Overall characteristics of participants of the selected studies
| Article | Design | Country | Number of PD patients | Number of healthy patients (control) | Age of PD patients (years) | Age of healthy patients (years) | Disease stage (Hoehn & Yahr) [20] | Disease severity (MDS UPDRS-III) [19] | Disease duration (years) |
|---|---|---|---|---|---|---|---|---|---|
| Baram et al. [21], 2020 | RCT | Denmark | 29 | - | 32–72 | - | 2.9 ± 0.4 | 20.6 ± 4.9 | 11.7 ± 5.0 |
| Baram et al. [22], 2023 | CS | Denmark | 6,874 | 34,285 | 71 (58–82) | 71 (58–82) | - | - | - |
| Barbe et al. [23], 2017 | CS | Germany | 100 | - | 71 ± 8.7 | - | - | 17.5 ± 8.6 | - |
| Barbe et al. [24], 2017 | CS | Germany | 26 | 26 | 69 ± 9.0 | 69 ± 9.0 | - | 13.0 ± 9.0 | 9.0 ± 4.0 |
| Barbe et al. [25], 2017 | CS | Germany | 30 | 30 | 69.3 ± 8.0 | 69.3 ± 7.9 | - | 12.0 ± 8.8 | - |
| Fleury et al. [26], 2021 | CS | Switzerland | 20 | 20 | 62.8 | 64.3 | 2 | 15 | 4.7 |
| García-De-La-Fuente et al. [27], 2022 | CS | Spain | 104 | 106 | 66.2 ± 9.2 | 59.2 ± 14.1 | - | - | - |
| García-de-la-Fuente et al. [28], 2023 | CS | Spain | 104 | 106 | 66.2 ± 9.2 | 66.2 ± 9.2 | - | - | - |
| John et al. [29], 2021 | CS | India | 32 | 42 | 58.4 ± 10.6 | 66.2 ± 9.2 | - | - | - |
| Lyra et al. [30], 2020 | CS | Portugal | 28 | - | 72.3 ± 8.1 | - | 2.7 | - | - |
| Pradeep et al. [31], 2015 | CS | India | 45 | 46 | 54.5 ± 9.1 | 63.9 ± 13.1 | - | - | - |
| Ribeiro et al. [32], 2016 | CS | Brazil | 17 | 20 | 69.41 ± 4.65 | 72.00 ± 5.69 | - | - | 6.76 ± 3.80 |
| Ribeiro et al. [33], 2017 | CS | Brazil | 17 | 17 | 69.4 ± 4.7 | 70.7 ± 4.7 | - | - | 6.76 ± 3.80 |
| Rozas et al. [34], 2021 | CS | USA | 30 | 30 | 69.2 ± 9.4 | 69.1 ± 8.4 | - | - | - |
| van Stiphout et al. [35], 2018 | CS | Netherlands | 74 | 74 | 70.2 ± 8.8 | 67.9 ± 10.1 | 2.4 | - | - |
| Verhoeff et al. [36], 2022 | CS | Netherlands | 341 | 411 | 65.5 ± 8.4 | 62.6 ± 5.3 | - | 11.5 ± 7.5 | 7.0 ± 5.5 |
-: unknown; CS: cross sectional; RCT: randomized clinical trial; MDS-UPDRS-III: Movement Disorders Society-Unified Parkinson Disease Rating scale-part III; PD: Parkinson’s disease. Age, disease stage/severity/duration are expressed in years as mean ± SD (standard deviation)