Conjoined twinning is one of the most fascinating human malformations and has also been reported in other animals—mammals, fishes, birds, reptiles, and amphibians. [1, 2, 3] Treating conjoined twins can be a daunting challenge for the surgeon. Furthermore, these cases often raise religious,  moral, ethical,  and legal [6, 7] issues.
Approximately 75% of conjoined twins are female, and 70% are fused at the thorax (thoracopagus) or abdomen (omphalopagus). The union can be in the frontal, transverse, or sagittal plane. The following two photographs show union in a transverse plane in the lower body area, so that the twins face each other.
The two main categories of conjoined twinning are as follows:
Symmetrical or equal conjoined twins (ie, two well-developed babies)
Asymmetrical or unequal conjoined twins (ie, a small part of the body is duplicated, or an incomplete twin is attached to a fully developed twin)
In broad terms, conjoined twins may be regarded as a doubling anomaly. The later the incomplete embryologic separation occurs, the higher the likelihood of a complicated fusion.
History of the Procedure
The term Siamese twins comes from Eng and Chang Bunker (1811-1874), the famous conjoined twins from Thailand (Siam). They settled in the United States, married healthy women (who were sisters), had children, and lived to age 63 years. P.T. Barnum, with whom they were affiliated at one time, was responsible for introducing the term Siamese twin; however, this term has fallen out of favor because it seems to convey that conjoined twins are circus freaks or monsters.
One of the earliest documented cases was that of Mary and Eliza Chulkhurst, who were born in 1100 in England. They have been described as being joined at the shoulders and hips, which is, in fact, unlikely based on the known types of conjoined twinning. Also known as the Biddenden Maids, the sisters lived for 34 years in Biddenden, County of Kent, England. The major types of conjoined twins were described in 1573 by the French renaissance surgeon Ambroise Pare.
Successful separation of a set of omphalopagus twins was first reported in 1689 in Basel, Switzerland.  While series of experience with conjoined twins have been reported in literature,  successful separation of large numbers of conjoined twins is limited to a few centers in the world (13 sets described by O'Neill et al from the Children's Hospital of Philadelphia in 1988,  14 sets reported by Mackenzie et al from the same hospital in 2002,  17 sets reported by Spitz and Kiely in the same year from the Great Ormond Street Hospital, London,  and 46 sets reported by Rode et al from Red Cross Children's Hospital, Cape Town, South Africa.  Surgical experience with conjoined twins has also been reported in other parts of the world, such as China, Saudi Arabia, and New Zealand. [14, 15, 16]
In England, the separation of a pair of conjoined twins from Malta, Jodie and Mary, born August 8, 2000, raised considerable ethical and legal issues.  The parents refused to grant permission for the surgery, and the matter was referred to court. The judges concluded that separation was in the best interests of both children, even though it meant the death of the weaker twin, Mary. In the United States, the decision of the parents might have been final.
In Singapore in 2003, skull and brain separation of 29-year-old craniopagus Iranian twins, Ladan and Laleh Bijani, was unsuccessful, and the sisters died of exsanguination on the operating table.
Craniopagus conjoined twins Mohamed and Ahmed Ibrahim were separated at the Children's Medical Center in Dallas, Texas. They were born in Egypt on June 2, 2001, and were separated on October 12, 2003, during a 34-hour operation. The operation involved skin grafting and separation of the venous sinuses. Later, they underwent skull reconstructive surgery, received rehabilitation therapy, and, finally, left for Egypt on November 19, 2005. A successfully staged separation of craniopagus twins (Carl and Clarence Aguirre) took place from 2003-2004 at Montefiore Medical Center in New York.
A support group for conjoined twins, Conjoined Twins International, was founded in 1996 in Prescott, Arizona, by the grandparents of a set of conjoined twins. Facebook also has a support circle of friends of conjoined twins. In addition, blog sites have been set up for certain sets of twins.
The term conjoined twinning refers to an incomplete splitting of monozygotic twins after 12 days of embryogenesis. See the diagram below.
The delivered fetuses are physically joined at some point as a result. The point of union is used to classify twins; the label used is the Greek word pagos, which means "that which is fixed." An even more rare condition occurs when one incompletely formed (ie, parasitic) twin is dependent on the well-formed one. This is known as heteropagus twinning. See the images below.
Another example of parasitic twin, in which the head is not visible, and the ethical decision is easier to make, is shown below.
A nonoperated case of parasitic twins, presenting late, is shown below.
The parasitic twin (sacrificed) after separation is shown below.
Multiple limbs attached to the thorax is shown below.
Twinning occurs in approximately 1 of every 87 live births. Monozygotic twins account for one third of twin births. Conjoined twins account for 1% of monozygotic twins. In the United States, the incidence is 1 per 33,000-165,000 births and 1 per 200,000 live births.  The stillbirth rate is 40%-60%. More cases are being reported now because of the routine use of fetal ultrasonography.
The condition is probably more common in Indian and African populations than in whites. Exact epidemiology across different races and nations is not known because of underreporting and lack of facilities for prenatal diagnosis.
When born live, females are affected more often than males, with a female-to-male ratio of 3:1 or greater. Stillborn conjoined twins are predominantly male.
