Objective
We sought to compare continence system function of black and white women in a population-based sample.
Study Design
As part of a cross-sectional population-based study, black and white women ages 35-64 years were invited to have pelvic floor testing to achieve prespecified groups of women with and without urinary incontinence. We analyzed data collected from 335 women classified as continent (n = 137) and stress (n = 102) and urge (n = 96) incontinent based on full bladder stress test and symptoms. Continence system functions were compared across racial and continence groups.
Results
Comparing black to white women, maximal urethral closure pressure (MUCP) was 22% higher in blacks than whites (68.0 vs 55.8 cm H 2 O, P < .0001). White and black women with stress incontinence had MUCP 19% and 23% lower than continent women. MUCP in urge incontinent white women was as low as stress incontinent whites, but blacks with urge had normal urethral function.
Conclusion
Black women have higher urethral closure pressures than white women. White women with urge incontinence, but not black women, have reduced MUCP.
Urinary incontinence is a common and distressing condition for which care costs $16 billion dollars each year. Clinical evaluation has suggested differences in incontinence prevalence between black and white women. Survey-based studies have confirmed these differences, indicating that black women are less likely than white women to experience urinary incontinence. In a recently conducted population-based investigation in Southeastern Michigan named Establishing the Prevalence of Incontinence (EPI) study, with adequate sampling of black women, we found the prevalence of urinary incontinence to be 14.6% for black women and 33.1% for white women confirming several other reports of lower incontinence rates in black women. A larger proportion of white women with incontinence reported symptoms of pure stress urinary incontinence (SUI) compared to black women (39.2% vs 25.0%), whereas a larger proportion of black women reported symptoms of pure urge urinary incontinence (UUI) compared to white women (23.8% vs 11.0%), confirming the observations of other studies. In the EPI study, the distribution of lifestyle and risk factors were generally similar by race. Therefore, the reason for higher prevalence of urinary incontinence, especially SUI, in whites remained unknown.
Stress continence depends on the strength of the continence system and the pressures to which it is subjected. The continence system consists of the urethral sphincters and their supports, including both endopelvic fascia and the levator ani muscles. In a recent study of stress incontinent women we found that poor urethral sphincteric function was the primary determinant of SUI, but the importance of urethral function in determining UUI was not examined. In a prior study we have shown how nulliparous black women have better urethral function than whites, and this observation may help explain the disparity in SUI symptoms by race. But the relative contributions of urethral function, support, and other factors in black and white continent and incontinent women with either SUI or urge incontinence are not known.
In this study, we compare continence system functions in a population-based sample of continent and incontinent black and white women to determine the relative contributions of urethral sphincteric function and urethral support to incontinence. Such knowledge should lead to a better understanding of reasons underlying disparities in incontinence and could have important implications not only for more targeted treatment but also to identify potentially modifiable risk factors.
Materials and Methods
The EPI study was designed in 2 phases. As previously described, the first phase of the study involved a telephone interview regarding self-reported incontinence drawn from a community-based sample of women residing in Southeastern Michigan. In brief, women aged 35-64 years were sampled from telephone records including 3 Southeastern Michigan counties with oversampling of black women to ensure adequate representation by race. Of the 12,541 telephone numbers purchased, 9199 (73.4%) were qualifying households that were contacted and screened. Of these, 3692 (40.1%) households had an eligible woman resident and 2814 completed the survey (1922 black, 892 white), for a 76.2% response rate. The telephone call was conducted by trained female interviewers from the Institute for Social Research at the University of Michigan. Women were asked to self-identify their race. If self-identifying as black or white race the interview progressed to questions about their demographic, health history, lifestyle, and obstetric/gynecologic characteristics as well as their urinary incontinence experience. Those who self-identified as other than of black or white race were excluded. In the second phase of the EPI study, the focus of this article, a subset of the women who participated in the telephone interview was invited to undergo urodynamic and pelvic floor testing in the clinic.
A priori sample size calculations conducted at the outset of the larger EPI study indicated need for 50-65 black and white women in each continence status (continent, SUI, UUI) to achieve power of 0.80 to detect effect sizes of 0.44-0.47 in comparing pelvic floor testing parameters. Recruitment was carried out to achieve groups of these sizes. Final group numbers differ somewhat from original targets because it is not possible to completely predict a subject’s continence status on urodynamic testing based on the telephone interview (ie, some subjects who described themselves as continent during the telephone interview reported being incontinent when they came in for their clinic visit).
