It was already known that in Parkinson’s disease the cells in the brain that produce dopamine slowly disappear. This is why patients are given extra dopamine as medication. But only a limited link has been found between the loss of those cells, and the severity of symptoms in Parkinson’s. Even if all the cells have disappeared, one person experiences mild symptoms, while another has many more symptoms. Researchers at the Radboudumc looked into whether something else might be going on.
They discovered that the outer layer of the brain, the cerebral cortex, can compensate for the loss of the cells that make dopamine, thereby delaying the worsening of symptoms. It turns out that the severity of symptoms is clearly related to compensation by the cerebral cortex. The more active it is in taking over tasks, the milder the slowness of movement and the better the thinking. Doctors have long suspected that such a mechanism of compensation exists, but it has now been demonstrated for the first time.
The conclusions are based on a study in 353 people with Parkinson’s and 60 healthy volunteers. They all took a test while in an MRI scanner. The test was a kind of computer game, where participants had to make both easy and difficult choices, which demands a lot from the brain. The researchers could use the MRI scanner to identify brain regions that were active during the game. They expected that making difficult choices stimulates compensation, and that the deployment of compensation differs between people.
First, they saw that the brain structure that strongly depends on dopamine, the basal ganglia, is indeed damaged and shows much less activity in people with Parkinson’s than in healthy volunteers. The basal ganglia lie deep in the brain, below the cerebral cortex. This brain area allows people to move and think smoothly. Hence, damage in this area leads to slowness of movement and thinking in Parkinson’s.
It was also found that in people with Parkinson’s there is a very clear relationship between the severity of symptoms and activity in the cerebral cortex. PhD candidate Martin Johansson: ‘People with mild symptoms showed much more activity in the cerebral cortex, especially in areas that are involved in controlling movement. These areas were even more active than in healthy volunteers, which demonstrates their compensatory role. In patients with severe symptoms, on the contrary, the cerebral cortex was much less active than in healthy volunteers.’
Martin E Johansson, Ivan Toni, Roy P C Kessels, Bastiaan R Bloem, Rick C Helmich. Clinical severity in Parkinson’s disease is determined by decline in cortical compensation. Brain, 2023; DOI: 10.1093/brain/awad325
Radboud University Medical Center. “Compensation by healthy brain parts alleviates Parkinson symptoms.” ScienceDaily. ScienceDaily, 28 September 2023. <www.sciencedaily.com/releases/2023/09/230928151931.htm>.
Photo by Pawel Czerwinski