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Risk Factors for Female Anal Incontinence: New Insight Through the Evanston-Northwestern Twin Sisters Study

From the Division of Urogynecology and Reconstructive Pelvic Surgery, Evanston Continence Center, Northwestern University, Feinberg School of Medicine, Evanston, Illinois; and The Center on Outcomes Research and Education (CORE), Evanston Northwestern Healthcare, Evanston, Illinois.

Address reprint requests to: Address correspondence to: Yoram Abramov, MD, Division of Urogynecology and Reconstructive Pelvic Surgery, Evanston Northwestern Healthcare, 1000 Central Street, Suite 730, Evanston, IL; e-mail: y-abramov{at}northwestern.edu.

	ABSTRACT

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OBJECTIVE: To evaluate risk factors for anal incontinence using an identical twin sisters study design to provide control over genetic variance.

METHODS: A total of 271 identical twin sister pairs (mean age 47 years) completed the validated Colorectal Anal Distress Inventory questionnaire detailing the presence and severity of anal incontinence. Data were analyzed using a stepwise logistic regression with repeated binary measures to account for correlated data within twin pairs. Three different statistical models were used to analyze nonobstetric as well as obstetric risk factors separately.

RESULTS: Significant risk factors for anal incontinence and higher Colorectal Anal Distress Inventory anal incontinence subscale scores included age 40 years or older (fecal: odds ratio [OR] 2.82, 95% confidence interval [CI] 1.21–6.0; flatal: OR 1.90, 95% CI 1.11–3.24), menopause (fecal: OR 2.10, 95% CI 1.15–3.8; flatal: OR 2.11, 95% CI 1.43–3.13), increasing parity (parity 2; fecal: OR 3.09, 95% CI 1.25–7.65; flatal: OR 2.72, 95% CI 1.65–4.51), and the presence of stress urinary incontinence (fecal: OR 2.11, 95% CI 1.12–3.98; flatal: OR 1.72, 95% CI 1.14–2.59). Obesity was associated with significantly higher Colorectal Anal Distress Inventory anal incontinence subscale scores (mean difference 5.18, P = .007). Cesarean delivery after initiation of labor was associated with a lower prevalence of anal incontinence than vaginal birth; however, this difference was not statistically significant (17% compared with 4%, P = .11). No anal incontinence was noted in women who had only elective cesarean deliveries.

CONCLUSION: Age, menopause, obesity, parity, and stress urinary incontinence are the major risk factors for female anal incontinence.

LEVEL OF EVIDENCE: II-2

	MATERIALS AND METHODS

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An extensive survey of incontinence symptoms was conducted at the world's largest annual gathering of twins at the 2003 and 2004 Twins Day Festival, in Twinsburg, Ohio, as part of the Evanston-Northwestern Twin Sisters Study. Two hundred seventy-one pairs of identical twin sisters (N = 542) completed the previously validated Colorectal Anal Distress Inventory questionnaire regarding the presence, frequency, and severity of anal incontinence.⁸ This 16-item questionnaire is part of a larger 46-item validated Pelvic Floor Distress Inventory questionnaire, in which each question receives a score from 0 to 4, where 0 indicates no symptom and 4 indicates the maximal severity of the symptom. The Colorectal Anal Distress Inventory is composed of 4 subscales of questions, namely, obstructive, incontinence, pain or irritation, and rectal prolapse. To calculate each subscale score, the average of the questions' scores is multiplied by 25. The Colorectal Anal Distress Inventory incontinence subscale, which was used for this study, is composed of the following 5 questions: 1) Do you usually experience loss of gas or stool as the result of physically stressful activities such as with exercise, coughing, sneezing, or hard laughing? 2) Do you usually experience loss of gas or stool after a sense of urgency or after another warning sensation? 3) Do you usually lose stool beyond your control if your stool is well formed? 4) Do you usually lose stool beyond your control if your stool is loose or liquid? 5) Do you usually lose gas from the rectum beyond your control? The Colorectal Anal Distress Inventory incontinence subscale scores range from 0 to 100, where 0 indicates no symptoms and 100 indicates maximal severity of symptoms. The mean differences in the total scores were calculated for each potential risk factor for anal incontinence.

