The Link Between Endometriosis and Asthma

A large-scale study presented at the 2018 American Thoracic Society Conference in San Diego, CA concludes that asthma is more often found in women with endometriosis than those without. The study, lead by Dr. Joe Zein of the Cleveland Clinic, examined over 3 million women, ages 20-40 and found that of the women without endometriosis, 13% have asthma, and in women with endometriosis, 24% also have asthma.¹

A connection between asthma and endometriosis has been investigated on and off for several decades now, but until recently, evidence was mixed. Early studies were small, relied on self reported diagnoses, and had confounding variables. Recently, two large-scale population studies and a few smaller studies report a connection between endometriosis and asthma.^2,3 In addition to the study lead by Dr. Zein, another large-scale study in Taiwanese women found an increased risk of endometriosis in women with asthma.²

A connection between oral contraceptives and asthma

Dr. Joe Zein also recently presented evidence at the 2018 American Thoracic Society Conference that showed a correlation between oral contraceptive use and a higher lifetime risk of asthma during reproductive years. This study looked at over 6 million women (age 20-50) and found asthma in 8.8% of the group not taking hormonal contraceptives. In the group taking oral contraceptives, asthma was found in 14.3% of the women.^4,5 While not a dramatic difference, these values were statistically significant when controlled for race, age, body mass index, and smoking status, and suggest that hormones may influence asthma.

Could there be a shared hormonal link?

Dr. Zein's earlier work also supports evidence of a hormonal component to asthma. His group found that in men and women, higher estrogen levels, and lower testosterone and progesterone levels are associated with patients with serious asthma.⁴

How hormones influence the lungs is not yet understood, but receptors for progesterone are present in our respiratory tract and control how quickly we can clear mucus from our lungs.

Researchers have also noted an age correlation with women and asthma. Before puberty, boys and girls have a similar rate of asthma, but after puberty, asthma is more commonly seen in the women and is more severe. After the age of menopause, asthma is again more severe in the male population.⁴ This gender and age dependent switching of the prevalence and severity of asthma symptoms that is correlated to the times when women experience hormonal changes suggest a causal relationship between hormone levels and asthma and warrant more investigation.

To treat endometriosis, hormone therapy is a commonly prescribed as it works to suppress growth of the endometrial tissue. Either a combination estrogen and progesterone treatment is prescribed, or a progesterone-only treatment is used to treat endometriosis.

Could there be a shared inflammatory link?

The connection between endometriosis and asthma is not yet understood, but scientists are investigating multiple ideas.

While asthma has long been understood to be an inflammatory disease and molecular pathways leading to asthma are fairly well understood, evidence for a role of the immune system in endometriosis is relatively new. Several recent studies have found higher levels of inflammatory cytokines in women with endometriosis.^2,6 These same cytokines cause endometriotic cells to divide, and this could explain the growth of the endometrial tissue. At least five inflammatory cytokines that are increased in endometriosis patients are the same cytokines involved in mediating asthma inflammation, supporting the theory of an immune connection.²

More research is needed to understand the role of immune system dysfunction in endometriosis before we can know if there is an immune connection between asthma and endometriosis, and what that connection might be.

How might future treatments impact both endometriosis and asthma?

We are only beginning to understand that endometriosis and asthma are connected. Future research is clearly needed to determine the nature of the connection. This research could more solidly establish a role for a shared immune system dysfunction, or it could yield clear evidence that elucidates a common role of sex hormones in both diseases. Alternatively, further research could suggest a more complex relationship where the two diseases are connected by both immune and hormonal components.

If future research shows strong evidence for cytokines playing a role in causing asthma and endometriosis, it is possible that some of the shared cytokine molecules could be targeted by future therapeutic treatments to treat both endometriosis and asthma.

In the event that future evidence favors the hormonal connection, it is difficult to say if a shared treatment could target both diseases, given the current evidence that oral contraceptives suppress endometriosis and are associated with higher rates of asthma.