Using a large research database with a sample size of over 20 million female patients aged 12-50, we were able to document an association between endometriosis and asthma in the United States patient cohort. For all of our age-stratified cohorts, there was a significant increase in the likelihood for endometriosis in patients with an asthma diagnosis after matching for age, race, ethnicity, female infertility, inflammatory diseases of the female pelvis, and leiomyoma of the uterus.

Our findings support the association documented by Peng et al. in Taiwan as similar findings were seen for women in the United States [2]. Peng et al. reported a 1.5-fold increase in endometriosis for patients with asthma, whereas we found an over3-fold increase in risk across all cohorts. Possible explanations for this difference are differing environmental factors in the United States compared to Taiwan or a difference in the genetics of the population that places American women at an increased risk for both conditions. Nevertheless, a 3-fold increase in the risk of endometriosis in 12–50-year-old women with asthma is a considerable finding, much larger than this observation arising by chance alone.

A study of the CDC National Health and Nutrition Examination Survey (NHANES) database was recently conducted by Pan et al [1]. This study also documented the association of endometriosis and asthma in the United States. However, the sample size of the NHANES study was only 5,556 and only included data from 1996-2006. Our study corroborates these findings on a much larger scale and also contains patient data as recent as the day of analysis. The implications for these findings in American women can be directed to the burden the combination of these two conditions can have on quality of life. On its own, asthma, especially when uncontrolled, takes a toll on patients’ lives with the need for numerous pharmacologic interventions and even hospitalizations to control the condition, while always having an existing risk of life-threatening asthma attacks [8]. The anxiety arising from this chronic state of disease uncertainty has been linked with even worse ratings of quality of life by patients [9]. Endometriosis is also not without its own burden on quality of life. Patients with endometriosis have been found to have 0.809 quality-adjusted life years per women due to the toll of endometriosis symptoms with the decrease in quality of life being a predictor of the healthcare costs associated with the condition.¹⁰ Therefore, the association between asthma and endometriosis may indicate a common factor in the pathophysiology of these disorders, warranting further research into what this commonality could be and even into the quality of life for patients faced with the burden of both of these  diagnoses.

The large sample size of our cohort is the main strength of our study, yielding large statistical power. However, it is not without its limitations. One limitation of the study is the fact the database is based on ICD-10 coding, thus relying on provider coding practices to reflect the actual diagnoses of these conditions. Another potential limitation of the study is that patients with asthma may have increased interaction with the healthcare system, increasing their likelihood of seeking out care for endometriosis-related symptoms.