 pg/ml versus 0.3+/-0.3 pg/ml; P<0.05). Maternal AVP levels were comparable between the TTTS and non-TTTS groups. In the non-TTTS twins, both plasma and amniotic fluid AVP levels were higher than those of the recipient twins (P<0.001) but lower than those of the donor twins (P<0.001). There was a significant association between amniotic fluid and plasma AVP levels both in the TTTS (r=0.78; P<0.001) and non-TTTS (r=0.70; P<0.01) infants.

Vasopressin concentrations in the donor twins were three times higher than their co-twins which suggests that oligohydramnios may occur as a consequence of AVP mediated reduction in fetal urine output.