What are the important ultrasound findings in evaluation of abnormalities in twins, and what are the treatment methods?
Both monochorionic and dichorionic twins are subject to the same anomalies that can affect a singleton fetus. However, for monochorionic twins, the abnormalities peculiar to these twins typically involve a discrepancy in the sizes of the fetuses or their amniotic fluid volumes (or both). For monochorionic twins, because of the risk for developing the complications discussed below, surveillance US studies are advised every two week after 16 weeks. Development of any of the complications will likely necessitate even more frequent follow-up.
In addition to the elements of the detailed extended obstetric ultrasound examination, evaluation of twins with a discordance in weight or fluid volume includes (where measurements and observations apply to both fetuses):
- • If not already done, establish whether the twins are mono- or dichorionic. The complications discussed in the following questions virtually all occur in monochorionic twins.
- • Assessment of the placenta: thickness and appearance in addition to location.
- • Measurement of the amniotic fluid of the fetuses. An amniotic fluid index (AFI) may be measured for each fetus, but it is not clear that the AFI method is appropriate for twins, so the single deepest vertical pocket is used for fluid volume assessment.
- • Assessment of the placental cord insertions and measurement of the distance between them.
- • Calculation of estimated fetal weights and the ratio of weights.
- • Umbilical artery, umbilical vein, and ductus venosus Doppler US evaluation.
- • Middle cerebral artery Doppler US evaluation.
- • Evaluation of the fetal bladders for degree of filling and evidence of voiding (cycling of the fetal bladder).
- • Evaluation of the fetal heart for cardiomegaly, pericardial effusion, and myocardial thickening.
- • Examination for hydropic changes (ascites, pleural or pericardial effusion, scalp edema).
- • Determination of locations of potential access to the amniotic sacs (areas free of the placenta)—this may be done by the maternal-fetal medicine physicians prior to treatment.
- • Fetal echocardiography for assessment of fetal cardiac function.
For twin to twin transfusion syndrome (TTTS) , stages are defined based on the ultrasound findings.
The below table shows the ultrasound findings and corresponding stages. Increasing stage is correlated with more severe TTTS and urgency for treatment. TTTS is believed to result from asymmetry in inter-twin blood flow between the two fetuses. The vascular connections between the two cords on the placental surface can lead one fetus (the “donor”) to pump blood into the circulation of the “recipient.” This results in one fetus being small with low amniotic fluid and the other being larger with an abundance of amniotic fluid. Left untreated, the donor twin becomes more severely growth restricted with the complications of oligohydramnios while the recipient twin develops hydrops from the volume overload. This can lead to the demise of both fetuses.
Stages of Twin to Twin Transfusion Syndrome (TTTS) (adapted from ).
|STAGE||POLY/OLIGOHYDRAMNIOS||ABSENT BLADDER IN DONOR||CRITICALLY ABNORMAL DOPPLER US||ASCITES, PERICARDIAL EFFUSION, SCALP EDEMA, OVERT HYDROPS||DEMISE OF ONE OR BOTH TWINS|
Treatment for TTTS is aimed at separating the circulations of the fetuses. If a reasonable plane between the two placental cord insertions can be found, a laser fiber is passed through one channel of a fetoscope and observed through a video camera attached to the other channel. The laser is used to photocoagulate the surface connections between the two fetal placental circulations. This treatment has been very successful with normalization of the criteria used to assess for TTTS. Survival of at least one twin is ≈80%. The survival rate for both fetuses ranges between 60% and 70%. Long-term complications include neurologic abnormalities in 6% to 11%.
Twin anemia-polycythemia sequence (TAPS) is a recognized complication of TTTS treatment, although spontaneous occurrences have also been reported. The syndrome resulting from TTTS is believed to be caused by very small residual arteriovenous connections between the twin circulations. These allow a very slow “transfusion” from one fetus to the other. This results in polycythemia in the recipient fetus and anemia in the donor. In spontaneous cases, the cause is believed to be the same, but not the result of previous TTTS treatment. The diagnosis is established through Doppler US evaluation of the middle cerebral arteries. In early stages, the middle cerebral artery peak systolic velocity (MCA PSV) is >1.5 multiples of the median (MoM) for the donor and <1.0 MoM for the recipient. Increasing stages are indicated by greater MCA PSV values and onset of hydrops in the donor or development of abnormal umbilical artery, umbilical vein, or ductus venosus Doppler US findings. As with TTTS, the highest stage is the demise of one or both fetuses.
