Twin Anaemia-Polycythaemia Sequence is a unique complication of monochorionic twin pregnancies, characterised by significant inter-twin haemoglobin discordance, with the characteristic imaging finding of discordant peak systolic velocity in the middle cerebral arteries of the twins on Doppler ultrasound.
Description
Twin Anaemia-Polycythaemia Sequence (TAPS) is a unique condition that affects monochorionic twin pregnancies (i.e. only seen in monozygotic twin pregnancy). It results in one twin (the donor) developing anaemia and the other twin (the recipient) developing polycythaemia. Unlike Twin-to-Twin Transfusion Syndrome (TTTS), TAPS occurs in the absence of significant amniotic fluid discordance and is characterised by slow and chronic transfusion through minuscule placental anastomoses.
Pathogenesis
TAPS results from slow and chronic inter-twin transfusion through tiny, unidirectional arterio-venous anastomoses within the shared placenta of monochorionic twins. This leads to an imbalance in haemoglobin levels between the twins, causing one twin to become anaemic (donor twin) and the other to become polycythaemic (recipient twin).
Subtypes
There are two types of TAPS:
- Spontaneous TAPS: Occurs randomly in about 5% of monochorionic twins.
- Post-laser TAPS: Occurs in approximately 16% of cases after laser ablation for TTTS.
Epidemiology, Risk Factors & Associations
- Monochorionic twin pregnancies are the primary risk factor.
- Prior laser therapy for TTTS increases the risk for post-laser TAPS.
Clinical Features
Clinical features are usually not apparent in utero and are mostly recognised after birth. They include:
- Donor twin: Anaemia, pallor.
- Recipient twin: Polycythaemia, plethoric or reddish appearance.
Complications
TAPS can result in significant morbidity, including intrauterine growth restriction, brain abnormalities, and death. Postnatal complications can include neurodevelopmental delay, especially in the anaemic twin.
Pathological Features
Histopathology
- Macroscopic: Characterised by the distinct colour difference between twins (pallor in the anaemic twin and plethoric appearance in the polycythaemic twin) immediately after birth.
- Microscopic: Not applicable.
Serology
- Marked inter-twin haemoglobin difference (>8 g/dL) and reticulocyte count ratio >1.7 are diagnostic.
Biochemistry
- No specific biochemical markers.
Radiological Features
General Features
- The characteristic imaging feature is the discordant peak systolic velocity (PSV) in the middle cerebral arteries (MCA) of the twins on Doppler ultrasound.
- Amniotic fluid volume should be normal:
- Greatest depth (devoid of umbilical cord and foetal parts) should be 3 – 8 cm.
- Estimate foetal weight (EFW) should be normal:
- Between 10 and 95th centile, with no difference between twins greater than 25%.
Ultrasound
- B-mode: Normal amniotic fluid volumes in both sacs.
- Colour Doppler: Discordant PSV in the MCA of the twins (>1.5 MoM in the donor and <1.0 MoM in the recipient).
- In the donor twin, the PSV in the MCA is greater than 1.5 times the median value for that parameter in a reference population (represented as >1.5 MoM).
- In the recipient twin, the PSV in the MCA is less than 1.0 times the median value for that parameter in a reference population (represented as <1.0 MoM).
Diagnosis
Diagnosis is mainly based on antenatal ultrasound findings demonstrating MCA-PSV discordance, and it is confirmed by the presence of significant inter-twin haemoglobin discordance after birth.
Differential Diagnosis
- Twin-to-Twin Transfusion Syndrome (TTTS): Can be differentiated by the presence of significant amniotic fluid discordance in TTTS, which is not seen in TAPS.
Management
Management options for TAPS include expectant management with close surveillance, intrauterine blood transfusion and/or partial exchange transfusion, or fetoscopic laser coagulation of the anastomoses. Postnatal treatment involves the management of anaemia in the donor twin and polycythaemia in the recipient twin. Referral to a specialist in maternal-fetal medicine and a neonatologist is recommended.