Racial Differences in the Structure and Function of the Stress Urinary Continence Mechanism

Abstract

Objective

To compare the structure and function of the urethral sphincter and the urethral support in nulliparous black and white women.

Methods

Eighteen black women (mean age 28.1 years) and 17 white women (mean age 31.3 years) completed this cross-sectional study. The following assessments were made: urethral function using multichannel cystometrics and urethral pressure profilometry, pelvic muscle strength using an instrumented speculum, urethral mobility using the cotton-swab test and perineal ultrasound, and pelvic muscle bulk using magnetic resonance imaging.

Results

Black women demonstrated a 29% higher average urethral closure pressure during a maximum pelvic muscle contraction (154 cm H₂O versus 119 cm H₂O in the white subjects; P = .008). Although not statistically significant, black women had a 14% higher maximum urethral closure .23) and pressure at rest (108 cm H₂O versus 95 cm H₂O; P = a 21% larger urethral volume (4818 mm³ versus 3977 mm³; P = .06). In addition, there was a 36% greater vesical neck mobility measured with the cotton-swab test (blacks 49° versus whites 36°; P = .02) and a 42% difference in ultrasonically measured vesical neck mobility during a maximum Valsalva effort (blacks = −17 mm versus whites −12 mm; P = .08).

Conclusion

Functional and morphologic differences exist in the urethral sphincteric and support system of nulliparous black and white women.

The prevalence of genuine stress urinary incontinence (GSI) is thought to be significantly lower in black women than in white women. This is based on information from cross-sectional studies. Bump¹ examined 200 consecutive patients referred to his urogynecology practice for prolapse or incontinence. Of the 54 blacks in this group, 27% were given a diagnosis of GSI compared with 61% of the white patients. He also found that the black women with GSI had a higher maximum urethral closure pressure and greater urethral mobility with stress than did the white subjects with the same diagnosis. Similarly, Peacock et al² retrospectively reviewed 159 black women and found the prevalence of GSI to be 28%. Although these two studies suggested that the prevalence of GSI is lower in black women, additional comparative studies are needed.

Stress continence is thought to be provided by two factors: urethral sphincter function and urethral support.^3–5 At present, it is not known whether racial differences exist in either one or both of these factors. Such differences, if they exist, might help explain racial differences in the prevalence of GSI and might help elucidate the pathophysiology of stress urinary incontinence. The aim of this study was to compare the structure and function of the sphincteric and urethral support mechanisms of nulliparous black and white women. Specifically, we tested the primary null hypotheses that no racial differences would be found in sphincter function as measured by passive and active urethral closure pressure, urethral mobility as measured by the cotton-swab test, levator ani muscle strength measured with the instrumented speculum, and urethral sphincter and levator ani muscle morphology as measured by the cross-sectional area. We also tested the secondary null hypotheses that no racial differences would be found in other measures of pelvic muscle morphology, such as the urethral sphincter and levator ani muscle thickness and volume, as well as urethral and levator ani functional measures such as the levator ani muscle resting tone and urethral mobility as measured by ultrasound.

Materials and Methods

Nulliparous black and white women were recruited for involvement in a larger study approved by the University of Michigan Institutional Review Board. Subjects from the Ann Arbor area and surrounding communities responded to both written and verbal advertisements. Written informed consent was obtained and subjects were reimbursed $200.00 for their time commitment. Subjects were included if they were between the ages of 18 and 45 years, were healthy, had never had a delivery, and had no symptoms of urinary incontinence. Exclusion criteria included recurrent or persistent urinary tract infections, presence of urogenital tract disease, or history of a neurologic disorder.

Eighteen black subjects and 17 white subjects completed the study. The data were collected over two separate visits. The ultrasound scans and urodynamic procedures were performed at the first visit, and magnetic resonance imaging (MRI) was performed on a second visit. Continence was determined by a standing stress test for urine leakage during a series of three hard coughs.⁶ After the pelvic examination was completed, an instrumented speculum, designed to quantify the force of a single pelvic muscle contraction, was placed in the vagina.⁷ Measurements were obtained at rest and during a maximum voluntary pelvic muscle contraction.

Post-void residual urine was obtained through urethral catheterization, and a urinalysis was performed on the specimen to ensure that none of the subjects had evidence of cystitis. Cotton-swab resting and strain angles were then obtained.⁸

An 8-French dual-tip Gaeltec catheter (Medical Measurements Inc., Hackensack, NJ) was inserted into the bladder and an Aquarius Urodynamic System (Laborie Medical Technologies Co, Williston, VT) was used to perform a cystometrogram and a urethral pressure profile. The bladder was filled at a rate of 100 mL/minute to a maximum of 300 mL or to maximum bladder capacity if this was less than 300 mL. A urethral pressure profile was then obtained, and the maximum urethral closure pressure at rest and the urethral pressure measured during a maximum pelvic muscle contraction (Kegel urethral closure pressure) were recorded.

