Asthma Prevalence Differs By Gender

If your child has been diagnosed with asthma, he or she is not alone. As the most common chronic illness in children today, asthma affects an estimated 9 million children in the United States under the age of 18. It is well known that boys are more likely than girls to be affected by the illness, but that those roles reverse by adulthood. And now scientists may be closer to understanding why this change occurs.

Researchers from Brigham and Women's Hospital and Harvard Medical School followed 1,041 children between the ages of 5 and 12 with mild to moderate asthma. Each year, the children underwent tests using methacholine, an airway-constricting agent that can trigger asthma. They observed almost no change in the girls' reactivity over the years, but boys seemed to become increasingly tolerant over time. By age 16, it took more than twice as much methacholine on average to provoke a 20 percent constriction in the boys' airway as with the girls'. By age 18, only 14 percent of the girls showed a significant degree of airway responsiveness (AR), compared to 27 percent of boys. "Persistence of airway responsiveness, commonly referred to as twitchy airways, is a major reason why asthma in girls may persist (or develop) past the onset of puberty," lead researcher Dr. Kelan G. Tantisira told Reuters Health.

The team is still not sure why twitchy airways persist in girls, but speculate it may be related to hormonal factors. They explain that progesterone, which increases in girls during puberty, has been shown to cause airways to be less likely to open up, whereas testosterone, which is increased in boys during puberty, has been linked to smooth muscle relaxation in animal research. "While our results were not unexpected, they do point to intriguing potential mechanisms to explain the gender differences in asthma incidence and severity. Especially intriguing is that the differences in gender begin at the time of transition into early puberty," Dr. Tantisira said.

In an editorial accompanying the study, Dr. Jorrit Gerritsen of Beatrix Children's Hospital in Gronengen, the Netherlands, called the results a "fascinating finding" and recommended that Dr. Tantisira and colleagues follow their cohort "for as long as possible since these individuals offer a unique opportunity that may provide insight in the natural course of asthma both in male and female subjects." And that is exactly what Dr. Tantisira intends to do. With 12 years of data, they are now looking into investigating the characteristics of those who attained clinically "normal" AR during follow-up. "Most of the original cohort has now reached adulthood," he said. "We are now able to perform a secondary analysis with an emphasis on those who have reached clinical 'normalcy.'"

Dr. Tantisira said that while "it is possible in the future that therapy may be developed that would be directed toward the mechanistic basis for persistence of airway responsiveness," these findings can assist clinicians now by helping them do a better job of managing asthma in adolescent patients. He suggests that for boys who report few or no episodes, physicians could try halting maintenance medication altogether, whereas "a more gradual approach" could be used for girls. "Obviously, much more work still needs to be done," he said.

The study was published in the second issue for August of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.