Association of Chronic Pain with Motor Symptom Severity in Parkinson’s Disease: An Exploratory Cross-Sectional Analysis

Background: Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor symptoms like bradykinesia, tremor, rigidity, and postural instability. Additionally, PD severely impacts physical abilities and independence. Chronic pain, affecting 67.6% of PD patients, varies in form and presentation, and it is often underdiagnosed. Objectives: This study investigated the association between chronic pain and motor symptom severity in PD patients. Methods: This analysis used data from a cross-sectional study on 52 Parkinson’s disease (PD) patients conducted at Jena University Hospital, Germany. The dataset, available on Dryad, included demographics; clinical reports; and assessments of coping strategies, quality of life, and pain. Descriptive statistics, a bivariate analysis, and an ordinal logistic regression model were executed to explore the association between pain and motor symptom severity (MSS). A direct acyclic graph was used to represent the relationship between variables and identify potential confounders, and an outcomes definition sensitivity analysis was used to assess the impact of using pain intensity as an outcome. The E-value was calculated to evaluate the strength of association needed by an unmeasured confounder to nullify the observed association. Results: A total of 50 Parkinson’s disease (PD) patients were included, with 64% being male, with an average age of 76.1 years. The sample included 20 patients without pain and 30 with chronic pain. The bivariate analysis did not identify significant differences in disease duration, cognitive function, and non-motor symptoms between pain and no-pain groups. However, significant differences (p-value < 0.05) emerged in motor symptom severity, coping strategies, and several SF-36 domains (Physical and Social Functioning, Role Functioning, Energy/Fatigue, Pain, General Health, and Health Change). The ordinal logistic regression showed a substantial association between chronic pain and MSS: patients with chronic pain had 3.52 times higher odds (95% CI: 1.40–8.84, effect size d ≈ 0.70, p = 0.02) of low to medium MSS and 5.44 times higher odds (95% CI: 2.03–14.60, effect size d ≈ 0.94, p = 0.01) of medium to severe MSS, indicating a dose–response relationship. Additionally, male patients had increased odds of higher MSS (OR 4.63, 95% CI: 1.15–18.58, effect size d ≈ 0.85, p = 0.03). Conclusions: Chronic pain is strongly associated with MSS in PD patients, with a more pronounced effect as MSS progresses from medium to severe, supporting a dose–response relationship. Effect sizes suggest a robust association, emphasizing the need for pain assessment in managing motor symptoms in PD.