URGE INCONTINENCE: DETERMINING ENVIRONMENTAL AND OBSTETRICAL RISK FACTORS USING AN IDENTICAL TWIN STUDY DESIGN

R. Goldberg, Y. Abramov, S. Botros, J. Miller, M. Rurak, P.K. Sand  

Evanston Continence Center, Evanston, IL; Northwestern University, Feinberg School of Medicine, Chicago, IL

OBJECTIVE:  Identical twin studies represent an ideal methodology for assessing environmental disease factors, providing ‘natural control’ over genetic variance that cannot be achieved within random samples of unrelated individuals.  We conducted an extensive survey of pelvic symptoms at the world’s largest annual gathering of twins, and investigated the main environmental determinants of urge urinary incontinence (UI).
MATERIALS AND METHODS:  The 67-item survey was completed by 271 identical twin sister pairs (n=542).  To account for correlated data within pairs, we implemented logistic regression for repeated binary measures. Numerous medical, obstetrical and demographic factors underwent univariate analysis, entering multivariate regression based on p<0.25.  Statistical analyses were performed using SAS.
RESULTS:  Demographics of the sample included mean age 47.5 (21-79), parity 1.8 (0-7), 45% menopausal, mean BMI 26.6, 90.3% Caucasian and 7.0% African American.  UI was reported by 35% of the cohort, and by 42.9% of parous women.  The risk of UI increased steadily with advancing age (OR 2.3 for age 40-49, 2.5 for 50-59, 2.7 for >60, p=0.001).  Parity represented the strongest environmental risk factor for UI, with the following odds ratios relative to nulliparous women: OR 2.4 for 1 birth, 4.4 for >2 births, p=0.001.  Obesity, defined as BMI>30, was the second factor significantly predictive of UI (OR 3.3, p=0.001) in the multivariate analysis. Among parous women, 83.8% had at least one vaginal birth and 16.2% by cesarean only.  Rates of UI in these groups were 45.1% and 27.9%, respectively.  The univariate analysis indicated a trend towards a lower risk of UI after ‘cesarean only’ versus vaginal birth (OR 0.53, p=0.09); however, this factor did not achieve statistical significance (OR 0.56, p=0.20) in the multivariate regression analysis of 173 (n=346) parous-parous twin pairs. Factors not associated with UI risk included: current or previous smoking, asthma, weight of largest baby, duration of 2nd labor stage, episiotomy, forceps, and hysterectomy.  
CONCLUSIONS:  Although the prevalence of urge incontinence is known to increase with age, the role of environmental and obstetrical risk factors has remained uncertain.  This study represents the first application of an identical twin research design to this question, providing nearly complete control over known and unknown genetic factors (‘nature’) that tend to confound the study of environmental disease factors (‘nurture’).  Parity and obesity were identified as the main environmental risk factors determining the risk of urge incontinence in women.  Vaginal birth mode may be associated with a higher risk of UI than cesarean; however total number of children appears to represent the more important obstetrical determinant.

Key Words:  urge incontinence, epidemiology, risk factors, identical twins

Disclosure - Consultant: R. Goldberg, Watson; Speakers Bureau: P.K. Sand, Alza/Ortho-McNeil, Watson, American Medical Systems; Advisory Board: P.K. Sand, Alza/Ortho-McNeil, Watson, American Medical Systems, Indevus, Yamanouchi, Roche, Pfizer, Bioform, Lilly, Boeringer-Ingelheim, Mentor, Novasys; Grant/Research: P.K. Sand, Alza/Ortho-McNeil, Watson, American Medical Systems, Indevus, Yamanouchi, Roche, Pfizer, Bioform, Lilly, Boeringer-Ingelheim, Mentor, Novasys.