Attention-deficit/hyperactivity disorder (ADHD) appears to be somehow linked to risk of dementia and Alzheimer's disease, a new multigenerational study has found.

Parents and grandparents of people with ADHD have a higher risk of Alzheimer's and dementia than people with no ADHD in their family, Swedish researchers said.

Specifically, parents of a child with ADHD have a 34% higher risk of dementia and 55% higher risk of Alzheimer's, the results showed. Grandparents have about an 11% increased risk of either condition.

"ADHD is associated with dementia across generations," said lead researcher Le Zhang, a doctoral candidate with the Karolinska Institute's department of medical epidemiology and biostatistics, in Stockholm. "Our study calls attention to advancing the understanding of ADHD and cognitive decline in older age."

However, it's unclear what might tie the two conditions together, the researchers said.

The largest genetic studies on ADHD and dementia to date "have failed to detect any genetic variant in common," Zhang explained.

But she noted that "there have been studies suggesting that certain genes may be implicated in both ADHD and dementia."

Another possibility is that outside influences on health might increase the risk of both diseases within a family, such as financial distress, obesity or substance use, Zhang added.

For example, the researchers said that ADHD in children and adults has been associated with excess weight, and at the same time middle-aged obesity has been tied to increased risk of dementia later in life.

Heather Snyder, vice president of medical and scientific relations at the Alzheimer's Association, 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. But it is interesting all the same."

For the study, Zhang and her colleagues analyzed data on more than 2 million people born in Sweden between 1980 and 2001. About 3% were diagnosed with ADHD, which is characterized by inattention, impulsivity and hyperactivity.

Using national registries, the research team linked the ADHD patients to more than 5 million biological relatives — parents, grandparents, aunts and uncles. They then checked to see whether these relatives had developed dementia or Alzheimer's disease.

Parents did have a significant increased risk of dementia, but the researchers noted that the risk decreased with the distance of family relation. Grandparents had a lower risk than parents, and aunts and uncles even less.

And even though parents of kids with ADHD had a significantly increased risk of dementia, their absolute risk of the degenerative brain condition remained low, the study authors said. Overall, fewer than 0.2% of the parents identified in the study actually wound up diagnosed with dementia.

"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," Snyder said.

The new study was published online Sept. 9 in Alzheimer's & Dementia: The Journal of the Alzheimer's Association.