CHICAGO - Nearly a third of children who stay in a pediatric intensive care unit have delusional memories, including vivid and frightening hallucinations, that put them at far higher risk of post traumatic stress disorder, British researchers said on Thursday.

They said children were five times more likely to report delusions or hallucinations if they were given strong sedatives such as opiates or benzodiazepines for more than two days.

“They reported seeing rats, cats and scorpions on the walls and in some cases crawling on the bed and a couple of children were convinced their parents had been replaced by impostors,” Gillian Colville, a clinical psychologist at St. George’s Hospital in London, said in an e-mail.

 “These strange experiences were reported more often by children who were admitted as emergencies ... and by those who had been sedated for over 2 days and were associated with higher rates of post-traumatic stress symptoms three months after discharge.”

Colville, whose study appears in the American Journal of Respiratory and Critical Care Medicine, said the work is among the first to examine children’s memories of their time in a pediatric intensive care unit.

She said it points to the need for more research on ways to help ease children’s fears in the already stressful settings.

For the study, Colville’s team evaluated 102 children ages 7 and older within three months discharge from the pediatric intensive care unit at Great Ormond Street Hospital for Children in London. They were interviewed about their memories and given a screening test for post-traumatic stress symptoms.

Overall, Colville said their factual memories were a bit spotty. Two in three children recalled something factual about their stay but half had only fragments, such as memories of a parent sitting by the bed.