What are the ultrasound (USG) signs of Atypical Twin-to-Twin Transfusion Syndrome (TTTS) in monochorionic diamniotic twins?

Ultrasound Signs of Atypical Twin-to-Twin Transfusion Syndrome

Atypical TTTS presents with Doppler abnormalities or fetal hydrops WITHOUT the classic oligohydramnios-polyhydramnios sequence, or with unusual progression patterns that skip Quintero stages, requiring heightened clinical suspicion and immediate referral to a fetal intervention center. 1

Core Diagnostic Principle

Classic TTTS requires both monochorionic diamniotic placentation AND the oligohydramnios-polyhydramnios sequence (MVP <2 cm in donor, >8 cm in recipient) 2, 3. Atypical TTTS deviates from this standard presentation in critical ways that can lead to misdiagnosis if not recognized 1.

Key Ultrasound Features of Atypical TTTS

Atypical Fluid Patterns

• Near-normal or borderline amniotic fluid volumes in one or both sacs despite hemodynamic imbalance, where MVP measurements may be 2-8 cm but other signs of transfusion are present 1
• Polyhydramnios in both sacs rather than the expected discordance, which can mask the underlying transfusion syndrome 1
• Stuck twin appearance WITHOUT severe oligohydramnios, where the donor twin appears trapped against the uterine wall despite MVP >2 cm 4

Non-Sequential Quintero Progression

• Direct progression to Stage III or IV without preceding Stage I or II findings on prior ultrasounds, presenting acutely with severe Doppler abnormalities or hydrops 1
• Absent or reversed end-diastolic flow in donor umbilical artery appearing before bladder becomes non-visible 2, 1
• Ductus venosus a-wave reversal or pulsatile umbilical vein flow in the recipient twin without the expected severe polyhydramnios 2, 1

Discordant Cardiac Findings

• Cardiomegaly in the recipient twin with only mild polyhydramnios (MVP 8-10 cm rather than >12 cm) 3, 5
• Tricuspid regurgitation or cardiac dysfunction appearing early without progression through typical staging 3, 1
• Hydrops fetalis in recipient with borderline amniotic fluid discordance 2, 3

Unusual Bladder Patterns

• Intermittently visible donor bladder that appears and disappears during extended observation, rather than persistent non-visualization 2, 1
• Paradoxically large recipient bladder with only moderate polyhydramnios 3, 5

Placental and Membrane Signs

• Marked contrast difference between placental territories on B-mode ultrasound, though this finding alone is unreliable for diagnosis 4
• Velamentous cord insertion in the donor twin, present in approximately one-third of TTTS cases and associated with atypical presentations 2, 6
• Intertwin membrane folding without meeting full Stage I criteria 2

Critical Doppler Findings in Atypical Cases

Donor Twin Doppler Abnormalities

• Excessive adrenal artery flow in the stuck twin, which has been associated with poor outcomes even without classic TTTS criteria 4
• Umbilical artery absent or reversed end-diastolic velocity appearing as the first sign of disease 2, 1

Recipient Twin Doppler Abnormalities

• Ductus venosus a-wave reversal occurring early in disease course 2, 6
• Pulsatile umbilical vein flow indicating cardiac compromise 2, 1

Important Clinical Caveats

The absence of classic oligohydramnios-polyhydramnios sequence does NOT exclude TTTS if other hemodynamic signs are present. 1 Up to 20% of TTTS cases may present atypically, and these require the same urgency in referral to fetal intervention centers 1, 7.

Growth discordance alone is NOT diagnostic of TTTS and can represent selective intrauterine growth restriction, which has different management implications 2, 3. However, when combined with any Doppler abnormalities or subtle fluid differences, atypical TTTS must be considered 1.

Serial surveillance every 2 weeks starting at 16 weeks is essential for all monochorionic diamniotic pregnancies, as atypical TTTS can develop rapidly between surveillance intervals 2, 6. When atypical features are identified, weekly or even more frequent surveillance is warranted 2, 6.

Differential Diagnosis Considerations

The following conditions can mimic atypical TTTS and must be excluded:

• Selective intrauterine growth restriction (sIUGR) with growth discordance ≥20% but without the hemodynamic instability of TTTS 2, 3
• Twin anemia-polycythemia sequence (TAPS) presenting with MCA-PSV >1.5 MoM in donor and <1.0 MoM in recipient WITHOUT oligohydramnios-polyhydramnios 2, 6
• Fetal anomaly in one twin causing isolated amniotic fluid abnormality 2, 3

When any doubt exists about atypical presentation, immediate referral to a maternal-fetal medicine specialist or fetal intervention center is mandatory for definitive diagnosis and consideration of fetoscopic laser photocoagulation 6, 1, 7.