Vessel Anastomoses Size in Twin Anemia-Polycythemia Sequence
The vessel anastomoses in TAPS are exclusively small-caliber arteriovenous connections measuring less than 1 millimeter in diameter, typically located near the placental edge. 1
Pathophysiologic Characteristics
The defining vascular feature of TAPS involves minuscule arteriovenous anastomoses (<1 mm) that facilitate slow, chronic blood transfusion between twins. 1, 2 These submillimeter connections are fundamentally different from the larger anastomoses seen in twin-twin transfusion syndrome (TTTS), which explains the distinct clinical presentation.
Blood Flow Dynamics
- The rate of blood flow through these tiny anastomoses is approximately 5 to 15 mL per day, which is substantially slower than in TTTS 1
- This slow transfusion rate allows for gradual hematologic compensation in earlier disease stages, explaining why amniotic fluid discordance is typically absent 1
- The small caliber and low flow rate result in chronic anemia in the donor twin and polycythemia in the recipient twin without the oligohydramnios-polyhydramnios sequence characteristic of TTTS 3, 2
Diagnostic Confirmation
**Postnatal diagnosis of TAPS requires identification of exclusively small-vessel anastomoses (<1 mm) upon placental pathology examination**, along with either an intertwin hemoglobin difference of ≥8 g/dL or a reticulocyte ratio >1.7 between donor and recipient. 1
Key Diagnostic Points
- The presence of only small-caliber anastomoses is pathognomonic for TAPS 2, 4
- Larger anastomoses or arterio-arterial connections are typically absent in TAPS, distinguishing it from other forms of fetofetal transfusion 5, 2
- These minuscule connections are commonly located near the placental edge rather than centrally 1
Clinical Implications
The submillimeter size of these anastomoses has important clinical ramifications:
- Prevention strategy: The Solomon laser technique, which involves equatorial dichorionization, reduces post-laser TAPS from 15.6% to 2.9% by more completely addressing these tiny residual anastomoses 1, 5
- Screening necessity: Because these anastomoses are too small to cause amniotic fluid abnormalities, TAPS requires specific MCA-PSV Doppler surveillance rather than relying on fluid discordance 1
- Pathophysiology: The small caliber explains why TAPS can develop at any time from early second trimester through third trimester, as the slow transfusion process takes time to manifest clinically 6
Important Caveat
The presence of small-caliber arterio-arterial anastomoses in addition to the arteriovenous connections may provide some protective hemodynamic balance, though when present in TAPS cases, these AA anastomoses are also of low caliber 4