People with intellectual disability (ID) are living longer lives than in generations past, and experience aging-related health issues, like Alzheimer's disease and related dementia (ADRD). Our objective was to document prevalence and age of diagnosis of ADRD in the population with ID (excluding Down syndrome) ≥30 years old in the United States from 2011 to 2022 enrolled in Medicaid and/or Medicare. We measured ADRD and ID using International Classification of Disease-9/10 claims algorithms and examined demographics and trends in ADRD. We examined age at incident ADRD diagnosis. Among 1,046,621 adults with ID ≥30 years old enrolled in Medicaid and/or Medicare between 2011 and 2022, 184,206 (17.6%) had claims for ADRD, corresponding to an 11-year prevalence of one case per 5.7 people. Yearly prevalence was lowest in 2011 (11.3%) and highest in 2019 (16.7%). There were few differences in prevalence for each age group between sexes, races, and ethnic group. Mean age at onset was 62.6 years (13.1 = SD). ADRD is common in people with ID and individuals, clinicians, and families should be prepared as individuals and populations age. As a heterogenous and often idiopathic group of conditions, further research is warranted to understand biologic and social risk factors for ADRD in this population.

BACKGROUND:Women with intellectual and developmental disability (IDD), including those with autism and/or intellectual disability, have historically been sterilised and institutionalised to limit their reproductive rights. As sterilisation laws have been repealed and protections against institutionalisation enacted, stigma persists but may be changing over time. OBJECTIVES:Our objective was to describe fertility rates among women with identified IDD (defined using ICD codes for intellectual disability or autism) using 11 years of data of all women aged 18-45 and enrolled in Medicaid and/or Medicare. METHODS:We identified a cohort of women with IDD enrolled in Medicaid and/or Medicare between January 2011 and December 2022. International Classification of Diseases (ICD) 9 and 10 codes were used to identify pregnancies and IDD diagnoses. We calculated general fertility rate (GFR), age-specific fertility rate (ASFR) and total fertility rate (TFR) and corresponding 95% confidence intervals (CI) for each year. RESULTS:From 2011 to 2022, we observed 50,562 livebirths among 33,457 women. There were 38,983 births to women with intellectual disability, 9360 with autism, and 2219 with autism and co-occurring intellectual disability. There were 1403 pregnancies to women with cerebral palsy and autism or intellectual disability. GFR in women with IDD was relatively stable over the years, from 9.3 per 1000 (95% CI 9.0, 9.6) in 2011 to 9.9 per 1000 (95% CI 9.6, 10.2) in 2022. ASFR for both women with autism-only and women with an intellectual disability-only was highest at 21-25 years. Total fertility rates were slightly higher for those with autism-only (ranging from 0.22 to 0.40 across years) than for women with intellectual disability-only (ranging from 0.25 to 0.27). Data also reveal that fertility rates for women with IDD have increased in most states. CONCLUSION:Trends in fertility rates among women with IDD were consistent over time. Incremental increase in fertility rates were observed among women with autism. Results warrant the need to further examine factors related to access, quality and continuity of care among women with IDD.

Background Many US children have congenital heart defects (CHD), yet data on their long-term outcomes have limitations. The Congenital Heart Survey To Recognize Outcomes, Needs, and well-beinG of KIDS (CHSTRONG KIDS) addresses these gaps by surveying caregivers of children with CHD identified through birth defect surveillance systems [BDSS]. Objectives To describe the CHSTRONG KIDS project design, characteristics of the eligible population, and the percentage not up-to-date on recommended cardiology care. Methods Children born 2006–2021 with CHD were identified using active, population-based BDSS in Atlanta, Georgia, Massachusetts, and Minnesota and linked to vital records. Caregivers of eligible (living) children with CHD were invited to complete surveys in 2024–2025. Characteristics were compared by site and response status using χ2 tests. We also estimated percentages of children in CHSTRONG KIDS who had not seen a cardiologist within the guideline-recommended timeframe for their specific defects. Results Among 7239 identified children with CHD, 6240 were eligible for survey recruitment. Of those,1841 (30%) had caregiver-reported survey data. Several characteristics, including CHD severity, birth year, and Trisomy 21 diagnosis, varied by site (p < 0.05). Survey response rates differed by site, CHD severity, maternal race, maternal education, and rurality (p < 0.05), prompting development of post-stratification weights. A weighted 21.6% were not up-to-date on their cardiology care. Conclusion With data on >7200 CHD cases and >1800 caregiver-reported surveys, CHSTRONG KIDS provides a population-based view of long-term outcomes among children with CHD. Notably, one in five were not up-to-date on their cardiology care and therefore may not be represented in clinical cohorts.

