Despite the odds, Johns Hopkins neurosurgeon Dr. Ben Carson said he was optimistic the surgery to separate Laleh and Ladan Bijani, 29 - the first attempt ever to separate adults joined at the head - will be successful.
"We don't have experience to go on, but I would give them a 50-50 chance," Carson said.
The surgeon, who has separated children joined at the head three times, said there also was a 50-50 chance of one or both of the twins surviving in a vegetative state. Carson, however, said he did not want to dwell on the negative.
"If I thought we weren't going to be successful, I wouldn't have agreed to it," Carson said.
The twins have separate brains but share a skull. The surgery to separate them will be long and tedious. Doctors estimate it could take up to four days, reports CBS Correspondent Dr. Sean Kenniff.
The risks include infection, clotting, bleeding and the fact that the adult brain does not have the same ability to reorganize as the brain of a child, he said.
The neurosurgeon, however, is accustomed to trying the untried.
He led a team in 1987 that separated 7-month-old boys from Germany, using for the first time a procedure in which their circulatory system was bypassed and their bodies were cooled to preserve brain function. In 1997, he led a team of South African doctors in the first successful separation of vertically conjoined twins.
In the upcoming operation in Singapore, Laleh and Ladan will be in a seated position for the operation, to be led by Carson and Dr. Keith Goh, a neurosurgeon at Raffles Hospital in Singapore, where the surgery will take place in early July.
Doctors have warned the twins the surgery could kill one or both, but they have decided to take the chance, Carson said.
Both studied law because Ladan wanted to become a lawyer, but Laleh wants to become a journalist. Such compromises have convinced the women, who are Iranian natives, that they need to be separated to live satisfying adult lives.
"They recognize their lives are moving in different directions ... For people who are rather sophisticated, recognizing they can't go in their own way could be a fate worse than death," Carson said.
The twins' brains do not share any vital portions, and they are not expected to suffer any brain damage if all goes well, Carson said.
"We're hopeful when they awaken and recover they will still have the same intellectual function," Carson said. "We are gearing everything we are doing, not just to survival, but to full intellectual function."
The brains, however, do share a major vein, and one sister will have to have a graft, probably a vein taken from a leg. Whether the surgery, which could take days, has been successful should be known within the first 24 hours after surgery, he said.