Conjoined Twins

These thoraco-omphalopagus twins were born at 33 weeks gestation via cesarean delivery. Both had low Apgar scores and hypoxemia that necessitated ventilatory support. Echocardiography showed separate hearts that were connected at the atrial level. Baby B had a perimembranous ventricular septal defect. MRI scans demonstrated that the twins shared a liver but had separate GI tracts that
were fused briefly at the duodenum forming an enterocele. Intraoperative cholangiography confirmed biliary drainage only into the GI tract of baby B.

The enterocele was partially resected in an attempt to maximize bowel length, but the intestinal tracts could not be completely separated. Postoperatively, both infants remained hypoxemic and became increasingly septic despite antibiotic therapy and critical life support. Support was ultimately withdrawn on the 65th day of life on parental request.

EMBRYOLOGY
Like monovular twins, conjoined (Siamese) twins are derived from a single blastocyst. Siamese
twins remain joined by some aberration in the duplication process that normally produces 2 identical twins.

Conjoined twins are monozygotic, monoamniotic, and monochorionic. The prevailing theory is
that the malformation occurs soon after the blastula stage of development. At this stage—between the 13th and 15th day after fertilization— the inner cell mass splits into equal halves, each capable of forming a normal individual. In conjoined twins, complete separation of the chorionic mass does not occur and non-separated parts of the otherwise normal twins remain fused throughout development.^1,2 In 1992, Spencer3 suggested an alternate theory. It holds that the unusual proximity of the 2 embryonic disks resulting from a complete division of a single fertilized ovum may cause secondary fusion into conjoined twins.⁴

Conjoined twins are classified according to the most prominent site of connection (Table).⁵ (The Greek root pagus means “something fixed.”)

EPIDEMIOLOGY
The true incidence of conjoined twinning is difficult to ascertain because the majority of these twins are either aborted or stillborn. A crude incidence of 10.25 per million births has been reported,⁶ but rough estimates range from 1 per 50,000 to 100,000 births.^2,4,7 Female conjoined twins are more common and account for approximately 70% to 95% of cases.^8,9 There have been reports of racial predilection—such as an increased incidence in some areas of Africa and Asia¹⁰—but no significant relationship with heredity, maternal age, or parity has been reported.

DIAGNOSIS AND OBSTETRICAL MANAGEMENT
Ultrasonographic evaluation is indicated for all pregnant women carrying more than a single fetus to rule out the possibility of conjoined twins. Koontz and coworkers¹¹ have summarized the sonographic findings associated with conjoined twins. These include:

• The lack of a separating membrane (all conjoined twins are monoamniotic; therefore, an interamniotic membrane cannot be identified).
• The inability to separate the fetal bodies.
• The presence of other anomalies.
• The presence of 3 or more vessels in the umbilical cord.
• Any of the classic radiographic signs described by Gray and associates.¹²
• Fetal heads persistently at the same level.
• A dorsiflexed cervical spine.
• A narrow space between the lower cervical and upper thoracic spine.
• No apparent change in fetal position after maternal manipulation.

If the diagnosis is not certain, other imaging techniques can be considered—including plain radiography or amniography.

When the diagnosis is made before the twins are viable, the option of pregnancy termination should be offered to the parents. After viability, serial examinations are indicated to monitor fetal growth, to detect the development of hydrops, and to reveal fetal demise. A 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 type of delivery is determined by the size of the twins, the nature of their fusion, the possibility of their survival, and parental wishes.⁵

After birth, evaluation of both twins should be conducted to assess the extent of organ system sharing. To this end, plain and contrasted radiography, echocardiography, angiography, ultrasonography, and CT scans have been employed. Further management is based on the type of conjunction and the presence of associated anomalies.

The prognosis for conjoined twins is generally poor. Roughly 40% of conjoined twins are stillborn, and approximately 35% die within the first day of life.¹³ In stillborn conjoined twins, males predominate. Survival depends on the type of conjunction and the presence of associated anomalies. Only 60% of surgically treated conjoined twins survive.¹⁴

SURGICAL MANAGEMENT
Preoperative diagnostic studies include radiographic examination of all systems—especially the urinary, reproductive, and GI systems. This may involve use of plain x-ray films, echocardiography, cystourethrograms, lower GI tract studies, CT scans, and MRI scans.

Surgical separation may be attempted in certain cases, depending on extent of fusion and, in particular, of visceral fusion. Surgery should be delayed for at least 3 to 6 months to allow the twins to stabilize and grow. A review of all attempted surgical separations of conjoined twins as
of 1987 revealed the following mortality rates: thoracopagus, 29%; omphalopagus, 70%; craniopagus, 33%; ischiopagus, 74%; and pygopagus, 56%.