More evidence needed to support link between ADHD, Parkinson’s

But attention deficit/hyperactivity may be a risk factor, per new review

Margarida Maia, PhD avatar

by Margarida Maia, PhD |

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While it appears that people with attention deficit hyperactivity disorder (ADHD) are more likely to develop Parkinson’s disease later in life than are individuals without the neurodevelopmental disorder, further proof is needed to establish a link, according to a new systematic review.

“Our review provides preliminary results that a diagnosis of ADHD may be a risk factor for the later development of a neurodegenerative disease or dementia,” the researchers wrote.

However, “the mechanism of how or why ADHD is associated with an increased risk of developing a neurocognitive disorder is still unclear and should be explored in future studies,” the team wrote.

The review, “Risk of neurodegenerative disease or dementia in adults with attention-deficit/hyperactivity disorder: a systematic review,” was published in Frontiers in Psychiatry.

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Study specifically focused on adult ADHD

Usually diagnosed in childhood, but with symptoms often lasting into adulthood, ADHD often makes it difficult for affected individuals to pay attention, stay organized, follow through on tasks, sit still, and control impulses. In children, ADHD symptoms may have a negative effect on schoolwork and homework. Adults with ADHD typically experience symptoms of inattention and/or hyperactivity-impulsivity.

What exactly causes ADHD isn’t clear, but it may be a combination of genetics and how the brain is structured and uses neurotransmitters, the chemical messengers that allow nerve cells (neurons) to communicate.

There’s some scattered evidence that having ADHD increases the risk of developing Parkinson’s and other neurodegenerative diseases or dementia later in life.

Now, a team of Canadian researchers aimed to get a clearer picture of the topic. To that end, they searched five databases for studies on ADHD and Parkinson’s, Alzheimer’s, mild cognitive impairment, and all-cause, Lewy body, or vascular dementia.

“This is the first systematic review to examine the relationship between adult ADHD and future development of a neurodegenerative disease or dementia,” the researchers wrote.

A total of seven studies were identified. Five were cohort studies, meaning they analyzed data collected from a group of individuals with a common characteristic and track their outcomes over time. Two were conducted in Sweden, two in the U.S., and one in Taiwan.

The other two studies were case-control studies: one from Taiwan and the other from Argentina. In case-control studies, researchers retrospectively assess people’s past exposures to potential risk factors to determine associations.

This is the first systematic review to examine the relationship between adult ADHD and future development of a neurodegenerative disease or dementia.

One study involved more than 190,000 people, with a mean age of 28.2, from the Utah Population Database. Of the 152 people who went on to develop Parkinson’s, 56 had received a diagnosis of ADHD.  Another retrospective study, which included data from nearly 5 million people in Sweden, with a median age of 47, also reported on Parkinson’s.

Overall, compared with controls, people with ADHD had a 1.5 to 2.6 times higher risk of developing Parkinson’s disease.

In a case-control study from Taiwan, which included 10,726 people with Parkinson’s and an equal number of individuals without the disease, the odds of ADHD occurring in those with Parkinson’s was 3.65 times higher than in individuals without Parkinson’s. Further, patients with Parkinson’s were 2.8 times more likely to have a prior ADHD diagnosis compared with those without a prior history of ADHD.

In general, all seven studies also suggested a link between ADHD and neurodegenerative diseases or dementia. However, the number of studies was too small to allow researchers to combine their data in a meta-analysis. That statistical technique is used to combine and analyze the results of multiple individual studies, this way increasing the statistical power and reliability of the findings.

“No meta-analysis of data was possible, and we were unable to determine the pooled risk for developing a neurodegenerative disease or dementia in people with ADHD,” the researchers wrote.

Because “the current literature on risk of developing a neurodegenerative disease in ADHD is limited,” there’s a “need for more stringent and well-defined studies,” they added.