This study was based on a random 5% of older (66 years or above) Medicare patients who were treated in intensive care in 2005 and whose health was followed for a further three years using Medicare claims data. Of the 25,368 patients included in the study 4,519 (17.8%) went on to receive a diagnosis of dementia during the three year follow up period.

Patients with previous indications of cognitive impairment for whom dementia could have been an escalation of a pre-existing condition were excluded from the study.

Increasing age was very strongly associated with diagnosis of dementia following ICU. The risk at 75 was more than double that of the 66 to 69 year olds. And this rose to more than five times the risk for those age 85 and older. Women had a marginally higher risk than men and, as other studies have shown, race was also important to risk. Length of stay in ICU was not a factor nor was the need for mechanical ventilation.

Three factors related to the critical illness were independently associated with an increased risk of a diagnosis of dementia: a critical illness with the presence of an infection which increased to a higher risk with more severe infection such as severe sepsis, having acute neurologic dysfunction during critical illness, including anoxic brain damage, encephalopathy, and transient mental disorders, and finally acute renal failure requiring dialysis. This last risk was time-dependent and only increased the risk 6 months after the patient had been discharged from hospital.

Dr Hannah Wunsch, from Columbia University Medical Center, lead author of the study commented, "Due to increasing life spans and better hospital care, millions of older people now survive a critical illness every year. Our study provides a greater understanding of the consequences of these hospitalizations on subsequent risk of receiving a diagnosis of dementia, and may allow for better planning and targeting future studies to high risk populations."