A new study shows that people with critical illnesses are more likely to develop post-traumatic stress disorder and other psychiatric illnesses than people who haven't experienced a life-threatening disease.

Studies show that people with diabetes and heart disease have greater than normal chances of suffering from depression, a mental illness that can interfere with daily life and any enjoyment of it.

But what about illnesses so severe that those who have them end up in hospital intensive care units?  

Dr. Derek Angus of the University of Pittsburgh Medical Center led a study that examined the relationship between critical illness and mental illness. Researchers reviewed the medical records of all patients admitted to intensive care units in Denmark over a three-year period.

“Denmark is important because for many years now they have been collecting national data on essentially the health and wellness of every person living in Denmark," said Angus.

“We were interested in people who got a new or sudden critical illness such as pneumonia or an episode of sepsis," said Angus.

For example, patients on ventilators admitted the intensive care unit, or ICU,  were compared to similar patients in other wards of the hospital and in the general population.    

“Patients who came into the ICU were, in fact, more likely to already have pre-existing psychiatric illness or to already be on psychiatric medications," said Angus.

But the researchers were curious about patients who had no previous mental illnesses and had not been on psychiatric medications.

"There was an absolute new risk of new psychiatric diagnoses and new psycho-active medication use. Even though it was less than one percent, it was 20-fold higher than the likelihood of getting a new psychiatric illness had you been a general population member with similar underlying characteristics," said Angus.

Dr. Angus says the risk is significant enough to tell patients and their families that depression, anxiety or post-traumatic stress disorder is a known complication of surviving critical illness.

The study found the risk was highest in the first three months after being discharged, and declined significantly as time went on. The study was published in the Journal of the American Medical Association.