Clinical examinations were performed with women in a semirecumbent position in an urodynamics chair at a 45-degree angle. Assessment of vaginal and uterine support was conducted using the Pelvic Organ Prolapse Quantification System, a technique that assesses the downward displacement of specific points along the vagina and cervix at maximal Valsalva. Urethral axis inclination measurements were made from the horizontal with a cotton-tipped swab at rest, during maximal Valsalva, and during attempt to contract the pelvic floor muscles (maximal contraction).
Urethral function was assessed with urethral profilometry. For each woman, 2 or 3 urethral pressure profile measurements were taken using an 8F Gaeltec dual-microtip urodynamics catheter (Medical Measurements Inc, Hackensack, NJ) with the transducer laterally oriented and averaged. Postvoid residual urine volume was measured by volume obtained during catheterization. First urge to urinate was noted as well as any detrusor contraction during bladder filling through a catheter to cystometric capacity using a medium fill rate. Cough and Valsalva leak point pressures were determined on 300-mL bladder volume (bladder volume was reduced to 300 mL through passive catheter drainage if first urge occurred at a higher volume during filling). Load on the system was quantified as highest cough pressure obtained during the leak point pressure testing. A positive full bladder standing stress test was conducted after removal of the catheter and resulting stress-associated urine leakage with cough or Valsalva was documented. Uroflow was performed after catheter removal. Levator ani muscle function was assessed with an instrumented vaginal speculum designed to measure vaginal closure force both at rest and during maximum voluntary contraction.
For the purposes of this study, classification of continence status was made using the following definitions.
SUI (n = 102)
All women who leaked urine during coughing on examination were classified as having the physical finding of SUI. Because the purpose of this portion of the project was to assess the relationship between continence mechanism structures and functional elements, we chose this objective evaluation over self-report of SUI. Thus, all of the SUI women analyzed had demonstrable leakage during cough. None had documented detrusor instability, but some were symptomatically positive for urge.
Urge incontinence (n = 96)
Women who were given a final clinical diagnosis of UUI, without SUI, if they had symptoms of UUI and negative stress test during examination.
Continent (n = 137)
Women who denied urinary incontinence ≥12 times per year and who did not demonstrate urinary incontinence during urodynamic testing were classified as continent.
Women excluded from analysis
Women who self-reported SUI as their only leakage symptoms, but in whom SUI could not be demonstrated on clinical examination (n = 22) were excluded from analysis because they could neither be properly classified as having demonstrable SUI, nor could they reasonably be considered continent. Women who demonstrated SUI only on Valsalva maneuver and never during coughing were also excluded (n = 29). This decision was made in recognition of the fact that healthy women can void by increasing their abdominal pressure while relaxing their pelvic floor muscles. In addition, 7 women with other forms of urinary incontinence were excluded; 1 reported the feeling of moisture but denied urge or stress symptoms, 1 reported only of urine loss at the end of micturition, and 5 had nocturnal enuresis but denied urge symptoms or demonstrable SUI during a cough.
Statistical methods
Sampling weights were applied to the data to adjust for oversampling for urinary incontinence and black race, for the purpose of projecting the clinical sample to the population from which the survey sample was drawn (ie, source population). Demographic characteristics, health history, lifestyle factors, and obstetric/gynecologic history were compared between black and white subjects within each of the continence status groups using χ 2 tests ( Table 1 ). Least squares mean measures of urethral function, urethrovaginal supports, and urodynamics were compared between white and black women, adjusted for age (continuous), body mass index (continuous), diabetes (yes, no), and vaginal parity (0, 1-2, ≥3) ( Table 2 ). Multivariable logistic regression analyses were conducted to determine factors best explaining SUI or UUI, separately for black and white women ( Table 3 ). All logistic regression models were adjusted for age (continuous), body mass index (continuous), diabetes (yes, no), and vaginal parity (0, 1-2, ≥3), and weighted to reflect the overall source population. For each model, goodness of fit was assessed using the Max-rescaled R 2 , and the area under the receiver operating characteristic curve. Within each racial group, pairwise comparisons of pelvic floor measures across continence groups were calculated using t tests, with an indication if the comparison remained statistically significant after Bonferroni adjustment for multiple inferences ( Figures 1-5 and Appendix ). P values < .05 were considered statistically significant. All analyses were conducted using software (SAS, version 9.1; SAS Institute Inc, Cary, NC).
