Key points

Introduction

Intrinsic sphincter deficiency (ISD) is a pathologic condition that can lead to female stress urinary incontinence (SUI), a condition that affects approximately 22% of women aged 45 to 64 years. SUI is currently defined by the International Continence Society (ICS) as “the complaint of involuntary loss of urine on effort or physical exertion or on sneezing or coughing.” Clinically, one will observe leakage of urine per urethra provoked by synchronous activity that increases intra-abdominal pressure (ie, coughing, Valsalva). In women, the causes of SUI are multifactorial, but to some degree are attributable to urethral hypermobility, impaired sphincteric function, or a combination of the two. Defining the causes of SUI was believed to be helpful when counseling patients about the available surgical options and response to therapy, but as will be discussed later, this article is no longer as important, due to advances in the surgical techniques.

Introduction

Intrinsic sphincter deficiency (ISD) is a pathologic condition that can lead to female stress urinary incontinence (SUI), a condition that affects approximately 22% of women aged 45 to 64 years. SUI is currently defined by the International Continence Society (ICS) as “the complaint of involuntary loss of urine on effort or physical exertion or on sneezing or coughing.” Clinically, one will observe leakage of urine per urethra provoked by synchronous activity that increases intra-abdominal pressure (ie, coughing, Valsalva). In women, the causes of SUI are multifactorial, but to some degree are attributable to urethral hypermobility, impaired sphincteric function, or a combination of the two. Defining the causes of SUI was believed to be helpful when counseling patients about the available surgical options and response to therapy, but as will be discussed later, this article is no longer as important, due to advances in the surgical techniques.

History

In an effort to facilitate appropriate diagnosis and treatment, SUI was classified by Green in 1962 based on the anatomic configuration of the bladder and the urethra. Type I SUI was defined as the loss of a normal posterior urethrovesical angle and support of the bladder neck. On fluoroscopy, one would visualize a straightening of the posterior urethrovesical angle to greater than 180° with straining. Urethral pressure measured by urodynamics would show a maximal urethral closure pressure (MUCP) in the proximal urethra to be greater than 20 cm H ₂ O. Women with Type II SUI demonstrated not only straightening of the posterior urethrovesical angle but also experienced hypermobility of the proximal urethra and bladder neck, which was seen as a shift down and back of greater than 3 cm (vs only 2–3 cm in those with Type I SUI) with straining. Again, the MUCP was measured greater than 20 cm H ₂ O. In both Type I and Type II SUI, a rise in intra-abdominal pressure is not equally distributed between the bladder and the posterior urethra once the urethra has descended out of its normal anatomic position. The increased pressure experienced by the bladder relative to the urethra produces a pressure differential and facilitates leakage. In the 1970s, McGuire reported an additional cause of SUI, which has since been labeled Type III. In these patients, there was no urethral hypermobility, but the bladder neck and proximal urethra did not function properly, which was thought to be associated with low closure pressures. Type III SUI was also referred to as ISD and is associated with an MUCP less than 20 cm H ₂ O.

Historically, the clinical categorization of SUI was considered an important factor in the determination of treatment, specifically which surgical intervention to use. Certain surgical interventions, like injection of urethral bulking agents, which are aimed at augmenting compression and increasing resistance of the mid- to proximal urethra, were found to improve symptoms in some women with Type III SUI, but still had higher failure rates in patients with both low Valsalva leak point pressures (less than 60 cm H ₂ O) and MUCPs (less than 20 cm H ₂ O). Retropubic urethral suspension procedures, which are aimed at preventing mobility of the bladder neck and urethra, were found to be most effective in those with Type I or Type II SUI. Mechanistically, these findings made conceptual sense and further supported the categorization of SUI. With the advent of the suburethral sling procedure, first placed at the bladder neck and more recently placed at the midurethra, the categorization of SUI has become less important as the sling has been deemed effective in all categories of SUI. However, there is continued interest in predicting response to surgery as some evidence suggests slightly improved dry rates following a sling for women with urethral hypermobility versus those with ISD.

