By some estimates, as many as 4 in 5 people diagnosed with Parkinson's disease go on to develop dementia in the next 15 years.

As intimidating as the figure might be, those predictions are often based on studies with numerous limitations and small sample sizes, calling into question just how closely linked these two neurological conditions might be.

New research carried out by researchers from the University of Pennsylvania, suggests that the chances of having dementia 15 years following a Parkinson's diagnosis are more like 50 per cent, only reaching 74 per cent after 20 years.

That's significantly lower than previous estimates: earlier studies from 2003 and 2008 had put the chance of developing dementia 15-20 years after a Parkinson's diagnosis at roughly 80 per cent on average.

"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 reevaluate these findings," says Daniel Weintraub, a psychiatrist from the University of Pennsylvania.

The figures are pulled from two separate, ongoing investigations, one involving 417 patients with an average age of 62 at the commencement of the analysis, and one of 389 people with an average age of 69 at the start of monitoring.

As well as finding lower rates of dementia than previous studies had identified, the new research suggests dementia takes longer to develop than experts thought: the first study with the lower average age put the chance of having dementia 10 years after diagnosis at 9 per cent, while in the second study with the higher average age, it was 27 per cent.

Just why each study disagreed so drastically isn't clear, though the researchers suspect it may have something to do with how recently each sample in the two studies had received their Parkinson's diagnosis, and the state of their treatment on recruitment.

In either case, the relatively slower rates of decline imply specialists might have more time for dementia treatments to take effect and for preparations to be put in place. It also means there's more hope for those with Parkinson's that their risk of also developing dementia is lower and less pressing than some studies suggest.

"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," says Weintraub.

This is of course a complex issue, and many different factors are involved in determining an individual's risk: the researchers found that an older age of Parkinson's diagnosis, being male, and having a lower education level all increased the risk of developing dementia, which only add to multiple other known factors.

Future research should look at bigger, more diverse groups of people, the authors of this latest study say, and the team is keen to see more data on cognitive abilities collected in people who are living with Parkinson's.

For now, it's an encouraging step forward in our understanding of this devastating disease, suggesting that the damage it causes in the brain isn't as closely linked to dementia as we previously thought.

"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," says Weintraub.

The research has been published in Neurology.