Dear Editor,

Socioeconomic status (SES) is associated with various health-related outcomes. Studies in patients with Parkinson’s disease (PD) have demonstrated associations between income level and mortality [1], as well as between the community deprivation index and motor severity [2]. However, the influence of SES on progression-related outcomes in PD remains to be explored.

Given the challenges of collecting comprehensive socioeconomic data in routine clinical practice, we utilized the nationwide claims database of the National Health Insurance Service (NHIS). In this study, disability registration and the use of cognitive-enhancing medications—reflecting significant functional impairment in activities of daily living—were employed as potential indicators for progression-related outcomes in PD. We analyzed the associations of income levels and residential areas with the risks of disability registration and cognitive impairment in patients with PD.

From January to December 2023, we identified newly diagnosed PD patients with the G20 code of the International Classification of Diseases, 10th Revision, Clinical Modification and the V124 code of the rare incurable disease classification in the NHIS database. The detailed study protocol is provided in the Supplementary Material. The NHIS claims data provide standardized NHI premium deciles (1–20) that correlate acceptably with actual income regardless of subscription type (i.e., workplace workers or regional subscribers) [3]. By including the population with medical aid, we classified the income levels of the enrollees into five categories: medical aid (monthly income <40% of the median; the lowest level) and NHI premium quartiles (Q1–Q4). Residential areas were categorized into capital (Seoul), metropolises (Busan, Incheon, Daejeon, Daegu, Gwangju, and Ulsan), other cities, and rural areas. To adjust for potential comorbidities, the Charlson Comorbidity Index was included in the statistical models [1]. The NHIS database contains records of disability registration due to brain lesions. In the present study, cognitive impairment was defined to be present in patients if they were prescribed a cholinesterase inhibitor or memantine. The dataset was analyzed between April and November 2025.

In total, data from 95,093 PD patients were analyzed. Detailed demographics are provided in Supplementary Table 1. During the study period, the medical aid group had fewer total days of supply compared to the NHIS premium groups (Supplementary Figure 1). Among 74,655 individuals not previously registered in the National Disability Registration System, 18,621 (24.9%) were newly registered as disabled. In a conditional logistic regression model using age, sex, and comorbidities as covariates, patients with NHI premiums had a significantly higher risk of disability registration than those receiving medical aid (Table 1 and Supplementary Table 2). Moreover, odds ratios (ORs) for NHI premiums showed an increasing trend in correlation with higher income levels. In addition, compared with PD patients living in Seoul and metropolitan cities, those in nonmetropolitan cities and rural areas had a significantly lower risk of disability registration (referenced by Seoul; other cities, OR 0.865, 95% confidence interval [CI] 0.822–0.911, p<0.001; rural areas, OR 0.726, 95% CI 0.677–0.778, p<0.001). In contrast, a Cox proportional hazards model using the same covariates demonstrated that low income level was associated with a high risk of cognitive impairment. Hazard ratios were comparable across the NHI premium groups. Regarding residential area, PD patients in Seoul had the lowest risk of cognitive impairment (Table 1 and Supplementary Table 3).

Epidemiologic studies have shown an association between SES and cognitive impairment in the general population [4-6]. A decrease in individual income levels is associated with an increased risk of dementia and a reduced duration of dementiafree periods across the lifespan [5]. Although SES factors are not completely independent, preserved community resources contribute to a slow decline in cognitive parameters [6]. High SES is associated not only with healthier lifestyles and superior medication adherence but also with greater volumes in cortical and subcortical structures, including the hippocampus, striatum, amygdala, and thalamus [4]. Our study confirms that the impact of SES factors on the risk of cognitive impairment remains valid in PD patients.

In contrast to previous studies of mortality among PD patients [1], our results indicated that high income and residence in metropolitan areas (i.e., Seoul and other metropolitan cities) were associated with a high risk of disability registration. Our dataset cannot establish a precise causal relationship; however, the benefits of the welfare system may offer meaningful advantages for PD patients who participate in social and occupational activities. In South Korea, disability registration provides tax benefits, employment support, and mobility assistance. In addition, Seoul and other metropolitan cities are better equipped with neurologists, imaging modalities, and other medical infrastructures. Although the risk of PD increases with age, metropolitan areas—despite their relatively younger demographics— report a higher prevalence of PD per 100,000 persons than nonmetropolitan areas [7]. Indeed, our supplementary analysis indicated that neurologist density per 100,000 persons was significantly correlated with PD prevalence (Supplementary Figure 2). These findings underscore that SES disparities, including the availability of healthcare infrastructures, may influence the frequency of both PD diagnosis and disability registration.

In claim data, PD diagnoses can be inaccurate, and NHI premiums represent overall economic capacity rather than individual earnings. Moreover, many elderly individuals utilize healthcare services as NHIS dependents. Despite the potential for misclassification, household-level SES remains a widely accepted and standardized indicator in nationwide database research. In light of these considerations, our results underscore the need for targeted public interventions, such as specialized cognitive screening and social support programs, to mitigate the risk of cognitive impairment in PD patients. Regarding disability registration, our results raise concern regarding a potential mismatch between clinical disability and policy utilization, suggesting that medical resources and welfare benefits might be disproportionately concentrated. Further research is warranted to directly observe the longitudinal effects of SES on PD progression.

Supplementary Materials

Notes

Ethics Statement

The Institutional Review Board of Pusan National University Hospital approved the study protocol and waived the need for informed consent because the study used public data (approval number: 2501-003-146).

Conflicts of Interest

The authors have no financial conflicts of interest.

Funding Statement

This study was supported by grants from the Korea Health Technology R&D Project through the Korean Healthy Industry Development Institute, funded by the Ministry of Health & Welfare, Republic of Korea (RS-2023-00265377) and Glocal University 30 Project at Pusan National University through the Institute for Regional System & Education in Busan Metropolitan City, funded by the Ministry of Education (MOE) and the Busan Metropolitan City, Republic of Korea (2025-glocal-02-004-812-01).

Acknowledgments

None

Author Contributions

Conceptualization: Myung Jun Lee, Jae-Hyeok Lee. Data curation: Dong Wook Kim, Kyoungjune Pak, Minkyeong Kim. Formal analysis: Myung Jun Lee, Dong Wook Kim. Funding acquisition: Myung Jun Lee, Jae-Hyeok Lee. Investigation: Myung Jun Lee, Dong Wook Kim. Methodology: Myung Jun Lee, Jae-Hyeok Lee. Supervision: Kyoungjune Pak, Minkyeong Kim, Jae-Hyeok Lee. Validation: Kyoungjune Pak, Minkyeong Kim, Jae-Hyeok Lee. Writing—original draft: Myung Jun Lee. Writing—review & editing: Dong Wook Kim, Kyoungjune Pak, Minkyeong Kim, Jae-Hyeok Lee.

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