Eyesight tests can predict which people with Parkinson’s disease will develop cognitive impairment and possible dementia 18 months later, new research suggests.

Scientists say the study, published in Movement Disorders, adds to evidence that vision changes precede the cognitive decline that occurs in many, but not all, people with Parkinson’s.

In another study published on Tuesday in Communications Biology, the same research team found that structural and functional connections of brain regions become decoupled throughout the entire brain in people with the disease.

According to the paper this was particularly the case among people with vision problems.

The two studies indicate how losses and changes to the brain’s wiring underlie the cognitive impairment experienced by many people with Parkinson’s disease.

Lead author Dr Angeliki Zarkali of the Dementia Research Centre, UCL Queen Square Institute of Neurology, said: “We have found that people with Parkinson’s disease who have visual problems are more likely to get dementia, and that appears to be explained by underlying changes to their brain wiring.

“Vision tests might provide us with a window of opportunity to predict Parkinson’s dementia before it begins, which may help us find ways to stop the cognitive decline before it’s too late.”

For the Movement Disorders paper, published earlier this month, 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.

The study also found that those who went on to develop Parkinson’s dementia had losses in the wiring of the brain, including in areas relating to vision and memory.

The Communications Biology 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.

Researchers say 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 scientists 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, the study found.

The research team also found changes to the levels of some neurotransmitters (chemical messengers) in people at risk of cognitive decline, suggesting that receptors for those transmitters may be potential targets for new drug treatments for Parkinson’s dementia.

Dr Zarkali said: “The two papers together help us to understand what’s going on in the brains of people with Parkinson’s who experience cognitive decline, as it appears to be driven by a breakdown in the wiring that connects different brain regions.”

Dr Rimona Weil of Dementia Research Centre, UCL Queen Square Institute of Neurology, senior author of both papers, said: “Our findings could be valuable for clinical trials, by showing that vision tests can help us identify who we should be targeting for trials of new drugs that might be able to slow Parkinson’s.

“And ultimately if effective treatments are found, then these simple tests may help us identify who will benefit from which treatments.”

The researchers were supported by Alzheimer’s Research UK, Wellcome, and the National Institute for Health Research UCLH Biomedical Research Centre.