Prevalence and associated factors of urinary incontinence in women living in China: a literature review

Background This review of studies on urinary incontinence (UI) was focused primarily on UI prevalence rates and associated factors across the adult lifecourse of Chinese women. UI is a urologic symptom that can have a significant impact on women's physical and mental health and quality of life. In addition, women with UI may experience socioeconomic burdens due to UI’s effect on their ability to work and function in society. Although researchers from many countries have reported prevalence rates and associated factors for UI, little is known about the prevalence of UI in China’s large female population. Language may act as a barrier to the inclusion of published studies in English-language journals. To overcome this barrier and to add to the global knowledge base about UI in women, the authors reviewed and discussed findings from epidemiological studies published in China and in Chinese language. Methods The authors retrieved research studies from the Wanfang database using the following search terms: "Subject: (Female) × Subject: (Urinary incontinence) × Subject: (Prevalence) × Date: 2013 to 2019". Searches employed the China National Knowledge Infrastructure Database, VIP Database for Chinese Technical Periodicals and China Biology Medicine Database. The authors also used PubMed to search English-language studies published in Chinese journals on UI in Chinese women. Results This literature review includes 48 articles published between January 2013 and December 2019. The overall UI prevalence rates reported in adult Chinese women ranged from 8.7 to 69.8%, representing 43–349 million women, respectively. For women aged 17–40 years, 41–59 years, and 60 years and older, prevalence rates ranged from 2.6–30.0, 8.7–47.7, to 16.9–61.6%, respectively. Significant associated factors for overall UI included age, body mass index, constipation, parity, and menopause. Despite the 17–40 age range being peak reproductive years, the literature revealed little focus on UI prevalence rates. For women aged 41–59 years, the main associated factors included those related to pregnancy and gynecologic diseases. For women 60 years and older, chronic diseases represented most of the associated factors. Conclusions About 43–349 million Chinese women may experience UI. Many of the identified associated factors could be mitigated to reduce UI incidence and prevalence rates. Little is known about the prevalence rates and associated factors for UI among young (aged 17–40) Chinese women. Future research should investigate UI in young women to improve bladder health across their lifecourse.

in women [2]. The three main types of UI are stress urinary incontinence (SUI), urgency urinary incontinence (UUI), and mixed urinary incontinence (MUI) [1]. The definition of each UI type is as follows: SUI is "the complaint of involuntary loss of urine on effort or physical exertion (e.g., sporting activities) or on sneezing or coughing"; UUI is the "observation of involuntary leakage from the urethra synchronous with the sensation of a sudden, compelling desire to void that is difficult to defer"; and MUI is the "complaint of involuntary loss of urine associated with urgency and also with effort or physical exertion or on sneezing or coughing" [1].
Prevalence rates of UI for women reported globally can differ as a result of variations in methods used in studies or reports, women's underreporting of their symptoms, and providers underdiagnosing the condition [3]. As an example, the UI prevalence rate for women between 45 and 60 years old living in Brazil was 23.6% [4], whereas the prevalence rates for adult women (over 18 years old) in Germany, Denmark, and Norway were 48.3%, 46.4% [5], and 18.7% [6], respectively. The number of women with UI in the United States has been estimated to be about 28.4 million [7]. Previous research suggests that 31.9% (approximately 160 million) women in China are affected by UI [8], indicating an urgent need for health and social resources to manage and treat UI.
Many factors are associated with UI [8], including unmodifiable factors (e.g., age, gender, menopause, history of vaginal delivery) and potentially modifiable factors (e.g., smoking, alcohol intake, toileting behaviors [9], constipation, and obesity). Besides UI's impact on women's physical and mental health, UI affects women's quality of life by limiting social activities [10] and interactions, interfering with the ability to work [11], and increasing the financial burden on women and society [12][13][14]. Therefore, UI should be viewed as both a women's health issue and a public health issue [15].
Studies of UI prevalence rates and associated factors often appear in English-language journals, but research findings published in non-English-language journals or English-language journals not published outside of China are seldom disseminated widely. The resultant knowledge gap could negatively affect potential research and clinical advances with regard to Chinese women's bladder health. This gap could also delay the development of culturally appropriate interventions to prevent and treat UI across women's lifecourse. Thus, the need to close the knowledge gap is important, especially considering China's large female population. For example, 650 million women were living in China in 2010, with more than 500 million women over 20 years old [16].
The aims of this study were to: (1) summarize findings from studies in non-English-language journals and English-language journals published in China that investigate UI prevalence in Chinese women, (2) categorize the findings by life stage (i.e., age range categories), and (3) facilitate dissemination of this existing information to researchers and clinicians to aid in their planning to prevent, manage, and treat female UI.

