Detecting and managing monochorionic twin complications
Approximately 20% of all twin pregnancies are monochorionic, with the fetuses sharing a single placenta. Although the majority of these pregnancies are uncomplicated, monochorionic twins are significantly more likely than dichorionic twins to incur complications that can threaten the life and health of one or both fetuses.
The death of one monochorionic twin leaves the other twin with a 15% risk of demise. Survival after the loss of a co-twin is also associated with a 25% incidence of neurologic injury, compared with a 2% incidence in dichorionic pregnancies. Additionally, monochorionic pregnancies carry the risk of unique complications such as twin-to-twin transfusion syndrome, selective fetal growth restriction, twin anemia polycythemia sequence, and twin reversed arterial perfusion.
,
Increased ultrasonographic surveillance recommended for monochorionic twin pregnancies has been outlined in a recent consensus statement from the North American Fetal Therapy Network (Obstet Gynecol. 2015 Jan;125[1]:118-23). Beginning at 16 weeks’ gestation, monochorionic twins should be assessed every 2 weeks using amniotic fluid balance, presence/absence of fluid within the fetal bladder, and with fetal Doppler (umbilical artery, middle cerebral artery, and ductus venosus) studies. Fetal growth should also be assessed at least every 4 weeks.
Since monochorionic twins are at increased risk for congenital heart disease, echocardiography is also performed between 18 and 22 weeks, with surveillance intervals of 2 weeks or shorter if potential complications are identified. Early detection of these and other complications allows for earlier intervention, earlier referral if necessary, and potentially better outcomes.
Twin-to-twin transfusion syndrome
Twin-to-twin transfusion syndrome (TTTS) is one of the most common and most serious complications, affecting approximately 10% of monochorionic pregnancies. Significant imbalances in blood-flow exchange lead to progressive cardiovascular decompensation that causes one twin to become a “donor” of blood volume, and the other twin to become a “recipient.” Without proper treatment between 16 and 26 weeks’ gestation, the perinatal mortality rate has been estimated to be 70% or higher.
Disease severity is classified according to the Quintero staging system. Stage I is characterized by amniotic fluid discordance. In stage II, the bladder of the donor twin is no longer visible sonographically. Stage III is marked by critically abnormal Doppler waveforms in either twin (absent/reverse end-diastolic velocity in the umbilical artery, reverse flow in the ductus venosus, or pulsatile flow in the umbilical vein). In stage IV, one of the twins has developed hydrops, and stage V is characterized by the death of one or both of the twins.
Amnioreduction to decrease intra-amniotic pressure had been the treatment of choice until a randomized controlled trial, published in 2004, demonstrated that fetoscopic laser coagulation of anastomoses was superior as a first-line treatment for severe TTTS that is diagnosed before 26 weeks. Perinatal mortality and morbidity were significantly lower after the laser treatment (N Engl J Med. 2004 Jul 8;351[2]:136-44).
Outcomes were further improved over the next decade as the laser surgery technique was modified to cover the entire vascular equator rather than selective components of the vasculature. In an open-label randomized controlled trial comparing the two approaches for severe TTTS, fetoscopic laser coagulation of the vascular equator (known as the Solomon technique) reduced the risk of twin anemia polycythemia sequence and recurrence of TTTS – the two main postoperative complications associated with residual anastomoses after selective coagulation (Lancet. 2014 Jun 21;383[9935]:2144-51).
The procedure has many challenges and can be impacted by one’s inability to see the entire vascular equator because of poor access, by the patient’s history of other interventions, and by the stage of TTTS.
Laser coagulation is regarded as the standard treatment for Quintero stage II-IV disease, and it is offered in some cases of stage I disease, such as those involving severe polyhydramnios and shortened cervix. Research currently underway is examining the outcomes of treatment for stage I disease, but data thus far suggest that intervening at stage I is generally better than expectant management.
With laser coagulation treatment, the survival rate in pregnancies complicated by TTTS is about 85%-90% for one fetus, and about 70% for both. TTTS sometimes causes one twin, particularly the recipient, to develop pulmonary valve stenosis, but this is generally a functional problem that resolves when the syndrome is treated.
After treatment, it is important to monitor for the development of twin anemia-polycythemia sequence, which may still occur if full visualization of the vascular equator was not possible or if a fine vessel was missed. Such monitoring involves weekly ultrasound surveillance with middle cerebral artery peak systolic velocity measurements.
Patients should also be monitored for abnormal neurologic development, ventriculomegaly, and other signs of abnormal brain development. Even “perfect” laser treatment with seemingly complete placental separation has been associated with abnormal neurologic development in about 10%-15% of cases.
Maternal complications with TTTS include placental abruption and preterm membrane rupture, the latter of which occurs about 15%-20% of the time.
Currently under much discussion is fetoscopic laser coagulation of TTTS placentas that have “proximate cord insertions.” The surgery in these cases – where the cords are less than 4 cm apart – is much more challenging because of technical difficulties in visualizing the vascular equator, and outcomes are being studied. Some centers will not perform laser surgery on placentas with proximate cord insertions, which fortunately are uncommon. However, the surgery is possible; I have completed three cases thus far, each with dual survival.
