Co-occurrence patterns and related risk factors of dementia and Parkinson’s disease among older adults across 204 countries and territories: a spatial correspondence and systematic analysis

BACKGROUND: Dementia and Parkinson's disease (PD) are among the most prevalent neurological disorders globally. Most previous research has focused on these two diseases in isolation; however, their co-occurrence has rarely been examined, limiting understanding of shared mechanisms and hindering integrated prevention and resource planning. This study aimed to characterize global co-occurrence patterns of dementia and PD from a spatial perspective and to identify the corresponding risk factors underlying this co-burden. METHODS: We extracted incidence rates of dementia and PD and exposure levels of 58 detailed risk factors among individuals aged ≥ 55 years from the Global Burden of Disease Study 2021 for 204 countries and territories. According to the quartiles of global incidence rates for both diseases, countries were categorized into three co-occurrence regions: consistent, dementia-dominant, and PD-dominant. Machine learning and negative binomial regression were used to screen and quantify key risk factors. A composite risk index was then constructed to assess the combined effects of these factors on global burden. RESULTS: Eighty-two countries were classified as consistent, 65 as dementia-dominant, and 57 as PD-dominant. The spatial distribution of these three regions overlapped substantially with exposure to health-related, dietary, and behavioral risk factors. Five risk factors were identified: high low-density lipoprotein (LDL) cholesterol, alcohol use, and smoking as common factors; kidney dysfunction specific to dementia; and diet high in sugar-sweetened beverages specific to PD. High LDL cholesterol exerted the strongest effect on both dementia (RR = 1.12, 95% CI 1.07-1.17) and PD (RR = 1.16, 95% CI 1.09-1.20). The composite risk index showed a right-skewed distribution, with the highest values concentrated in Europe (e.g., Germany, Czech Republic, Slovakia) and the USA, and the lowest in East Africa (e.g., Uganda, Zambia). This pattern closely paralleled the spatial distribution of incidence rates for dementia and PD. CONCLUSIONS: The co-burden of dementia and PD represents a growing global health concern. Geographic disparities are driven by both shared and disease-specific modifiable risk factors. Strengthening collaborative prevention strategies and optimizing resource allocation targeting these factors may effectively reduce the worldwide burden of both conditions.