Conjoined twins


Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births to one in 200,000 births, with a somewhat higher incidence in southwest Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.
Two possible explanations of the cause of conjoined twins have been proposed. The one that is generally accepted is fission, in which the fertilized egg splits partially. The other explanation, no longer believed to be accurate, is fusion, in which the fertilized egg completely separates, but stem cells find similar stem cells on the other twin and fuse the twins together. Conjoined twins and some monozygotic, but not conjoined, twins share a single common chorion, placenta, and amniotic sac in utero.
Chang and Eng Bunker were brothers born in Siam who traveled widely for many years and were known internationally as the Siamese Twins. Chang and Eng were joined at the torso by a band of flesh and cartilage, and by their fused livers. In modern times, they could easily have been separated. Due to the brothers' fame and the rarity of the condition, the term Siamese twins came to be associated with conjoined twins.

Causes

There are two hypotheses about the development of conjoined twins.
  1. a single fertilized egg does not fully split during the process of forming identical twins. If the zygote division occurs after two weeks of the development of the embryonic disc, it results in the formation of conjoined twins.
  2. fusion of two fertilized eggs occurs early in development.
Partial splitting of the primitive node and streak may result in the formation of conjoined twins. These twins are classified according to the nature and degree of their union. Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. The type of twins formed depends on when and to what extent abnormalities of the node and streak occurred. Misexpression of genes, such as goosecoid, may also result in conjoined twins. Goosecoid activates inhibitors of BMP4 and contributes to regulation of head development. Over- or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of conjoined twins.

Types

Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:
  • Thoracopagus : Two bodies fused from the upper chest to the lower chest. These twins usually share a heart and may also share the liver or part of the digestive system. Survival rate is poor.
  • Thoraco-omphalopagus : Two bodies fused from the upper chest to lower belly. The heart is always shared in these cases., twins who share a heart have not been able to both survive separation; a designated twin who is allotted the heart may survive if the other twin is sacrificed.
  • Omphalopagus : Two bodies fused at the lower abdomen. Unlike thoracopagus, the heart is not shared; however, the twins often share a liver, a digestive system, a diaphragm and other organs. Survival rate is 82%.
  • Heteropagus : Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin's organs for survival.
  • Craniopagus : Fused skulls, but separate bodies. These twins' heads may be conjoined at the back, front, or side of the head, but not on the face or at the base of the skull. Survival rate is poor.
Other, less common types of conjoined twins include:
  • Cephalopagus: Two faces on opposite sides of a single, conjoined head; the upper portion of the body is fused while the bottom portions are separate. These twins generally cannot survive due to severe malformations of the brain. This is also known as janiceps.
  • Syncephalus: One head with a single face but four ears and two bodies.
  • Cephalothoracopagus: Bodies fused at the head and thorax, with two faces facing in opposite directions, or sometimes with a single face and an enlarged skull.
  • Xiphopagus: Two bodies fused in the xiphoid cartilage, which extends approximately from the navel to the lower breastbone. These twins almost never share any vital organs, with the exception of the liver. A famous example is Chang and Eng Bunker.
  • Ischiopagus: Fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle. These twins have four arms; one, two, three or four legs; and typically one set of external genitalia and one anus. Survival rate is 63%.
  • Omphalo-ischiopagus: Fused in a similar fashion to ischiopagus twins, but facing each other, with a joined abdomen, akin to omphalopagus. These twins have four arms, and two, three, or four legs.
  • Parapagus: Fused side by side with a shared pelvis. Those that are dithoracic parapagus are fused at the abdomen and pelvis, but not at the thorax. Those that are diprosopic parapagus have one trunk and two faces. Those that are dicephalic parapagus have one trunk and two heads, and may have two, three, or four arms. Survival rate is poor.
  • Craniopagus parasiticus: Like craniopagus, but with a second bodiless head attached to the dominant head.
  • Pygopagus or Iliopagus: Two bodies joined at the pelvis. Survival rate is 68%.
  • Rachipagus: Twins joined along the back of their bodies, with fusion of the vertebral arches and the soft tissue from the head to the buttocks. Twins of this type cannot be separated.
  • Tricephalus : Extremely rare conjoining of 3 fetuses. Very few confirmed cases, both human and animal, are known.

