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Articles » Multiple gestations » Twins, conjoined » Conjoined twins, thoraco-omhalopagus

1994-08-12-11 Twins, conjoined, thoraco-omphalopagus © Norris


Twins, conjoined, thoraco-omphalopagus

Cheryl D. Norris, BS, Harris J. Finberg, MD, William Peoples, MD, Mary L. Nielsen, MD#, George R. Tiller, MD


MESH Twins; conjoint BDE 0202 MIM 27641 ICD9 759.4 CDC 759.430

Address correspondence to Cheryl Norris, BS, Vanderbilt University Medical Center, Department of Radiology, 21st and Garland Avenue, Nashville, TN 37232-2675. Ph: 615-322-0888, Fax 615-343-4890. ¶Phoenix Perinatal Associates, Phoenix, AZ. §Arizona Pediatric Heart Specialists, #Pathology Consultants, Wichita, KS, ¤Womens Health, Wichita, KS.


Conjoined twinning is a rare anomaly, occurring in 0.2:10,000 births3,21, with 75% occurring in females1. Thoraco-omphalopagus occurs most frequently, and the prognosis is generally poor18. Conjoined twins are derived from a single ovum and therefore have the same genetic and chromosomal constitution. We present a second trimester ultrasound diagnosis of thoraco-omphalopagus twins.

Case report

A 22-year-old woman G2P0010 was referred for ultrasound evaluation at 22 weeks gestation for suspected conjoined twins. The patient was not exposed to any medications or any possible teratogenic exposures, and her personal and family history were unremarkable. The ultrasound revealed a live, thoraco-omphalopagus twin gestation with moderate polyhydramnios without fetal gastrointestinal obstruction. Both twins were in breech presentation. Heads, spines, and extremities appeared normal on both twins. The abdominal walls were fused but otherwise normal, and the external genitalia were female. The liver was shared but the stomachs (fig. 1) and bladders separated. The abnormal heart was shared, and the patient was referred for a fetal cardiac evaluation (fig 2-4). A single umbilical cord contained one vein and three arteries (fig. 5).

Figure 1: Section at the level of the stomachs demonstrating correct position.
Figure 2: Ultrasound image of conjoined thorax.

Figure 3: View of the conjoined heart. The hypoplastic right heart of the left twin is seen (HRV), as well as the intercardiac communication (arrow), the left ventricles (LV) and the dextropositioned apex of the heart of the right fetus (A).

Figure 4: View of the common atrium (CA) and ascending aorta of the left fetus.

Figure 5: Four-vessel cord containing one vein and three arteries.

The echocardiogram demonstrated extensive cardiac fusion at both the atrial and ventricular levels. The right-sided twin had the morphology of tricuspid atresia and the left-sided twin had a morphology of a double outlet single ventricle. There were large communications between the atria and ventricles of the two twins, and the ventricular mass was a single muscular mass. The single cardiac rhythm was shared between the two hearts. All the inferior vena cava and hepatic venous return came to the right-sided twin"s heart.

The patient was counseled about therapeutic options, including expectant management and surgical or medical abortion. Since the ventricles were shared, the twins were considered unseparable21,22. The patient opted for termination of the pregnancy.

Premature delivery of a stillborn was the method of termination of pregnancy selected by the patient. The outpatient procedure included mechanical dilatation of the cervix, repeated insertion of osmotic dilators, and the application of intrafetal digoxin. Twilight anesthesia was employed for the delivery, and a quadruple footling breech technique was used after one and a half hours of labor. No maternal lacerations or complications were experienced.


The autopsy revealed the bodies of female twins facing each other, weighing a total of 939g and joined at the thorax and upper abdomen (fig. 6-7).

Figure 6: The X-ray of the twins

Figure 7: The conjoined twins at delivery.

There was a common umbilical cord, with two umbilical arteries from fetus #1, and right umbilical artery from fetus #2, and a single umbilical vein. There were conjoined livers, with both hepatic veins joining inferior vena cava of fetus #1. There were common thoracic, pericardial and abdominal cavities. The heads appeared normal as well as ears, noses, lips and palates. The mouths and tongues were of normal size, and the chins and necks appeared normal as well. The following organs and systems were separate: neck organs, respiratory tracts and lungs, thymus, gastrointestinal tracts, pancreas, spleens, adrenal glands, kidneys and urinary tracts, and internal genitalia. The gastrointestinal tract was dextrorotated and of normal caliber. The backs were both mildly scoliotic but had no visible or palpable defects.

The complex malformation of conjoined cardiovascular system is reported in Table 1 and figure 8.

Figure 8: Schematic drawing of the flow and anatomy of the conjoined heart.

