Onset and progression factors in Parkinson's disease: A systematic review

- Parkinson's disease (PD) is a neurodegenerative disorder marked by tremor, muscular rigidity, and slow, imprecise movement mainly affecting individuals over age 65. It is characterized by the death of dopaminergic neurons in the midbrain, which results in dopamine deficiency and impaired motor and non-motor functions.
- PD is the second most common neurodegenerative disorder in the world after Alzheimer's disease with a prevalence of 1 per 300 people in Canada with incidence increasing with age.
- Pesticide exposure, head injury, rural living, well-water drinking, farming occupation, and family history of PD were identified as risk factors for PD onset.
- Constipation, depression, rapid eye movement (REM) sleep behavior disorder (RBD), erectile dysfunction, hip fractures, and olfaction are recognized as prodromal markers of this disease.
- Interventions aimed at reducing pesticide exposure and head injuries may reduce PD incidence.

Current research has identified several factors thought to be associated with the onset and progression of Parkinson's Disease (PD); however, whether certain factors contribute to or are protective against PD remains unclear. As such, a systematic search of the literature was performed using variations of MeSH and keyword search terms to identify and summarize systematic reviews and primary studies pertaining to factors associated with the onset and progression of PD. Factors referred to both traditional risk factors and prodromal markers. The following databases were searched: MEDLINE, MEDLINE In-Process, EMBASE, PsycINFO, Scopus, Web of Science, Cochrane Database of Systematic Reviews, Cumulative Index to Nursing and Allied Health Literature (CINAHL), ProQuest Dissertations & Theses, AARP AgeLine, and PDGene. A quality assessment of included systematic reviews was completed using the validated Assessment of the Methodological Quality of Systematic Reviews (AMSTAR) tool. Data extraction targeted reported factors, risk estimates, and 95% confidence intervals (CI). Findings identified 11 systematic reviews of sufficient quality reporting factors for PD onset, and no systematic reviews reporting factors for PD progression. In addition, 93 primary articles were identified, of which, 89 articles addressed factors related to PD onset and 4 articles addressed factors related to the PD progression. Pesticide exposure, rural living, well-water drinking, and farming occupation were consistently found to be positively associated with the onset of PD. Moreover, family history and polymorphisms to key genes were also found to be positively associated with the onset of PD. Conversely, coffee consumption, cigarette smoking, and some polymorphisms were consistently found to be negatively associated with the onset of PD. Urate was the only identified factor linked to the progression of PD; it was mostly found to be negatively associated with PD. In sum, the evidence was systematically found and summarized in the literature pertaining to factors related to the onset and progression of PD.