    Treatment

Separation

Surgery to separate conjoined twins may range from very easy to very difficult depending on the point of attachment and the internal parts that are shared. Most cases of separation are extremely risky and life-threatening. Though there have been a number of successful separations throughout history, in many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head or share a vital organ. This makes the ethics of surgical separation, where the twins can survive if not separated, contentious. Alice Dreger of Northwestern University found the quality of life of twins who remain conjoined to be higher than is commonly supposed. Lori and George Schappell and Abby and Brittany Hensel are notable examples.
The first recorded separation of conjoined twins took place in the Byzantine Empire in the 900s. One of the conjoined twins had already died, so the doctors of the town attempted to separate the dead twin from the surviving twin. The result was briefly successful, as the remaining twin lived for three days after separation. The next recorded case of separating conjoined twins was several centuries later, in Germany, in 1689. The first recorded successful separation of conjoined twins was performed in 1689 by Johannes Fatio. Around this same time Dr. Böhm of Gunzenhausen separated his own children, a pair of omphalopagus or xiphopagus twins; the feebler twin died four days later, but the healthier one was still alive and well at age five, when the case was reported. In 1955, neurosurgeon Harold Voris and his team at Mercy Hospital in Chicago performed the first successful operation to separate craniopagus twins, which resulted in long-term survival for both. The larger girl was reported in 1963 as developing normally, but the smaller girl was permanently impaired.
In 1957, Bertram Katz and his surgical team made international medical history performing the world's first successful separation of conjoined twins sharing a vital organ. Omphalopagus twins John Nelson and James Edward Freeman were born in Youngstown, Ohio, on April 27, 1956. The boys shared a liver but had separate hearts and were successfully separated at North Side Hospital in Youngstown, Ohio, by Bertram Katz. The operation was funded by the Ohio Crippled Children's Service Society.
Other successful separations of conjoined twins include that of the separation of Ganga and Jamuna Shrestha in 2001, who were born in Kathmandu, Nepal, in 2000. The 97-hour surgery on the pair of craniopagus twins was a landmark one which took place in Singapore; the team was led by neurosurgeons Chumpon Chan and Keith Goh. The surgery left Ganga with brain damage and Jamuna unable to walk. Seven years later, Ganga Shrestha died at the Model Hospital in Kathmandu in July 2009, at the age of eight, three days after being admitted for treatment of a severe chest infection.
Infants Rose and Grace Attard, conjoined twins from Malta, were separated in the United Kingdom by court order Re A over the religious objections of their parents, Michaelangelo and Rina Attard. The twins were attached at the lower abdomen and spine. The surgery took place in November 2000, at St Mary's Hospital in Manchester. The operation was controversial because Rose, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Grace's. However, if the operation had not taken place, it was certain that both twins would die. Grace survived to enjoy a normal childhood.
In 2003, two 29-year-old women from Iran, Ladan and Laleh Bijani, who were joined at the head but had separate brains, were surgically separated in Singapore, despite surgeons' warnings that the operation could be fatal to one or both. Their complex case was accepted only because technologically advanced graphical imagery and modeling would allow the medical team to plan the risky surgery. However, an undetected major vein hidden from the scans was discovered during the operation. The separation was completed but both women died while still in surgery.
In 2019, Safa and Marwa Ullah were separated at Great Ormond Street Hospital in London, England. The twins, born January 2017, were joined at the top of the head with separate brains and a cylindrical shared skull with the twins each facing in opposite directions to one another. The surgery was jointly led by neurosurgeon Owase Jeelani and plastic surgeon Professor David Dunaway. The surgery presented particular difficulties due to a number of shared veins and a distortion in the shape of the girls' brains, causing them to overlap. The distortion would need to be corrected in order for the separation to go ahead. The surgery utilized a team of more than 100 including bio engineers, 3D modelers and a virtual reality designer. The separation was completed in February 2019 following a total of 52 hours of surgery over three separate operations. The family returned to their home in Pakistan in October 2020, though one of the twins had a stroke during surgery which could make her unable to walk for the rest of her life.