Researchers here note that in many people visual decline precedes the more evident worsening of Parkinson's disease as it progresses. Similar mechanisms of neurodegeneration contribute to both manifestations of aging. Neurodegenerative conditions are the result of many interacting processes that collectively harm function in the brain, from the structural issues resulting from vascular aging, to failing mitochondrial function, to the formation of protein aggregates. These processes give rise to numerous distinct forms of loss of function, and thus people who exhibit any one of those losses are more likely to develop the others.
A new study adds to evidence that vision changes precede the cognitive decline that occurs in many, but not all, people with Parkinson's. A second new study found that structural and functional connections of brain regions become decoupled throughout the entire brain in people with Parkinson's disease, particularly among people with vision problems. The two studies together show how losses and changes to the brain's wiring underlie the cognitive impairment experienced by many people with Parkinson's disease.
In the first study, researchers studied 77 people with Parkinson's disease and found that simple vision tests predicted who would go on to get dementia after a year and a half. Dementia is a common, debilitating aspect of Parkinson's disease, estimated to affect roughly 50% of people within 10 years of a Parkinson's diagnosis. These longitudinal findings add weight to previous studies that were done at one time point, which had suggested that performance in vision tests was linked to the risk of cognitive decline. Those who went on to develop Parkinson's dementia had white matter damage to some of the long-distance wiring connecting the front and back of the brain, which helps the brain to function as a cohesive whole network.
The second study involved 88 people with Parkinson's disease (33 of whom had visual dysfunction and were thus judged to have a high risk of dementia) and 30 healthy adults as a control group, whose brains were imaged using MRI scans. In the healthy brain, there is a correlation between how strong the structural (physical) connections between two regions are, and how much those two regions are connected functionally. That coupling is not uniform across the brain, as there is some degree of decoupling in the healthy brain, particularly in areas involved in higher-order processing, which might provide the flexibility to enable abstract reasoning. Too much decoupling appears to be linked to poor outcomes.
The researchers found that people with Parkinson's disease exhibited a higher degree of decoupling across the whole brain. Areas at the back of the brain, and less specialised areas, had the most decoupling in Parkinson's patients. Parkinson's patients with visual dysfunction had more decoupling in some, but not all brain regions, particularly in memory-related regions in the temporal lobe.