Association between multimorbidity and young-onset dementia: a prospective study

Multimorbidity contributes to an increased risk of late-onset dementia, but the multimorbidity effect on young-onset dementia (YOD, defined as dementia diagnosed before age 65) remains unclear. We aimed to explore the associations between multimorbidity (including its burden, pattern, and trajectory) and YOD.

We utilized the China Health and Retirement Longitudinal Study in China. Multimorbidity burden, patterns and trajectories were assessed based on 12 chronic diseases from 2011 to 2015. Patterns of multimorbidity were analysed by hierarchical clustering in 2015. Group-based trajectory modeling (GBTM) was utilized to identify the trajectories of the number of newly developed chronic conditions from 2011 to 2015. YOD and probable YOD were assessed by self-reported physician diagnosis of YOD and cognitive assessments from 2015 to 2020. Cox regression was used to estimate associations between multimorbidity and YOD.

Among 4,874 participants in 2015, 47.21% had multimorbidity; overall, 189 individuals developed YOD. The number of chronic conditions was associated with incident YOD (hazard ratio (HR) = 1.42, 95% confidence interval (95% CI):1.32-1.53). Three distinct multimorbidity patterns were found to be associated with YOD: the cardiometabolic cluster (HR = 3.57, 95% CI: 2.40-5.31), the gastric-arthritis cluster (HR = 2.43, 95% CI: 1.66-3.54), and the mixed cluster (HR = 4.05, 95% CI: 2.76-5.94). Multimorbidity trajectories were classified as "no new condition", "slow growth", and "rapid growth" by the GBTM. Compared to the "no new condition" group, both the "slow growth" (HR = 1.42, 95% CI: 1.04-1.94) and "rapid growth" group (HR = 2.67, 95% CI: 1.76-4.04) were associated with YOD.

The multimorbidity burden, patterns, and growth trajectories were associated with the risk of YOD. The study highlights the importance of addressing multimorbidity in reducing YOD risk and improving public health outcomes.