results from a large, multigenerational study show.
“The findings suggest there are common genetic and/or environmental contributions to the association between ADHD and dementia,” study investigator Zheng Chang, PhD, from the department of medical epidemiology and biostatistics at Karolinska Institute, Stockholm, said in a statement.
“There have been few studies previously on the link between ADHD and dementia, all with limited sample size,” Dr. Chang said in an interview.
“This is the first study to look at ADHD and dementia within extended families. It’s a large population-based study including over 2 million individuals and their over 5 million biological relatives,” he noted.
The study was published online Sept. 9, 2021, in the journal Alzheimer’s & Dementia.
Shared familial risk
The researchers identified roughly 2.1 million people born in Sweden between 1980 and 2001. Overall, 3.2% of the cohort had a diagnosis of ADHD.
Using national registries, they linked these individuals to more than 5 million of their biological relatives including parents, grandparents, uncles, and aunts and determined which of these relatives developed dementia over time.
In adjusted analyses, parents of individuals with ADHD had 34% higher risk for any dementia than parents of those without ADHD (hazard ratio, 1.34; 95% CI, 1.11-1.63).
The risk for AD, the most common type of dementia, was 55% higher in parents of individuals with ADHD (HR, 1.55; 95% CI, 1.26-1.89).
Individuals with ADHD were more likely to have parents with early-onset dementia rather than late-onset dementia. However, the absolute risk for dementia was low for the parent cohort: Only 0.17% of the parents were diagnosed with dementia during follow-up.
The association between ADHD and dementia was not as strong for second-degree relatives of individuals with ADHD. For example, grandparents of individuals with ADHD had a 10% increased risk for dementia, compared with grandparents of individuals without ADHD.
The finding of attenuated associations with decreasing genetic relatedness (parents > grandparents and uncles/aunts), points to shared familial risk between ADHD and AD, the researchers said.
There could be “undiscovered genetic variants that contribute to either traits or family-wide environmental risk factors, such as socioeconomic status, that may have an impact on the association,” Dr. Chang said in the news release.
“There are no direct clinical implications from this study, but research like this could lead to further research with goals for improved detection, prevention, and treatment,” he said in an interview.
More questions than answers
Heather Snyder, PhD, vice president of medical and scientific relations for the Alzheimer’s Association that the way different brain diseases are linked “is a question the Alzheimer’s Association is often asked, and it is a part of our funding portfolio to get that question answered.”
This study looking at ADHD and dementia is “intriguing,” Dr. Snyder said, “because, right now, there is limited information available. That said, this is an association study; it shows that two things are somehow connected. Because of how the study was conducted, it does not – and cannot – prove causation,” Dr. Snyder said. “But it is interesting all the same. More research is needed to uncover specifically why and how these two diseases are related. That might eventually give us insight into how to manage risk or even improve treatment.”
The study was supported by grants from the Swedish Council for Health, Working Life and Welfare, the Swedish Research Council, the Swedish Brain Foundation, the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie, the Fredrik & Ingrid Thurings Stiftelse, and the Karolinska Institutet Research Foundation. Dr. Chang and Dr. Snyder disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.