As they age, those affected by Down syndrome have a greatly increased risk of developing a type of dementia that's either the same as or very similar to Alzheimer's disease.
About Down syndrome and Alzheimer's
Down syndrome is a condition in which a person is born with extra genetic material from chromosome 21, one of the 23 human chromosomes. Most people with Down syndrome have a full extra copy of chromosome 21, and so they have three copies instead of the usual two. In ways that scientists don't yet understand, the extra copies of genes present in Down syndrome cause developmental problems and health issues. Scientists think that the increased risk of dementia in individuals with Down syndrome may also result from the extra gene.
As with all adults, advancing age also increases the chances a person with Down syndrome will develop Alzheimer's disease. Because people with Down syndrome live, on average, 55 to 60 years, they are more likely to develop younger-onset Alzheimer’s (Alzheimer’s occurring before age 65) than older-onset Alzheimer’s (Alzheimer’s occurring at age 65 or older). According to the Down Syndrome Society, about 30 percent of people with Down syndrome who are in their 50s have Alzheimer’s. Fifty percent or more of people with Down syndrome will develop Alzheimer’s as they age.
Autopsy studies show that by age 40, the brains of almost all individuals with Down syndrome have significant levels of plaques and tangles, abnormal protein deposits considered Alzheimer's hallmarks. But despite the presence of these brain changes, not everyone with the syndrome develops Alzheimer's symptoms.
One of the many questions researchers hope to answer about Down syndrome is why some people develop dementia symptoms and others don't. Researchers are working to answer a similar key question about those who don't have Down syndrome: Why do some people with levels of brain changes characteristic of Alzheimer's never show symptoms of the disease?
In people with Down syndrome, changes in overall function, personality and behavior may be more common early signs of Alzheimer's than memory loss and forgetfulness.
Early symptoms may include:
- Reduced interest in being sociable, conversing or expressing thoughts.
- Decreased enthusiasm for usual activities.
- Decline in ability to pay attention.
- Sadness, fearfulness or anxiety.
- Irritability, uncooperativeness or aggression.
- Restlessness or sleep disturbances.
- Seizures that begin in adulthood.
- Changes in coordination and walking.
- Increased noisiness or excitability.
Memory loss also may occur. Researchers don't yet know why early Alzheimer's symptom patterns may tend to differ among those with and without Down syndrome.
Diagnosing dementia in a person with Down syndrome can be difficult because of the challenges involved in assessing thinking-skill changes in those with intellectual disabilities. Experts recommend the following:
- Document baseline adult function by age 35. By age 35, each individual's medical record should ideally include detailed information on his or her adult abilities, and intellectual, social and behavioral function.
- Watch for changes in day-to-day function. Reduced enthusiasm for daily activities, loss of interest in social interactions, and changes in personality and behavior are often early signs of an underlying decline in thinking skills.
- Consider professional assessment by a dementia expert. A variety of cognitive tests have been used to evaluate thinking changes in adults with Down syndrome. Experts caution that cognitive tests should never be used as the only benchmark to diagnose dementia in a person with Down syndrome.
- Rule out other causes of symptoms. Thyroid problems, depression, chronic ear and sinus infections, and sleep apnea can also cause changes in a person's thinking and functioning.
Causes and risks
Scientists think that the increased risk of dementia — like other health issues associated with Down syndrome — results from the extra genes present. One of the chromosome 21 genes of greatest interest in the Down syndrome-Alzheimer's connection codes amyloid precursor protein (APP).
Scientists don't yet know APP's function, but they've learned that day-to-day brain activity involves continuous "processing" of APP into shorter pieces. One of the brain's APP processing pathways produces beta-amyloid, a fragment that's the chief component of plaques and a prime suspect in Alzheimer's-related brain changes
. Having an extra copy of the APP gene may increase production of beta-amyloid, triggering the chain of biological events leading to Alzheimer's.
The APP gene is also linked to Alzheimer's through small variations in the gene's chemical code that cause rare, inherited forms of Alzheimer's. The fact that APP is strongly implicated in Alzheimer's through two different mechanisms — one involving a whole extra copy of the normal gene and the other involving specific minor coding changes — makes the intersection of Down syndrome and Alzheimer's a strong focus of research.
Treatment and outcomes
The U.S. Food and Drug Administration (FDA) has not approved any drugs specifically to treat dementia associated with Down syndrome. In the United Kingdom (U.K.), cholinesterase inhibitors, a class of drugs approved in the United States and many other countries to treat Alzheimer's, are approved to treat dementia in people with Down syndrome. However, international experts evaluating treatment effectiveness found that there isn't enough evidence to reach a conclusive judgment about the benefit of cholinesterase inhibitors for people with Down syndrome.
An international clinical study has shown no benefit for the Alzheimer's drug memantine in adults with Down syndrome. Experts urge more research and clinical studies to identify effective treatments for dementia in those with Down syndrome.
Down syndrome remains a condition that shortens lifespan. People with Down syndrome experience earlier-than-usual onset of a variety of conditions linked to aging in addition to Alzheimer's disease. People with Down syndrome currently live, on average, about 55 to 60 years, although some live into their seventies and, rarely, into their eighties.