Objective
We calculated incidence rates of urinary incontinence by incontinence frequency and type over 4 years in Asian, black, and white women in the United States.
Study Design
Prospective analyses included 76,724 participants aged 37–79 years in the Nurses’ Health Study cohorts with no incontinence at baseline.
Results
The 4-year incidence of incontinence at least monthly was higher in white women (7.3/100 person-years) compared with Asian (5.7/100 person-years; P = .003) and black women (4.8/100 person-years; P < .001). The incidence of at least weekly stress incontinence was significantly lower in black compared with white women (0.1 vs 0.8 per 100 person-years; P < .001). The difference between black and white women in the incidence of any incontinence and stress incontinence remained significant after adjusting for known risk factors ( P < .001 for both).
Conclusion
Urinary incontinence incidence differs by race. Studies to confirm these results and better understand underlying mechanisms are needed.
Growing scientific evidence indicates that the burden of urinary incontinence (UI) may vary across racial groups. Several cross-sectional studies, including our own, have reported a lower prevalence of overall incontinence in black and Asian women compared with white women, and cross-sectional studies have consistently found a higher prevalence of stress incontinence in white vs black women. In addition, biologic data suggest that differences in pelvic floor anatomy between white and black women may explain differences in UI prevalence.
However, because prevalence is a function of both incidence and duration of incontinence, these differences in prevalence may or may not reflect incontinence incidence. Better characterization of racial variations in incontinence incidence could help further understanding of the natural history and the cause of incontinence and incontinence types. However, few prospective studies have examined incontinence incidence in different racial groups, and, among the limited studies, only 1 collected data on incontinence type. Thus, we examined incontinence incidence rates, by incontinence frequency and type, in Asian, black, and white female health professionals enrolled in 2 large US prospective cohort studies, the Nurses’ Health Study (NHS) and the Nurses’ Health Study II (NHSII).
Materials and Methods
The NHS and NHSII
The NHS was initiated in 1976, when 121,700 female nurses aged 30–55 years completed a mailed questionnaire about their medical history and health behaviors. In 1989, the NHSII was established, when 116,430 female nurses aged 25–42 years returned a similar mailed questionnaire. Return of the questionnaire implied informed consent. Updated information on participants is obtained using biennial questionnaires. During each questionnaire cycle, a full-length questionnaire is sent for initial mailings, after which an abbreviated version is sent to nonresponders to maximize participation. The Institutional Review Board of Brigham and Women’s Hospital approved these studies.
Study population
Questions about urinary incontinence were included on the full-length questionnaires mailed in 2000, 2002, and 2004 for the NHS and 2001, 2003, and 2005 for the NHSII. In the NHS, 83,996 women answered the incontinence questions on the 2000 questionnaire. Responders were identical to the entire NHS cohort in mean age, mean body mass index (BMI), and parity. In addition, the racial distribution of responders was very similar to the entire cohort (0.7% vs 0.8% Asian; 1.2% vs 1.5% black; 94% vs 92% white, respectively). In the NHSII, responders to the incontinence questions in 2001 (n = 85,503) were identical to the entire NHSII cohort in mean age and BMI and highly similar to the entire cohort in parity (18% vs 15% nulliparous, respectively) and racial distribution (95% vs 94% white; 1.4% vs 1.8% black; 1.3% vs 1.7% Asian, respectively).
For these analyses, we defined baseline as 2000 in the NHS and 2001 in the NHSII. In each cohort, we excluded prevalent cases of incontinence at least once per month or incontinence of more than a few drops less than once per month at baseline (NHS, n = 40,807; NHSII, n = 43,923). In addition, we excluded women missing incontinence information on both the first and second follow-up questionnaires (NHS, n = 2920; NHSII, n = 2848); women missing information on race (NHS, n = 430; NHSII, n = 47); and women who self-identified as 2 or more races or as a race other than Asian, black, or white (NHS, n = 1047; NHSII, n = 753). Thus, analyses included 76,724 women (NHS, n = 38,792; NHSII, n = 37,932), or 94% of all Asian, black, or white women who were at risk for incident incontinence at baseline and received at least 1 follow-up questionnaire.
Measurement of UI
Participants in the NHS and NHSII were asked on the baseline and follow-up questionnaires, “During the last 12 months, how often have you leaked or lost control of your urine?” Response options were never, less than once per month, once per month, 2–3 times per month, about once per week, and almost every day. In reliability testing, response to this question was highly reproducible among these nurses.
Incident incontinence during follow-up was defined as incontinence occurring at least once per month. Among incident cases, occasional incontinence was defined as incontinence occurring 1–3 times per month, and frequent incontinence was defined as incontinence occurring at least once per week.
Measurement of UI type
We believed that women with at least weekly incontinence were likely better able to describe the precipitants of their incontinence than women with less frequent incontinence. Therefore, we defined incontinence type only among cases with frequent incontinence. At the first follow-up, incontinence type was assessed on a supplementary questionnaire mailed to women with frequent incontinence. Because of the large number of incident cases with frequent incontinence in the NHS, the supplementary questionnaire was mailed to a random sample of 80% of the cases (n = 2171) and completed by 84% of these. In the NHSII, the same supplementary questionnaire was mailed to 98% of cases (n = 1222; 19 cases identified late in the questionnaire cycle did not receive a questionnaire) and completed by 79%. Important incontinence risk factors, including mean age, mean BMI, parity, and the racial distribution, were similar in incident cases with frequent incontinence who did and those who did not provide incontinence type information. For the second follow-up period, data on incontinence type were collected directly from the main questionnaire, and thus information on incontinence type was available from 99% of women with frequent incontinence.
