Cognitive difficulties are among the most troubling and disabling non-motor symptoms of Parkinson’s disease (PD). Unfortunately, it is now recognized that they can also be very common as PD progresses. Some people experience mild changes even before a PD diagnosis; by 20 years after diagnosis, up to 80 percent of people with PD develop dementia.
A paper published in the September 1 online edition of npj Parkinson’s Diseasereviews the complex causes of cognitive impairment in PD and the potential for using neuroimaging and cognitive tests to detect these difficulties earlier and more accurately so they can be treated.
For people with PD, cognitive changes may include difficulty organizing and carrying out tasks, or changes in memory, speech and vision. There are many different kinds of brain changes that underlie these symptoms. The complexity in brain changes makes it difficult for scientists to describe how cognitive difficulties begin and progress in PD. It also makes it difficult for them to focus their work and to identify biomarkers (such as a blood tests) that would help to diagnose and monitor cognitive changes in PD. The authors reviewed studies published to date about cognitive changes in PD and summarized the key findings.
The brain changes in PD that affect cognition include:
- Loss of dopamine, the chemical messenger involved in controlling the body’s movement. However, medications that restore dopamine can affect different areas of the brain unevenly, leading to cognitive changes.
- Loss of other brain chemicals, including noradrenaline, serotonin and acetylcholine, may play a role.
- Lewy bodies — the toxic clumps of a protein called alpha-synuclein that are the hallmark of PD — may be involved. They seem to not only form in areas of the brain associated with PD, but also in other areas, where changes may result in different cognitive symptoms.
- A person who develops dementia within a year of a PD diagnosis has a parkinsonism known as Dementia with Lewy bodies (DLB).
- Plaques of beta-amyloid protein and tau neurofibrillary tangles — the changes that lead to Alzheimer's disease — can also occur in people with PD and are frequently observed in those diagnosed with DLB.
What Does It Mean?
Dementia causes significant disability in PD. It often can lead to a person with PD to move into a nursing home or assisted living facility. Recognizing people with PD who are at risk for cognitive impairment could help researchers design and test new therapies. Unfortunately, current testing and imaging scans cannot indicate who with PD will or will not likely develop dementia.
To this end, the study authors propose that PD researchers develop a unified view of cognitive changes in people with PD and dementia with Lewy bodies.
The authors indicate that both diseases include accumulation of the protein alpha-synuclein which is the heart of Lewy bodies. New studies should combine neuroimaging with sensitive cognitive tests to follow a cohort of people with PD over time. This would help clarify the interplay between the various brain changes that herald the development of dementia. They also point out that changes associated with Alzheimer’s disease can also contribute to dementia in people with PD, and suggest that a cognitive scale widely used to monitor Alzheimer’s (the Mini-Mental State Examination (MMSE)) could help in identifying people with PD who are at risk for dementia.
Biundo R, Weis L, Antonini A. (2016). Cognitive Decline in Parkinson’s Disease: The Complex Picture. npj Parkinson’s Disease. DOI: http://dx.doi.org/10.1038/npjparkd.2016.18