Literature search
The authors searched the relevant literature using five databases: the Wanfang full-text database (a Chinese professional academic database covering journals, meeting minutes, papers, academic achievements and academic conference papers), China National Knowledge Infrastructure (CNKI) Database (the largest continuously updated China journal full-text database in the world. It contains more than 9100 important journals in China, mainly including academic, technical, policy guidance, higher science popularization and education, and some basic education, popular science and technology, popular culture and literature and art works, covering natural science, engineering technology, agriculture, philosophy, medicine, humanities and Social Sciences and other fields, there are more than 32.52 million full-text documents), VIP Database (it analyzes the contents and citations of more than 14,000 kinds of science and technology periodicals and 57 million fulltext periodicals published in China), and China Biology Medicine database (it involves basic medicine, clinical medicine, preventive medicine, pharmacy, traditional Chinese medicine, traditional Chinese medicine and other biomedical fields. It is an important retrieval tool for medical literature in China at present) for Chinese Technical Periodicals, and PubMed for English-language articles. The Wanfang full-text database search expression was Subject (Female/Women) × Subject (Urinary incontinence) × Subject (Prevalence) × Date: 2013-2019 or Subject: (Lower urinary tract symptoms) × Subject (Prevalence) × Date: 2013 to2019. The authors used the same search strategy for the other four databases: China National Knowledge Infrastructure, VIP, China Biology Medicine, and PubMed. After the electronic retrieval of relevant articles, the authors obtained further studies from the references cited in those articles. The search was conducted in two phases: initially the review included studies published between January 2013 and December 2017, and was subsequently updated to include literature published between January 2018 and December 2019.

Eligibility criteria
The inclusion criteria for this review were: (1) studies must be a cross-sectional research design; (2) study participants were adult women (≥ 17 years old) living in China; (3) studies discussed prevalence rates and/or associated factors of UI; and (4) sample sizes were greater than or equal to 100 women. The exclusion criteria were (1) narrative or systematic reviews, meta-analyses, or clinical guidelines; (2) case-control studies of UI treatment or care; and (3) studies focused on UI mechanisms. (4) Study participants were currently pregnant or up to 3 months postpartum.

Study selection
Two native Chinese-speaking reviewers (proficient in English) independently screened the article titles and abstracts. Duplicate articles were excluded. Full texts were obtained for the selected studies to assess their eligibility and their reference lists were scanned for further relevant articles. Any disagreement that arose between the reviewers regarding the inclusion or exclusion of articles was resolved through discussion. See Fig. 1.

Data extraction and analysis
The study team developed a standardized abstraction table. Data extraction was performed by two reviewers.
One author independently read the included studies and extracted data from them, but consulted with the other authors to resolve ambiguities. The studies were described and then summarized using a narrative descriptive approach. Data in the abstraction table were double checked by reviewers.
The age ranges of the participants differed among the studies selected for review. Thus, we adopted the following age range categories to examine the studies more closely in terms of participant age: 17-40 years old for young women, 41-59 years old for middle-aged women, and 60 years old and over for older women. Two reviewers independently extracted data onto a data extraction summary sheet regarding prevalence rates and associated factors for UI in young, middleaged, and older Chinese women. Factors that were significantly associated with urinary incontinence (p < 0.05) were included in the review.

duplicates removed
263 Chinese-language articles did not report UI prevalence. 718 English-language articles did not report UI prevalence.
32 Chinese-language articles did not report prevalence data or reported the same data. 10 English-language articles 588 Chinese-language and 736 English-language references imported for screening 335 Chinese-language articles and 736 English-language articles screened against title and abstract 72 Chinese-language articles and 18 English-language articles assessed for full-text eligibility 40Chinese-language articles 8 English-language articles

