**5. Parkinson's Disease**

Compared with AD and MD, any correlation between PD and periodontitis has, traditionally, been less e ffectively understood. Nevertheless, a few studies have noted an increased prevalence of periodontal disease among individuals with PD relative to age-matched controls [69,70]. PD causes motor disturbance (due to tremor, rigidity, akinesia, and involuntary movement), apathy, and cognitive impairment, all of which appear to make it di fficult for patients to maintain daily oral hygiene. Therefore, periodontitis can be considered as a consequence of the poor oral hygiene related to clinical symptoms of PD. Recent epidemiological studies have investigated whether periodontitis increases the risk for developing PD. Chen et al. (2017) conducted a population-based retrospective matched-cohort study and reported that individuals with newly diagnosed periodontitis (*n* = 5396) had an increased risk of subsequent PD compared to individuals without periodontitis (*n* = 10,792), regardless of sex, age, comorbidities, and urbanization levels (HR 1.431, 95% CI 1.141–1.794) [71]. The authors also examined the e ffect of periodontal treatment on developing PD. Their population-based nested case-control study demonstrated that among individuals without periodontitis aged 40–69 (*n* = 5552), dental scaling over five consecutive years showed a protective e ffect against PD development, relative to individuals who did not undergo dental scaling (OR 0.204, 95% CI 0.047–0.886) [72]. Moreover, among individuals with periodontitis aged ≥70 (*n* = 3377), the discontinued scaling (i.e., not five consecutive years) or no treatment were significant risk factors for developing PD [35]. These findings sugges<sup>t</sup> that early and consecutive dental scaling could prevent the development of PD. Although these seminal epidemiological studies imply a feasible causal link of periodontitis to PD and a preventive e ffect of periodontal treatment on PD development, experimental studies for verifying these concepts are lacking at the present time. Therefore, additional epidemiological studies and experimental studies along this line are required.
