Clinical Review

URINARY INCONTINENCE

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The past year has seen the publication of much useful evidence regarding urinary incontinence, from both epidemiologic studies and clinical trials. Research into the pathophysiology of incontinence continues to move forward, slowly but surely, measured not in breakthroughs but in gradually increasing knowledge of how the urethra and bladder function in the continent person and how that function can break down, leading to incontinence and other urinary symptoms.

Highlighted here are four notable studies from 2007, as well as progress notes on a trial mentioned early this year in Examining the Evidence (January issue).

New data clarify incidence and uncloak the effect of weight gain

Townsend MK, Danforth KN, Liffort KL, et al. Incidence and remission of urinary incontinence in middle-aged women. Am J Obstet Gynecol. 2007;197:167.e1–167.e5.

Townsend MK, Danforth KN, Liffort KL, et al. Body mass index, weight gain, and incident urinary incontinence in middle-aged women. Obstet Gynecol. 2007;110:346–353.

Studies of urinary incontinence in numerous populations have reported its prevalence—i.e., the percentage of people who have the condition at any point in time—but few have attempted to define its incidence—i.e., the rate at which it develops during a defined period.

Incidence is a true rate, described with a unit of time in the denominator. Prevalence is not a rate (although it is commonly referred to as such) and is described as a percentage only, without time in the denominator. With that distinction in mind, it is easy to see why prevalence data greatly outnumber incidence data: Prevalence can be obtained by means of cross-sectional study, with one-time collection of data. In contrast, incidence data require a population that is free of the condition of interest at baseline; that population is then followed to determine how many people who were initially free of the condition go on to develop it.

Lack of a standard definition makes it hard to measure incontinence

Reported prevalence can range from less than 10% to more than 90%, depending on how incontinence is defined:

  • Very low prevalence is found when the definition is limited to persons with the greatest severity or frequency of symptoms
  • at the other end of the spectrum, very high prevalence—even approaching 100%—can be found using a definition that includes people who have “ever” leaked urine.

The same issues complicate estimates of incidence. Because there is no consensus over what constitutes a clinically significant threshold for incontinence, investigators are forced to develop their own definitions.

In a pair of studies, Townsend and colleagues neatly circumvent this problem. Using data from the Nurses’ Health Study II, they used a series of definitions of incontinence, ranging from less severe to more severe, to describe their findings in ways that are easily transferred to clinical practice. They focused their attention on women aged 36 to 55 years to estimate the incidence of incontinence over a 2-year period. At baseline, women were considered at risk of incident incontinence if they reported never leaking or leaking only a few drops less than once a month. Three categories of incontinence were then defined, based on symptoms 2 years later:

  • incident incontinence: any urine loss, defined as leaking 1–3 times a month
  • frequent incontinence: urine loss at least once a week
  • severe incontinence: urine loss at least once a week of sufficient volume to at least wet underwear.

Incidence rose with BMI, weight gain

In almost 34,000 continent women from 2001 to follow-up in 2003, the overall (average) incidence of urinary incontinence was 6.9 women for every 100 woman-years. Frequent incontinence developed in, on average, 1.8 women for every 100 woman-years; severe incontinence, in 0.6 women for every 100 woman-years.

Using multivariable logistic regression models, the authors analyzed the likelihood of incident incontinence by body mass index (BMI) and estimated weight gain from the age of 18 until 2001. For either variable, odds ratios (OR) showed a highly significant trend (P<.001) for an increased risk of incident incontinence.

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