Although individuals with Down syndrome (DS) remain highly vulnerable to severe infections, vaccination remains underutilized. Here we review, specific to people with DS, the safety and efficacy of vaccination, drivers of susceptibility to infection, and existing and emerging opportunities to improve vaccine response. We find that vaccines are generally safe and immunogenic in individuals with DS, although continued research is essential to improve vaccine efficacy and health outcomes.

Our objective was to describe how distance to specialty clinics and availability of primary care providers affects health and service use for adults with Down syndrome in the United States. We used 2019 data from a cohort of Medicaid enrollees ≥18 years with Down syndrome. We identified specialty clinics and mapped distance from individual's zip codes. We created quantiles by distance and primary care provider density. Our cohort of 89,382 adults showed differences in health outcomes and costs by their geographic proximity to clinics and their access to primary care providers. After adjustment, mortality rates in Quartile III of distance to the specialty clinics were 1.16 times (95% CI: 1.00, 1.34) and Quartile IV was 1.27 times (95% CI: 1.08, 1.49) that of Quartile I. Hospitalization rates were quantitatively similar across groups. Ensuring equitable access to both primary and specialized care for adults with Down syndrome remains a significant challenge.

People with Down syndrome have higher age-specific mortality rates compared to the general population as well as peers with other intellectual and developmental disabilities. While a large proportion of mortality is attributable to Alzheimer's disease, many die prior to Alzheimer's diagnosis and some live to old ages, dying without Alzheimer's. Our objectives were to use 11 years of Medicaid and Medicare data to describe characteristics and factors related to death in adults with Down syndrome and use machine learning to identify which conditions most strongly predict death in the full population and stratified by age. We identified death using Center for Medicare and Medicaid Systems reported date of death health conditions using ICD 9 and 10 codes. We used a case-control design with risk set sampling to have that controls to mimic the distribution of times of incident Alzheimer's disease. We trained gradient boosted trees to identify strongest predictors. Our cohort included 137,293 adults with Down syndrome. Among those, 30,894 (22.5%) died during the study period. Mean age at death among those who died was 55 years (SD=10). Mean age of death in those with Alzheimer's disease was 59 (SD=7) and those without was 52 (SD=12). The most influential predictors of mortality were any claim for dementia, any claim for pneumonia, re-occurring claim for cardiovascular disease three years before index death, and any claim for heart failure and epilepsy. Our results align with previous clinical work and highlight intervenable areas to reduce mortality in the Down syndrome population.

Background Home and Community Based Services (HCBS) are Medicaid funded services that support independence, person-centered care, and connection to community for disabled people. With drastic Medicaid cuts on the horizon due to the 2025 Budget Reconciliation Bill, HCBS will likely be impacted. Objective To describe HCBS among adults in Medicaid with intellectual and developmental disabilities (IDD) in 2022. Methods We used Medicaid data from all with claims for autism, intellectual disability, and Down syndrome and identified HCBS use using established algorithms. We described differences by state, IDD type, and race/ethnicity and used multi-level models to account for confounding and state-level differences. Results Of 1,400,630 adults with IDD, 68.3% (N = 957,220) received HCBS in 2022. The most used HCBS types were case management (35.4%), home-based services (34.6%) and non-medical transportation (19.8%). There were limited differences by race and ethnicity in overall HCBS use. Asian Americans were more likely to use home-based services and less likely to use case-management compared to white peers. After adjustment, Autistic individuals without intellectual disability were 18.3 percentage points less likely and autistic people with intellectual disability were 4.6 percentage points more likely to receive any HCBS compared to people with intellectual disability without autism. Conclusion Disabled people have care needs that exist beyond the scope of typical healthcare system. HCBS are widely used optional services that are not mandated to be covered. With pending Medicaid cuts, HCBS loss may be an area where services are lost, which will disproportionately harm the IDD community.

