Study identifies brain process that may help differentiate tremor disorder from Parkinson's

A study has identified a process involving dopamine and serotonin chemicals in the brain that could help distinguish essential tremor in a movement disorder from that experienced as part of Parkinson's disease.

Findings published in the journal Nature Communications suggest that dopamine-producing processes, commonly thought to be impacted in Parkinson's disease, may not be the only ones affected.

Shivering of the hands is the most prominent symptom of essential tremor disorder. However, tremors in limbs, along with problems in maintaining balance, are among the symptoms that affect one's movement in Parkinson's disease, an ageing-related neurological disorder.

Studies have shown that the brain's dopamine-producing processes are affected in Parkinson's disease, resulting in lower levels of the chemical known to be important for feeling motivation and pleasure.

However, the study's findings show that the lack of rise-and-fall in serotonin and dopamine levels "turned out to be the clearest difference between Parkinson's and essential tremor", senior author William Howe, an assistant professor at Virginia Polytechnic Institute and State University's school of neuroscience, USA, said.

Serotonin, known to help regulate one's moods, does not commonly feature in processes thought to contribute towards Parkinson's disease and, therefore, the study could open a new view and potentially powerful clinical insights, researchers said.

The team monitored activity in the 'caudate of the striatum', a brain region that helps in decision-making and processing rewards, as patients with Parkinson's disease and essential tremor played a game involving fair and unfair offers.

How the patients formed and adjusted expectations during the game was analysed using a computational model.

Among the patients with essential tremor, offers made in the game that went against expectations triggered a "seesaw pattern — dopamine levels rose, while serotonin dropped", the researchers said.

In a 2018 study, the team published that levels of dopamine and serotonin simultaneously fluctuate when a human is engaged in decision-making.

However, among the patients of Parkinson's disease, the simultaneous ups-and-downs in dopamine and serotonin amounts were found to be absent.

"It wasn't just that dopamine was disrupted. It was that the normal back-and-forth between dopamine and serotonin was gone," Howe said.

Mismatches between what the patients expected and what they received were found to trigger changes in serotonin levels, which the researchers said were strong indicators of which disease the patient had.

"There's neither the serotonin dip nor the dopamine rise. It's not just one system being disrupted —it's the lack of that dynamic interaction that turned out to be the clearest difference between Parkinson's and essential tremor," the senior author said.

The researchers "show that violations in the expected value of monetary offers are encoded by opponent patterns of dopamine and serotonin release in essential tremor, but not Parkinson's disease, patients".

Linking moment-to-moment changes in one's internal beliefs to measurable chemical levels in the brain is powerful and "opens a new window into how deeply human cognitive processes, like social evaluation, are shaped by disease", said author Dan Bang, an associate professor at the center of functionally integrative neuroscience at Aarhus University, Denmark.