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Older adults who develop sepsis — a serious, widespread bacterial infection -- are at risk of declining both mentally and physically in subsequent years, a new study has found.

The researchers say their findings suggest the long-term effects of sepsis are under-recognized and could account for a number of the cases of dementia that are diagnoses each year.

Sepsis is a serious condition that's sometimes called blood poisoning. It can start as a reaction to an infection such as pneumonia, but eventually causes the entire immune system to begin attacking the body's organs and tissues. It affects thousands of patients each year, and is a leading cause of death in hospital ICUs.

About 40 per cent of sepsis patients will die. But for those who survive -- usually with the use of antibiotics -- it had been assumed that most make a full recovery.

Yet this new study in the Journal of the American Medical Association has found that older sepsis patients have a much higher risk of developing a significant mental and physical decline afterward.

The study, led by Dr. Theodore J. Iwashyna, of the University of Michigan Medical School in Ann Arbor, Mich., looked at the medical information from 516 people over age 50 who survived severe sepsis, comparing them to 4,517 older adults hospitalized for other conditions. The average age of survivors at hospitalization was 76.9 years.

The participants were assessed for cognitive problems as well as physical disabilities four years before their illness and then again about eight years afterward.

They found that three out of five of the sepsis patients, or about 60 per cent, experienced worsening cognitive or physical function, or both, after their infection.

Nearly 17 per cent showed signs of moderate to severe cognitive impairment. About 40 per cent of sepsis patients later had trouble walking, and 20 per cent needed assistance with everyday activities, such as going to the grocery store or preparing a meal.

The researchers estimate that sepsis survivors have a threefold increase in life-altering mental declines after surviving sepsis. The patients with no history of sepsis showed no increased risk over the course of the study.

"These data argue that the burden of sepsis survivorship is a substantial, under-recognized public health problem with major implications for patients, families, and the health care system," the researchers write.

The researchers estimate that sepsis may be responsible for 20,000 new cases of dementia among people aged 65 or older each year in the U.S.

"An episode of severe sepsis, even when survived, may represent a sentinel event in the lives of patients and their families, resulting in new and often persistent disability, in some cases even resembling dementia," they write.

The authors say while they can't prove that the sepsis led to the mental and physical declines they recorded, there are a number of ways that sepsis -- and perhaps its treatment -- could lead to mental and physical decline.

They note that sepsis typically causes a significant drop in blood pressure as well as full body inflammation, both of which could lead to brain damage that leads to decline.

Sepsis patients also frequently become delirious, a state known to be associated with Alzheimer's disease.

"Future research to identify mechanisms leading from sepsis to cognitive impairment and functional disability -— and interventions to prevent or slow these accelerated declines -— is especially important now given the aging of the population," the authors write.