Twin-to-twin transfusion syndrome fetal ultrasound

Twin-to-twin transfusion syndrome (TTTS) is a potential complication that can occur in a monochorionic (either MCDA or MCMA) twin pregnancy. 

Epidemiology

This complication can occur in ~10% (range 15-25%) of monochorionic pregnancies, giving an estimated prevalence of ~1:2000 of all pregnancies.

Clinical presentation

Marked abdominal distension has been described as a clinical feature.

Pathology

TTTS results from unbalanced vascular (arteriovenous and arterioarterial) anastomoses in the placenta – that is, placental circulation is directed predominantly towards one twin and away from the other.

The resultant hypovolemia and hypoperfusion in one twin and hypervolemia and hypertension in the other create a cascade of hormonal changes including the renin-angiotensin system. This in turn leads to chronic tubulopathy and oliguria in the hypovolemic (donor) twin with consequent oligohydramnios, and polyuria and consequent polyhydramnios in the hypervolemic (recipient) twin.

Staging

The extent of the syndrome can be staged according to severity. One method proposed by Quintero et al. is as ³:

• stage I: oligohydramnios/polyhydramnios
• stage II: bladder not visible in donor twin
• stage III: abnormal Dopplers in either twin
• stage IV: hydrops fetalis in either twin *
• stage V: in utero demise of either twin

* Almost always in the recipient; rarely in the donor if there is coexistent TAPS (see below).

Variants

Twin anemia-polycythemia sequence (TAPS) is a relatively recently described subset of TTTS, which is discussed separately.

Radiographic features

Antenatal ultrasound

Monochorionicity is a prerequisite at any stage of gestation. 

Growth discordance is often but not invariably present, and does not contribute to diagnostic criteria.

First trimester

The following findings on a first-trimester ultrasound of a monochorionic pregnancy have been associated with an increased risk of developing TTTS, although predictive value remains poor ⁸:

• NT discordance: 20% or more
• CRL discordance: 10% or more
• ductus venosus reversed a-wave
• folding of the inter-twin membrane as an early sign due to a disparity in amniotic fluid volumes (amniotic fluid discordance)

Second and third trimesters

Features that may be noted individually in each twin include:

• recipient twin
  • polyhydramnios (deepest vertical pocket (DVP) >8 cm)
  • large or constantly cycling urinary bladder
  • features of cardiac overload
    • fetal hydrops
    • fetal cardiomegaly
      • in certain cases, fetal echocardiography may also show AV valve incompetence
    • Doppler abnormalities: predominantly venous
• donor twin
  • oligohydramnios (deepest vertical pocket <2 cm), which may result in the twin appearing pinned to the side of the gestational sac: “stuck twin“
  • small or non-visualized urinary bladder
  • Doppler abnormalities: predominantly umbilical artery +/- venous
  • evidence of fetal anemia

Color Doppler

• absent or reversed diastolic flow in the umbilical artery is an indication of worsening twin-to-twin transfusion syndrome
• abnormal ductus venosus waveform pattern suggests the possibility of cardiac diastolic dysfunction