For a one-year period starting in 2012, an anesthetist at every hospital in the United Kingdom and Ireland recorded every case where a patient told a staff member that he had been awake during surgery. Prompted by these reports, the researchers investigated 300 cases, interviewing the patient and doctors involved.

One of the most striking findings, says the study’s lead author, Jaideep Pandit of Oxford University Hospitals, was that pain was not generally the worst part of the experience: It was paralysis. For some operations, paralyzing drugs are given to relax muscles and stop reflex movements. “Pain was something they understood, but very few of us have experienced what it’s like to be paralyzed,” Pandit says. “They thought they had been buried alive.”

“I thought I was about to die,” says Sandra, who regained consciousness but was unable to move during a dental operation when she was 12 years old. “It felt as though nothing would ever work again — as though the anesthetist had removed everything apart from my soul.”