Surgeons remove parasitic twin from teenager who had legs growing out of his chest

Surgeons in India have removed a parasitic twin from a teenager who had an extra set of legs, buttocks and genitals attached to his chest.

The parasitic twin condition occurs when a conjoined twin stops developing but stays attached to its surviving twin. In this case, the parasitic twin was protruding from the abdomen of a 17-year-old boy and weighed almost 33 pounds (15 kilograms), BBC News reported.

Doctors at the All India Institute Of Medical Science hospital in Delhi detached the parasitic twin during a successful two-hour-long surgery on Feb. 8, according to The Indian Express. Parasitic twins are extremely rare, and this surgery was an exceptional case because they're usually removed when the surviving twin is very young.

"Only 40 to 50 cases of parasitic twins have been documented in world medical literature and, in those cases, the surgery had been attempted on children," Dr. Asuri Krishna, who led the team of doctors in Delhi, told BBC News.

Related: In extremely rare case, doctors remove fetus from brain of 1-year-old

The precise rate at which they occur is unknown, but they're estimated to affect less than one in a million births, according to a 2018 study published in the Journal of Pediatric Surgery Case Reports.

Researchers aren't sure why parasitic twins develop, but they have two main theories based around embryonic fission and fusion — not to be confused with the nuclear reactions of the same name. The fission theory suggests that 13 to 15 days after fertilization, the cluster of cells that form the embryo splits apart, according to a 2010 study published in the Journal of Pediatric Surgery. This is the same process that creates identical twins, except instead of total separation, the two embryos stay partially connected.

The fusion theory argues that two initially distinct inner cell masses join together at a later stage of development, according to the study. Either way, a parasitic twin forms when one embryo stops developing but remains attached to the other twin.

A parasitic twin doesn't survive the pregnancy while the surviving twin fully develops, according to the Cleveland Clinic, a nonprofit medical center in Ohio. In this latest case, the parasitic limbs sensed pain and changes in temperature, BBC News reported. The surviving twin's blood supply feeds the parasitic twin, and the surviving twin can be susceptible to disease and other complications from having to support it, according to the Cleveland Clinic.

Scans revealed that the parasitic twin was attached to the teenager's breastbone with a blood supply coming from a vessel in his chest. The surgeons carefully removed the parasitic twin and then a large cyst that was found in the boy's abdomen.

The teenager's blood pressure dropped dramatically during the surgery because up to 40% of his blood flowed to the parasitic twin. However, the surgeons quickly stabilized him and he left hospital four days after being admitted, BBC News reported.

Living with the parasitic twin was difficult for the teenager, who had dropped out of school. He is now looking forward to a normal life.

"I couldn't travel anywhere or do any physical activity," the teenager told The Indian Express. "Now a new world has opened before me. I hope to study and get a job."