Using urodynamics to evaluate SUI

When it is important for the clinician to establish the cause and type of SUI, urodynamics is commonly utilized. The goal of urodynamics in a patient presenting with SUI is to objectively measure, in a way that is quantifiable, the lower urinary tract function of each patient and to find a physiologic reason for their subjective symptoms. The 2002 ICS “Good Urodynamic Practices” report recommends first obtaining a clear history, including a voiding diary, and performing a thorough physical examination, which could include a cough stress test. The clinician may also want to obtain noninvasive urinary flow rate and post-void residual urine volume measurement if indicated by the clinical scenario. However, if attempting to define the types of SUI, invasive urodynamics (filling cystometry, pressure flow) would be needed. Invasive urodynamics uses a pressure transducer in the bladder and the rectum (or vagina) along with electromyography, which is most commonly obtained by surface electrode patches on the anal sphincter, measures lower urinary tract function, and helps identify the pathology behind the lower urinary tract symptoms. Urodynamic measures include bladder capacity; bladder compliance; abdominal leak point pressure (ALPP) (which can be induced by Valsalva leak point pressure or cough [cough leak point pressure]); MUCP; and the presence of observations such as detrusor overactivity, bladder outlet obstruction, detrusor sphincter dyssynergia, and underactive detrusor contraction.

To standardize measurements during urodynamics, the ICS recommends establishing a detrusor pressure of zero at the start of the study by equalizing the vesicle and abdominal pressure transducers. For fluid-filled systems, a reference height is established at a height that is equal to that of the upper edge of the pubic symphysis (as opposed to air charged, which do not require this reference point). A dual lumen transurethral catheter is usually used to measure intravesical pressure and allow for simultaneous bladder filling. A rectal (or vaginal) catheter is used for the measurement of abdominal pressure. It is also recommended that the display include 3 measurement channels: a method of displaying and storing pressure data; the ability to accurately measure pressure from 0 to 250 cm H ₂ O, flow from 0 to 50 mL/s, and volume up to 1000 mL; and a standard scale for displaying data. All equipments should be appropriately and regularly calibrated. To help achieve accurate measurements, the ICS recommends assuring that resting values are appropriate, abdominal and intravesical pressures vary similarly with breath or movement, and that coughs are regularly used to be sure of equivalent abdominal and intravesical response. Finally, it is suggested that urodynamic testing should be repeated to confirm reproducibility of the results before initiating treatment.

Even with these guidelines in place, urodynamic testing can be challenging. There are many additional factors that can be altered during testing. These alterations undoubtedly affect results and are without standardized guidelines from the ICS currently. These will be discussed in greater detail later with specific relation to ISD. Additionally, the reliability and reproducibility of urodynamics has been discussed by the ICS. During the evaluation of SUI, and specifically ISD, careful attention is paid to the values of ALPP and MUCP. There are no standard recommendations for positioning during urodynamics, and in a 2002 study out of UCLA, the effect of positioning on leak point pressure was evaluated. Thirty-seven patients with SUI and 4 with mixed incontinence underwent measurement of leak point pressures while in supine, semirecumbent, and standing positions. They found that leaking occurred with less intravesical pressure as the patient went from supine to standing position.

It has been noted in the literature that there are differences in MUCP depending on the type of catheter used. In a 2008 prospective study by Zehnder, 64 women were randomized to undergo MUCP measurements first with either an air-charged catheter or a microtip catheter used. MUCP was then measured again with the other type of catheter. Mean MUCP was significantly higher with the use of an air-charged catheter, but the repeatability of measurements was stable for both catheters. Therefore, although the measurement of MUCP may be trusted to be reproducible for a given woman, it is challenging to establish reference ranges and compare data across studies without standardization of catheter type.

