What is the significance of an increasing umbilical vein diameter in monochorionic diamniotic (MoDi) twins with known twin anemia polycythemia syndrome (TAPS)?

Increasing Umbilical Vein Diameter in TAPS: A Critical Warning Sign

An increasing umbilical vein diameter in monochorionic diamniotic twins with known TAPS indicates severe disease progression with cardiac compromise in the donor twin, signaling the need for immediate escalation of care including consideration for fetoscopic laser surgery or delivery depending on gestational age.

Pathophysiologic Significance

The umbilical vein diameter increase reflects critical hemodynamic decompensation in TAPS:

• Pulsatile umbilical vein flow represents one of the defining features of Stage 3 TAPS, indicating cardiac compromise of the donor twin 1
• This finding, along with absent or reversed end-diastolic flow in the umbilical artery and reversed a-wave in the ductus venosus, signifies severe cardiovascular dysfunction 1
• The donor twin's severe anemia leads to high-output cardiac failure, causing increased venous pressures that manifest as umbilical vein dilation and pulsatility 1

Disease Staging and Prognosis

The presence of critical umbilical vein Doppler abnormalities upgrades TAPS severity:

• Stage 3 TAPS is defined by MCA-PSV criteria (Stage 1 or 2) plus cardiac compromise of the donor, which includes pulsatile flow in the umbilical vein 1
• Stage 4 TAPS involves ascites or hydrops of the donor twin, representing further progression 1
• These findings typically manifest in the donor twin and indicate severe disease with substantially increased risk of fetal demise 1

The natural history shows concerning outcomes:

• Perinatal mortality in TAPS reaches 15% overall, with donor twins at particularly high risk (22% mortality versus 7% in recipients) 1
• Severe neonatal morbidity occurs in 33% of affected twins 1
• Outcomes vary from low morbidity in late preterm births to double fetal demise in severe early presentations 1

Management Implications

Immediate care escalation is warranted when umbilical vein abnormalities are detected:

• For Stage 3 TAPS presenting before 32-34 weeks, fetoscopic laser coagulation should be strongly considered rather than expectant management 1
• All patients with TAPS qualifying for laser therapy should be referred to a fetal intervention center for evaluation 1
• If near term (≥32-34 weeks), delivery should be expedited given the signs of severe cardiovascular compromise 1

Surveillance Protocol

Enhanced monitoring is critical once umbilical vein changes are identified:

• Weekly or more frequent ultrasound surveillance with comprehensive Doppler assessment (umbilical artery, MCA-PSV, umbilical vein, and ductus venosus) 1
• Assessment for progression to hydrops or ascites, which would indicate Stage 4 disease 1
• Evaluation of the recipient twin for cardiomegaly and tricuspid regurgitation, which occur in up to 86% of TAPS cases 1

Common Pitfalls to Avoid

Do not rely solely on MCA-PSV values when umbilical vein abnormalities are present:

• The presence of critical Doppler findings in the umbilical vein already indicates advanced disease regardless of MCA-PSV thresholds 1
• False reassurance from stable MCA-PSV values can occur while cardiovascular compromise progresses 1

Do not confuse with post-laser hemodynamic adaptation:

• MCA-PSV abnormalities and Doppler changes can occur as fetal hemodynamic adaptation following laser surgery, mimicking TAPS 1
• However, progressive umbilical vein dilation in established TAPS represents true disease worsening, not adaptation 1

Recognize that amniotic fluid volumes remain normal in TAPS:

• Unlike TTTS, TAPS does not present with oligohydramnios-polyhydramnios sequence 1, 2
• The absence of fluid discordance should not provide false reassurance when umbilical vein abnormalities are present 1