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Aug. 15, 2008 -- New research may help explain why more boys than girls have asthma in childhood, yet more adult women than men are afflicted with asthma.
More than 9 million American children have asthma, with a 30% higher prevalence among boys than girls. This trend switches in adulthood, when women with asthma outnumber men by 40%, according to the CDC.
The newly published study was among the first to examine airway responsiveness over time in children with asthma, and researchers found key differences in boys and girls beginning around the time of puberty.
This suggests a possible role for sex hormones in mediating asthma severity, says lead author Kelan G. Tantisira, MD, MPH, of Boston's Brigham and Women's Hospital and Harvard Medical School.
"This study is the first to show that the changes that occur between genders happen right around the time of puberty," he tells WebMD. "The speculation was certainly there, but it had never before been demonstrated."
The study included 1,041 children with mild to moderate persistent asthma enrolled in a larger, ongoing asthma treatment study.
All the children were between the ages of 5 and 12 at enrollment, and all underwent yearly testing to measure airway responsiveness, which is a marker for asthma severity.
This was done using a diagnostic test designed to measure response when patients are challenged with the inhaled airway-constricting agent methacholine.
The study participants were followed for an average of 8.6 years, during which time they had eight to nine annual methacholine challenges.
The amount of methacholine that it took for airway constriction did not change much over time in girls. But boys became increasingly tolerant to the challenges, with larger and larger doses of the bronchoconstrictor needed to provoke a reaction.
By the age of 16, boys needed, on average, more than twice as much methacholine as girls needed to cause a 20% airway constriction.
By age 18, almost twice as many boys as girls (27% vs. 14%) failed to respond to the challenge, indicating that they no longer had asthma.
The study appears in the August issue of the American Journal of Respiratory and Critical Care Medicine.
In an accompanying editorial, childhood asthma researcher Jorrit Gerritsen, MD, PhD, writes that although sex hormones like estrogen, progesterone, and testosterone may explain the gender differences in asthma progression, other factors may also be at play.
Another possible mechanism: Studies show that from around age 16 females have smaller and narrower airways in proportion to lung volume than males of the same age. This might also explain why many boys with asthma become increasingly asymptomatic as they get older.
Gerritsen concludes that by continuing to follow the children in the study into adulthood, Tantisira and colleagues could further the understanding of the natural course of asthma in males and females.
Those insights could lead to better treatments for asthma patients, American Thoracic Society past-president John Heffner, MD, tells WebMD.
"The next step is to try to understand the factors that may be diminishing asthma around puberty to see if we might leverage those factors into better treatments," he says.