Cognitive decline following Parkinson’s disease
Parkinson’s disease dementia (PDD) is a decline in thinking, memory and other cognitive skills significant enough to affect daily independence. It develops in people who have had Parkinson’s disease for some years.
It is caused by the same underlying protein changes as Parkinson’s disease, which spread over time into the brain regions responsible for thinking and behaviour.
PDD typically appears after movement symptoms have been present for a year or more – this timing is what distinguishes it from the closely related condition, dementia with Lewy bodies (DLB), where cognitive changes appear earlier, alongside or before movement symptoms.
What is Parkinson’s disease dementia?
Parkinson’s disease dementia is diagnosed when someone with an established diagnosis of Parkinson’s disease develops significant difficulties with thinking, memory or other cognitive skills that start to interfere with everyday life, work or social activities. Movement symptoms such as slowness, stiffness and tremor are usually well established, often for a year or longer, before dementia develops.
Common features include:
- executive dysfunction – difficulty planning, organising, multitasking or solving problems
- slowed thinking – reduced attention, concentration or processing speed
- visuospatial difficulty – trouble judging distances, navigating or recognising spatial relationships
- memory changes – often less prominent early on than in Alzheimer’s disease, though memory difficulty can develop as the condition progresses
- behavioural and psychiatric features – visual hallucinations, fluctuating alertness, apathy, depression and sleep disturbance (including acting out dreams).
Over time, PDD typically affects:
- attention, concentration and processing speed
- behaviour and mood, including apathy, depression and hallucinations
- independence in daily activities
- memory, particularly later in the condition
- planning, organisation and problem-solving
- visuospatial skills, such as judging distance or navigating.
What happens in the brain?
Parkinson’s disease dementia is linked to progressive loss of brain cells with the spread of abnormal alpha-synuclein protein deposits, known as Lewy bodies, beyond the movement-control areas of the brain affected in Parkinson’s disease, into regions responsible for thinking, memory and behaviour, including the cortex.
Because the same abnormal protein is involved in Parkinson’s disease, Parkinson’s disease dementia (PDD) and dementia with Lewy bodies (DLB), these three conditions are grouped together as ‘Lewy body diseases’, with PDD and DLB being collectively referred to as ‘Lewy Body dementias’. The timing of when cognitive symptoms appear relative to movement symptoms is what helps distinguish between them.
What increases the risk of Parkinson’s disease dementia?
Not everyone with Parkinson’s disease develops dementia, but several factors are associated with a higher likelihood of doing so.
Factors that can’t be changed:
- age – both older age at Parkinson’s disease diagnosis and older current age increase the risk
- duration of Parkinson’s disease – the longer someone has had the condition, the higher the cumulative risk of developing dementia
- genetics – certain genetic factors may slightly increase risk, though most cases are not directly inherited
- motor symptom pattern – people with prominent slowness, stiffness and gait difficulty (rather than tremor-dominant symptoms) tend to have a higher risk.
Factors that may be associated with higher risk, some of which may be partly modifiable through medical care:
- cardiovascular health – vascular risk factors such as high blood pressure and diabetes may contribute to cognitive decline in people with Parkinson’s disease, similar to their broader role in other dementias
- depression – the presence of depression in Parkinson’s disease has been associated with cognitive decline
- early hallucinations or psychiatric symptoms – developing these earlier in the course of Parkinson’s disease is associated with increased risk
- REM sleep behaviour disorder – this sleep disorder, when present alongside Parkinson’s disease, is linked to a higher likelihood of later cognitive decline.
The evidence base for directly modifiable risk factors in PDD is still developing, so ongoing specialist review of both movement and non-movement symptoms remains one of the most important ways to monitor for early signs and plan care.
Common patient questions
No. Not everyone with Parkinson’s disease develops dementia, though the risk increases with longer disease duration and older age. Many people live with Parkinson’s disease for years without significant cognitive decline.
Early signs commonly involve slowed thinking, difficulty planning or multitasking, or visuospatial difficulty, rather than memory loss alone – this is a key difference from how Alzheimer’s disease typically first presents.
Most cases are not inherited. Some genetic factors may influence risk, but PDD is not typically considered a directly inherited condition.
There is currently no treatment that reverses the underlying condition. Management focuses on optimising quality of life through:
- medication (some treatments used in other Lewy body or Alzheimer’s-related dementias may help certain symptoms)
- managing movement symptoms
- treating any depression or sleep disturbance
- support from a multidisciplinary team – including occupational therapy for daily support and carer support services.