In addition, each subject was asked the following 4 questions: 1) Do you lose control of liquid stool? 2) Do you lose control of solid stool? 3) Do you lose control of passing gas from your rectum? 4) Do you leak urine with coughing, straining, laughing, physical activity or exercise? Fecal incontinence to liquid or solid stool was defined by a patient answering "yes" to questions 1 or 2, respectively. Flatal incontinence was defined by a patient answering "yes" to question 3. Anal incontinence was used as a general term for both fecal and flatal incontinence. Stress urinary incontinence was defined by a patient answering "yes" to question 4. The definitions for fecal incontinence (incontinence of stool) flatal incontinence (incontinence of gas), and stress urinary incontinence in this study conformed to the recommendations from the National Institutes of Health Terminology Workshop for Researchers in Female Pelvic Floor Disorders.⁹ Sociodemographic data as well as obstetric and surgical histories were also obtained. Each participant anonymously completed the survey without verbal assistance by the physicians administering the study.

We used a clustered stepwise logistic regression model with repeated binary measures to account for correlated data within twin pairs with regard to a large number of demographic, medical, and obstetric factors.¹⁰ To maintain statistically valid reference groups, we used 3 different statistical models: The first model concentrated on nonobstetric risk factors for anal incontinence and included all pairs of identical twins (N = 542). The second aimed to assess the effect of the mode of delivery (ie, vaginal compared with cesarean delivery) on anal incontinence and included all twin pairs where both sisters underwent at least 1 childbirth either vaginally or by a cesarean delivery (n = 346). The third model was designed to evaluate risk factors specific to the vaginal birth mode (such as instrumental delivery, episiotomy, birth weight, etc.) and included pairs where both sisters had at least 1 previous vaginal delivery (n = 274). We performed the stepwise multivariate analyses using Microsoft Excel 2002) (Microsoft Corp., Redmond, WA) and SAS (SAS Institute, Cary, NC). P < .05 were considered statistically significant. Odds ratios were adjusted for potential confounding factors, including: age, race, parity, body mass index (BMI), menopausal status, and mode of delivery. The study protocol was approved by the Investigational Review Board of the Evanston Northwestern Healthcare Research Institute.

	RESULTS

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Response rate for the survey was 72% of all eligible twin pairs. Demographic and obstetric characteristics of the study population are presented in Table 1. Ninety percent of the women were white and 46% were postmenopausal, with a mean age of 47 years (range 26–86 years). Seventy-three percent of respondents had at least 1 previous childbirth, and 14% had cesarean deliveries, of which 9 (16%), were elective (Table 1). Any type of anal incontinence (flatal or fecal) was reported in 32% of all women (Table 2). The prevalence of flatal incontinence was substantially higher than that of fecal incontinence (28% compared with 11%). The prevalence of both fecal (12% compared with 8%) and flatal (34% compared with 16%) incontinence were higher in parous than in nulliparous women (odds ratio [OR] 3.09, 95% confidence interval [CI] 1.25–7.65 for fecal and OR 2.72, 95% CI 1.65–4.51 for flatal incontinence in women after 2 childbirths) (Tables 2^, 3^, and 4). Parous women also scored higher in the Colorectal Anal Distress Inventory incontinence subscale (mean difference 6.57, P = .002 for women after 2 childbirths) (Table 5).

Additional significant risk factors for both fecal and flatal incontinence included age 40 years or older (OR 2.82, 95% CI 1.21–6.0 for fecal incontinence and OR 1.90, 95% CI 1.11–3.24 for flatal incontinence), menopause (OR 2.10, 95% CI 1.15–3.8 for fecal incontinence and OR 2.11, 95% CI 1.43–3.13 for flatal incontinence), and presence of stress urinary incontinence (OR 2.11, 95% CI 1.12–3.98 for fecal incontinence and OR 1.72, 95% CI 1.14–2.59 for flatal incontinence) (Tables 3 and 4). These risk factors were also associated with significantly higher scores in the Colorectal Anal Distress Inventory incontinence subscale (Table 5).

