Original text 1999-05-23 Philippe Jeanty, MD, PhD & Sandra R Silva, MD

Synonym: Stuck twin.

Definition: Twin to twin transfusion syndrome is a congenital entity unique to monochorionic diamniotic twin pregnancies, characterized by an imbalanced blood exchange between the twins circulation, which occur through interplacental anastomosis, and may cause fetal compromise varying from growth discordance to death of both fetuses. The first description was made by Schatz in late 1800s.

Etiology: Vascular anastomosis.

Recurrence risk: Unknown, but very low, considering the small chance to develop a second monochorionic diamniotic twin pregnancy.

Incidence: Twin to twin transfusion complicates about 15% of monochorial twin gestations and is responsible for 17% of the perinatal mortality in multiple pregnancies.

Diagnosis: Ultrasonographic criteria for the diagnosis of twin to twin transfusion syndrome include monochorionic placentation, with visualization of a separating membrane, fetus of the same sex, mid-pregnancy polyhydramnios-oligohydramnios sequence (polyhydramnios at the recipient's sac and oligohydramnios at the donor's sac), in the absence of other causes of abnormal amniotic fluid volume, and marked growth discordance. Significant weight discordance is considered a difference between the twin's size equal or over 20%. Other findings that might be observed include non visualization of the donor's bladder with enlarged recipient's bladder, abnormal Doppler S/D ratio at the umbilical cord, hydrops or evidence of congestive heart failure of either twin (more commonly found at the recipient twin). Milder forms of the disease are more difficult to diagnose due to the lack of uniform criteria, however, one should suspect twin to twin transfusion in the presence of amniotic fluid discrepancy between the cavities.

Figure 1: Twin-to-twin transfusion syndrome. The twin on the left side is at the folding membrane stage, before the stuck twin. The discrepancy in amniotic fluid quantity is already marked. The fluid appears more echogenic. 2 weeks later both twins died.

Pathogenesis: If embryonic splitting occurs before day three after fertilization, two independent fetuses with separate placentas will result.  A single placenta with two amniotic cavities occurs if splitting takes place between days four and seven.  If division of the embryoblast occurs after about eight days, the twins share a single placenta and amniotic cavity (monochorionic-monoamniotic twins). Division beyond day 12 results in conjoint twins.
When two fetuses share the same placenta, vascular anastomoses develop between their circulations. These anastomoses can be of three types: vein-to-vein, artery-to-artery, and artery-to-vein. Even when there are multiple vascular connections within a single placenta, no transfusion should occur provided the anastomoses are balanced.  Placentas from pregnancies with twin to twin transfusion syndrome have fewer anastomoses, which are more likely to be solitary and of deep arteriovenous type than those without twin to twin transfusion syndrome.  Provided transfusion occurs, the donor or "pump†twin becomes hypovolemic due to blood loss.  Hypoxia develops because of placental insufficiency, which is also responsible for intrauterine growth retardation.  Poor renal perfusion leads to oligohydramnios. This latter feature, when severe, is responsible for the classical appearance of the stuck twin: the amniotic sac becomes too small, the amniotic membrane comes in close contact with the body of the "pump†twin and the fetus appears trapped to the uterine wall. Hypervolemia with increased renal perfusion leads to polyhydramnios in the sac of the recipient twin. Since there is no loss of protein or cellular components from its circulation, colloid osmotic pressure draws water from the maternal compartment across the placenta, establishing a vicious cycle of hypervolemia, polyuria and hyperosmolarity leading to high output cardiac failure, hydrops and polyhydramnios.
In the most severe forms, the diagnosis should not be difficult: a single placenta, massive polyhydramnios in the sac of the recipient twin, a stuck donor twin attached to the uterine wall with poor mobility and obvious growth discordance. Milder forms of the disease are more difficult to diagnose due to the lack of uniform criteria, however, one should suspect twin to twin transfusion in the presence of amniotic fluid discrepancy between the cavities, regardless of the amount of weight discordance between the twins. An intertwin hemoglobin difference >2.4 gm/dl in fetal blood obtained by cordocentesis has been shown to be consistent with stuck twin syndrome.

Associated anomalies: The over distension of the uterus caused by the polyhydramnios can causes preterm labor, amniorrhexis, abruptio placentae, respiratory and abdominal discomfort. Death of one twin can cause embolic phenomena (twin embolization syndrome) and coagulation problems in the remaining twin, and sequelae such as neurological, cardiac and renal diseases are common among survivors.

Differential diagnosis: Differential diagnosis should be mainly concerned with twins of discordant size without transfusion as the underlying pathophysiologic mechanism for the problem. Some authors have proposed a new entity called twin oligohydramnios-polyhydramnios sequence, in which twin-twin transfusion is included . Histopathological studies of the placenta are required to differentiate twin-twin transfusion from the other conditions included in twin oligohydramnios-polyhydramnios sequence. Isolated intra-uterine growth restriction can be considered if the growth discrepancy is minimal (less than 15%) and the other features of the syndrome are not present. Dichorionic twin pregnancy with fused placentas and growth restriction of one of the fetuses is another condition that can leads to misdiagnosis. This can be excluded if the twins have different sexes or after birth, by histopathological analysis of the placenta.

Prognosis: When the disease manifests during the second trimester, there is a higher risk of perinatal morbidity and mortality.  Intrauterine hypoxia, preterm delivery, and death of one fetus (usually the donor) with subsequent death or hypoxic-ischemic sequelae (see twin embolization syndrome above) in the surviving twin are the most common complications to watch for in these pregnancies. The absent end diastolic flow in the donor's umbilical artery accompanied by venous pulsation in the recipient's umbilical vein are usually associated with a poor prognosis. Hydrops or evidence of congestive heart failure of either twin are also associated with ominous prognosis. These signs are found more commonly at the recipient twin.

Management: Although efficacy of the therapies available is still controversial, the elevated mortality rate (which can be as high as 100%) when expectant management is opted for, imposes the necessity of invasive therapy. Treatment includes serial amniocentesis to drain polyhydramnios in the sac of the recipient twin and, more recently, ablation of communicating vessels on the placental surface by neodimium: YAG laser guided by fetoscopy and umbilical cord ligation. Intensive monitoring with weekly non stress test, alternating with biophysical profiles is recommended from the time of diagnosis to delivery.

References:

[1] Schatz F. Arch Gynaekol 1875, 7: 336.

[2] Wittmann BK, Baldwin VJ, Nichol B. Antenatal diagnosis of twin transfusion syndrome by ultrasound. Obstet Gynecol, 1981, 58: 123-127.

[3] Bruner JP, et al. Placental pathophysiology of the twin oligohydramnios-polyhydramnios sequence and the twin-twin transfusion syndrome. Placenta. 1998 Jan; 19(1): 81-86.

[4] Saunders NJ, Snijders RJM, Nicolaides KH. Therapeutic amniocantesis in twin-twin transfusion syndrome appearing in the second trimester of pregnancy. Am J Obstet Gynecol 1992, 820-824. 

[5] De Lia JE, Cruikshank DP, Keye WR. Fetoscopic neodymium: YAG laser occlusion of placental vessels in severe twin-twin transfusion syndrome . Obstet Gynecol 1990, 75: 1046-1053.

                    [6] Quintero RA, Romero R, Reich H, Goncalves L, Johnson MP. In utero percutaneous umbilical cord ligation in the management of complicated monochorionic multiple gestations. Ultrasound Obstet Gynecol, 1996, 8: 16-22.