Your father gets confused while in the hospital. Maybe he insists that your mother or his brother visited him that day - even though they passed away 20 or 30 years ago. Maybe he tells you all about the fishing trip he took today with his buddies - when he hasn't been fishing for years. Maybe he believes that he's in a different city and a different decade than he is actually in.

Some of these fictions may seem amusing. But don't be misled by the seemingly creative and imaginative stories that people come out with when they suddenly lose touch with reality during a hospital stay. This mental shift reflects a life-threatening medical emergency called delirium, no less destructive because it sometimes involves entertaining tales.

The issue isn't simply that people experience a temporary break from reality for a few days while getting intensive medical care. If they returned to normal afterward, that short-term flight from the real world wouldn't matter very much. But people who develop delirium in the hospital often end up with long-term or even permanent damage to their ability to think and function independently.

These problems arise even if their illnesses are treated successfully and even if they weren't having any problems thinking clearly before they were hospitalized. People who experience delirium in the hospital are more likely to end up in nursing homes and are more likely to die sooner than people whose medical conditions are otherwise similar.

Think about how often you've heard people say something like, "Aunt Matilda just wasn't the same after she got out of the hospital. The people in the rehab facility did a great job with her, and physically, there's nothing much really wrong with her. But she just couldn't manage on her own any more. She never even went home. She's been in a nursing home ever since, and we had to sell her house."

A well-designed study described in a recent issue of the New England Journal of Medicine analyzed the experiences of more than 800 adults with trouble breathing, heart failure, or life-threatening infections who were admitted to ICUs (intensive care units) at two highly respected hospitals. The researchers wanted to find out how many of them would develop delirium - and how well their minds would work after they got out of the hospital.

They used a series of tests to identify anyone who might have had impaired thinking or dementia when they arrived at the hospital. They checked the patients every day they were in the hospital, using carefully designed methods, to see if they had delirium as defined medically. You might be surprised at some of their findings.

The average age of the patients was 61. Only 6 percent of them arrived at the hospital with dementia or other cognitive problems. Seventy-four percent of them developed delirium while in the hospital, and on average the delirium lasted for four days. That's all - just four days.

But three months after they got out of the hospital, patients' scores on tests of mental functioning showed that they had mild cognitive impairment. In fact, 40 percent scored worse than people who had moderate traumatic brain injury; 26 percent scored worse than people with mild Alzheimer's. And the longer their delirium had lasted, the more likely they were to have more severe mental deterioration.

Even 12 months after they got out of the hospital, these numbers didn't improve very much: 34 percent still scored worse than people with moderate traumatic brain injury, and 24 percent worse than people with mild Alzheimer's.

Even patients who were under 50 years old and who didn't have medical problems other than the one that landed them in the hospital experienced the same deterioration initially, although they did a little bit better over time. At 12 months, 34 percent scored as badly as people with moderate traumatic brain injury, and 20 percent scored like those with Alzheimer's.

Next week's column will offer action steps you can take to help keep delirium from destroying a loved one's life.