Parkinson's disease dementia accounts for around 2 per cent of all cases of dementia in the UK.
People with Parkinson's disease have a higher-than-average risk of developing dementia, although around two thirds of people are unaffected. When dementia does occur, it is typically not until late in the course of the illness.
Symptoms of dementia associated with Parkinson's disease vary from person to person. The most common are memory loss and loss of the ability to think quickly and carry out everyday tasks. The person may become obsessive, and there may be a loss of emotional control, with sudden outbursts of anger or distress. Visual hallucinations - seeing things which are not really there - may also occur. The person's symptoms vary and can seem better or worse at different times.
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It is thought that Parkinson's disease dementia results from microscopic deposits known as Lewy bodies, located in nerve cells in the brain stem (the lower part of the brain just above the spinal cord). As Parkinson's disease progresses, Lewy bodies spread beyond the brain stem into other parts of the brain, causing dementia.
Lewy bodies are also seen in dementia with Lewy bodies (DLB). Parkinson's disease dementia appears very similar to dementia with Lewy bodies. The main difference is that problems with movement occur before cognitive symptoms in dementia associated with Parkinson's disease. In dementia with Lewy bodies, cognitive symptoms occur before, or at the same time as, problems with movement .
The side-effects of certain drugs for Parkinson's may make symptoms of dementia worse, so adjusting a person's medication accordingly can sometimes be of benefit. Some of the drugs used in Alzheimer's disease may also be helpful, especially if the person is experiencing hallucinations or delusions.