Treatment has been aimed at correcting the anemia through fetal transfusion or prevention through the use of a modified laser photocoagulation technique in treatment of TTTS. The Solomon technique involves extending the laser photocoagulation to create a true separation of the placental circulations, not just eliminating the vascular connections on the fetal surface of the placenta. This process is also known as dichorionization because it effectively creates two placentas. If a separation plane is not clearly visible or the placental portions are very asymmetric, this technique is not applicable because it is likely to result in the demise of the fetus with the smaller placental portion.
Selective intrauterine growth restriction (sIUGR) is a complication of monochorionic twins resulting from the twins having unequal portions of the placenta. This might result, for example, from one twin having a placental cord insertion that is central and the other having a cord insertion that is marginal or velamentous. The unequal size twins that result can mimic TTTS, but there are ultrasound characteristics that can help differentiate sIUGR from TTTS. These include:
- • Normal amniotic fluid volume in the larger fetus or both fetuses (no polyhydramnios-oligohydramnios; may have normal-normal, or normal-oligohydramnios).
- • Both have urinary bladders that cycle (empty and fill) during the study.
- • Large weight discordance (typically ≥25%) with the smaller fetus at <10th percentile weight for gestational age and the other in the normal weight range.
Evaluation of the umbilical artery with Doppler US is critical for staging. In Type I sIUGR, the smaller fetus has positive end-diastolic flow in the umbilical artery; in Type II sIUGR, the smaller fetus has persistently absent or persistently reversed end-diastolic flow in the umbilical artery; and in Type III sIUGR, the smaller fetus has intermittently absent or reversed end-diastolic flow in the umbilical artery. Stage III sIUGR carries a poor prognosis with demise of the smaller twin and potential neurologic sequelae (or demise) of the larger twin. Treatment for type I is usually expectant management with weekly or twice-weekly ultrasound evaluation for growth and umbilical artery assessment. For types II and III, the laser photocoagulation therapy used for TTTS will not work for sIUGR because of the marked differences in placental proportions. To prevent the neurologic complications for the larger fetus, the usual treatment for types II and III is cord coagulation of the smaller twin. This results in demise of the smaller twin but reduces the risk of neurologic problems for the larger twin.
In the twin reversed arterial perfusion (TRAP) sequence , either fetus dies early in development, or succumbs to severe anomalies. Because of the vascular connections between the placental circulations of monochorionic twins, the normal fetus (“pump twin”) can pump blood into the abnormal (acardiac or parabiotic) twin. The acardiac twin, as the term suggests, has no intrinsic cardiac activity (although we have seen a case with an “acardiac” twin that had cardiac activity, but with severe bradycardia and maldevelopment). Blood flow becomes reversed in the umbilical artery and vein of the acardiac twin. Because the oxygenated blood is entering the acardiac fetus via the umbilical arteries, the most oxygenated blood circulates first through the pelvis and lower extremities. This likely accounts for the appearance of most acardiac fetuses with more lower than upper body development. Ultrasound diagnosis is not difficult, but aspects are important for therapy planning. The ultrasound examination includes:
- • Evaluation of the umbilical cords of both fetuses; the acardiac fetus usually has a single umbilical artery. It is important to determine how the cords insert on the placenta because this may affect later treatment.
- • Documentation of reversed flow in the umbilical artery and vein of the acardiac twin
- • Measurement of the acardiac twin to estimate the weight. There are formulae to estimate the weight based on the length, but we use a method of measuring the major elements (body and whatever limbs are present) and adding the individual volumes estimated from the prolate ellipsoid formula. An assumption of 1 mL of tissue weighing 1 gram is used for conversion of volume to weight.
- • Evaluation of the pump twin for any evidence of cardiac decompensation (development of hydrops); an echocardiogram is also usually done.
- • Evaluation of the pump twin for any anomalies; anomalies of the pump twin have been reported in up to 9% of the “normal” twins.
Because the acardiac twin cannot survive on its own and continuing to allow the pump twin to supply blood to it can result in cardiac overload, treatment is usually occlusion of the umbilical cord of the acardiac fetus. This was initially done by ligation, but more recently is performed using radiofrequency ablation via a fetoscope.