Next, a midsagittal perineal ultrasound (Sonoline SI-400; Siemens AG, Erlangen, Germany) was performed in the standing position using a 5-MHz curved-array probe. The ultrasound measurements were based on the method developed by Schaer et al.⁹ The position of the vesical neck was measured using a rectangular coordinate system with origin at the inferior border of the symphysis. Vesical neck locations were measured at rest, during a maximum Valsalva effort, and during a maximum cough effort while intravaginal pressures were measured in the proximal vagina (Figure 1).

Figure 1.

Vesical neck movement as measured in relation to the symphysis. D (dark arrow) shows the total distance the vesical neck moved from rest to its location at peak Valsalva pressure.

Finally, each subject underwent MRI of her pelvis (Signa; General Electric Medical Systems, Milwaukee, WI). Transverse, coronal, and sagittal proton density-weighted images (TR/TE 4000/15) were made of the pelvic regions using a slice thickness of 4 mm separated by a slice gap of 1 mm. A 160 × 160-mm field of view and an imaging matrix of 256 × 256 were used. Morphologic measurements were standardized and carried out with Advantage Windows SdC AW 2.0.18 (General Electric Medical Systems). The coronal cross-sectional areas of the levator ani muscle from five different levels were used to calculate its volume. Similarly, the urethral sphincter muscle volume was calculated using the product of its transverse cross-sectional area at the level of the proximal urethra times its midsagittal urethral length. The urethral volume was calculated from the product of its proximal cross-sectional area times its midsagittal length. All MRI measurements were performed by one of the authors. In ten of the subjects, the measurements were performed twice with the investigator masked to the initial results. The upper and lower limits of agreement were calculated from the mean of the paired differences plus two standard deviations (SDs), and a plot was made of the paired differences between the values against the mean.¹⁰ No significant differences in the measurement were found.

The data were analyzed using the Mann-Whitney U test for pairwise comparisons with the StatView statistical package (Abacus Concepts Inc., Berkeley, CA) and linear regression with the SPSS 9.0 statistical package (SPSS Inc., Chicago, IL).

Results

There was no significant difference in the mean (± SD) age of the black women (28.1 ± 3.5 years; range 20–34) and white women (31.3 ± 5.6 years; range 22–42) (P = .08). The groups also had comparable body mass index (blacks 26 ± 7, 20–50 kg/m² versus whites 25 ± 5, 20–43 kg/m²; P = .35). One black woman was unable to complete the MRI because of claustrophobic symptoms. One white woman experienced uninhibited contractions on bladder filling; all other examinations were normal.

Morphologically, the black subjects had a 21% greater urethral volume as measured by MRI (4818 ± 1469, range 2444–8091 mm³ versus 3977 ± 1005, range 2622–6550 mm³; P = .06). The urethral cross-sectional area comparison was not significant. The primary urethral function comparisons indicated that the mean Kegel urethral closure pressure was 29% higher in the black subjects (154 ± 29, 108–202 cm H2O versus 119 ± 37, 76–194 cm H₂O; P = .008) (Figure 2). Similarly, although the difference did not reach statistical significance, the mean maximum urethral closure pressure at rest for the black subjects was 14% higher (108 ± 21, 62–148 cm H₂O versus 95 ± 30, 49–147 cm H₂O; P = .23).

Figure 2.

Mean resting urethral closure pressure of both groups and amount of pressure increase achieved during a maximum pelvic muscle contraction, with standard deviations.

Measures of the morphology and functional capacity of the contractile structures known to contribute to urethral support revealed that black women had a 17% greater levator ani muscle cross-sectional area (579 mm² compared with 493 mm²; P = .08) at the level of the midurethra. Black women also had 24% greater pelvic floor muscle strength, although that difference did not reach statistical significance (P = .3) (Table 1).

Table 1.

Levator Ani Morphology and Function^*

Variable	Blacks (n = 18)	Whites (n = 17)	P
Cross-sectional area (mm2)	579 ± 151 (308–800)	493 ± 134 (308–785)	.08
Resting tone (N)	3.9 ± 1.0 (1.8–6.1)	3.9 ± 2.0 (0.5–6.8)	.86
Maximum voluntary contraction force (N)	8.2 ± 4.0 (0.9–17.0)	6.6 ± 4.7 (2.2–16.7)	.30

Data are presented as mean ± standard deviation (range).

^*Levator ani muscle morphology as quantified by measuring its cross-sectional area at the level of the midurethra, and function as quantified by its resting tone and the maximal volitional contractile force.

Functional measures of urethral support during stress also revealed group differences (Table 2). Although there were no differences during cough, black women had 36% greater vesical neck excursion during a maximum Valsalva effort as measured by the cotton-swab test (blacks 49° versus whites 36°; P = .02). This finding was consistent with the 42% greater displacement of the vesical neck measured with ultrasound during the Valsalva maneuver (blacks −17 mm versus whites −12 mm; P = .08). There were no significant pressure differences between the groups during the cough or Valsalva maneuvers (Table 2). There were also no group differences in other secondary measures, such as those involving vesical neck resting location.

Table 2.