Results
The articles initially retrieved included 335 Chineselanguage and 736 English-language articles, after removing 253 duplicates. Of those articles, 72 Chineselanguage articles and 18 English-language articles were retained after screening titles and abstracts. We then reviewed the full texts of each article and identified 40 Chinese-language and 8 English-language articles for final analyses (Fig. 1). Among the Chinese-language articles, there were four Master's thesis [17][18][19][20].
Based on the eligibility criteria, all the included articles were cross-sectional research design. The sample sizes range from 146 to 18,992. Almost all of the studies reported that trained interviewers surveyed face to face with standardized questionnaire and some of the researchers also conducted gynecological examination for participants [32][33][34]. All the studies focused on the associated factors of any UI or SUI. Two article focused on the associated factors of the other type (UUI) [19,22]. No articles that focused on the associated factors of MUI were located.
The prevalence rates and associated factors for female UI in China are presented in the "Appendix". The reported prevalence rates of overall UI (overall UI includes all types of UI) in Chinese adult women ranged from 8.7 [34] to 69.8% [35]. Where prevalence of specific UI types were reported, the following ranges were: SUI prevalence rates ranged from 6.7 [34]   UI of older women (≥ 60 or older). No prevalence of UI reported. Two studies including women with 17 years old: * [20]. ** [40] prevalence rates ranged from 1.2 [17] to 21.0% [22], and MUI prevalence rates ranged from 1.5 [17] to 15.7% [36]).
Definitions for UI, SUI, UUI, and MUI differed across some of the studies. Most of the authors used the International Continence Society (ICS) definition of UI: "complaint of involuntary loss of urine" [1]. Five studies [37][38][39][40][41] did not include a definition for UI. Other definitions used varied slightly from the ICS definition, including: (1) UUI was defined as, "the occurrence of urinary frequency, urgency, increased frequency of nocturnal discharge and decreased urine output; or cannot control urine leaking out, waited too late to urinate leading to leakage of urine" [42,43]; and (2) UUI was defined as, "urinating without any warning or a weak or faint amount of early warning, sudden urge sensation resulting in uncontrolled urine outflow" [22]. SUI was defined as "urine leaks out when exercising" [22]. The "Appendix" includes the UI definitions used in the reviewed studies.

Discussion
The studies selected for this review revealed a wide range of prevalence rates for overall UI and the specific types of UI, which may be due, in part, to the country's size. China is a large country with 9.6 million square kilometers and it includes numerous cultures in urban and rural locations. Genetic factors, diet, lifestyle, local environment, climate, economic development level, occupation types, and toileting behaviors may differ across these regions. These factors could act as determinants of UI, and thus affect variation in UI prevalence rates. Other important reasons for the variations in UI prevalence rates and UI types are the different research definitions and statistical and sampling methods used in the selected studies. For example, researchers used different methods when creating samples (i.e., based on general outpatient [40] or gynecological clinics [38] or physical examination [61] or nursing institutions [60]). The study participants had different occupations (i.e., medical personnel [62], nurses [22] and railway workers [63]), and different

