Conjoined Twins ( Siamese twins )

(A painting of Chang and Eng Bunker, circa 1836)

Conjoined twins  are identical twins whose bodies are joined together since birth. It is a rare phenomenon. The occurrence is estimated to range from 1 in 50,000 births to 1 in 100,000 births.

Two contradicting theories exist to explain the origins of conjoined twins. The older theory is fission, in which the fertilized egg splits partially. The second theory is of fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. 

The most famous pair of conjoined twins was Chang and Eng Bunker(1811–1874) shown in the picture above. They were born in Siam, now Thailand. They traveled with P.T. Barnum's circus for many years and were billed as the Siamese Twins. Due to the brothers' fame and the rarity of the condition, the term "Siamese twins" came to be used as a synonym for conjoined twins.

The earliest known documented case of conjoined twins dates from the year 942, when a pair of conjoined twin brothers from Armenia were brought to Constantinople for medical evaluation.

Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:

• Thoraco-omphalopagus : 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.
• Thoracopagus :Two bodies fused from the upper thorax to lower belly. The heart is always involved in these cases.
• Omphalopagus : Two bodies fused at the lower chest. Unlike thoracopagus, the heart is never involved in these cases; however, the twins often share a liver, digestive system, diaphragmand other organs.
• Parasitic twins : Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin for survival.
• Craniopagus : Fused skulls, but separate bodies. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.

Surgery can be tried to separate conjoined twins which may range from relatively simple to extremely complex, depending on the point of attachment and the internal parts that are shared. In many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head. 

Recent successful separations of conjoined twins include that of the separation of Ganga & Jamuna Shreshta 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, Dr. Chumpon Chan and Dr. Keith Goh. Ganga Shrestha died at the Model Hospital in Katmandu in July 2009, at the age of 8, three days after being admitted for treatment of a severe chest infection.

In 2003 two women from Iran, Ladan and Laleh Bijani, who were joined at the head but had separate brains (craniopagus) were surgically separated in Singapore, despite surgeons' warnings that the operation could be fatal to one or both. Both women died during surgery on July 8, 2003.

A case of particular interest was that of Rosie and Gracie Attard, two conjoined twins from Malta who were separated by court order in Great Britain over the religious objections of their parents, Michaelangelo and Rina Attard. The surgery took place in November, 2000, at St Mary's Hospital in Manchester. The operation was controversial because Rosie, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Gracie's.However, if the operation had not taken place, it was certain that both twins would die.