The prevalence of fecal incontinence was higher in women who had vaginal births than in those who had cesarean deliveries only (17% compared with 4%); however, this difference did not reach statistical significance (OR 0.39, P = .11) (Tables 2 and 3). Prevalence rates of flatal incontinence were not significantly different among women who had cesarean delivery only and those who had at least 1 vaginal birth (27% compared with 35%, OR 0.92, P = .71) (Tables 2 and 4). Of the 9 women who had planned (elective) cesarean delivery only, none had either fecal or flatal incontinence. Colorectal Anal Distress Inventory anal incontinence subscale scores were lower in women who underwent cesarean deliveries only, however this difference did not reach statistical significance (mean difference –3.18, P = .41) (Table 5).

Episiotomy, forceps delivery, prolonged second stage of labor, high birth weight, previous hysterectomy, and smoking habits did not significantly affect the risk for anal incontinence (Tables 3 and 4). These factors were also not associated with statistically significant differences in Colorectal Anal Distress Inventory anal incontinence subscale scores (Table 5). Although the risk for overt anal incontinence was similar between obese (BMI > 30) and nonobese women (OR 1.35, P = .34 for fecal and OR 1.40, P = .13 for flatal incontinence), Colorectal Anal Distress Inventory anal incontinence subscale scores were significantly higher in obese subjects (mean difference 5.18, P = .007) ().

	DISCUSSION

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Among all environmental factors studied, parity and obesity demonstrated the greatest effect on anal incontinence risk, with roughly 3-fold higher rates among women with 2 births or more. The risk of anal incontinence also increased significantly with age, menopause, and the presence of stress urinary incontinence.

The primary focus of our identical twin study design was on "environmental" risk factors. Among younger women, vaginal childbirth has been implicated as a significant "environmental" predisposing factor for anal incontinence due to mechanical trauma to the anal sphincter as well as neurologic trauma to the pelvic floor.^1,2,11,12 Overt sphincter damage due to a third-degree or fourth-degree tear occurs in approximately 0.7% of women undergoing vaginal delivery.¹³ In addition, occult sphincter defects occurring during vaginal birth in up to 35% of women may predispose women to anal incontinence.^1,2 Decreased anal squeeze pressures have been reported after vaginal delivery.¹⁴ Pudendal nerve conduction abnormalities, reflecting denervation injuries to the pubococcygeus and external anal sphincter muscles occur in the majority of vaginal deliveries.¹² Although most previous studies have concentrated on the short-term effects of childbirth on anal sphincter function, the current study assessed childbirth's long-term effects on anal incontinence symptoms and quality of life in postreproductive twin sisters. The study identified a significantly higher risk for fecal incontinence after 1 childbirth, increasing further (more than 3-fold) with higher parity. These findings suggest that childbirth has a cumulative and long-standing detrimental effect on anal sphincter function and that many cases of anal incontinence can be accurately considered a postreproductive women's health disorder.

Because women are thought to sustain both mechanical and neurologic trauma during vaginal delivery, previous studies have investigated a possible protective role of cesarean delivery for anal incontinence, with conflicting results.^7,15 Some authors have suggested that this protective effect may be more discernible in elective cesarean delivery, because the damage to the pelvic floor may occur as early as the first or the beginning of the second stage of labor.¹⁵ In the current study, although cesarean delivery performed during labor seemed to confer some protection against fecal incontinence, this trend did not reach statistical significance. Nine women who underwent only elective cesarean delivery reported neither fecal nor flatal incontinence, suggesting a protective effect; however, definitive conclusions would require a larger sample size.

Previous studies have reported on increased risk for anal sphincter tears and anal incontinence after forceps delivery.^1,7,16–18 The current study did not identify an increased risk for either fecal or flatal incontinence in women who underwent forceps delivery. The explanation for this discrepancy may be that many of the previous studies concentrated on the risk for anal sphincter tears and not necessarily on anal incontinence symptoms. Those that focused on anal incontinence symptoms reported on their findings shortly (up to 18 months) after childbirth. In contrast, the current study investigated incontinence symptoms several years after the last childbirth. Therefore, the possible effects of forceps intervention and resultant sphincter tears may have been washed out by more significant and long-lasting risk factors such as age and additional vaginal deliveries. In a 30-year retrospective cohort study, Nygaard et al¹⁹ concluded that the increased risk of anal incontinence associated with known anal sphincter disruption is not sustained 30 years after delivery.

