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Key Points
Conjoined twins are rare, and they may be joined at a variety of sites. The nomenclature in use to describe conjoined twins is derived from the most prominent site of conjunction.
The most common type of conjoined twins include thoracopagus, xiphagus or omphalopagus, pygopagus, ischiopagus, and craniopagus.
Conjoined twins can be diagnosed with ultrasound examination, and suspicion should arise when a dividing membrane cannot be visualized.
Congenital anomalies are common even in organs that are not shared.
The antenatal natural history of conjoined twins is not well known due to the fact that cases are rare and many patients elect termination.
Successful surgical separation is possible, and the prognosis for the surgery depends on the type of conjunction.
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Although a rare event, the birth of conjoined twins has always fascinated both the physician and the layperson. The first well-documented case was reported in A.D. 1100 and described the Biddenden maids who were joined at the hips and shoulders. In 1134, when the maids had lived together for 34 years, Mary was suddenly taken ill and died. Eliza, her sister, died six hours later (Bondeson, 1992). The most famous conjoined twins were Eng and Chang Bunker, born in Siam in 1811. The inappropriate term “Siamese twins” was coined by P.T. Barnum, who promoted the exhibition of Chang and Eng Bunker. An early medical description of these most famous conjoined twins, who lived unseparated until they died at age 63, can be found in the works of Warren (1829). Many conjoined twins are stillborn. In one series 40% of conjoined twins were stillborn and an additional 35% survived only 1 day (Edmonds and Layde, 1982). Konig recorded the first successful separation of conjoined twins in 1689. These twins were joined at the umbilicus, and the division was accomplished by necrosing the band of tissue between the two children with a constricting ligature. Kiesewetter (1966) reviewed 24 surgical attempts at separation that appeared in the literature from 1689 to 1962. There are now over 100 reports of successful separations in the medical literature or lay press. Conjoined twins may be joined at a variety of anatomic sites, and classifications have been developed to describe all the possibilities (Guttmacher and Nichols, 1967). The nomenclature in use clinically is derived from the most prominent site of conjunction. The common twin types include thoracopagus, xiphopagus or omphalopagus, pygopagus, ischiopagus, and craniopagus.
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Thoracopagus is the most common type of conjoined twin and with omphalopagus (or xiphopagus) represents about 75% of cases reported (Malone and D’Alton, 2000). The two individuals lie face to face and share a common sternum, diaphragm, and upper abdominal wall from xiphoid to umbilicus. An extensive review of the anatomyofthoracopa-gus twins has been published by Nichols et al. (1967). In data from 32 cases, 75% have conjoined hearts. Because of the ...