TWIN-TWIN TRANSFUSION SYNDROME (TTTS)

Definition, General Comments and Vascular Anastomoses

ULTRASOUND IN THE FIRST TRIMESTER

TTTS is a slowly progressive disease. Initial presentation as early as 13 weeks' gestation has been reported, but it usually occurs in the second trimester. Routine obstetric ultrasonography will usually allow for the visualization of TTTS at 17 to 26 weeks.

Diagnostic criteria include;

Monochorionic gestation .
Nuchal translucency measurement >3 mm at 10-14 weeks' gestation (with normal chromosomes).
Poor crown-rump length growth in one fetus.
Membrane folding at 10-13 weeks' gestation .

ULTRASOUND IN THE SECOND AND THIRD TRIMESTERS

Abnormal vascular connections and the resultant disparate sharing of blood flow occur in monochorionic twins. The twin-twin transfusion syndrome results from arterio-venous connections (one of the many vascular anastomoses) that occur in monochorionic placentas and may lead to a physically "stuck" twin. The vast majority of monochorionic placentas have vascular anastomoses, however only 15-30% actually result in the twin-twin transfusion syndrome.

Monochorionic twins.
Fetuses have the same sex (9,10).
Biometric disparities on a first or second trimester scan.
Different fetal sac sizes.
Different quantity of amniotic fluid surrounding each fetus (One sac with oligohydramnios, deepest vertical pocket 2.0 cm
One sac with polyhydramnios, deepest vertical pocket 8.0 cm).
Identification of placental confluence (9). Discordant echogenicity of the donor and recipient placental areas has been reported (7).
Persistent Urinary Bladder Findings

Small or no bladder visualized in twin with oligohydramnios.
Large bladder visualized in twin with polyhydramnios.

Appearance of a stuck twin
Hydrops fetalis (presence of one or more of the following in either twin): skin edema (5-mm thickness), pericardial effusion, pleural effusion, ascites.
Membrane folding at 14-17 weeks' gestation
Inter-twin hemoglobin difference of 5 g/dl or more (9).
Inter-twin birth weight difference of greater than 15-20% (5).
Different size umbilical cords. Doppler velocimetry of the umbilical artery with an inter-twin difference in S/D ratio of greater than 0.4 (9).
Artery-to-vein shunt involving the fetal surface placental vessels.

Fetal hydrops or cardiac failure may be present.
Increased nuchal translucency
Most workers seem to agree that the above diagnostic criteria may be present, but the diagnosis should be restricted to monochorionic twins with gross discordance of amniotic fluid volume in the second or early third trimester (7).

Donor Twin

Recipient Twin

Single placenta

Thin membrane

Same sex fetuses

Discordant growth

Oligohydramnios (60%)

Polyhydramnios - moderate to severe

Small / empty bladder

Large bladder

"Stuck twin" - pinned to the side
of the gestational sac by the amniotic membrane and lack of amniotic fluid

Cardiomegaly due to increased perfusion and volume overload
May develop non-immune hydrops

No free movement in the gestational sac due to the oligohydramnios
Motion of fetal extremities should not exclude the diagnosis.

Moves freely in the hydramniotic gestational sac

Amnion may not be seen as it lies in contact with the fetal body parts and can't be distinguished as a membrane separating twins.

Anemia and hypovolemia

Polycythemia and plethora

Morphologically normal

Fetal papyraceus

Velamentous cord insertion (64%)

Cord Edema

Fetal hydrops rare

Fetal hydrops in 10-25%

Vascular Anastomoses in the Placenta

“Coccoon sign” – this is thought to be a variant of the classic stuck twin, characterized by a donor fetus with severe oligohydramnios being enveloped by intact, collapsed dividing membranes, yet located away from the periphery of the intrauterine cavity. The fetus is anchored to the periphery by a tether of folded membranes, similar to a cocoon (14).

Placental Vascular Anastomoses in Monochorionic Twin Pregnancies
Study	Subjects (n)	Conclusions
Bajoria et al (11)	MC/TTTS (10); MC/no TTTS (10)	TTTS was characterized by A-V vascular anastomoses running from donor to recipient deep within the placenta (uncompensated by A-V anastomoses running in the reverse direction)
Machin et al (12)	MC (69)	Worst clinical outcomes were associated with A-V anastomoses in the direction from donor to recipient
Bajoria (13)	MC/MA (6) MC/DA (12)	Compared with MC/MA pregnancies, MC/DA pregnancies had fewer anastomoses of all types
MC = monochorionic; TTTS = twin-to-twin transfusion syndrome; A-V = arteriovenous; MA = monoamniotic; DA = diamniotic.

STAGING OF TTTS BASED ON TREATMENT (8)

TTTS has been defined as a deep vertical pocket in the recipient twin of >8 cm and in the donor twin <2 cm.

Stage 1 – donor bladder visible

Stage 2 – donor bladder not visible (normal dopplers: UA, UV, Ductus venosus).

Stage 3 – critically abnormal dopplers (absent or reversed diastolic flow in the UA, UV or intrahepativ vein pulsations, reversal of a wave of ductus venosus).

Stage 4 - hydrops

Stage 5 - demise

DOPPLER STUDIES

Doppler investigations of the arterial vessels and ductus venosus, IVC and right hepatic vein, tricuspid and mitral ventricular inflow performed on the venous side revealed decreased blood flow velocities.

Mean values of atrioventricular flow velocities showed a significant decrease in the donor group (1).

Donor Twin

Recipient Twin

DIFFERENTIAL DIAGNOSIS

IUGR of one twin. To distinguish this from the twin-twin transfusion syndrome the recipient twin does not usually have polyhydramnios or congestive cardiac failure. IUGR may occur in dichorionic pregnancies whereas the twin-twin transfusion syndrome only occurs in monochorionic pregnancies.

OUTCOME

Perinatal mortality of 71% (when diagnosed prior to 26 weeks).

REFERENCES

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Chescheir NC, Seeds JW. Polyhydramnios and oligohydramnios in twin gestations. Obstet Gynecol 1988;7:882-884.
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Brennan JN, Diwan RW, Rosen V et.al. Feto-fetal transfusion syndrome: prenatal ultrasonographic diagnosis. Radiology 1982;143:535-536.
Bajoria R, Wigglesworth J, Fisk NM. Angioarchitecture of monochorionic placentas in relation to the twin-twin transfusion syndrome. Am J Obstet Gynecol. 1995;172:856-863.
Machin G, Still K, Lalani T. Correlations of placental vascular and clinical outcomes in 69 monochorionic twin pregnancies. Am J Med Genet. 1996;61:229-236.
Bajoria R. Abundant vascular anastomoses in monoamniotic versus diamniotic monochorionic placentas. Am J Obstet Gynecol. 1998;179:788-793.
Quintero RA, Chmait RH. The cocoon sign: a potential sonographic pitfall in the diagnosis of twin-twin transfusion syndrome. Ultrasound Obstet Gynecol 2004;23:38-41.