Using urodynamics to diagnose ISD

ISD is believed to be due to deficiencies in the urethral and periurethral tissues that result in weakness of the sphincteric mechanism. These weaknesses can result from age, pregnancy, and childbirth; the development of a neurologic injury; or from previous surgical or radiation therapies. Clinically, urethral function is objectively measured by ALPP and MUCP. ALPP is defined by the ICS as the intravesical pressure at which urine leakage occurs due to increased abdominal pressure in the absence of a detrusor contraction. MUCP is the highest pressure, relative to bladder pressure, generated along the length of the urethra. The literature has suggested that both a low MUCP and ALPP be used to diagnose ISD, and the cutoff values most commonly recommended are an ALPP less than 60 cm H ₂ O and a MUCP less that 20 cm.

Part of the challenge with reliably diagnosing ISD with urodynamics is a lack of true consensus on how exactly the ALPP and MUCP are measured. McGuire initially described ISD as being a loss of tone that manifested itself as a low proximal urethral pressure. He advocated using fluoroscopy during urodynamics to appropriately position a catheter in the proximal urethra while measuring urethral pressures, as this was the “part of the urethra concerned with resistance to abdominal pressure as an expulsive force.” In the early 1990s, McGuire introduced a new urodynamic measure that he believed was better correlated with ISD, the ALPP.

McGuire’s technique for testing ALPP is as follows: first the flow pressure of the system must be zeroed. Then, the bladder is filled at 60 mL/minute until the bladder contains approximately 200 to 250 mL of fluid. The patient is placed in an upright position, maintaining the transducer at the height of the pubic symphysis, and is asked to perform a slow, progressive Valsalva until leakage occurs. He recommends repeating the study several times to obtain an average abdominal pressure at the moment of leakage.

As mentioned, a diagnosis of ISD is based on abnormal measurements of ALPP and MUCP. Hosker published a recent review of the literature focusing on the methods used to obtain these values clinically. From the studies he referenced it is clear that there is no set consensus on how exactly urodynamics is to be performed and interpreted when trying to identify ISD. Although it appears that the urologic community agrees that ALPP of 60 H ₂ O is a requirement for diagnosing ISD, there were different definitions for the cutoff for MUCP. Most studies used an MUCP of 20 cm H ₂ O, but some used 15 H ₂ O or 30 H ₂ O as their parameter for ISD.

Furthermore, the methodologies for measuring MUCP and ALPP varied significantly. They were measured with an empty bladder, with the bladder at capacity up to 500 mL, with150 mL in the bladder, with 250 mL in the bladder, and with a bladder volume between 200 and 300 mL. Patient positioning and catheter size were also not standardized. Measurements were obtained in the semirecumbent, standing, and upright sitting positions. Catheter sizes ranged from 7 to 10 French. Further divergence in technique could develop when considering the other components of urodynamic testing such as catheter withdrawal speed, viscosity of bladder fluid, rate of infusion, and the use of a Valsalva or a cough.

Typically, MUCP is measured at rest, as opposed to ALPP, which is measured during an increase in intra-abdominal pressure, which simulates the real world scenario that leads to leakage. MUCP may reflect underlying deficiencies in the sphincteric function, but may miss deficiencies that are only notable during increases in intra-abdominal pressure. In addition to the association with degree of incontinence, MUCP has been found to correlate strongly with age, which suggests that the aging process weakens the intrinsic function of the urethra even in continent women. MUCP is also significantly decreased in women who have previously had anti-incontinence surgery.

In Table 1 , examples of normative urodynamic values of MUCP in women of various ages who are either continent or with stress incontinence are displayed. In both groups, MUCP is seen to decrease with age, but there is a wide range of MUCP in each age group, making it difficult to interpret and establish diagnoses. Of note, there are women who are continent and yet have MUCP less than 20 mm H ₂ O. Given this evidence that MUCP can vary with age and degree of incontinence, it complicates the interpretation of these data for diagnosis of ISD.

Table 1

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