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Reward processing disrupted in Parkinson’s disease, tied to dopamine therapy, psychiatric disorders

Journal of Neurology, Neurosurgery and Psychiatry
Reuters Health - 30/12/2021 - Reward processing impairment in Parkinson's disease (PD) differs depending on the type of reward and the patient's dopamine medication state (on or off), and may underlie neuropsychiatric disorders such as PD-related depression, a systematic review and meta-analysis suggests.

"How our brains process positive events ('rewards') is believed to be a key process driving several psychiatric disorders and mediated by dopamine," Dr. Harry Costello of University College London, UK, told Reuters Health by email. "Our analysis aimed to establish whether reward processing was impaired in Parkinson's and how this related to psychiatric symptoms and medication."

"Two findings surprised me," he said. "Firstly, how few studies of this kind had looked at specific psychiatric syndromes such as depression in Parkinson's, and secondly, how the effect of dopamine medications seemed to be dependent on what kind of reward processing was being tested."

"For example," he said, "patients off their dopamine medication were impaired in using expectations about rewards to make choices, but were relatively spared in terms of learning about rewards."

As reported in the Journal of Neurology, Neurosurgery and Psychiatry, Dr. Costello and colleagues searched the literature through November 5, 2020 for studies reporting reward processing task performance by PD patients versus controls.

Fifty-five studies involving 1,638 PD patients and 940 controls were included. The studies assessed three subcomponent categories of reward processing tasks: option valuation (12 studies), reinforcement learning (37) and reward response vigor (6).

Across studies, patients with PD on medication had a small-to-medium impairment versus controls (standardized mean difference, 0.34), with greater impairments seen off dopaminergic medication in within-subjects designs (SMD, 0.43).

Within-subjects subcomponent analyses showed impaired processing off medication on option valuation (SMD, 0.57) and reward response vigor (SMD, 0.36) tasks.

By contrast, relative to controls, reinforcement learning was impaired on medication (SMD, 0.45) but not off medication (SMD, 0.28).

Impulse control disorder (ICD) was the only neuropsychiatric syndrome with sufficient studies for a meta-analysis; however, no significant impairment was identified compared to non-ICD patients (SMD, −0.02).

Study limitations included a lack of data in a majority of studies on participants' psychiatric medication use or PD medications and treatment regimens, and inclusion of case-control studies that can't show a causal relationship among reward processing impairment, PD, and associated neuropsychiatric syndromes over the long term.

Dr. Costello said, "The take-home message is that PD is not just a movement disorder, but has clear effects on cognitive processing from early on in the disease. We know this has a big impact on patients' quality of life, and that Parkinson's medication can have both positive and negative impacts."

"This suggests that dopamine's role in reward processing could be a key mechanism and treatment target for psychiatric symptoms in Parkinson's in the future, but this requires further study," he concluded.

His next step, he said, is to investigate the role of dopamine and reward processing in Parkinson's depression and whether it differs from depression in the general population.

SOURCE: https://bit.ly/3qA9pR5 Journal of Neurology, Neurosurgery and Psychiatry, online December 20, 2021.

By Marilynn Larkin

© 2023 The Author(s). Published by Medicom Medical Publishers.
User license: Creative Commons Attribution – NonCommercial (CC BY-NC 4.0)

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