A study, published in the journal JAMA Psychiatry, is the first to show that delirium can worsen cognitive decline in people with dementia.
The new research was carried out by UCL and the University of Cambridge and was funded by the Wellcome Trust.
When hospitalised, people can become very confused and disorientated. This condition is known as delirium and affects a quarter of older patients. This study finds delirium may have long-lasting consequences, including accelerating declines in memory and thinking caused by dementia.
Researchers looked at three populations in Finland, Cambridge and the UK, examining brain specimens in 987 people over the age of aged 65. Each person's memory, thinking and experience of delirium had been recorded over 10 years towards the end of their life.
People who had experienced delirium or who had dementia-related changes in their brains showed greater declines in memory and thinking performance than those without. However, those with dementia-related changes who also experienced delirium had the most severe declines in their cognition over the 10 years prior to death.
Dr Clare Walton, Research Manager at Alzheimer's Society said:
'Delirium is a temporary state of confusion and disorientation that is quite common among older people, especially those in hospital or living with dementia.
'Growing evidence shows that a case of delirium can predict worsening memory and thinking problems or the onset of dementia. This study suggests that delirium is not just a result of dementia-related changes in the brain but might independently cause problems with cognition. We don't understand why yet, but future research should look at the long term impact of delirium on the brain.
'We often hear of people who have developed memory and thinking problems or dementia after a stay in hospital. Understanding how delirium is involved and whether it can be prevented or treated is a pressing issue.'