Although there was some heterogeneity in results between older and younger women, changes in UI frequency over two years generally varied by race – independent of any differences in health and lifestyle factors related to UI across the racial groups. Specifically, incontinence remission was more common in black women and any incontinence improvement was more common in Asian women versus white women. In addition, among older women, black women were more likely to report any improvement of incontinence than white women. Interestingly, proportions with UI progression appeared fairly similar across Asian, black, and white women, although when considering risk factor differences across races, progression also was lower in black compared with white women.
In general, previous studies of the natural history of UI in largely white populations have reported stable or increasing incontinence frequency over time for the majority of women with incontinence, but there is spontaneous improvement or remission of symptoms for a small, but significant, proportion of women,8,19,23–27
which may occur in the absence of treatment. For example, Samuelsson et al. observed a mean annual remission rate of 6% among 90 women with incontinence followed for 5 years, none of whom had sought treatment for UI.25
We also observed this overall pattern among the Asian, black, and white women in our study, although we did not specifically exclude those who had sought treatment.
Basic biologic studies have found differences in pelvic floor anatomy and function between black and white women that are consistent with our finding of more remission of UI in black versus white women. For example, Hoyte et al. found a higher mean levator ani volume, a longer mean distance between the levator and pubic symphysis, and a wider mean pubic arch angle in 12 black women compared with 10 white women, possibly indicating more protection against injury during childbirth in black women.9
In addition, there is evidence of a smaller pelvic floor cross-sectional area10
and higher urethral closure pressure11,12
in black versus white women. Together, these findings suggest that, in general, the ability to recover from insults to the pelvic floor may differ between black and white women. Little is known regarding potential differences in the pelvic floor between Asian and white women, and studies are needed to explore whether anatomical or functional differences might explain the higher odds of UI improvement we observed in Asian versus white women.
Several cross-sectional studies have found differences in UI prevalence by race. For example, studies have reported UI prevalence proportions that were 50–70% lower in black versus white women13–15,17
and 30–40% lower in Asian versus white women15–17
. More limited research also indicates that the incidence of UI is lower in black and Asian women18–20
Longitudinal data on potential racial differences in UI progression and remission are scarce. However, in a prospective study of 11,591 women aged 50 years and older in the Health and Retirement Study (HRS), Komesu et al. generally observed higher rates of UI remission and improvement in black women compared with white women, consistent with our findings.19
For example, among women aged 50–79 years, over four years, average annual UI remission rates ranged from 10–13% in black women compared with 8–9% in white women and average annual improvement of severe incontinence (defined as UI >15 days/month) ranged from 11–20% in black women compared with 11–14% in white women. Among the NHS participants (aged 54–79 years), average annual remission rates were 3% in black women and 1.5% in white women and average annual improvement from frequent UI was 14% in black women and 10% in white women. It is unclear why remission was less common in our study than in the HRS; however, in the HRS, the annual rate was averaged from four years of follow-up rather than two in our study, allowing more time for incontinence resolution.
In contrast to our findings, two studies did not observe differences in UI improvement across races after adjusting for potential confounding factors.19,26
For example, among 2,415 women aged 42–52 years in the SWAN study, the likelihood of decreasing incontinence frequency over 6 years was virtually identical in Chinese women and in Japanese women compared with white women.26
However, the observation of similar odds of improvement in black versus white women in SWAN is consistent with our finding among the younger NHSII participants. Nonetheless, additional data are clearly needed to try to elucidate the findings across the small number of existing studies.
Regarding progression of incontinence, findings from previous studies have been mixed. In the SWAN study, incontinence worsening was not significantly different in Chinese or Japanese women compared with white women, consistent with our findings.26
There was also relatively little difference in progression for black versus white women, although in contrast to our results, worsening UI appeared somewhat more common in black women. In the HRS, the multivariable-adjusted odds of progression, defined as UI incidence or worsening of existing UI, were 43% lower in black versus white women.19
UI progression was similarly lower in black compared with white women in our study. Again, an explanation for the inconsistent findings among these few studies on UI progression is unclear and additional prospective studies are needed to further explore potential differences in changes in UI across races.
Several limitations of our study should be considered. First, UI frequency was self-reported and, thus, rates of change in UI frequency may be subject to error. However, several studies28,29
, including our own22
, have found reasonable short-term reproducibility of UI frequency reports. Moreover, we combined categories of UI frequency (e.g., occasional incontinence combined 1 leaking episode per month and 2–3 episodes per month), to reduce misclassification of the exact number of leaking episodes.
Second, because over 90% of NHS and NHSII participants are white, some measurements of changes in UI in Asian and black women were limited by small numbers, and should be interpreted with caution. Nonetheless, the total number of black women in our cohort was 120% higher than the total number of black women in the HRS19
, and just 26% lower than the total number of black women in SWAN26
. Also, because we did not collect information on Asian ethnicity, our findings for Asian women are not directly comparable to those for Chinese and Japanese women in SWAN.
Finally, we did not collect information on treatment for UI among women who reported incontinence on the baseline questionnaire. Thus, we could not determine the impact of treatment on estimates of UI remission and improvement. However, data indicate that a minority of women with UI seek treatment and that treatment-seeking does not appear to vary significantly by race.30–32
In addition, in our cohorts of health professionals with equivalent access to health care and healthcare knowledge, any potential racial differences in treatment-seeking would likely be minimized. Thus, it does not seem likely that lack of data on treatment meaningfully affected our ability to compare changes in UI frequency across races.
In conclusion, over two years of follow-up, Asian and black women with UI were generally more likely to experience remission or improvement of their urinary symptoms than white women and these differences could not largely be explained by a variety of health and lifestyle factors. Thus, clinicians should be aware that, while UI affects women of all racial backgrounds, the natural history of the condition may differ in women of different races. In addition, since data indicate that women of different races are similarly unlikely to seek treatment for incontinence30
, clinicians should be encouraged to initiate discussions about UI so that their patients can take advantage of available behavioral, pharmacologic, and surgical therapies.33