Urethral Mobility

Variable	Blacks (n = 18)	Whites (n = 17)	P
Cotton swab-test: strain angle (°)	49 ± 20 (−10 to 80)	36 ± 18 (10 to 70)	.02
Cough distance* (mm)	−8.5 ± 3.8 (−15 to −2.3)	−8.2 ± 4.1 (−16 to −3.1)	.60
Valsalva distance* (mm)	−17 ± 8 (−31 to −6)	−12 ± 5 (−22 to −5)	.08
Cough pressure† (cm H2O)	160 ± 45 (48 to 209)	160 ± 35 (82 to 212)	.75
Valsalva pressure† (cm H2O)	116 ± 43 (42 to 198)	111 ± 25 (72 to 162)	.86

Data are presented as mean ± standard deviation (range).

^*Distance = distance that bladder neck moved during a maximum cough or Valsalva effort.

^†Pressure = intra-abdominal pressure generated during a maximum cough or Valsalva effort.

No significant group differences were found in secondary morphologic measures of urethral and levator ani muscle thickness, area, or volume. There was no correlation found between age and any of the functional or structural measures of the continence control mechanism.

Race continued to have an independent effect when linear regression was used to control for interactive and confounding effects of the dependent variables that were found to be significant in paired comparisons. When the cotton-swab strain angle was used as the dependent variable, both age (P = .02) and black race (P = .009) had an independent effect. When the Kegel urethral closure pressure was used as the dependent variable, age had no effect (P = .7), but black race continued to contribute significantly to the differences found (P = .05).

Discussion

This study seeks to explain the observed differences in the prevalence of GSI in the black and white female population by examining the continence mechanism in nulliparous women. We elected to examine the urinary continence mechanism’s structure and function in healthy, young, nulliparous women to avoid the possibility of confounding introduced by associated factors such as birth, age, or disease.

Our morphologic and functional results suggest that young nulliparous black women have greater urethral sphincter capacity than white women, as evidenced by larger urethral volume and significantly higher urethral closure pressures during a pelvic muscle contraction. A greater bulk of urethral striated muscle or a higher density of urethral striated muscle fibers could explain this difference. This is consistent with the fact that black women have a higher appendicular skeletal mass than their white counterparts.^11,12 In addition, although not significant, the black subjects had a larger levator ani muscle cross-sectional area and greater levator ani strength. The variance in our data for levator ani strength is not surprising given the known difficulty that women can experience in voluntarily contracting the levator ani muscle.¹³

Our sphincteric function findings corroborate with those of others. They are consistent with the findings of Bump,¹ as mentioned in the Introduction. Likewise, in a comparison of multiparous black and Indian South Africans, Knobel¹⁴ found that black women had a 21% higher urethral closure pressure and a 56% greater contractile power of the pelvic muscle.

Stress incontinence has been presumed to result from urethral hypermobility,¹⁵ with a lesser emphasis on sphincteric function. However, paradoxically, we found greater vesical neck mobility during a Valsalva maneuver in black women, a population thought to have a lower prevalence of stress incontinence. Because the Valsalva intra-abdominal pressures did not differ significantly in the two groups, the compliance of the vesical neck support must be greater in black women. The fact that black women were able to relax their pelvic muscles more successfully as instructed during a Valsalva maneuver might explain this finding. However, the vesical neck compliance during the hard cough was similar in both groups. Higher urethral closure pressures in black women suggest that sphincter function might play a greater role than support in maintaining continence during stress. Indeed, Fantl et al¹⁶ showed that urethral mobility was not predictive of urethral function, further emphasizing the importance of the sphincter mechanism in maintaining continence.

The present study has limitations. The black women were younger than the white subjects, although this difference was not significant. We know that maximum urethral closure pressure decreases with age; however, this decrease becomes significant only after the age of 36 years, with a 2–4% decrease in function up to age 40 years.^17,18 In addition, correlation analysis of maximum urethral closure pressure on age revealed that age explained only 2.4% of the variation in the maximum urethral closure pressure. Linear regression also confirmed an independent effect of race on both the cotton-swab strain angle and the urethral closure pressure during a maximum pelvic muscle contraction. Therefore, this slight age difference between groups does not explain the differences demonstrated in sphincter function. Although the study was prospective, we cannot exclude the possibility of selection bias because the subjects were volunteers.

Other limitations include the small group sizes and lack of power. In performing power calculations, we discovered that the power of the study was limited in some areas. The power to distinguish a significant difference in the urethral volume was 47%. As far as functional measures were concerned, the power of our study to distinguish a difference in the maximum urethral closure pressure was 29%. There are likely other significant biologic differences in the structure and function of the continence mechanism between these groups that we were unable to demonstrate. This study provides the pilot data needed to estimate the adequate sample sizes required to demonstrate these suggested differences.

An advantage of this study is that the accuracy and reliability of the measurement methods have been demonstrated elsewhere.^6,7,9

The insights provided by this study further emphasize the importance of understanding the biology of the continence mechanism to further define the pathogenesis of GSI. Knowledge of the factors that place individuals at increased risk allows the development of treatment strategies. Future studies might focus on further defining the prevalence of this problem in specific populations. Additionally, the influence of age and parturition in these groups deserves investigation. Histologic examination of the urethras of different populations might further clarify the role of striated muscle density and bulk. The biologic differences between the continence mechanisms noted in these women deserve further investigation.