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Endometriosis is the presence of functioning endometrial glands and stroma outside the uterine cavity, most often in the pelvic peritoneal cavity. Women with endometriosis commonly have dysmenorrhea, dyspareunia, pain, menorrhagia and/or metrorrhagia; disease complications can include adhesions, chronic pain, and infertility. This exploratory case-control study investigated the relationship between lifetime occupational history and surgically confirmed endometriosis in a population-based sample.
Interviews were conducted with cases, all reproductive-aged female enrollees of a large health-maintenance organization first diagnosed with surgically confirmed endometriosis between April 1, 1996 and March 31, 2001 and randomly selected controls from the reproductive-aged female enrollee list from the same time period. Each reported job was coded using US Census Occupations and Industries codes, and jobs were classed into categories. Having ever worked an occupation in a given job class was compared to never having done so using unconditional logistic regression.
Having ever worked as a flight attendant, service station attendant, or health worker, particularly as a nurse or health aide, was associated with increased risk of endometriosis (flight attendant: OR 9.80, 95% CI 1.08 - 89.02; service station attendant: OR 5.77, 95% CI 1.03 -32.43; health worker: OR 1.49, 95% CI 1.03 - 2.15). Income and education did not make a difference in the odds ratio estimates for the occupations examined.
This exploratory study suggests that having ever worked as a flight attendant, service station attendant, or health worker, particularly as a nurse, may be associated with an increased risk of endometriosis.
Endometriosis, which affects 5-10% of US women of reproductive age, is the presence of functioning endometrial glands and stroma outside the uterine cavity, most often in the pelvic peritoneal cavity (1). Symptomatic disease commonly features dysmenorrhea, dyspareunia, pain, menorrhagia and/or metrorrhagia; complications can include adhesions, chronic pain, and infertility. Endometriosis is associated with increased overall cancer risk (2), with particular elevation in risk of ovarian cancer (3-5).
Endometriosis has been studied as a potential outcome of occupational exposure to dioxins and included as a secondary outcome in general studies of reduced fertility among workers in certain industries and occupations. These studies have found increased risk of endometriosis-associated infertility among workers exposed to formaldehyde (6), video display terminals (7), chemical dusts (7), or organic solvents (6, 7); and among workers in agricultural industries and occupations, in particular farmworkers (8). To our knowledge there has been no examination to date of the full spectrum of occupational risk of endometriotic disease. In the United States, 72% of women between the ages of 25 and 64 work in the civilian labor force, and the majority of these work full-time, year-round (9). If occupational risks for endometriosis do exist, such exposures will affect many women. This analysis explores the relationship between occupation and endometriosis in a large population-based case-control study.
As described elsewhere (10), all female enrollees of Group Health Cooperative (GH), a large health-maintenance organization (HMO) in Washington State, USA, between 18 and 49 years of age and first diagnosed with surgically confirmed endometriosis between April 1, 1996 and March 31, 2001 were invited to participate as cases (n = 467.) Potential cases’ reference dates were the month and year of the first GH visit for symptoms ultimately leading to the diagnosis of endometriosis. Potential controls (n = 1016) were randomly selected from a list of women enrolled in GH during the same time period, frequency-matched on five-year age group, and assigned reference dates corresponding to the distribution of case reference dates.
A total of 341 (73.0%) cases and 742 (73.0%) controls participated in the study. A face-to-face or telephone interview using a structured questionnaire collected information concerning exposures prior to the reference date, as well as demographics, prior medical conditions, reproductive history, and other characteristics. Case in-patient and out-patient medical records were reviewed and abstracted for type and severity of symptoms (e.g. dysmenorrhea, menorrhagia), and endometriosis characteristics (e.g. dimensions, presence of adhesions). Case definitions were further refined and cases whose disease did not meet the Holt-Weiss standards for definite or probable disease were excluded from this analysis (11). Briefly, under these criteria, ovarian endometriomas, pelvic endometriotic lesions over 5 mm deep, and pelvic endometriotic lesions with adhesions not attributable to other causes are classed as “definite disease”, and other endometriotic implants with at least one major endometriosis symptom (infertility, moderate to severe dysmenorrhea, moderate to severe dyspareunia, or moderate to severe pelvic pain) are classed as “probable disease”; thus, cases without surgical evidence of disease and asymptomatic cases with superficial or ambiguous pelvic lesions were excluded (n=12). Additionally, three cases of extrapelvic scar endometriosis were excluded.