Alzheimer's disease and related dementias (ADRD) are burdensome and lethal conditions that have been hypothesized to be related to autism through shared genetic etiologies and environmental risk factors. Our objective was to use longitudinal Medicaid and Medicare data to describe the epidemiology of ADRD in publicly insured autistic adults. We used all claims and encounters from 2011 to 2019 to identify autism and ADRD. We calculated prevalence, incidence, age at onset, and created survival curves. There were 90,229 autistic adults ≥ 30 years of age and enrolled for at least 1 year in Medicaid and/or Medicare and 267 ADRD cases. Prevalence of ADRD was 2.09% (95% CI: 1.99%, 2.20%) in 2011 and 8.11% (95% CI: 7.92%, 8.30%) in 2019. Mean age at ADRD onset was 59.3 years (SD: 14.2). Mean age among men was 58.3 years (SD: 13.8) and 61.0 years among females. Incidence of ADRD was higher in autistic adults with intellectual disability with no difference by sex. ADRD is a prevalent condition in middle- and older-aged adults identified with autism in the Medicaid and Medicare system. Understanding the diagnostic process and phenotype of ADRD will be important to improve identification and treatment.

Home and Community Based Services (HCBS) are Medicaid funded services that support independence, person-centered care, and connection to community for disabled people enrolled in Medicaid. With drastic Medicaid cuts on the horizon due to the 2025 Budget Reconciliation Bill, HCBS will likely be impacted. Our objective was to describe HCBS use by service type category among all adults enrolled in Medicaid with intellectual and developmental disabilities (IDD) diagnoses in 2022. We evaluated differences by state, IDD type, and race and ethnicity. We used Medicaid data from all adult enrollees with claims for autism, intellectual disability and Down syndrome and identified HCBS use using established algorithms. Of 1,519,852 adults with IDD, 63.1% (N=958,437) received any HCBS in 2022. The most used HCBS types were case management (32.5%), home based services (32.0%) and non-medical transportation (18.2%). There were limited differences by race and ethnicity, even after adjustment by state. With large Medicaid cuts forthcoming HCBS are optional services that are not mandated to be covered and a possible area for cuts that will disproportionately harm the IDD community. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was funded by the National Institute on Aging (R01AG073179) and the Eunice Kennedy Shriver National Institute on Child Health and Human Development (1R01HD106948). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The IRB of Boston University waived ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes The data used in this study are Medicaid claims data obtained from the Centers for Medicare & Medicaid Services (CMS) under a data use agreement. These data are not publicly available due to privacy and legal restrictions. Access to the data may be granted to qualified researchers through application to CMS via the Research Data Assistance Center (ResDAC) and completion of the necessary data use agreements.

Background:Co-research methods have improved the inclusion of people with intellectual and developmental disabilities on research teams. This article presents the evaluation of the DS-TO-THE-MAX Co-Research Program to assess program process and outcomes. The team consists of co-researchers with Down syndrome and academic researchers who collaborate on health research projects important to the Down syndrome community. Methods:We developed a logic model to map program components and outcomes. From the model, we identified and assessed five key areas. We interviewed seven co-researchers with Down syndrome and conducted a focus group with research assistants based on key areas. We analysed using thematic analysis. Findings:We found that mutual recognition of contributions and multiple communication modes facilitated engagement and team connectedness. Co-researchers reported their interests and experiences drove contributions, and they saw benefits from collaborating with each other. Co-researchers noticed accessible team practices and described self-advocating. Though co-researchers did not explicitly identify as researchers, they reported making decisions and feeling heard and valued. They shared barriers such as difficult vocabulary. Conclusions:Our evaluation showed that relationships and co-researchers' experiences and interests were central to research engagement. Co-research team evaluations are feasible mechanisms to improve inclusive collaboration for research team members with intellectual and developmental disabilities. Accessible Abstract:People with intellectual and developmental disabilities are working on research teams as co-researchers to study health.We looked at DS-TO-THE-MAX Co-Research Team to understand the program's goals and the experiences of co-researchers' with Down syndrome.We interviewed seven co-researchers with Down syndrome and did a focus group with research assistants to understand their experiences. We asked questions about the program's organisation and how co-researchers participate and feel included in research projects.Team connections and good communication were helpful for co-researchers, but sometimes difficult vocabulary made it hard to participate. Co-researchers did not see themselves as researchers but said they felt included and participated more when the project matched their interests.It is important to understand how co-research programs work. We can improve programs like ours to help team members with intellectual and developmental disabilities participate.