Table 1: Heart and great vessel findings at autopsy:

Right-sided fetus (#1)

Left-sided fetus (#2)

1 left superior vena cava

1 entrance of inferior vena cava into right atrium of fetus #1 after coursing through diaphragm adjacent to aorta and across thoracic cavity

1 entrance of inferior venae cavae of both fetuses into right atrium

1 small smooth-walled right atrium with absent appendage, receiving only superior vena cava

1 long, predominantly smooth-walled right atrium with absent appendage

1 tricuspid valve atresia

1 right atrial communication with right atrium of fetus #2

1 absence of interatrial foramen

1 tricuspid valve atresia

1 right atrial communication with right atrium of fetus #1

1 absence of atrial septum

1 rudimentary right ventricle

1 ventricular septal defect, membranous

1 truncus arteriosus Type I

1 hypoplastic right ventricle

1 pulmonary artery stenosis

1 pulmonary stenosis

1 left ventricular communication with left ventricle of fetus #1

1 left ventricular communication with left ventricle of fetus #2

1 ductus arteriosus not identified

1 small ductus arteriosus

1 relative hypoplasia of lungs of fetus #1: combined weight 9.35g,

compared to 12.15g for lungs of fetus #2



The earliest record of conjoined twins is that of the Biddenden maids born in 1100 CE, in England, who were united from the hips to the shoulders with only one pair of shared upper and lower extremities and lived to the age of 3431. The most famous ones are the "siamese twins" Chang and Eng Bunker, who were xiphopagus twins exhibited worldwide and died at the age of 6313. They were born in 1811 in Siam (now Thailand). The twins married sisters and fathered 21 children12. The first report of successful antenatal diagnosis by ultrasonography was reported in 197726. The earliest reported diagnosis of the presence of thoracopagus twins has been possible at the 13th week of gestation5. The history of surgical separation dates to the 15th century, and the first successful separation of conjoined twins, with both twins surviving, was recorded in 1689. The first successful operation for thoraco-omphalopagus, with at least one twin surviving, was done in 19004.

Ultrasound diagnosis

Until the 1950s, the diagnosis of conjoined twins was only made at delivery, generally because of dystocia18. In 1950, Gray et al. 29 described four signs seen on plain radiographs that suggested the presence of thoraco-omphalopagus:

  • both heads at the same level and in the same plane,
  • two hyperextended spines,
  • both heads and spines abnormally close to each other, and
  • constant relationship regardless of active movements and external attempts at moving the twins.

In the past, conjoined twins were only occasionally suspected prenatally by means of radiographic studies, whereas at present, the diagnosis is usually made by ultrasound13 by visualizing regions of anatomic fusion between the twins. In the absence of clearcut signs of fusion, additional sonographic findings summarized by Koontz et al include24:

  • the lack of a separating membrane,
  • inability to separate the fetal bodies,
  • detection of other anomalies in a twin gestation,
  • more than three vessels present in the umbilical cord,
  • both fetal heads persistently at the same level,
  • backward flexion of the cervical and upper thoracic spine, and
  • no change in the relative positions of the fetuses despite attempts at manual manipulation of the twins24.

The bifid appearance of the first-trimester fetal pole should be added to the list of sonographic features of conjoined twins. The diagnosis of conjoined twins is usually clear, but several pitfalls should be avoided. Inseparable skin contact sites must be persistent, and at symmetric body zones, to avoid a false-positive diagnosis. A relatively small zone of fusion may be pliable and permit the twins to rotate as much as 180° , so that a vertex-breech presentation does not necessarily exclude conjoined twinning. With a vertex-vertex presentation, engagement of one head within the pelvis may alter the apparent symmetry, thwarting diagnosis. With extreme degrees of fusion, the twins may be mistaken for a singleton33. Sonography allows a complete anatomical examination and search for associated lethal malformations. A detailed ultrasound examination to exclude the possibility of conjoined twins is mandatory in all multiple pregnancies.


Conjoined twins are monozygotic, monochorionic twins. It is believed that the malformation occurs soon after the blastula stage (fig. 7594-7-8, p7594-7). At this stage of development, between the 13th and 15th days after fertilization, the inner cell mass is split into equal halves, each capable of forming a normal individual. In conjoined twins, complete separation of the inner cell mass within the chorionic mass does not occur, and nonseparated parts of the otherwise normal twins remain fused throughout development4. When the failed separation occurs later (heteropagus twin), one of the twins is incomplete and attached to the more normal twin (autosite). If the separation is further delayed, then an infant with only duplication of one region is formed: a duplicata incompleta.


Conjoined twinning is classified by the most prominent site of connection, plus the Greek root pagoz: ice, and, by extension "frozen in place". Such sites include the thorax (thoracopagus), abdomen (omphalopagus), pelvis (ischiopagus), sacrum (pyopagus) or skull (craniopagus). Very extensive zones of fusion may be named by the prefix di- (meaning two), followed by the portion of the twins that is unfused. Examples include dicephalus (two heads on one body), and dipygus (single head and torso with separated pelves and four legs). Asymmetric forms, termed heteropagus, are exceedingly rare and have a parasitic attachment of a variably sized portion of anatomy appended to or even within any region of the body33. The types and frequency of conjoined twins, as well as the organs shared, are illustrated in Table 230. Thoracopagus is the most common, accounting for 40%30. Some authors group thoracopagus and omphalopagus as thoraco-omphalopagus, therefor accounting for 74%.

