Epidemiology of Dementia in Down Syndrome and Other Developmental Disabilities

Background and Purpose: We have seen a profound increase in lifespan for individuals with Down syndrome (DS) in the past few decades leading to a large and somewhat understudied population of middle- and older aged adults with DS. Individuals with DS are at higher risk for earlier onset of dementia given the trisomy of chromosome 21 where the amyloid precursor protein (APP) gene is located. Beta-amyloid (produced by APP proteins) is hypothesized to increase risk for Alzheimer’s dementia. While neuropsychological testing and imaging data confirms the increased risk of dementia in convenience samples of adults with DS, little epidemiological work has clarified the extent of this public health crises. Our objective was to describe prevalence and incidence of dementia in DS and compare results to other intellectual and developmental disabilities (IDD) in which we can now assess later life outcomes.

Methods: We assessed Medicaid claims for adults (≥ 21 years) who had two claims for IDD on two different days ever during their Medicaid enrollment. Participants were included if they had ICD-9 or ICD-10 codes for autism spectrum disorder (ASD; N=5860), intellectual disability (ID; N=13482), or DS (N=2698). We analyzed claims from 2008-2018. Dementia claims were extracted from codes from Center for Medicaid Studies Chronic Conditions Data Warehouse and used previously validated procedures and that  recommend a minimum of 3 years enrollment to ensure diagnostic validity. We categorized our case groups into exclusive categories (DS, ASD without ID, ASD with ID, and ID not DS) and created three age groups to account for differences in age distribution between case groups (age at first year enrolled of <40, 40-54, or ≥55).  We used a 1-year washout period to assess incident dementia claims and created Kaplan Meier curves that accounted for administrative censoring.

Results: In beneficiaries with DS, 20.5% had a claim of dementia over the ten-year follow-up. By age category, 50.0% of beneficiaries with DS ≥55 years had a dementia claim, more than twice the prevalence of other IDD groups.  Probability of an incident dementia claim was 0.4 over ten years enrollment for the 40-54 DS age group and 0.6 for the ≥55 DS age group. We found differences between prevalent dementia claims across age groups comparing ASD without and ASD with ID (post-hoc logistic regression adjusting for age; odds ratio 0.53, 95% CI 0.4, 0.7). Incidence did not differ between the ASD, ASD+ID, and ID groups in the older two age groups based on log-rank tests having P values >0.05.

Conclusions and Implications: Our findings from a state-wide health service system illustrate that dementia is highly prevalent in community-dwelling adults with DS, confirming past clinical and convenience samples. Adults with ASD+ID were at higher risk compared to adults with ASD without ID, highlighting areas for future analyses. High prevalence of dementia in adult Medicaid beneficiaries with DS suggests the need to develop specific services and supports for adults with DS and dementia, particularly as adults with DS age and reside in Medicaid-funded assisted living and/or skilled nursing facilities.