Previously diagnosed asthma is a risk factor for developing obstructive sleep apnea, according to a study published in the Journal of the American Medical Association (JAMA).

Previous studies suggested some relationship between the two respiratory conditions, but researchers weren’t sure which was starting the vicious interplay between the two.

The new study examined the prospective relationship between pre-existing asthma and new obstructive sleep apnea (OSA) in the Wisconsin Sleep Cohort Study, which consists of randomly selected Wisconsin state employees that, at the time of enrollment in 1988, ranged between 30 and 60 years of age and were free of OSA. Every 4 years, they took part in an overnight sleep study and completed health-related questionnaires.

The primary finding from the study: participants with pre-existing asthma were 40% more likely to develop OSA than those without pre-existing asthma.

“We didn’t know up to this point that asthma may lead to obstructive sleep apnea, but what we did know was that once established, OSA tends to worsen asthma control, so we’re looking at this vicious cycle of relationships between OSA and asthma,” says Mihaela Teodorescu,  associate professor of medicine (pulmonary, critical care, and sleep medicine) at the University of Wisconsin School of Medicine and Public Health and lead author of the study, in a release. “Finding out that asthma promotes the development of OSA gives us some understanding that we really need to periodically screen patients with asthma for OSA because they may develop OSA, which in turn can worsen asthma control.”

According to the paper’s senior author Paul Peppard, associate professor of population health sciences and principal investigator for the Wisconsin Sleep Cohort Study, sleep apnea is associated with increased risk for cardiovascular disease, stroke, cognitive deficits, depression, lower overall quality of life, and even increased mortality rate, so identifying risk factors for sleep apnea, such as asthma, is an important area of investigation.

“We really need to pursue mechanistic studies to find out how we can prevent or at least mitigate this risk of developing OSA in patients with asthma,” Teodorescu says. “As is always the case with research, our study opens up more questions than answers.”

The study also found there is a dose dependent relationship with asthma duration in adults—the longer they had asthma, the more risk they had of developing OSA. Asthma often starts early in life, but participants in this study were adults, so a new, long-term study seeking to understand the development of asthma in children and its relationship to OSA as participants age could be beneficial.

This study was funded by two of the National Institutes of Health, including the National Heart, Lung and Blood Institute and the National Institute on Aging, and with additional resources from the William S. Middleton Memorial Veterans Hospital in Madison. The sleep studies were performed at the Clinical Research Unit at UW Hospital in Madison.