The Important Connection

By Erica K. Salamida 

Few people realize that Down syndrome and Alzheimer’s disease share an important biological connection. Individuals with Down syndrome face a significantly elevated risk of developing Alzheimer’s disease, with research indicating a lifetime risk exceeding 90%. While Alzheimer’s typically affects people later in life, individuals with Down syndrome often experience Alzheimer’s-related changes in the brain decades before symptoms appear. In fact, dementia has become a leading cause of death for these individuals.  

Down syndrome, also called trisomy 21, occurs when a person is born with an extra copy of chromosome 21. That extra chromosome is key to understanding the link to Alzheimer’s. Chromosome 21 carries the gene for the amyloid precursor protein (APP). In the brain, APP is processed into smaller pieces, including beta-amyloid. Beta-amyloid can accumulate and form plaques, a hallmark brain change associated with Alzheimer’s disease. Because people with Down syndrome have an extra copy of chromosome 21, they also have an extra copy of the APP gene, which may increase beta-amyloid production and contribute to the development of Alzheimer’s. 

By age 40, most people with Down syndrome have significant levels of beta-amyloid plaques and tau tangles, the two hallmark brain changes associated with Alzheimer’s. However, these brain changes do not necessarily mean someone has dementia. Some individuals remain symptom-free for years, and not everyone with Down syndrome develops Alzheimer’s dementia. 

When Do Symptoms Typically Appear?

People with Down syndrome tend to develop Alzheimer’s dementia much earlier than those without the condition. A large study cited in the Alzheimer’s Association’s 2026 Alzheimer’s Disease Facts and Figures reported an average age of dementia onset of about 54.5 years.  

Early signs may also present differently. Rather than memory loss being the first noticeable symptom, families and caregivers may notice changes in personality, behavior, and everyday functioning. These can include reduced interest in socializing or favorite activities, difficulty concentrating, anxiety or irritability, sleep disturbances, changes in walking or coordination, or seizures that begin in adulthood. 

Because abilities vary among people with Down syndrome, recognizing a change from an individual’s usual functioning is especially important. Experts recommend documenting a baseline of adult intellectual, behavioral, and social abilities by age 35 to help identify changes more readily later. However, not every change indicates Alzheimer’s. Thyroid problems, depression, vision or hearing problems, sleep apnea, and other conditions can affect thinking and behavior and should be evaluated. 

The link between Down syndrome and Alzheimer’s is also helping researchers better understand Alzheimer’s. Why do some people develop brain changes yet remain free of dementia symptoms? What might protect them? Answering these questions could ultimately benefit everyone at risk for Alzheimer’s disease. 

Erica K. Salamida is the Director of Community Engagement for the Coalition of New York State Alzheimer’s Association Chapters. To learn more about Down syndrome and Alzheimer’s disease or to find support and resources, visit alz.org or call the Alzheimer’s Association 24/7 Helpline at 800-272-3900.