(Reuters Health) - There may be a strong connection between asthma and attention deficit hyperactivity disorder (ADHD) that makes people with one condition more likely to develop the other one, a recent study suggests.

Researchers examined data from 49 smaller studies that included a total of 210,363 people with ADHD and more than 3.1 million individuals without ADHD. Almost 17 percent of people with ADHD had asthma, compared with 11.5 percent of those without ADHD, the analysis found. And, 8.8 percent of people with asthma had ADHD, compared with 5.6 percent of people without asthma.

The study team also did a separate analysis of almost 1.6 million individuals in the Swedish population, including 259,253 with asthma and 57,957 with ADHD. In this analysis 24.8 percent of people with ADHD had asthma, compared with 16.1 percent of people without ADHD. And, 5.5 percent of people with asthma had ADHD, compared with 3.3 percent of people without asthma.

Overall, the two analyses found that having either asthma or ADHD increased the risk of having the other condition by about 45 percent to 53 percent.

“So now we know for sure that patients with asthma have higher risk of ADHD and vice versa,” said lead study author Dr. Samuele Cortese of the University of Southampton in the UK.

What we don’t know is why, Cortese said by email. Neither analysis examined data from controlled experiments designed to prove whether or how one condition might cause the other, or a third factor might cause both.

“It is possible that there are common inflammatory alterations that make the brain of some children more prone to develop both ADHD and asthma,” Cortese said. “It is also possible that sleep alterations, such as sleep disordered breathing, increase the likelihood of having both asthma and ADHD. We need to test these and other possible explanations.”

Asthma is the most common chronic respiratory disease, affecting an estimated 358 million people worldwide, including up to 5 percent of residents in low-income countries and more than 20 percent of the population in high-income countries, researchers note in The Lancet Psychiatry.

ADHD, meanwhile, affects about 5 percent of school-age children and 2.5 percent of adults worldwide.

The connection between these two disorders persisted even after researchers accounted for other factors that can influence the risk of developing asthma and ADHD such as mothers’ age at birth, parents’ income and education levels, babies’ gestational age and weight at birth, and childhood eczema.

Based on the results, it makes sense for clinicians treating asthma patients to assess people who struggle with focus and concentration for ADHD, the study authors conclude. It also makes sense for clinicians treating ADHD patients to test for asthma in people who complain about chronic wheezing, coughing or shortness of breath.

It’s possible that symptoms of one disease might make people more prone to developing the other condition, said Jessica Agnew-Blais of King’s College London, author of an accompanying editorial.

“Individuals with asthma may experience nighttime coughing, wheezing and breathlessness that lead to poor sleep, which can result in ADHD symptoms,” Agnew-Blais said by email. “However, it is possible that psychosocial stress related to ADHD symptoms may lead to dysregulation of immune function, which could trigger or exacerbate asthma symptoms.”

Another possibility is not that one disorder leads to the other per se, but that something else leads to both asthma and ADHD, she said.

“Both asthma and ADHD have large genetic components, and it may be that people who are more genetically susceptible to asthma are also more genetically susceptible to ADHD,” Agnew-Blais added.

SOURCE: bit.ly/2OESyYQ and bit.ly/2L3l58a The Lancet Psychiatry, online July 24, 2018.