When Jeri Sharp, 62, was sedated and restrained while hospitalized for ARDS from H1N1, or swine flu, in 2016, she also had frightening delusions. The proportion of intensive-care patients who experience such ICU delirium is anywhere from 20 to 87 percent, depending on the study, though it appears to be more common in patients with ventilators than in those without. Sharp remembers at one point being strapped to a bed and her legs being spread apart. “I thought I was being molested,” she says. The memory has some basis in reality: She really was restrained in bed, and a nurse was probably placing a catheter. But in her delirium, it took on a sinister cast. Other patients have reported experiencing being taken to the MRI machine as being put into an oven or misinterpreting overhead conversations as plans to kill them—then lying awake for hours trying to escape.

These delusions are experienced at the time as real, and like genuine traumatic memories, they can rewire the emotional circuits of the brain. They “can lead to PTSD just like something a person literally and really experiences can lead to PTSD,” says Shawniqua Williams Roberson, a neurologist at Vanderbilt. The drugs used in sedation alter chemicals in the brain too. These factors, Williams Roberson says, in addition to the interrupted sleep, inflammation, lack of oxygen, and toxins from kidney or liver failure that are part of critical illness, may all play a role in psychological and cognitive changes after the ICU.