In Parkinson's, dementia may occur less often, or later, than previously thought

There's some good news for people with Parkinson's disease: The risk of developing dementia may be lower than previously thought, or dementia may occur later in the course of the disease than previously reported, according to a study published in the August 7, 2024, online issue of Neurology.

"The development of dementia is feared by people with Parkinson's, and the combination of both a movement disorder and a cognitive disorder can be devastating to them and their loved ones," said study author Daniel Weintraub, MD, of the University of Pennsylvania in Philadelphia.

"These results provide more hopeful estimates of the long-term risk of dementia for people with Parkinson's disease, suggesting that there is a longer window to intervene to prevent or delay cognitive decline."

Previous studies had indicated that about 80% of people with Parkinson's disease would develop dementia within 15–20 years after being diagnosed with Parkinson's.

"While these studies were important in highlighting the issue of cognitive decline in Parkinson's disease, the studies were conducted many years ago, were relatively small and had other limitations, so we wanted to re-evaluate these findings," Weintraub said.

For the study, researchers analyzed data from two large, prospective studies. An international study involved 417 participants with an average age of 62 who were newly diagnosed with Parkinson's disease and had not yet received treatment for the disease at study enrollment.

A study at the University of Pennsylvania involved 389 people with Parkinson's with an average age of 69 who had been diagnosed with Parkinson's an average of six years before the start of the study. The participants were followed to see whether they developed dementia.

The international study showed an estimated probability of being diagnosed with dementia 10 years after being diagnosed with Parkinson's disease of 9%. For the Pennsylvania study, the probability of being diagnosed with dementia 10 years after a Parkinson's diagnosis was 27%. For the latter study, researchers found an estimated risk of dementia at 50%, 15 years after a Parkinson's diagnosis and 74%, 20 years after diagnosis.

Factors that increased the risk of dementia in the Pennsylvania study included being older when Parkinson's was diagnosed, being male and having a lower level of education.

A limitation of both studies is that participants were highly educated, mainly white people, and were recruited for participation in a research study, so they may not represent the general population.