Table 1 Prevalence Rates and Factors Associated with Urinary Incontinence in Chinese Women
UI urinary incontinence, SUI stress urinary incontinence, UUI urgency urinary incontinence, MUI mixed urinary incontinence, BMI Body Mass Index, POP pelvic organ prolapse living conditions (i.e., rural [64] and urban [45]). Also, in some studies, clinicians conducted physical examinations and documented medical histories, which may have yielded different results from women's self-reported questionnaires.
Researchers identified several factors associated with overall UI in Chinese women. Some of these factors are modifiable, such as weight, BMI, education, smoking and drinking alcohol. Some factors can be remedied or controlled (i.e., constipation, hypertension, chronic cough, diabetes, respiratory diseases, and vaginitis). Lastly, although some of the identified factors are unmodifiable (i.e., age, vaginal delivery, cesarean section delivery, and menopause), interventions can nonetheless be designed and tested to promote bladder health and help delay the onset or slow worsening of UI.
Age is often associated with UI risk factors such as education level, number of pregnancies and deliveries, menopause, and chronic conditions such as hypertension, diabetes, and respiratory diseases. Thus, studying clusters of factors that increase UI risk across the lifecourse is important. Moreover, using a lifecourse perspective is advocated in bladder health research [65].
Chinese scholars have largely confined their research on prevalence of UI in adult women (≥ 17 years old), but in their findings of subgroup analyses for discrete age groups were not reported. Despite this limitation, when using the prevalence rate range of 2.6 [45]-30% [46] for young women aged 17-40 years old, the number of Chinese women in this age group who are affected by UI is estimated to be between 6.6 million and 75.8 million. This finding alone indicates that screening young women for risk factors, especially modifiable ones, and taking actions to minimize or eliminate the effects of these factors could potentially prevent or delay incident cases of UI throughout the lifecourse and especially later in life.
In a prospective cluster-randomized controlled trial of UI among young women (18-40 years), SUI prevalence was 14.3% [66]; 49.5% of these women had SUI during pregnancy, 43.6% had postpartum SUI, and 6.9% had SUI before pregnancy. Because muscle, connective, and nervous system pelvic structures are subjected to anatomical, morphological, functional, and hormonal changes during pregnancy, clinicians should initiate primary prevention interventions [67]. The pelvic floor also undergoes an enormous amount of stretching to allow the passage of a newborn during vaginal delivery [68]. Evident or hidden injuries to the pelvic floor may manifest as urinary and fecal incontinence, prolapse symptoms, or sexual dysfunction, all of which have a considerable impact on quality of life. Because pregnancy and childbirth can put young women at risk of developing UI [69], research and clinical attention should be focused on understanding the underlying mechanisms of UI as well as developing effective strategies (perform pelvic floor muscle training, maintain normal weight, avoid constipation [70]) to preserve bladder health for young Chinese nulligravid women.
In our research, there was only one study focused on the young women (18-26 years old) and it found that age (21-26 years old comparing with 18-20 years old), constipation, alcohol consumption, often/always delayed urination, and/or often/always strained to urinate were associated with UI. The prevalence of UI in this study was 23.6% [44]. Evidence from studies of young women who live outside of China also provides compelling evidence that UI in young women should be investigated further and intentionally. For example, the prevalence rate of UI for Swedish women (N = 653) between the ages of 18 and 30 years was 12% [71]. In nulligravid Australian women aged 16-30 years (average age 22.5 ± 3.2 years), the prevalence of overall UI was 12.6% [72], and women  20:159 who were sexually active and those who were not using oral contraceptives had the highest rates of UI [72]. Mishra et al. 's study found that the UI prevalence rate for Australian women aged 22-27 years was 6.8% at baseline and increased to 16.5% nine years later [73]. These researchers also reported that women with depressive symptoms or a history of depression were more likely than those without depressive symptoms to report subsequent UI symptoms [73]. For nulligravid women living in Italy between 15 and 25 years old, age, BMI, depression/ anxiety/panic attacks, eating disorders, and constipation were risk factors for UI [74]. Participating in organized sports that involves high-volume exercise for competition also increased the risk of developing UI (OR = 2.53, 95% CI = 1.3-2.7) [75]. Other studies conducted outside of China showed that UI is an issue for many nulliparous female athletes [76].
Cultural differences may be evident with regard to UI risk and associated factors in China and abroad. A relatively new factor under investigation is toileting behaviors, i.e., actions women take immediately prior to and during urination [77][78][79][80]. Toileting behaviors play a role in developing or worsening urinary symptoms, but more research is required, especially studies that focus on young women in China [44]. It is often during youth and young adulthood when women develop habitual behaviors and form beliefs and attitudes about bladder health for themselves and their children. This period in women's lifecourse may be pivotal in influencing prevalence rates because evidence is mounting that research to prevent or reduce UI in this age stage is important [65].