| Demographic | Overall, % | Continent | Stress urinary incontinent | Urge urinary incontinent | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Black, % | White, % | P value a | Black, % | White, % | P value a | Black, % | White, % | P value a | ||
| Age, y | ||||||||||
| 35-44 | 37.9 | 36.7 | 52.7 | .11 | 23.5 | 19.6 | .40 | 35.3 | 36.4 | .45 |
| 45-54 | 34.9 | 41.5 | 27.6 | 55.3 | 42.5 | 37.9 | 24.5 | |||
| ≥55 | 27.2 | 21.8 | 19.7 | 21.2 | 37.9 | 26.8 | 39.1 | |||
| Education level, y completed | ||||||||||
| <12 | 6.6 | 7.3 | 0.0 | .003 a | 8.5 | 8.1 | .68 | 3.8 | 16.8 | .006 a |
| 12 | 21.4 | 18.0 | 11.0 | 36.5 | 32.5 | 17.3 | 28.3 | |||
| 13-15 | 34.2 | 42.3 | 34.3 | 37.4 | 28.4 | 57.0 | 18.6 | |||
| ≥16 | 37.8 | 32.5 | 54.7 | 17.6 | 31.0 | 21.9 | 36.2 | |||
| Currently working for pay | 66.1 | 62.0 | 71.3 | .20 | 74.0 | 60.8 | .27 | 59.1 | 68.0 | .44 |
| Household income, $ | ||||||||||
| <35,000 | 28.1 | 27.1 | 14.9 | .14 | 42.9 | 36.4 | .75 | 21.6 | 43.1 | .15 |
| 35,000-69,999 | 18.5 | 17.9 | 17.7 | 26.3 | 22.9 | 21.9 | 11.1 | |||
| ≥70,000 | 53.4 | 55.0 | 67.4 | 30.8 | 40.7 | 56.5 | 45.8 | |||
| Marital status | ||||||||||
| Married/living together | 51.1 | 44.5 | 55.6 | .02 a | 26.9 | 54.2 | .19 | 39.6 | 62.3 | .09 |
| Never married | 14.8 | 17.0 | 20.3 | 14.8 | 10.8 | 15.3 | 6.1 | |||
| Divorced/separated | 31.6 | 30.7 | 24.1 | 55.3 | 33.8 | 39.4 | 31.6 | |||
| Widowed | 2.5 | 7.8 | 0.0 | 2.9 | 1.1 | 5.7 | 0.0 | |||
| Health history | ||||||||||
| Diabetes | 13.0 | 19.7 | 1.9 | < .0001 a | 9.7 | 17.0 | .42 | 19.3 | 18.7 | .95 |
| Mobility impairment | 11.4 | 9.5 | 5.9 | .36 | 18.0 | 19.1 | .91 | 19.9 | 8.6 | .15 |
| Constipation | 89.3 | 93.0 | 97.1 | .19 | 88.0 | 83.4 | .63 | 84.9 | 79.6 | .58 |
| Urinary tract infection | 12.0 | 10.4 | 11.1 | .88 | 13.8 | 8.9 | .52 | 14.3 | 17.4 | .73 |
| Chronic lung disease | 24.6 | 19.5 | 26.3 | .30 | 10.6 | 33.6 | .04 | 13.9 | 28.2 | .17 |
| Body mass index, kg/m 2 | ||||||||||
| ≤25 | 22.8 | 13.2 | 29.1 | .02 a | 17.1 | 21.9 | .62 | 6.5 | 31.1 | .03 a |
| 26-35 | 47.7 | 55.1 | 53.4 | 43.1 | 49.7 | 38.5 | 34.4 | |||
| ≥36 | 29.5 | 31.7 | 17.4 | 39.8 | 28.5 | 55.0 | 34.5 | |||
| Depressive symptoms b | 45.3 | 45.4 | 36.0 | .21 | 46.8 | 41.4 | .67 | 58.1 | 60.7 | .83 |
| Lifestyle factors | ||||||||||
| Exercise involving bouncing at least once/wk | 34.1 | 35.8 | 46.8 | .15 | 20.5 | 25.7 | .63 | 24.3 | 27.5 | .76 |
| Lift or carry ≥30 lb >once/wk | 69.9 | 66.2 | 78.1 | .08 | 57.3 | 79.1 | .15 | 50.4 | 62.6 | .31 |
| Current cigarette smoking | 26.7 | 23.8 | 15.0 | .14 | 24.1 | 41.7 | .15 | 26.8 | 35.9 | .43 |
| Drink >8 glasses fluid/d | 37.5 | 37.4 | 28.2 | .20 | 26.4 | 52.0 | .04 a | 38.2 | 42.0 | .75 |