Conjoined twins show characteristic points of attachment. They are classified according to the site of union, with the following frequency:
Thoracoomphalopagus (ie, joined at the chest, abdomen, or both) - 74%
- Thoracopagus or xiphopagus (ie, joined at the chest) - 40%
- Omphalopagus (ie, joined at the abdomen) - 34%
Pygopagus (ie, joined at the buttocks) - 18%
Ischiopagus (ie, joined at the ischium) - 6%
Craniopagus (ie, joined at the head) - 2%
See omphalopagus in the photos below.
See an example of craniopagus twins in the image below.
A radiograph of the skull, showing the conjoined heads in the case above is shown below.
An MRI or CT may show the brain structure, while a magnetic resonance angiography can show the extent of vascular connections. See the image below.
The incomplete anatomic separation between monozygotic twins occurs sporadically, with no increased risk in future pregnancies.
Seven cases (2 published) have been reported in which conjoined twinning occurred with the use of clomiphene for induction of ovulation. Two cases of thoracopagus have been reported in which conjoined twinning occurred with periconceptional maternal griseofulvin intake. Spina bifida is associated with conjoined twinning, and one case of conjoined twinning after maternal exposure to valproic acid has been reported.
No gene mapping or linkage analysis currently exists for the malformation. Some investigators implicate abnormal X inactivation. The latter may be related to the increased incidence in female twins. Other studies refute abnormal X-chromosome inactivation.
The morula becomes a blastocyst on day 6 after the ovum is fertilized. An inner cell mass develops at one end within this vesicle. The inner cell mass can form a whole fetus. Conjoined twins are produced when this inner cell mass, derived from a single zygote, incompletely splits late, after the 12th day of gestational life.
Cephalothoracopagus twinning is characterized by the anterior union of the upper half of the body, with two faces angulated variably on a conjoined head. The anomaly is occasionally known as janiceps, named after the 2-faced Roman god Janus. The prognosis is extremely poor because surgical separation is not an option, as a single brain and heart are present and the GI tracts are fused. This malformation is extremely rare (see the image below).
Craniopagus occurs in 2% of conjoined twins. In this variety, the twins have cranial fusion (see the images below). Stone and Goodrich have recently subclassified craniopagus into 4 varieties, depending on whether a significantly shared dural venous sinus system (total vs partial) is present, and whether the inter-twin longitudinal angulation is below 140 degrees. 
Bicephalus or dicephalus means that 2 heads are present on a single trunk (see the images below).
In omphalopagus, the anterior abdomens are united (see the image below). Omphalopagus (34%) is considered a subset of thoracoomphalopagus. Its incidence is usually combined with that of thoracopagus (40%) to provide an incidence of thoracoomphalopagus (74%). Pure omphalopagus twins have no cardiac union.
A case of thoracoomphalopagus twins, after separation, is shown below.
In the condition parapagus, (see the image below) or diprosopus,  twins have lateral union of their trunks so that both faces are forward looking in the same plane.
Pygopagus (see the image below) is the term used when the twins face in opposite directions. The sacra are fused, and the twins may share a portion of the spinal cord. In addition, the rectum and perineal structures are usually fused.
In rachipagus (see the image below), the twins are joined back-to-back at any point, usually above the lumbar spine. They may have extensive vertebral fusion in the dorsal midline and may have meningocele, neural connection, or both. One of the embryos generally fails to survive, leaving some body part attached to the other twin's dorsal column. In these cases, rachipagus must be differentiated from fetus in feto (within a body cavity) and teratoma (no mature organ). 
Thoracopagus is the most common variety, occurring in 40% of conjoined twins. The chests are joined, and the hearts are almost always fused in some way (see the images below). As noted above, thoracopagus is often combined with omphalopagus.
In ischiopagus twins (see the image below), the lower abdomen and the pelves are fused. [22, 23] The twins may have 3 legs (ie, tripus) or 4 legs (ie, tetrapus). The genitourinary system and the rectum are shared; the liver may also be fused.
Of all types of conjoined twins, omphalopagus twins are the most favorable candidates for elective surgery because of good survival rates. 
Historically, conjoined twins in general have been placed into 3 groups:
Group 1 - Those who do not survive delivery plus those who die shortly after birth
Group 2 - Those who survive to undergo an elective procedure
Group 3 - Those in whom an emergent procedure is required
Emergent conditions may arise at any time and include intestinal obstruction, rupture of an omphalocele, congestive cardiac failure, severe degree of respiratory compromise, and terminal illness in one of the twins.
Harper and Kenigsberg suggest that the abdominal cavities grow as the twins age, whereas the bridge connecting the omphalopagus twins does not grow in diameter.  Hence, abdominal wall closure can be performed more easily at approximately age 1 year.
The moral and ethical aspects of separation must be considered, especially in the following circumstances:
A choice must be made concerning single organ systems. The twin who receives the organ system will live and thrive, whereas the other twin will suffer or die. A similar problem arises when unequal limbs are present.
The twins have conjoined hearts. Surgical separation of the cardiac complex has been mostly unsuccessful. In some cases, one twin is allowed to live with the entire cardiac complex. The availability of 2 heart transplants at the same time may improve the options.
The twins are craniopagus and have complete brain junction. These twins are usually inseparable.
The larger the connecting bridge, the more complex its contents.
Ethical questions may arise in conjoined twins. For example, will the surgery be successful? Is sacrificing one twin to save the other justified? A conjoined heart is a contraindication because the heart complex is usually inseparable. Postoperatively, the divided heart often goes into congestive cardiac failure. Cephalopagus twins with extensive brain union cannot be separated. Furthermore, in the United States, parental refusal is a contraindication to surgery.
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