Ladan and Laleh Bijani were craniopagus twins - joined at the brain. The attempt to separate them was the first time such surgery had been carried out on adults.

It's particularly difficult to separate conjoined twins who are joined at the brain or heart. Laden and Laleh shared a blood supply to their brains, which lowered their chances of survival. Starving the brain of a blood supply, even for a few seconds, can cause brain damage and eventually death.