INTRODUCTION:The Down syndrome-associated Alzheimer's disease (DSAD) autosomal dominant Alzheimer's disease (ADAD) 2024 Conference in Barcelona, convened under an Alzheimer's Association International Society to Advance Alzheimer's Research and Treatment (ISTAART) grant through the Down syndrome and Alzheimer's disease (AD) Professional Interest Area (PIA), brought together global researchers to foster collaboration and knowledge exchange between the fields of DSAD and ADAD. METHODS:This article provides a synthesis review of the conference proceedings, summarizing key discussions on biomarkers, natural history models, clinical trials, and ethical considerations in anti-amyloid therapies. RESULTS:A total of 211 attendees from 16 countries joined the meeting. Global researchers presented on disease mechanisms, therapeutic developments, and patient care strategies. Discussions focused on challenges and opportunities unique to DSAD and ADAD. Experts emphasized the urgent need for tailored clinical trials for ADAD and DSAD and debated the safety and efficacy of anti-amyloid treatments. Ethical considerations highlighted equitable access to therapies and the crucial role of patient and caregiver involvement. DISCUSSION:The conference highlighted the importance of inclusive research and collaboration across the genetic forms of AD. HIGHLIGHTS:Biomarker research and natural history models developed in Down syndrome-associated Alzheimer's disease (DSAD) and autosomal dominant Alzheimer's disease (ADAD) enable the prediction of disease progression not only for DSAD and ADAD, but also for sporadic Alzheimer's disease (AD). -Collaboration and knowledge exchange among researchers across these genetic forms of AD will accelerate our understanding of the pathophysiology and advance preventive trials in DSAD and ADAD. -Tailored clinical trials for DSAD are urgently needed to address specific safety and efficacy concerns. -Inclusive research practices are crucial for advancing treatments and understanding of DSAD and ADAD.

BACKGROUND:At least half of children and adults with Down syndrome have a major mental health concern during their life but few studies ask people with Down syndrome directly about their experience. We used a co-research model to explore anxiety, stress, and coping in adults with Down syndrome. METHODS:Our group of researchers and adults with Down syndrome conducted an online survey on mental health for adults with Down syndrome. We analysed quantitative data and thematically grouped coping mechanisms. RESULTS:Sixty adults with Down syndrome completed the survey, mean age was 30 years, and 55% of respondents had some employment. Approximately 80% of respondents reported experiencing stress and 75% reported experiencing anxiety. Employed respondents were more likely to use social coping mechanisms. CONCLUSION:Soliciting responses from adults with Down syndrome about their mental health can provide valuable insights. Mental health is a concern for people with Down syndrome that should be addressed.

BACKGROUND:For people with intellectual and developmental disabilities, other's perceptions of them based on their condition often begin before birth and go on to impact relationships, opportunities, and self perception across the life course. Search engine results and news media, which may portray these conditions stereotypically or in poor light, are often a key source in these perceptions. Our purpose was to understand how search engine results and available news media can shape perceptions on certain intellectual and developmental disabilities. METHODS:We developed an online Likert-scale survey to measure differences in perceptions based off first available search engine results, images, and news headlines of four intellectual and developmental disabilities: cerebral palsy, Down syndrome, Prader-Willi syndrome, and Angelman syndrome. These four conditions were selected to compare less prevalent (Prader-Willi and Angelman) and more prevalent conditions (Down syndrome and cerebral palsy). Perception questions addressed general impression and aspects of the disability experience expected to be impacted by perception from others. We recruited via multiple social media platforms, flyers posted in the Boston area, and word of mouth to local communities and friends. FINDINGS:229 individuals opened the survey, and 125 responses were used in analysis. Mean responses to Prader-Willi syndrome were significantly more negative than responses to cerebral palsy, Down syndrome, and Angelman syndrome across all variables. Responses to Angelman syndrome were also more negative than responses to Down syndrome. Significant differences between conditions found when treating the data as continuous were confirmed when treating the data as ordinal. CONCLUSION:Lesser-known intellectual and developmental disabilities, such as Prader-Willi syndrome and Angelman syndrome, are subject to more negative portrayal in media, leading to more negative perception, which may impact social opportunity and quality of life. Combined with our finding that the perception of Prader-Willi syndrome follows the ideals of the medical model of disability more closely than the social model, a need for social model of disability training and education for physicians and other medical providers is clear.