The same explanation may apply to the effects of episiotomy, increased birth weight, and prolonged second stage of labor on the risk for anal incontinence in postreproductive women. Previous studies have reported conflicting data regarding the importance of these risk factors in the pathophysiology of anal incontinence.^2,17,20 Most of these studies reported on anal sphincter rupture or on incontinence symptoms shortly after childbirth. Our study did not find a significant increase in the risk for anal incontinence with any of these factors, suggesting that other risk factors, such as age and repeated deliveries may become more important several years after labor.

In the current study we found aging to be a major risk factor for anal incontinence. This finding conforms to previous reports by Nygaard et al,¹⁹ and Goldberg et al,²¹ identifying aging as an important determinant in the development of anal incontinence symptoms during the postreproductive years. The contribution of obesity to female anal incontinence has been a matter of debate. Although some studies reported increased risk for anal incontinence in obese women,²² others have failed to find such an association.²³ Our twin sisters study found a statistically significant detrimental effect of obesity (BMI > 30) on the Colorectal Anal Distress Inventory anal incontinence subscale, suggesting that obesity may in fact increase the risk for anal incontinence.

Another interesting observation was a strong association between urinary and anal incontinence. Because urinary and anal incontinence share many causative factors, including obstetric trauma, the aging process, and neurologic disorders, it seems plausible that these 2 conditions would coexist. Previous studies have shown high prevalence of anal incontinence among women referred to a urogynecology unit. Meschia et al²² reported a prevalence of anal incontinence of up to 20% in a referral population presenting with symptoms of urinary incontinence and pelvic organ prolapse. Gordon et al²⁴ reported a 29% prevalence of anal incontinence among urogynecologic patients. To the best of our knowledge, and after MEDLINE search of the English-language literature since 1966, the current study is the first to evaluate this association in a cross-sectional analysis of community-dwelling women. We found a significantly increased risk for both fecal and flatal incontinence as well as a significant detrimental effect on Colorectal Anal Distress Inventory anal incontinence subscale scores in women with stress urinary incontinence, confirming that anal and urinary incontinence tend to coexist in community-dwelling women. This finding may have implications for symptom screening in the general female population.

Certain limitations of this study should be considered to interpret these results properly. First, the study provides only subjective data on incontinence symptoms, because objective assessment of symptoms was not part of its design. All data were obtained retrospectively and were therefore liable to recall bias. As a result, we did not obtain information regarding anal sphincter tears during childbirth, because we assumed that the majority of women would not be able to provide it accurately. Finally, because more severe cases of anal incontinence may not have attended the twins gathering, this could result in a selection bias against women with more severe symptoms. However, because the prevalence of anal incontinence in this study conformed to the prevalence reported in previous community-based studies, it would be reasonable to assume that this selection bias did not skew our results.

Notwithstanding these limitations, this identical twin-sisters study provides substantial strengths inherent to its unique design. We performed a MEDLINE search of the English-language literature since 1966 using the terms "twins" and "incontinence," and found no previous reports regarding the use of twins models in the research on anal incontinence. The Evanston-Northwestern Twin Sisters Study is the first to use an identical twin-sisters study design to evaluate risk factors for fecal and flatal incontinence. Whereas previous studies may have been biased by genetic risk factors, the current study provides almost absolute control over genetic variance, therefore allowing for optimal assessment of environmental risk factors. In contrast to previous studies that have used various definitions for fecal, flatal, and urinary incontinence, the current study used the definitions recommended by the National Institutes of Health Terminology Workshop for Researchers in Female Pelvic Floor Disorders.⁹ The use of a validated colorectal effect questionnaire added further credibility to the study's findings. As primary preventive strategies for anal incontinence begin to evolve, these findings may provide valuable insight.

	Footnotes

 
Reprints are not available.

doi:10.1097/01.AOG.0000161367.65261.16

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