Twin-to-Twin Transfusion Syndrome (TTTS) is a serious complication of monozygotic twin pregnancies, characterised by the ‘stuck twin’ appearance on ultrasound with oligohydramnios in the donor twin’s sac and polyhydramnios in the recipient twin’s sac.
Description
Twin-to-Twin Transfusion Syndrome (TTTS) is a condition specific to monochorionic twin pregnancies (monozygotic or identical twins sharing a single placenta), where there is a unidirectional arteriovenous shunt from one twin (the donor) to the other (the recipient) via placental vascular anastomoses. This leads to a discordance in amniotic fluid volumes and, if untreated, can lead to significant morbidity and mortality for both twins.
Pathogenesis
The pathogenesis of TTTS involves the presence of abnormal vascular anastomoses in the shared placenta of monochorionic twins. These anastomoses allow for blood to flow disproportionately from the donor twin to the recipient twin, causing the donor twin to become hypovolaemic and the recipient twin to become hypervolaemic. This blood flow imbalance can cause a series of secondary complications for both twins, such as cardiac dysfunction, renal impairment, and growth restriction in the donor twin, and polycythaemia, congestive heart failure, and hydrops in the recipient twin.
Subtypes
There are no recognised subtypes of Twin-to-Twin Transfusion Syndrome.
Epidemiology, Risk Factors & Associations
- Most commonly occurs in monochorionic diamniotic twin pregnancies, affecting approximately 10-15% of these pregnancies.
- No specific risk factors have been identified apart from the type of pregnancy (monochorionic). The condition is not thought to be associated with maternal age, ethnicity, or parity.
Clinical Features
- Polyhydramnios in the recipient’s sac leading to maternal discomfort, dyspnoea, and preterm labour.
- Discrepant size between the twins.
- Discordant amniotic fluid volumes with oligohydramnios in the donor’s sac.
- Structural heart defects
Complications
Complications of TTTS can be severe and impact both the donor and recipient twin. These include:
- Foetal death of one or both twins, with a mortality rate of up to 80-100% if left untreated.
- Neurological abnormalities due to hypoxia, including cerebral palsy.
- Cardiac dysfunction, renal impairment, and growth restriction in the donor twin.
- Polycythaemia, congestive heart failure, and hydrops in the recipient twin.
Pathological Features
Histopathology
- Macroscopic: Discrepancy in the size of the twins with the donor twin being significantly smaller.
- Microscopic: No specific microscopic findings are associated with TTTS.
Serology
- No specific serologic markers exist for TTTS.
Biochemistry
- Biochemical differences between twins are not commonly used in diagnosis or management of TTTS.
Radiological Features
General Features
- TTTS is characterised by the stuck twin appearance, where the donor twin is compressed against the uterine wall with oligohydramnios, while the recipient twin lies in a polyhydramniotic sac.
- Discordant amniotic fluid volumes with deep maximum vertical pocket (MVP) of amniotic fluid less than 2 cm in the donor’s sac and more than 8 cm in the recipient’s sac.
- Twin growth is usually discordant but lesser degrees of discrepancy does not exclude diagnosis
- Associated features
- Congenital heart disease
Ultrasound
- B-mode: Can identify the characteristic findings of TTTS, including polyhydramnios in the recipient twin’s sac, oligohydramnios in the donor twin’s sac, and discrepancy in twin sizes.
- Adherence of intertwin membrane and uterine wall to donor twin creates a stuck twin appearance.
- Doppler: May show abnormal blood flow in umbilical arteries and veins, ductus venosus, and middle cerebral artery reflecting the cardiovascular imbalances in the twins.
Grading and Staging
The Quintero Staging System is used to stage TTTS1:
- Stage I: Donor bladder visible. Discordant amniotic fluid (deep MVP <2 cm in donor and >8 cm in recipient).
- Stage II: Donor bladder empty. Normal doppler.
- Stage III: Donor bladder empty. Abnormal Doppler findings in;
- Umbilical arteries: absent or reversed end diastolic flow
- Umbilical vein: pulsatile flow
- Ductus venosus: reversed flow
- Stage IV: Hydrops foetalis in either twin.
- Stage V: Death of either or both twins.
Diagnosis
Diagnosis is primarily based on ultrasound findings in the context of a monochorionic diamniotic twin pregnancy. The characteristic stuck twin appearance, along with oligohydramnios in the donor twin’s sac and polyhydramnios in the recipient twin’s sac, is pathognomonic of TTTS.
Differential Diagnosis
- Twin Anaemia Polycythemia Sequence (TAPS): A rare condition that can occur spontaneously or post-laser therapy for TTTS, characterised by large inter-twin haemoglobin differences without significant discordance in amniotic fluid volumes.
- Selective Intrauterine Growth Restriction (sIUGR): Presents with size discordance between the twins (i.e. greater than 25%), but without the characteristic amniotic fluid discordance seen in TTTS.
- Premature rupture of membranes
Management
- Initial management may involve conservative measures, such as bed rest and serial ultrasound examinations.
- More severe cases (Stage II or higher) may require invasive treatments like foetoscopic laser photocoagulation, which aims to disrupt the abnormal placental vascular anastomoses, or amnioreduction to manage polyhydramnios.
- Referral to a perinatologist with expertise in high-risk pregnancies and neonatal intensive care support is essential.
References