Down syndrome is a condition caused by trisomy of chromosome 21 and is the most common genetic cause of intellectual disability. Due to distinct body and facial morphology, people with Down syndrome appear to be at increased risk for obstructive sleep apnea (OSA). Additionally, adults with Down syndrome are at increased risk for Alzheimer dementia at younger ages than the general population, and OSA has been identified as a risk factor for Alzheimer dementia in the general population. This study aims to explore the prevalence of diagnosed OSA, as well as the association between OSA and Alzheimer dementia, in adults with Down syndrome using Medicaid claims data from2011 to 2019. Of 118,539 adults with Down syndrome who met inclusion criteria, 23,785 had at least one OSA claim from2011 to 2019 (20.1%, 95% CI 19.8%-20.3%). After weighting for age, sex, dual enrollment, race, ethnicity, and region, adults with Down syndrome and OSA claims had 1.08 times the hazard of having a claim for Alzheimer dementia compared to those without OSA claims (95% CI 1.05-1.10). OSA is common in adults, and our findings have clinical implications for its evaluation and treatment in those with Down syndrome.

Down syndrome (DS) or trisomy 21 (T21) is present in a significant number of children and adults around the world and is associated with cognitive and medical challenges. Through research, the T21 Research Society (T21RS), established in 2014, unites a worldwide community dedicated to understanding the impact of T21 on biological systems and improving the quality of life of people with DS across the lifespan. T21RS hosts an international conference every two years to support collaboration, dissemination, and information sharing for this goal. In 2022, T21RS hosted an international conference in Long Beach, California, from June 9 to 12. The conference, attended by 483 people including scientists, families, self-advocates, and industry representatives from 17 countries, was a dynamic and interactive meeting that shared discoveries from international research teams. This summary highlights the scientific discoveries shared at the 4th T21RS meeting with the Imagine, Discover, Inspire theme.

INTRODUCTION:As life expectancy improves for people with Down syndrome (DS) in Africa, the risk of developing DS-associated Alzheimer's disease (DSAD) will rise. There is a pressing need to plan for this emerging challenge, particularly in the context of existing health and social disparities. METHODS:This work emerged from a pan-African collaboration, including discussions at the Brain Ageing and Dementia in Low- and Middle-Income Countries conference held in Nairobi in 2024, where stakeholders identified regional priorities for DS and dementia care. RESULTS:Limited epidemiological, cognitive, biomarker data, delayed diagnoses, and gaps in specialized services may impact access to care. However, innovative solutions, such as mobile biomarker sampling and culturally adapted cognitive assessments, offer promising strategies. DISCUSSION:Integrating global advances in DSAD research with Africa's strengths in community-based care offers opportunities. By prioritizing research, capacity building, and health system integration, this work advocates for the inclusion of DS in Africa's dementia strategies. HIGHLIGHTS:Projected increases in life expectancy for individuals with Down syndrome (DS) in Africa will lead to a substantial rise in DS-associated Alzheimer's disease (DSAD), necessitating urgent planning and response. There is a critical lack of epidemiological, cognitive, and biomarker data on adults with DS in Africa, hindering accurate diagnosis, care planning, and inclusion in global research. Innovative, scalable solutions-such as mobile biomarker sampling and culturally adapted cognitive assessments-offer an opportunity to integrate scientific advances with Africa's strengths in community-based care. Investment in research and capacity building is essential to address current gaps, reduce disparities, and ensure equitable access to emerging diagnostics and treatments for DSAD across the continent.

Background Most adults with Down syndrome will develop Alzheimer's disease (AD) due to the triplication of the amyloid precursor protein in the 21(st) chromosome. Predictors of condition onset are less known. Objective We used Medicaid and Medicare data and machine learning to identify which co-occurring conditions predict incident AD in United States adults with Down syndrome. Methods We examined adults with Down syndrome enrolled in Medicaid and/or Medicare between 2011 and 2019. We identified AD and other conditions using ICD 9 and 10 codes. We used a case-control design with risk set sampling to have that controls to mimic the distribution of times of incident AD. We trained gradient boosted trees to identify strongest predictors. Results The cohort had a mean age at entry of 44.6 years, 46.2% were male, and 73.7% were white non-Hispanic. 16,398 had incident AD diagnoses over the study period. The machine learning model had an area under the curve of 0.86 and high positive predictive value. Strongest predictors of increased probability of AD were age, dual Medicaid/Medicare enrollment; incident epilepsy or incident ulcer three years before index date; any hypothyroidism, schizophrenia, or hyperlipidemia. We found synergistic interaction between epilepsy and enrollment by age. Conclusions Predictors aligned with known predictors in the general population and characteristics that signal AD symptom onset. New onset epilepsy may be a relevant clinical sign. Identifying these predictors highlights areas for further etiologic inquiry and intervention.