| Obstetric/gynecologic history | ||||||||||
| Vaginal parity, no. of vaginal births | ||||||||||
| 0 | 31.3 | 20.0 | 54.5 | < .0001 a | 11.5 | 22.8 | .10 | 16.7 | 27.8 | .32 |
| 1-2 | 37.1 | 44.2 | 30.1 | 30.9 | 45.5 | 46.1 | 29.6 | |||
| ≥3 | 31.6 | 36.7 | 15.4 | 57.6 | 31.7 | 37.2 | 42.6 | |||
| Current estrogen use | 13.1 | 8.5 | 6.9 | .68 | 13.6 | 18.0 | .64 | 20.0 | 21.5 | .88 |
| Prior surgery for prolapse or urinary incontinence | 7.7 | 1.5 | 6.4 | .12 | 7.6 | 8.9 | .86 | 7.3 | 17.3 | .24 |
| Menopause | 42.7 | 41.6 | 28.7 | .08 | 37.3 | 50.6 | .29 | 47.3 | 61.0 | .26 |
| Prior hysterectomy | 21.6 | 23.6 | 8.5 | .006 a | 23.5 | 27.0 | .75 | 30.3 | 32.6 | .84 |
a χ 2 P value for comparison between black and white women within continence category;
b Self-reported feelings of sadness, depression, and/or loneliness in prior week.
| Variable | All white (n = 145) | All black (n = 190) | P value | ||
|---|---|---|---|---|---|
| LS mean | 95% CI | LS mean | 95% CI | ||
| Urethral function | |||||
| Maximal closure pressure | 55.8 | (52.4–59.2) | 68.0 | (63.4–72.6) | < .0001 a |
| Pressure increase with maximal contraction | 17.6 | (15.7–19.6) | 20.4 | (17.7–23.1) | .11 |
| Urethrovaginal supports | |||||
| Urethral support; cotton-tipped swab angle | |||||
| Rest | −0.95 | (−2.8 to 0.85) | −2.3 | (−4.8 to 0.19) | .39 |
| Valsalva | 24.7 | (21.9–27.4) | 23.4 | (19.7–27.1) | .59 |
| Pelvic muscle contraction | −14.8 | (−16.7 to −12.9) | −14.9 | (−17.5 to −12.2) | .99 |
| Uterovaginal support | |||||
| Anterior wall (point Aa) | −1.2 | (−1.5 to −0.99) | −1.0 | (−1.3 to −0.68) | .27 |
| Apex (point C) | −6.4 | (−6.7 to −6.2) | −6.6 | (−6.9 to −6.2) | .53 |
| Posterior wall (point B) | −1.2 | (−1.3 to −1.1) | −1.1 | (−1.3 to −0.93) | .57 |
| Hiatus measurements | |||||
| Genital hiatus at rest | 2.9 | (2.7–3.0) | 2.9 | (2.7–3.1) | .66 |
| Genital hiatus with Valsalva | 3.4 | (3.2–3.5) | 3.5 | (3.3–3.6) | .48 |
| Vaginal closure force | |||||
| Rest | 3.6 | (3.3–4.0) | 4.1 | (3.6–4.6) | .15 |
| Maximal contraction | 3.0 | (2.7–3.3) | 2.9 | (2.5–3.2) | .50 |
| Urodynamics | |||||
| Bladder pressure | |||||
| Rest | 21.2 | (20.0–22.4) | 23.6 | (22.0–25.3) | .02 a |
| Maximal cough | 155.9 | (149.9–161.9) | 174.4 | (166.2–182.6) | .0004 a |
| PVR | 40.7 | (34.8–46.6) | 22.9 | (14.9–31.0) | .0005 a |
| CMG first urge | 198.4 | (185.5–211.3) | 198.6 | (181.2–216.1) | .98 |
| CMG maximum | 396.2 | (381.5–410.9) | 376.1 | (356.0–396.2) | .11 |
| Maximum flow | 29.9 | (27.9–31.9) | 30.9 | (28.2–33.7) | .54 |
| Average flow | 17.1 | (15.6–18.6) | 19.2 | (17.2–21.3) | .09 |
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