Introduction
• Monochorionic twin complication caused by intrauterine transfusion of blood from donor twin (D) to recipient twin (R) via arteriovenous (AV) placental anastomoses.
• Staging based on fluid volume, bladder size, Doppler findings, presence of hydrops.
Epidemiology
• This complication can occur in ~10% (range 15-25%) of monochorionic pregnancies giving an estimated prevalence of ~1:2000 of all pregnancies.
Pathology
• TTTS results from unbalanced vascular (arteriovenous and arterioarterial) anastomoses in the placenta - that is, placental circulation is directed predominantly towards one twin and away from the other.
Fig: Close-up view of the placenta from a case complicated by TTTS shows a large arteriovenous anastomosis from the donor artery to the recipient vein. These deep placental anastomoses manifest as "nose-to-nose" vessels on the placental surface.
Diagnostic Criteria
• Monochorionic (MC) twins.
• Oligohydramnios in one sac + polyhydramnios in other.
- Seen only in MC diamniotic twins (MDT); diagnosis in monoamniotic twins is more challenging.
Ultrasound Findings
• The syndrome is usually first identified between 16 and 26 weeks’ gestation.
• Serial ultrasounds of all monochorionic gestations are recommended to monitor for the development of TTTS, with fluid and bladder checks every 2 weeks.
Donor
• Oligohydramnios defined as maximum vertical pocket (MVP) ≤ 2 cm.
• 'Stuck twin' describes severe oligohydramnios with smaller, donor twin in fixed position by uterine wall.
– Variant 'cocoon' or intrauterine sling; D cocooned in membranes, sling reflects back to uterine wall.
– D suspended by sling, floats in pool of R fluid.
– Look for differing echogenicity of fluid; D urine more concentrated → fluid more echogenic.
• Echogenic bowel described as sign of hypoxia in donor.
• Doppler abnormalities in D usually involve umbilical artery (UA).
– Absent (AEDF) or reversed end diastolic flow (REDF).
Fig: Cocoon
Fig: Stuck Twin
Recipient
• Polyhydramnios defined as MVP ≥ 8 cm at < 20 weeks, > 10 cm at > 20 weeks.
• Doppler abnormalities in R more likely to involve ductus venosus (DV) or umbilical vein (UV).
– Look for increased pulsatility or reversed A wave in DV.
– Look for pulsatile UV flow.
□ Indicates imminent hydrops.
• Cardiomyopathy due to volume overload.
– Cardiomegaly, tricuspid regurgitation, impaired ventricular function, biventricular hypertrophy.
• Pulmonary atresia, pulmonary stenosis (PS) incidence as high as 9.6%.
– Isolated PS seen in 0.2% uncomplicated MDT, 2.9% of TTTS cases treated with laser.
– 18% regressed after laser, 65% required neonatal valve dilation in series published in 2015.
Other Features
• Discordant twin growth not mandatory feature.
○ Only 20% of donor twins met criteria for selective growth restriction (sFGR) in one large series of TTTS.
• Umbilical cords may differ in size.
○ R larger than donor D.
Fig: Postnatal examination of the placenta demonstrating the discordant sizes of the donor (D) and recipient (R) cords.
Fig: Early Twin-Twin Transfusion Syndrome.
Stage I: Monochorionic twins at 16 weeks’ gestational age. A discrepancy in abdominal diameters is apparent. The smaller twin on the maternal left (L) has much less amniotic fluid than the larger twin on the maternal right (R).
The very thin separating amnion is visible between the twins (arrowhead).
Fig: Twin-Twin Transfusion; Stuck Twin.
Monochorionic twins at 16 weeks’ gestational age. One twin (O) is ‘stuck’ to the anterior uterine wall as a result of severe oligohydramnios. Its twin (P) is far posterior, freely moving in a large amount of amniotic fluid.
The membrane (arrowhead) is barely visible surrounding a tiny amount of echogenic amniotic fluid between the legs of the ‘stuck’ twin.
Fig: Continued shunting from donor to recipient causes volume overload, which can, as in this case, lead to hydrops. Note the skin thickening and ascites. The cord Doppler is also abnormal with absent end diastolic flow and pulsatile umbilical vein flow. This is stage 4 TTTS.
Fig: In another case, pulsed Doppler ultrasound shows tricuspid regurgitation in the recipient as well as thick myocardium and pericardial effusion. These are all signs of cardiac decompensation.
Fig: Abnormal Doppler findings are used to stage TTTS. In this case, the donor (A) Umb A shows either absent or reversed end diastolic flow and pulsatile Umb V flow.
The recipient (B) has normal cord flow. This is stage III-D TTTS.
Staging
Cardiovascular Profile Scoring (CVPS)
○ Points based on hydrops, DV/UV/UA Doppler, cardiothoracic ratio, cardiac function based on ventricular systolic function and atrioventricular valve regurgitation
Children's Hospital of Philadelphia (CHOP)
○ Points based on 4 Doppler and 9 echo parameters (including heart size, ventricular and valve function, venous Doppler, great artery size, pulmonary insufficiency)
Quintero System
• Most established
Stage I: oligohydramnios/polyhydramnios
Stage II: bladder not visible in donor twin
Stage III: abnormal Dopplers in either twin
Stage IV: hydrops fetalis in either twin (almost always in the recipient; rarely in the donor)
Stage V: in-utero demise of either twin
Complications
• Donor
- Oligohydramnios, may be “stuck”
- IUGR, small size
- Small/nonvisualized bladder
- If live born, has better outcome
• Recipient
- Polyhydramnios
- Possible hydrops
- Dilated bladder ± dilated renal pelves
- Large size
- Cardiomegaly
- Struggles postnatally
Treatment
• Fetoscopic laser surgery performed at specialized centers worldwide; a less invasive alternative is serial amniocentesis to equalize luid volumes and decrease the risk of preterm labor.
Fig: Graphic illustrates endoscopic laser coagulation of the chorionic anastomoses that cause TTTS. Access to the placenta is via the large amount of fluid in the recipient's sac. The "stuck" donor twin is seen on the left.
Laser was performed for TTTS 16 weeks prior to delivery in this case. The vascular equator is devoid of intertwin communications as a result of "dichorionization." All intertwin vascular connections along the equator are lasered with this technique.
• Other management options include:
- Conservative management with surveillance for Quintero stage 1 TTT.
- Serial amnioreduction, where laser treatment is not available .
Courtesy
• Diagnostic Ultrasound, 5th Ed; Carol M. Rumack
• Diagnostic Imaging: Obstetrics, 3rd Ed; Paula J. Woodward, Anne Kennedy, Roya Sohaey
• https://radiopaedia.org/articles/twin-to-twin-transfusion-syndrome-1
• Case courtesy of Dr Matt Skalski, Radiopaedia.org, rID: 68846
• Ultrasound Obstet Gynecol 2005; 25: 307–311
• Radiology Channel (Youtube)
• Wiley InterScience (www.interscience.wiley.com) DOI:10.1002/uog.5233