Cases (n=13) and controls (n=14) who reported a previous surgically confirmed endometriosis diagnosis were excluded from this analysis as well. After all exclusions, 313 cases and 727 controls remained for this analysis. This study was approved by the institutional review boards of Fred Hutchinson Cancer Research Center and GH. Each participant gave written informed consent to participate in the study, and was compensated $20 for her time.
All participants were asked about each paid job, part-time or full-time, held for six months or longer between 18 years of age and the reference date. Information collected included the job title, type of business or industry, average hours worked per week, and years worked at the jobs. Jobs were coded using 1980 Census - Occupations and Industries codes (12, 13). These codes were grouped into 56 categories by the method of Schnitzer et al. (14). To improve statistical power, multiple Schnitzer categories were merged to form two broader classes, “wood-and paper-related jobs” and “health workers”. The Schnitzer categories “forestry and logging workers”, “sawmill workers”, “paper workers” and “carpenters, woodworkers”, which respectively had 7, 4, 3, and 6 participants, were combined into a new category “wood- and paper-related jobs” (n=19). [Numbers do not sum because a respondent could work more than one job class during her history.] A new category, “health workers”, consisted of the Schnitzer categories “dentists and dental technicians” (n=13, all of whom were dental technicians), “physicians and surgeons” (n=3), and “nurses, health technicians” (n=149.). Within the “health worker” category, subcategories were created for 1) physicians/surgeons (job code 084), 2) registered nurses (RN)/licensed practical nurses (LPN) (job codes 095 and 207), 3) radiologic technician (job code 206), 4) nursing/health aides (job codes 446 and 447) and 5) all other health workers. The Schnitzer category “aircraft operators”, which included all occupation codes 226 and 465), was renamed “flight attendants” after examination of the individual jobs revealed that all of the respondents in this category listed their jobs as flight attendants, occupation code 465. These changes reflect the adaptation of the Schnitzer tool to an exclusively female population of workers, rather than the mixed-gender group used by Schnitzer et al, or the all-male population of the Canadian system from which the instrument arose.
We used unconditional logistic regression to calculate odds ratios and 95% confidence intervals (CIs) for the association between endometriosis and occupation using STATA 8.2 (College Station, TX). Those who had ever worked in an occupation in a Schnitzer category were compared with all those who had never worked in that category. Using a low-stringency standard of α<0.15, occupational groups that were significantly associated with endometriosis were examined more closely, and divided or further aggregated to obtain reasonably-sized and meaningful categories for further analysis.
Potential confounding factors considered in the creation of logistic regression models were race/ethnicity, household income, education, marital status, gravidity, parity and use of alcohol, cigarettes and oral contraceptives. Each potential confounder, treated categorically, was evaluated individually in a model containing each re-aggregated occupational group and the frequency-matching variables (age and reference year), to determine which, if any, changed the estimate of the association between the occupation and endometriosis by 10% or more. Those confounders were then included in a multivariate model to determine a final, conservatively adjusted estimate of association.
As women who seek treatment for infertility may have endometriosis discovered incidentally as part of the diagnostic process, rather than because of symptomatic disease, we also considered separately cases who reported seeking care only for reasons other than infertility (n=260). Presentation of endometriosis was determined by case participants’ answers to the question, “On or before (the reference date), what was the reason you first visited a doctor or other health care practitioner that led to your recent diagnosis of endometriosis?” and their sexual intercourse histories. Those who listed infertility as a reason (n=36) or the only reason (n=17) for their initial visit were classed as having infertility-associated endometriosis. Premenopausal women who reported 12 or more consecutive months of unprotected sexual intercourse with a male partner without a pregnancy immediately before their reference date and who had no other reason for their initial visit were also classed as having infertility-associated endometriosis.
Demographic characteristics are shown in Table 1. Cases and controls were similarly distributed in terms of age, race/ethnicity, marital status, education, income, and use of oral contraceptives. Cases had fewer pregnancies and fewer births than controls. Cases were more likely than controls to use tobacco or alcohol at the reference date.
Cases and controls were equally likely ever to have worked outside the home (305 (97.4%) cases, 710 (97.7%) controls, odds ratio (OR) 1.01, 95% confidence interval (CI) 0.41 - 2.48, after adjustment for reference age and reference year.)
