PET Scan Findings in TRAP Syndrome
In patients with Twin Reversed Arterial Perfusion (TRAP) syndrome, PET scans are not routinely indicated as ultrasound is the primary imaging modality for diagnosis and monitoring.
Understanding TRAP Syndrome
TRAP sequence is a rare complication unique to monochorionic twin pregnancies, occurring in approximately 1 in 30,000 pregnancies and in 0.3% of all monozygotic twin gestations 1. The condition is characterized by:
- A structurally normal "pump" twin
- An acardiac "parasitic" twin without cardiac activity
- Reversed arterial perfusion via anomalous arterio-arterial placental anastomoses
Primary Imaging Modality for TRAP Syndrome
Ultrasound
- Transabdominal ultrasound is the primary and most appropriate imaging modality for diagnosis and monitoring of TRAP syndrome 1
- Key ultrasound findings include:
- Structurally normal pump twin
- Acardiac twin with reversed flow in its umbilical artery
- Markedly anomalous acardiac twin (often acephalic, lacking upper extremities, with marked edema)
- Arterio-arterial placental anastomoses
Monitoring Parameters on Ultrasound
- Volume/weight of the acardiac twin should be calculated at each imaging session 1
- Ratio of acardiac twin weight to pump twin weight (critical threshold >50%)
- Signs of cardiac decompensation in the pump twin 1
- Doppler assessment of umbilical artery and middle cerebral artery of the pump twin
Role of PET Scanning in TRAP Syndrome
PET scanning has no established role in the diagnosis or management of TRAP syndrome. The American College of Radiology guidelines do not mention PET scanning for TRAP syndrome evaluation 1.
Other Imaging Modalities in TRAP Syndrome
MRI
- May provide additional anatomical detail of the acardiac twin 2
- Can help visualize the vascular connections between twins
- Not routinely required unless ultrasound findings are equivocal
3D and Color Doppler Ultrasound
- Adds value for both diagnosis and detection of poor prognostic features 3
- Helps visualize the reversed arterial flow pattern
Management Considerations
The primary concern in TRAP syndrome is the high mortality rate (approximately 50%) of the pump twin due to cardiac overload 1. Intervention is indicated when:
- The estimated weight of the acardiac twin is ≥50% of the pump twin
- Features of cardiac decompensation are seen in the pump twin
- Treatment involves interruption of flow in the communicating vessels or umbilical cord of the anomalous twin
Conclusion
While PET scanning has important applications in other conditions such as prosthetic heart valve endocarditis 1, 4, it has no established role in TRAP syndrome diagnosis or management. Ultrasound remains the gold standard imaging modality for this rare condition, with MRI serving as a complementary imaging technique when additional anatomical detail is needed.