This review also found that UI prevalence rates for middle-aged Chinese women ranged from 8.7 [34] to 47.7% [36], which represents 15.5-85.0 million middle-aged women with UI. The UI prevalence rate for women between 45 and 60 years old living in Brazil was 23.6% [4]. The associated factors relate mainly to obstetrics-related ones, such as parity, perineal laceration, and postpartum UI, and gynecological factors, such as menstrual disorder, menopause, pelvic organ prolapse, pelvic operation history, and hormone replacement therapy (see Table 1). UI prevalence rate was found to be significantly higher in a postmenopausal group than a premenopausal age group [81], which may be related to the change of hormone levels in postmenopausal women [82]. Research conducted in China found that the protective effect of cesarean section delivery compared with vaginal delivery was more obvious at five years postpartum than at one year postpartum [83]. Vaginal delivery appears to aggravate pelvic floor structure injuries. Compared to this study, studies conducted in other countries had similar findings. In Norway, a survey of middle-aged women (average age 47 years) who had delivered either vaginally or by Caesarian sections 15-23 years previously had 46.9% UI prevalence. In addition, caesarian section delivery was a protective factor as UI prevalence was lower than in this group of women as compared to women who delivered vaginally. [84]. Further research is needed to determine the mechanism(s) of injury during childbirth and identify associated factors and interventions that prevent or lessen adverse effects of childbirth on bladder health.
Although UI is common across the lifecourse, its prevalence peaks in the older age group of women [3]. China's older female UI prevalence rate ranged from 16.9 [57] to 61.6% [60], which translates to more than 12.8-46.7 million older Chinese women living with UI. Scholars outside of China have found similar UI prevalence rates for older women. A survey of 622 older women living in Brazil (average age 64 years) revealed that the UI prevalence rate was 52.3% [85] and for women over 65 years old living in Turkey the UI prevalence was 51.6% [86]. In China, risk factors associated with UI include being 80 years old and over, BMI, malnutrition, low educational level, sleep disorders [58], unspecified number and types of medications, history of pregnancy, urine leakage during pregnancy, chronic respiratory disease, cardiovascular disease, gynecological diseases, diabetes, urinary tract diseases [31], chronic pain, constipation, and restricted activity [41] (see Table1). Compared to middle-aged women, older women have a higher prevalence of chronic diseases (i.e., respiratory disease, hypertension, diabetes, etc.), limited physical activity, malnutrition, and other factors that could be contributing risk factors for UI. A longitudinal study of older women (baseline ages of 51-74 years) conducted in the United States years who did not have UI found the incidence of UI was 37.2% ten years later [87]. This study also found that UI prevalence in later in life had strong associations with obesity, functional ability, and medical comorbidities, but not with parity [87].
Although we used rigorous methods to conduct this epidemiological review, some limitations are noted. First, although we searched one English database (i.e., PubMed), and the PRISMA review process was followed, we may have inadvertently overlooked eligible articles. Second, research into UI prevalence rates has not been conducted in all 34 provinces in China (e.g., 22 of the 34 provinces are represented in the included studies), which could result in an incomplete picture of the distribution of UI across the Chinese female adult population. The quality of the included studies was not formally assessed in order to include a broad representation of the literature. It is recommended that future studies include this important element. This paper, however, provides important information and raises awareness about prevalence of UI in women living in China.

Conclusions
Most of the information in this review has been previously unavailable to researchers in countries outside of China. Thus, in addition to adding to the specific knowledge about UI in Chinese women, this review adds to the global knowledge base about female UI. Findings from the reviewed studies revealed that UI prevalence rates for Chinese women range from 8.7 to 69.8%. Most of the studies focused on middle-aged and older women. Little information about UI prevalence for women in their teens and twenties was found. Given the prevalence of UI and size of China's female population, research is needed to better understand the risk, and protective, factors for UI across the lifecourse of women. This information should stimulate interest in further multi-country comparisons and the development and testing of evidence-based interventions to prevent UI.                  [88] SUI*: Urine leakage during exercise SUI**: In the past year, loss of urine more than twice when abdominal pressure is increased (including stress, laughing, coughing, etc.) UUI*: Urinating without any warning or a small amount of early warning, sudden stimulation results in uncontrolled urine outflow UUI**: Occurrence of urinary frequency, urgency, increased frequency of nocturnal discharge and decreased urine output, or cannot control, too late, leading to urine leakage MUI*: SUI and UUI exist at the same time