How do vessel size anastomoses differ in management between Twin-to-Twin Transfusion Syndrome (TTTS) and Twin Anemia Polycythemia Sequence (TAPS)?

Vessel Size Anastomoses in TAPS Compared to TTTS

Key Pathophysiologic Distinction

TAPS is characterized by tiny submillimeter arteriovenous (AV) anastomoses (<1 mm diameter), typically located near the placental edge, whereas TTTS involves larger AV anastomoses that are uncompensated by protective arterio-arterial (AA) anastomoses. 1, 2, 3

Vascular Architecture Differences

TTTS Anastomoses

• Large unidirectional AV anastomoses (90-95% of monochorionic placentas have AV connections) that allow significant volume transfer 1
• The actual anastomotic connections occur deep within cotyledons, linked through large capillary beds 1
• Absence or inadequacy of compensatory AA anastomoses is the critical factor—mortality is 42% without AA versus 15% with AA present 1, 4
• AA anastomoses (present in 85-90% of monochorionic placentas) are direct superficial connections with bidirectional flow that can compensate for AV imbalance 1
• When present, AA anastomoses protect against TTTS in only 25-30% of cases, indicating incomplete compensation 1

TAPS Anastomoses

• Submillimeter AV anastomoses (<1 mm) that allow only slow red blood cell transfusion 1, 2, 3
• Blood flow rate through these tiny connections is approximately 5-15 mL per day, compared to the much larger volume transfers in TTTS 1
• These minuscule anastomoses are commonly located near the placental edge 1
• May include low-rate, small-caliber AA anastomoses that are insufficient to prevent the condition 3

Clinical Implications of Vessel Size Differences

TTTS Presentation

• Rapid volume imbalance leading to twin oligopolyhydramnios sequence (donor with oligohydramnios, recipient with polyhydramnios) 1, 2
• Significant amniotic fluid discordance is the hallmark (MVP <2 cm in one sac, >8 cm in the other) 1
• Recipient develops volume overload with polyuria, biventricular hypertrophy, and potential hydrops 1
• Donor develops volume depletion with oliguria and absent bladder visualization 1

TAPS Presentation

• Chronic, insidious hemoglobin imbalance without significant amniotic fluid discordance 1, 2, 3
• The slow transfusion rate allows fetal compensation, particularly in early stages 1
• Anemic donor and polycythemic recipient develop gradually 1, 2
• Amniotic fluid volumes typically remain normal in both sacs 1, 2

Management Implications Based on Vessel Architecture

TTTS Management

• Fetoscopic laser surgery is standard treatment for stage II-IV TTTS at 16-26 weeks 1, 5, 6
• The Solomon technique involves linear photocoagulation along the entire vascular equator to eliminate all anastomoses, including small ones 1, 7
• This technique reduces residual anastomoses and decreases post-laser TAPS from 4.2-15.6% (selective technique) to 0-2.9% 7
• Dual survival rates: 50-70% with experienced centers 1, 5

TAPS Management

• No clear optimal management exists—options include expectant management, intrauterine transfusion with/or without partial exchange transfusion, or fetoscopic laser surgery 1, 8
• Stage I TAPS can be managed with weekly local surveillance without fetal care center referral 1
• Referral to fetal care center recommended for stage ≥II before 32 weeks or when TTTS coexists 1
• Perinatal mortality rates are similar across management approaches (7-18%), with no significant difference between laser, transfusion, or expectant management 1

Critical Pitfall

The presence of tiny anastomoses in TAPS means incomplete laser surgery in TTTS can create iatrogenic TAPS (2-16% incidence post-laser) by leaving behind submillimeter connections that were not visualized or adequately coagulated 1, 2, 3. The Solomon technique specifically addresses this by coagulating the entire vascular equator rather than just visible anastomoses 1, 7.