We examined relationships between measures of adaptive behavior, cognitive ability, and autism symptom severity in 1458 preschool-aged children with autism from the Study to Explore Early Development. While publications commonly describe autistic children as "low-" or "high-functioning" based on cognitive ability, relying solely on cognitive scores may obscure meaningful variation in functioning. We found significant heterogeneity in adaptive behavior scores of children with cognitive scores both above and below the threshold of two or more standard deviations below the population mean specified in the diagnostic criteria for intellectual disability (ID). Although cognitive and adaptive behavior scores were strongly associated in our sample, considerable variation in overall adaptive behavior and more than half in socialization and motor skills was unaccounted for by cognitive ability, autism symptom severity, and other covariates. Among children who could be designated "low-functioning" based on cognitive scores, 39.7% had composite adaptive behavior scores indicating no significant delays, while among those who might be designated "high-functioning," 9.0% had significant delays in overall adaptive behavior and 22.2% in socialization. These results suggest adaptive behavior scores capture variations in the autism phenotype not accounted for by other measures we considered.Lay Abstract Autistic people are often described as "low-" or "high-functioning" based on their scores on cognitive tests. These terms are common in publications and in everyday communication. However, recent research and feedback from the autistic community suggests that relying on cognitive ability alone to describe functioning may miss meaningful differences in the abilities of autistic children and adults and in the kinds of support they may need. Additional methods are needed to describe "functioning" in autistic children. We examined whether scores from a test measuring adaptive behaviors would provide information on the functional abilities of children with autism that is different from cognitive ability and autism symptom severity. Adaptive behaviors include age-appropriate skills that allow people to function in their everyday lives and social interactions. We found that a large amount of the variation in adaptive behavior scores was not explained by cognitive development, autism symptom severity, and behavioral and emotional problems. In addition, there was a wide range of adaptive ability levels in children with autism in our study, including in those with low, average, or high cognitive scores. Our results suggest that adaptive behavior scores could provide useful information about the strengths and support needs of autistic children above and beyond measures of cognitive ability and autism symptom severity. Adaptive behavior scores provide important information on the needs of autistic people.

Importance:With the advancement in administrative data as a research tool and the reliance on public health insurance for individuals with Down syndrome, population-level trends in Alzheimer dementia in this population are beginning to be understood. Objective:To comprehensively describe the epidemiology of Alzheimer dementia in adults with Down syndrome in a full US Medicare and Medicaid sample. Design, Setting, and Participants:This cohort study included 132 720 adults aged 18 years or older with Medicaid and/or Medicare claims data with an International Statistical Classification of Diseases and Related Health Problems code for Down syndrome. Data were collected from January 1, 2011, to December 31, 2019, and analyzed from August 2023 to May 2024. Main Outcomes and Measures:The main outcome was prevalence of Alzheimer dementia in each calendar year and during the 9-year period. Alzheimer dementia incidence rates by calendar year and age and stratified for race or ethnicity as well as time to death after Alzheimer dementia diagnosis were also assessed. Results:There were 132 720 unique adults with Down syndrome from 2011 to 2019: 79 578 (53.2%) were male, 17 090 (11.7%) were non-Hispanic Black, 20 777 (15.7%) were Hispanic, 101 120 (68.8%) were non-Hispanic White, and 47 692 (23.3%) had ever had an Alzheimer dementia diagnosis. Incidence was 22.4 cases per 1000 person-years. The probability of an incident Alzheimer dementia diagnosis over 8 years was 0.63 (95% CI, 0.62-0.64) for those entering the study between ages 55 to 64 years. Mean (SD) age at incident diagnosis was 54.5 (7.4) years and median (IQR) age was 54.6 (9.3) years. Mean (SD) age at death among those with Alzheimer dementia was 59.2 (6.9) years (median [IQR], 59.0 [8.0] years). The mean (SD) age at onset for the Hispanic group was 54.2 (9.2) years, 52.4 (7.8) years for the American Indian or Alaska Native group, and 52.8 (8.2) years for the mixed race groups compared with 55.0 (7.8) years for the White non-Hispanic group. For age at death, there were no differences by sex. The mean (SD) age at death was later for the White non-Hispanic group (59.3 [6.8] years) compared with the Hispanic group (58.5 [7.8] years), Native American group (57.8 [7.1] years), and mixed race group (58.2 [7.0] years). Conclusions and Relevance:In this cohort study of adults with Down syndrome who were enrolled in Medicaid and Medicare, Alzheimer dementia occurred at high rates. Consistency with clinical studies of dementia in Down syndrome supports the use of administrative data in Down syndrome-Alzheimer dementia research.