ADHD a New Risk Factor for Alzheimer's?

There is a link between attention deficit hyperactivity disorder (ADHD) and risk for Alzheimer's disease (AD) and other dementia types, 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, Sweden, said in a statement.

"There have been few studies previously on the link between ADHD and dementia, all with limited sample size," Chang told Medscape Medical News.

"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 September 9 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 [HR] 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 say.

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," 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 told Medscape Medical News.

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