Table 2: Type and frequency of anomalies



Organs shared



Heart, liver, GI



Liver, GI



Spine, GU, lower GI



Pelvis, GU, GI, liver




Associated anomalies

Conjoined twins, in most cases, have more malformations than separate monozygotic twins. The malformations occurring in conjoined twins are often more severe and major. Some of the malformations are related to the junction regions, but others are distant from the junctional sites and not related to the twinning process itself2. Neural tube defects, orofacial clefts, imperforate anus, and diaphragmatic hernia are the most common defects not associated with fusion1. Congenital heart disease is found in 75% of patients. In 90%, there is some degree of fusion of the pericardial sac. The most frequent abnormality is a conjoined heart, with two ventricles and a varying number of atria (1 to 4). Ventricular septal defects are found in virtually all patients with other ventricular deformities1.

Nichols et al. 42 presented a comprehensive review on the findings in 45 sets of thoraco-omphalopagus twins. The cardiovascular findings were as follows: in 75% of thoraco-omphalopagus twins the hearts are conjoined, and in 90% the pericardium is common. In all except one case, the ventricles intercommunicated. The ventricular structure of the 45 sets of thoraco-omphalopagus twins is described in figure 929.

It was common, as in our case, that specific cardiac malformations were present in each twin. These anomalies included failure of the pulmonary artery to form or failure of development of the ventricular septum.

Nichols et al. report that a conjoined liver was present in each, a conjoined biliary tract in 22%, and conjoined gastrointestinal tract in 46%. A single umbilicus, as in our case, is usual, as is fusion of the sternum.

Figure 9: Cardiac ventricular structure in a series of 45 thoracopagus twins. The four possibilities are: one ventricle shared by both twins, two ventricles (a single ventricle for each twin), three ventricles (one for one twin and two for the other), and four ventricles (two for each twin).


Thirty-nine percent of conjoined twins are stillborn, and 34% die within the first day of life1. Survival depends upon the type of conjunction and the presence of associated anomalies. In thoraco-omphalopagus, the degree of fusion of the heart determines the prognosis. When a common heart is present, the chances for a successful surgical separation are negligible (see Treatment).


After the diagnosis of conjoined twins, timing and mode of delivery should be planned on the basis of possibility of survival, size, nature of the fusion, and parental wishes27. Early diagnosis by ultrasound allows elective pregnancy termination. After viability, serial examinations are indicated to monitor fetal growth and the development of hydrops, and to detect fetal demise. Scheduled delivery in a tertiary care center is ideal so that procedures required to evaluate the twins can be carried out shortly after birth. The method of delivery depends upon the prenatal assessment of the likelihood of survival. Cesarean section is recommended in most third-trimester deliveries because of the high incidence of dystocia and resultant fetal damage12. Vaginal delivery should be reserved for stillbirths and for forms of conjoined twins that are incompatible with life. There may also be a risk of birth canal trauma with large conjoined twins and destructive fetal procedures may be performed to deliver vaginally. After birth, evaluation of both twins should be conducted to assess the extent of organ system sharing.


Options for the treatment of thoraco-omphalopagus conjoined twins are largely dependent on the anatomy of the cardiovascular system. Separation of the liver and gastrointestinal tracts can be managed readily in most cases. The number and nature of the hearts, and the presence of any vasculature intercommunications, are of particular importance if surgical separation is to be considered in conjoined twins19. Seo et al. has introduced a new classification of the cardiovascular system in conjoined twins15. The classification has 5 types and is based on the degree of fusion and the symmetry of the hearts and great vessels (see table 2). Types I and II are easily separable. Separation of hearts with interatrial fusion (Type IIIa and IIIb) may be possible, and the analysis of the conduction system of the cases is important. The heart with atrioventricular fusion (Type IV) and the single hearts (Type V) are inoperable15.

Hershlag et al. separated the cardiac findings into a simpler three category system: 1) pericardial union only; 2) atrial connection; and 3) ventricular connection, usually associated with multiple cardiac defects. Separation of twins with pericardial and atria union is feasible, while cases with ventricular communication are inoperable10 (Table 3).

Table 3: The Seo (top) and Herslag (bottom) classifications of conjoined hearts and their surgical separability


Degree of fusion



No significant fusion



Fusion of the great vessels





Atrial fusion

Mirror image right atrial fusion

Other type of atrial fusion





Atrioventricular fusion

Not possible


Single heart in one of the twins

Not possible



Degree of fusion



Pericardial union only



Atrial connection



Ventricular connection

Not possible

Before 1975, the perioperative mortality rate for thoraco-omphalopagus twins was 55.6%. Between 1975 and 1979, the perioperative mortality rate was essentially unchanged at 58.3%; however, from 1980 to 1987, it decreased to 26%, a 32% decrease4. The current outcomes, even in difficult cases of conjoined twinning, suggest that separation should be considered. Delay of surgical separation is preferred to allow time for adequate diagnostic studies of the major organ systems and maturation of lungs, liver, and soft tissues. Separation is best delayed until infants are relatively mature (6-12 months of age). Operative survival was 50% in those operated on in the neonatal period, but 90% in those over 4 months of age9. Earlier surgical intervention may, however, be necessitated by ruptured omphalocele, intestinal obstruction, or deterioration of one or both twins.


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