What is the rate of conjoined twin birth? What is their chance of survival? How urgent is the surgery?

How are conjoined twins classified? How does their classification impact their prognosis?

Describe the nature of the preoperative evaluation for conjoined twins.

Conjoined twins are a relatively rare occurrence with an incidence of approximately 1 in 50,000 to 1 in 200,000 births. They are always monozygotic and monochorionic twins. Forty percent of these pregnancies end in stillbirth. Of the 60 % that are born, only about 20 % live to be eligible for separation. The defect leading to conjoined twins is likely a fusing of overlapping or closely contiguous twin embryonic axis formative fields within a single embryonic disc. It is thought that these factors are responsible for the failure of twins to separate after the 13th day after fertilization. Conjoined twins have been created in amphibians by simply constricting the embryo so that two embryos form, one on each side of the constriction. Most often this surgery is not urgent, and time can be taken to optimize the planning for the environment as well as the underlying health of the twins. Many of these cases are performed when the twins are a year or more of age. On rare occasions, based on the shared anatomy, the health of one or both twins may be adversely impacted by their conjoined nature – in which case the need for surgery becomes more time sensitive.

Conjoined twins are classified according to the region by which they are joined. Craniopagus twins are joined at the head. Thoracopagus twins are joined at the upper half of the trunk. (This is the most common form of conjoined twins making up 35–40 %.) Omphalopagus twins are joined at the chest or abdomen – the second most common form of the anomaly at 30 % of all cases. Most often these patients share a liver and prognosis for separation is generally good. Pygopagus are joined at the sacrum and constitute 19 % of all cases. Separation is most straightforward in these cases since they do not generally share vital organs and survival is high.