"If you're admitted to the ICU and you develop brain dysfunction, your risk of not surviving your hospital stay is doubled," says Dr. Stevens.
Delirium - characterized by a sudden onset, fluctuating mental status, inattention and confusion - was identified in 31.8% of critically ill patients (5,280 of 16 ,595).
These figures come from the 42 studies that the researchers narrowed down to after reviewing 10,000 published reports. Their findings have been reported in The BMJ.
As well as identifying the development of delirium in around a third of all patients admitted to the intensive care unit (ICU), the study discovered that the presence of such brain dysfunction leads to a higher risk of death.
Compared with patients without delirium, those developing it had significantly higher mortality during admission.
The risk ratio was calculated at 2.19 - meaning over twice the risk of death was found in the group with delirium versus that without.
The researchers conducted a meta-analysis to find that this was the higher risk of mortality even after accounting for severity of illness.
Delirium was also associated with a greater chance of a longer need for mechanical ventilation to provide breathing. Lengths of stay in the unit or other hospital departments also increased for these patients.
The risk ratio for longer duration of mechanical ventilation was 1.79, and for longer lengths of stay in the ICU, 1.38.
"Every patient who develops delirium will on average remain in the hospital at least 1 day longer," says Dr. Robert Stevens, one of the authors and a specialist in critical care as well as an associate professor at the Johns Hopkins University School of Medicine in Baltimore, MD.
Worse, adds Dr. Stevens:
"If you're admitted to the intensive care unit and you develop brain dysfunction, your risk of not surviving your hospital stay is doubled."
The authors also found that "available studies indicated an association between delirium and cognitive impairment after discharge" - among patients who develop delirium, the risk of long-term cognitive decline increases by up to 30%.
Further research should determine what causes delirium in critically ill patients
Most of the studies reviewed used a diagnostic tool for spotting delirium known as CAM-ICU. This involves first identifying that there has been a sudden (acute) change from a baseline mental status, or that the patient's mental status has fluctuated over the past 24 hours.
If this has been the case, the clinician looks for inattention by, for example, asking the patient to squeeze the doctor's hand when they say the letter A during a spoken sequence of letters.
The level of consciousness is also tested along the Richmond Agitation-Sedation Scale, with any state other than "alert and calm" contributing to the delirium diagnosis.
Even if the patient is alert and calm, delirium will be diagnosed if the course of symptoms and inattention are also accompanied by disorganized thinking, which is tested through responses to commands, such as to hold up the same number of fingers as the clinician, or to questions such as, "Will a stone float on water?" or, "Are there fish in the sea?"
The researchers cite sedatives and other medications used in the ICU as the best-known causes of delirium. Disorientation and confusion may result from the use of benzodiazepine, for example - often used to help patients calm down and sleep.
Dr. Stevens says there can be commonsense approaches to reducing the need for such drugs, with nighttime interruptions being kept minimal, for example - to ensure patients get rested without needing sedatives.
The study paper concludes with remarks about a better understanding of the other causes of delirium in the critical medical setting. "Research is needed to unravel the biological mechanisms governing these relations" between delirium and death and other risks, "and to discover strategies and treatments that will reduce the burden of acute and long-term brain dysfunction in critically ill populations."
Other causes of delirium might be harder to address than the use of sedatives, say the researchers.
Another is that delirium is related to blood flow changes to the brain, "sometimes resulting in strokes that are not recognized as such."
Dr. Stevens sums up the relevance of his team's work:
"We're seeing that even though you may have a very severe illness or injury and you're lucky enough to survive, you're still not quite out of the woods.
We need to think about the measures we can put into place to decrease these long-term burdens."