The level of cognitive decline in patients with long-term Parkinson’s disease (PD) is significantly less than previously suggested, an Australian and UK study has shown. 

Clinical assessment in a small cohort of 36 patients with PD for over 20 years duration found only 19% met Movement Disorder Society Level 1 criteria for a diagnosis of probable dementia in Parkinson disease (PDD). 

This contrasts markedly with earlier results from the 2008 Sydney Multicenter Study of PD which found 83% of patients had developed dementia after 20 years of living with PD. 

However the researchers acknowledged their PDD rate of 19% was likely to be an underestimate as it included only patients attending the research clinic or receiving home visits as part of a brain donation program but did not include long-term PD patients residing in nursing homes.

The current study found the frequency of PDD was similar in patients older and younger than 70 years. There was also no significant difference in the age of onset of PD between those who subsequently developed PDD and those who didn’t.

Patients with dementia had a lower level of education attainment than those without dementia. 

The study, led by researchers from the Parkinson’s Disease Research Clinic at the University of Sydney’s Brain and Mind Centre, also found PDD patients had more severe motor symptoms than those PD patients who didn’t have dementia. 

In particular, individuals with PDD exhibited a higher tremor dominant/non-tremor dominant (NT/NTD) ratio compared to those PD patients without dementia.

“Taken together, considering the role of altered cholinergic neurotransmission on motor and cognitive functioning in PD, the more severe level of motor impairment and TD/NTD ratio observed in the PDD group may reflect a more widespread pathology that includes additional cholinergic denervation.”

The study said the findings reinforce the view that there is likely to be a more profound neuropathology associated with PDD.

“Findings from the present study highlight the need to investigate factors that might affect the trajectory of cognitive decline in long-term PD patients, which may lead to the determination of potential modulating factors in the development of dementia in these patients.”

Future work would also include genetic screening focussed on mutations known to be associated with rapid cognitive decline in PD, the researchers said.