The distribution of Schnitzer occupational categories among cases and controls is reported in Table 2. Under the non-stringent criterion of α<=0.15, the categories “biological scientists”, “teachers, librarians”, “aircraft operators”, and “service station attendants” were significantly associated with endometriosis case status after adjustment for matching variables age and reference year. These categories were further examined in multivariate models, as were the categories that were reconstructed to add meaning and power (Table 3). Potential confounders that made a 10% or greater difference in any ever/never risk estimate were alcohol use, oral contraceptive use, gravidity and nulliparity. All ever/never models were thus adjusted for these variables as well as age and reference year. In these multivariate models, health workers, service station attendants, and flight attendants were at increased endometriosis risk, (respectively, OR 1.49, 95% CI 1.03 - 2.15; OR 5.77, 95% CI 1.03 - 32.43; OR 9.80, 95% CI 1.08 - 89.02). Teachers/librarians were at nonsignificantly decreased endometriosis risk (OR 0.74, 95% CI 0.51 - 1.05), while working in a wood/paper-related job or a biological scientist was associated with a nonsignificantly increased risk of disease (respectively, OR 2.16, 95% CI 0.83 - 5.66; OR 1.57, 95% CI 0.73 - 3.36). Working at an occupation for five years or longer did not appear to strengthen negative or positive associations with that occupation.
In analyses of subcategories of health care workers (Table 4), non-significant elevations in disease risk were seen among physicians, RN/LPN, and nursing aides/health aides (respectively, OR 7.80, 95% CI 0.62 - 97.83; OR 1.72, 95% CI 0.93 - 3.19; OR 1.57, 95% CI 0.99 - 2.47).
The associations between selected occupations and case status by presentation are shown in Table 5. In general, associations between occupation and case status were more pronounced among those presenting for reasons other than infertility than among all cases. Those whose primary disease manifestation was not infertility were more likely to have worked as service station attendants (OR 5.98, 1.07 - 33.45), flight attendants (OR 10.18, 95% CI 1.12 - 92.14), or health workers (OR 1.53, 95% CI 1.06 - 2.22), including RN/LPN (OR 1.80, 95% CI 0.97 - 3.34), and nursing/health aide (OR 1.60, 95% CI 1.00 - 2.55).
The present exploratory study suggests that having ever worked as a flight attendant, service station attendant, or health worker, particularly as a nurse, may be associated with increased endometriosis risk. There is some indication that having worked as a teacher or librarian is associated with a decreased risk, although the association was not statistically significant. Some potential exposures found among one or more of the vulnerable jobs but uncommon in school or library settings include radiation (health workers and flight attendants), fuel and exhaust components (service station and flight attendants), night work (all), and circadian/sleep disruption (all). Another study of this population did suggest that endometriosis may be related to night and evening shift work (10), but small numbers prevented examination of risk by job title in that analysis.
Measuring exposures is preferable to using job titles in occupational health research (15), but since little is known concerning occupation and endometriosis, and the present study did not allow the measurement of a spectrum of exposures, using job categories provided an alternative exporatory approach to generate initial occupational hypotheses. Occupations include a broad array of exposures, and may also capture some aspects of socioeconomic standing. In this study, income and education did not affect the odds ratio estimates for the occupations examined, suggesting that physical exposures are more likely than sociodemography to be responsible for associations seen. We also found that occupational duration of five years or longer did not appear to strengthen the associations within the occupational classes studied. The absence of a duration dose response might suggest that a shared chemical exposure is not responsible for the association, but larger studies are required to address this hypothesis more definitively. We did not find that women with endometriosis were less likely to work than women without the disease, but if women with severe symptoms were less likely to belong to the HMO under study (perhaps they were not working at all) associations between disease risk and occupations would be attenuated.
The most commonly studied compounds likely to be occupationally related to endometriosis are dioxins and other organochlorine compounds. Associations between endometriosis and these substances have been controversial, with most studies involving simultaneous measurement of body burden and disease status (16-18), or measurements after industrial disasters (19), rather than routine occupational exposures. Previous occupational studies of infertility in relation to certain industries and occupations have evaluated endometriosis as a secondary outcome. Taskinen et al found substantially elevated endometriosis risks among woodworkers exposed to formaldehyde and organic solvents (formaldehyde OR 4.5, 95% CI 1.0 - 20.0; organic solvents OR 14.7, 95% CI 3.1 - 70.0), but the presence of endometriosis was ascertained from self-report in that study, and temporal relationships between exposure and outcome could not be determined (6). The suggestive but non-significant association we found between endometriosis and wood/paper-related jobs may support these findings.
