Polyhydramnios in sac of parasitic twin: atypical manifestation of twin reversed arterial perfusion sequence

A 30-year-old pregnant woman with a monochorionic diamniotic twin pregnancy was referred to our center at 18 + 1 weeks of gestation for the presence of gross abnormalities in one fetus. On ultrasound examination, twin reversed arterial perfusion (TRAP) sequence was diagnosed. The pump twin showed normal anatomy and growth, no signs of cardiac overload or polyhydramnios and the umbilical cord was inserted on the intertwin membrane. The parasitic twin appeared as a mass, less than half the size of the normal twin, partially attached to the placenta directly. There was a rudimentary beating heart with intracardiac flow, an anomalous lower limb and a poorly-defined head. The parasitic twin had no clearly identifiable umbilical cord and covered the placental surface between its attachment to the placenta and the intertwin membrane; the aorta was not seen clearly. Given the relatively low risk of cardiac failure in the pump twin and the difficult approach for both intrafetal and fetoscopic treatment, conservative management was chosen. Sequential scans performed weekly showed slow growth of the parasitic twin and progressive polyhydramnios within its sac (Figure 1). At 29 + 1 weeks' gestation, Doppler studies showed increased pulsatility in the umbilical artery and ductus venosus of the pump twin, with symptomatic polyhydramnios in the sac of the parasitic twin (deepest vertical pocket of 12 cm); amniodrainage was performed. Amniotic fluid re-accumulated gradually around the parasitic twin, with the pump twin in a stable condition. At 33 + 1 weeks' gestation, fetal heart-rate abnormalities of the pump twin were detected and an emergency Cesarean section was performed. A female weighing 1997 g was delivered, with Apgar scores of 3 and 9 at 5 and 10 min, respectively. The postnatal course was uneventful and the infant was well at 2-year follow-up. On pathological examination of the 400-g parasitic twin, it was found to be attached directly to the placental surface with no intervening umbilical cord (Figure 2). Proboscis, cyclopia and a rudimentary mouth were observed and a single body cavity was found containing rudimentary organs such as heart, pancreas, lung, thyroid and bowel, however there was no evidence of kidneys or bladder. We describe an atypical case of TRAP sequence, in which clinically significant polyhydramnios developed in the amniotic sac of the parasitic twin, instead of its more common appearance in the sac of the pump twin. The amniotic fluid was not of urinary origin, as the kidneys and excretory organs could not be identified. Fluid can accumulate from fetal lung secretions, but we are not aware of any reports on the occurrence of polyhydramnios in the absence of urine production. The membranes and placenta also play a role in determining amniotic fluid balance, but this is usually in the context of fluid absorption, rather than inflow into the amniotic cavity1, 2. Another peculiarity of our case was the presence of a rudimentary beating heart, however, this is less uncommon as it has been reported in up to 14% of TRAP-sequence cases3. D. Marella†, F. Prefumo*†, A. Valcamonico†, C. M. Donzelli‡, T. Frusca† and A. Fichera† †Department of Obstetrics and Gynaecology, University of Brescia, Piazzale Spedali Civili 1, Brescia 25123, Italy; ‡Department of Pathology, Spedali Civili di Brescia, Brescia, Italy *Correspondence. (e-mail: federico.prefumo@gmail.com)