Stress incontinence was defined as leaking primarily with coughing or sneezing, lifting things, laughing, brisk walking, or exercise. Urgency incontinence was defined as primarily leaking accompanied by an urge to urinate or a sudden feeling of bladder fullness. Incontinence type was classified as mixed when women reported that stress and urgency incontinence symptoms were equally common. Incontinence type was classified as “other” when leaking occurred in circumstances other than those described previously.
Measurement of race
Women were asked to indicate their race and ethnicity on the NHS and NHSII questionnaires. We classified women as Asian, black, or white if they marked their race as only Asian, only black or African American, or only white, respectively. Women of both Hispanic and non-Hispanic ethnicity were included within each racial category (only 1% of women reported Hispanic ethnicity).
Statistical analysis
We calculated race-specific incontinence incidence rates and their 95% confidence intervals (CIs) by incontinence frequency (any, occasional, frequent) and incontinence type (stress, urgency, mixed). Because we allowed women missing incontinence information on either the first or second follow-up questionnaire (but not both) to remain in the study population, we calculated incidence rates using observed person-years of follow-up. This method allowed us to account for changes in follow-up status and take advantage of all available data, as opposed to cumulative incidence proportions, which assume a uniform population throughout the study period. Thus, for example, a woman at risk for incident incontinence at baseline who reported no incontinence at follow-up 1 and was missing incontinence information at follow-up 2 contributed person-time during the first follow-up period only. Race-specific incidence rates per 100 person-years were obtained by dividing the total number of incident cases over the 4-year follow-up period by the total observed person-years and multiplying by 100 within each racial category.
We used the score statistic to test whether rates of incontinence in black and Asian women were significantly different from rates in white women. To assess associations between race and incontinence incidence adjusting for potential confounding factors, we used Cox proportional hazards models, controlled for age in months, to calculate multivariable-adjusted hazards ratios (HRs) and 95% CIs. For these analyses, we excluded 2007 women missing information on BMI or parity, because these are such important covariates. In addition to age, all multivariable models included the following covariates, identified as incontinence risk factors in previous studies among these women: BMI (continuous), parity (0, 1–2, 3+ births), cigarette smoking (never, past, current), physical activity (metabolic equivalent hours/week, continuous), type 2 diabetes, and postmenopausal hormone use (premenopausal, postmenopausal-never user, past user, current user, unknown hormone use status). Additional control for hypertension, diuretic use, hysterectomy, major neurologic disease (defined as stroke, multiple sclerosis, or Parkinson’s disease), and functional limitation (defined as a significant limitation in climbing 1 flight of stairs, walking 1 block, bathing, or dressing) did not affect the HRs and thus were not included in models. All covariates were updated to reflect participant status as of the beginning of each 2-year risk period.
We also conducted a secondary analysis among the subset of women with UI data at both follow-up periods (n = 10,850) to explore potential racial differences in the persistence of incontinence at the second follow-up among women with incident UI at follow-up 1. For this analysis, we used the χ 2 test to compare the proportion of Asian or black vs white women with persistent incident UI.
Results
Baseline characteristics
In 2000, the NHS participants were aged 54–79 years, and, in 2001, the NHSII participants were aged 37–54 years. In both cohorts, Asian women had the lowest mean BMI and black women were the most likely to have been diagnosed with type 2 diabetes ( Table 1 ). White women were the most likely to be current smokers.
| NHS | NHSII | |||||
|---|---|---|---|---|---|---|
| Asian | Black | White | Asian | Black | White | |
| Variable | (n = 287) | (n = 590) | (n = 37,915) | (n = 565) | (n = 548) | (n = 36,819) |
| Age, y [mean (SD)] | 66.1 (6.4) | 65.9 (6.1) | 65.5 (7.0) | 46.7 (4.3) | 47.2 (4.5) | 46.0 (4.7) |
| BMI, kg/m 2 [mean (SD)] a | 23.5 (3.5) | 28.4 (5.2) | 26.0 (4.9) | 23.3 (3.5) | 29.4 (6.3) | 25.6 (5.5) |
| Physical activity, MET-h/wk [mean (SD)] | 20.1 (18.2) | 14.8 (14.2) | 18.8 (17.3) | 19.2 (21.1) | 20.5 (26.1) | 23.1 (22.4) |
| Parity, % | ||||||
| 0 | 4.9 | 6.4 | 6.1 | 29.0 | 25.6 | 21.0 |
| 1–2 | 40.1 | 50.2 | 35.6 | 51.2 | 57.1 | 51.3 |
| 3+ | 49.1 | 40.5 | 56.9 | 17.4 | 16.4 | 26.8 |
| Missing | 5.9 | 2.9 | 1.4 | 2.5 | 0.9 | 1.0 |
| Premenopausal, % | 0.7 | 0.9 | 1.3 | 74.9 | 61.3 | 72.2 |
| Postmenopausal hormone use, % b | ||||||
| Never | 21.8 | 30.6 | 26.1 | 26.1 | 23.1 | 17.1 |
| Past | 28.8 | 33.7 | 27.7 | 18.3 | 21.2 | 18.5 |
| Current | 43.9 | 22.9 | 40.5 | 47.9 | 41.5 | 60.7 |
| Missing | 5.6 | 12.8 | 5.7 | 7.8 | 14.2 | 3.7 |
| Cigarette smoking, % | ||||||
| Never | 67.9 | 53.6 | 44.6 | 89.9 | 74.5 | 66.3 |
| Past | 28.9 | 41.4 | 45.9 | 7.8 | 18.3 | 25.4 |
| Current | 3.1 | 5.1 | 9.5 | 2.3 | 7.3 | 8.3 |
| Diabetes, % | 9.1 | 15.6 | 7.0 | 2.8 | 6.6 | 2.1 |
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