In contrast to our results, Fuortes et al. found strong associations between endometriosis and working in agricultural industries (OR 6.9, 95% CI 1.6 - 30.4) and occupations (OR 8.1, 95% CI 1.3 - 52.5) when comparing women with medically-confirmed infertility caused by endometriosis to postpartum women (8). The present work compared women with surgically confirmed endometriosis (with and without infertility) to a general population of female HMO enrollees. The different control populations affect the interpretation of the odds ratios - one reflects the relative risk of exposure among those who are infertile because of endometriosis versus those who are fertile, and the other reflects exposure among those with and without endometriosis irrespective of fertility. One possible explanation of the difference in findings is that endometriosis might be equally likely to occur among women who did and did not work in agricultural occupations, but more likely to be diagnosed as the cause of infertility, or to cause infertility, among those who had agricultural occupations.
This is the first systematic survey of occupational risk of endometriosis. This study has a number of important strengths. The study design allowed nearly complete case ascertainment. Instead of the potentially biased samples obtained by recruiting patients from gynecology or infertility clinics, with this design the full spectrum of disease diagnosed within a large health maintenance organization is represented. For the current analysis, we excluded cases without surgical confirmation, in contrast to previous occupational studies that relied on self-report. We also excluded all those who reported prior surgically confirmed endometriosis to capture the first diagnosis and look at exposures in the correct timeframe. Disease features and course were evaluated by experts directly from medical records rather than relying on self-report, and we used a well-defined set of criteria to evaluate the certainty of diagnosis, enhancing the reproducibility of our findings. We did rely on self-reported occupational histories. If inaccurate, these reports could produce a biased risk estimate, particularly if cases and controls recalled occupations differently, but such histories have been found to have high validity and reliability (20), and the interview covered many topics, so occupation was not likely to be seen as a special exposure under study and recalled in more detail by either group.
As well, investigators seeking to evaluate the effects of occupational exposures on reproductive system health may wish to take into consideration the infertile worker effect, whereby women raising families are likely to spend less time in the workforce than their counterparts so that paid employment may be falsely associated with reproductive disease risk (21). To address this issue, we adjusted risk estimates for gravidity and nulliparity, and evaluated non-infertile cases separately. These adjustments did not substantively affect the general relationships between ever being employed in a particular job class and endometriosis. The infertile worker effect may have a more substantial impact on the relationship between reproductive disease and overall employment or duration of employment, rather than ever employment in a particular type of job.
In this study, we used the 1980 Census occupational codes. While the codes are dated and somewhat arbitrarily numerically arranged, we chose to use them because they formed the basis for the Schnitzer aggregation scheme, which overcomes the irregularities of grouping by, for instance, classing all health care professionals together. This cannot overcome the limitation of aggregating disparate occupations into single codes, such as the inclusion in code 095 of registered nurses, nurses in advanced practice occupations requiring graduate and postgraduate education (nurse midwife and nurse anesthetist), and exclusion of administrative nursing positions, nor the absence of codes for new but well-populated professions, such as web designer, network engineer, and help desk, and industries such as information technology and biotechnology. These codes, however, are broadly used in studies of population-level occupational data (22) and the Schnitzer categorization was explicitly developed for use with this version of the codes in studies of workplace exposures.
This exploratory study suggests that having ever worked as a flight attendant, service station attendant, or health worker, particularly as a nurse, may be associated with an increased risk of endometriosis. Examination of endometriosis risk in existing occupational cohorts may be a beneficial next step to better understanding the role of occupational exposures in the etiology of endometriosis.
Thanks to the participants of the study; Dean Nancy Fugate Woods for comments on the manuscript; Berta Nicol-Blades and Dr. Anneclaire De Roos for technical assistance; and Georgia Green for administrative support.
Support: NIH grants R01 HD33792, F31 NR009164, P30 CA015704, T32 HD052462; Maternal and Child Health Dissertation Award from the Maternal and Child Health Leadership Training Program at the University of Washington School of Public Health and Community Medicine; Woodrow Wilson-Johnson&Johnson Dissertation Grant in Women’s Health.
Contributions All authors jointly conceived this investigation using the WREN data. JLM conducted the analysis and prepared the manuscript. VLH conceieved WREN and its ancillary studies, designed the WREN data collection instruments, and supervised all aspects of data collection. VLH, CC, and SD provided guidance and assistance in conducting the analysis and edited the manuscript.