Parkinson's Disease Dementia
Parkinson's disease is best known for affecting movement, causing tremor, stiffness and slowness. But as the disease progresses, it can also affect thinking, memory and perception. When these problems become severe enough to interfere with everyday life, and they start at least a year after the movement symptoms, it's called Parkinson's disease dementia.
How common is it?
Many people with Parkinson's have some changes in thinking at some point, but not everyone develops dementia. Estimates vary between studies:
- The Alzheimer's Society says that around a third of people with Parkinson's eventually develop dementia.
- Studies that follow people over their whole illness suggest the proportion is higher in those who live with Parkinson's for many years. One large study found about three-quarters of people who had Parkinson's for more than 10 years developed dementia.
The biggest risk factors are older age and having had Parkinson's for longer. On average, dementia develops around 10 years after Parkinson's begins, though this varies a lot.
Parkinson's disease dementia and dementia with Lewy bodies
Parkinson's disease dementia is closely related to dementia with Lewy bodies. Both are caused by clumps of a protein called alpha-synuclein (Lewy bodies) in brain cells, and many scientists see them as the same disease starting in different parts of the brain. Together they're called Lewy body dementia.
The difference is mainly the order in which symptoms start:
| Parkinson's disease dementia | Dementia with Lewy bodies | |
|---|---|---|
| Which comes first | Movement symptoms, then dementia at least a year later (usually many years) | Dementia before, at the same time as, or within a year of movement symptoms |
| Movement symptoms | Always present, usually for years | May be mild, come later, or never develop |
This "one-year rule" is somewhat arbitrary, but it's still used to make the diagnosis.
Early signs
Thinking problems usually start gradually. Before dementia, many people go through a stage of mild cognitive impairment, which doesn't always progress to dementia. Early signs include:
- trouble concentrating and paying attention
- difficulty planning, starting or finishing tasks, and making decisions
- slower thinking
- problems interpreting what they see, such as spotting an object in a cluttered space or judging distances
- difficulty finding words or understanding complicated sentences
Memory loss is often less prominent early on than in Alzheimer's disease.
Symptoms as it progresses
As dementia develops, symptoms often change from hour to hour or day to day, with the person sometimes alert and sometimes very confused. Common symptoms include:
- worsening memory, concentration and judgement
- visual hallucinations: seeing people, animals or objects that aren't there
- delusions, especially paranoid ideas
- excessive daytime sleepiness, very disturbed sleep and acting out dreams (REM sleep behaviour disorder)
- low mood, anxiety, irritability and apathy
- muffled or softer speech
- changes in appetite
The movement symptoms of Parkinson's continue to progress at the same time, which increases the risk of falls and, later, problems with swallowing.
Stages
There isn't a separate staging system for Parkinson's disease dementia. Like other dementias, it's usually described as moving from mild (managing with some support) to moderate (needing help with many daily tasks) to severe (needing full-time care). The speed varies widely. Because Parkinson's itself is also progressing, people often need more physical care than in some other dementias. The onset of dementia is often what leads to a move into a care home. Read more about the stages of dementia and end stage dementia.
Treatment
There's no cure, but treatment can help with symptoms:
- Dementia medicines. Donepezil and similar medicines can help with some symptoms of dementia in Parkinson's disease, such as forgetfulness and confusion.
- Reviewing Parkinson's medicines. Parkinson's drugs tend to become less effective over time and can make hallucinations and delusions worse. A specialist may need to balance movement control against these mental side effects.
- Caution with other medicines. Always tell doctors about the diagnosis before any new medicine is started, especially for hallucinations, agitation or sleep, as some medicines can make symptoms worse.
- Support and therapies. Physiotherapy, occupational therapy, speech and language therapy, a Parkinson's nurse, and treatment for depression, anxiety and sleep problems all help.
Tell the person's specialist team about new hallucinations, confusion or a sudden change. A sudden change can also be caused by an infection, dehydration or a medicine.
Life expectancy
Parkinson's disease dementia gets worse over time and shortens life, but the speed varies widely between people. People usually die from complications, such as pneumonia linked to swallowing problems, or falls, rather than from the dementia itself. See dementia life expectancy for figures for other types.
Getting help
If someone with Parkinson's has new problems with thinking, memory, hallucinations or confusion, speak to their GP, Parkinson's nurse or specialist. Some causes of confusion can be treated, and an accurate diagnosis helps with planning care and finding the right support.
References
- Parkinson's disease dementia. Alzheimer's Society. Accessed 1 November 2026.
- Parkinson's Disease Dementia. Alzheimer's Association. Accessed 1 November 2026.
- Parkinson's Disease Dementia. Cleveland Clinic. Accessed 1 November 2026.
- What you need to know about Parkinson's and dementia. Dementia UK. Accessed 1 November 2026.
- About donepezil. NHS. Accessed 1 November 2026.
- Advances in dementia with Lewy bodies. PubMed Central. Accessed 1 November 2026.