Urodynamic Assessment: Leak Point Pressures and Urethral Pressure Profile
WHEC Practice Bulletin and Clinical Management Guidelines for healthcare providers. Educational grant provided by Women's Health and Education Center (WHEC).
Attempts to evaluate and quality of urethra's role in storage and voiding disorders have led to the development and use of urethral pressure profilometry and leak point pressure studies. Leak point pressure (LPP) is a measurement of the amount abdominal pressure or detrusor pressure required to overcome outlet resistance and produce incontinence. Urethral pressure profilometry is a graphic representation of pressure within the urethra at successive points along its length. There is no general consensus on how best to evaluate urethral function in women with lower urinary tract dysfunction. Ideally, urethral function is assessed quantitatively in the hope of providing objective parameters that can be used to make important clinical decisions. Urinary continence depends on the pressure in the urethra exceeding the pressure in the bladder at all times, even with increases in abdominal pressure. Contributing to normal urethral compliance and pressure are smooth and striated urethral muscles; fibroelastic tissue of the urethral wall; vascular tension caused by the rich, spongy network around the urethra; and extrinsic compression from surrounding pelvic floor musculature. The urethra lies on the supportive layer composed of the endopelvic fascia and the anterior vaginal wall. This layer is structurally stable because of its bilateral attachment (as a hammock) to the arcus tendineus fasciae pelvis and the levator ani. Intraabdominal pressures compress the urethra to close its lumen; that is, urethral closure pressure during stress rises because the urethra is compressed.
The purpose of this document is to review the definitions, methodology, interpretation variables, and clinical applications of these tests. One of the most important concepts to be put forth in recent years is that "adequate storage at low intravesical pressure" will avoid deleterious upper urinary tract changes in patients with bladder outlet obstruction and/or neuromuscular lower urinary tract dysfunction. Application of this concept to patients with storage problems caused by decreased compliance has also resulted in the concept of the "leak point" as a significant piece of urodynamic data. This "detrusor leak point pressure" is not the same as the "abdominal/coughing Valsalva leak point pressure". The latter parameter refers to the vesical pressure produced by straining, which is necessary to overcome sphincteric resistance and produce incontinence.
Leak Point Pressures (LPPs)
An alternative method to evaluate or objectively assess urethral resistance and function is the leak point pressure. Stress incontinence is abdominal pressure-induced leakage. A measure of the abdominal pressure required to cause leakage is the abdominal or stress leak point pressure. Abdominal and detrusor leak point pressures determine urethral resistance to two different expulsive forces. As far as urethra is concerned, these two forces (abdominal and detrusor pressure) are the sum total of the forces acting on the urethra to cause leakage.
The leak point pressure is the precise pressure at which leakage occurs, requiring close attention to observe the moment of leakage and the bladder pressure at that time. It tests the amount of abdominal pressure required to drive fluid across the urethral sphincter (the urethral "opening" pressure). The study is used to differentiate between genuine stress incontinence (GSI) caused by bladder neck hypermobility versus that caused by intrinsic sphincter deficiency (ISD). The Valsalva Leak Point Pressure (VLPP) study measures the competency of the urethral closure mechanism. The test is dependent on the patient's ability to bear down slowly at an increasing force until appropriate pressures are generated. Leak point pressures, abdominal and bladder, actually reflect outlet resistance at the time these forces individually induce leakage. Neither leak point pressure reflects the strength or character of the detrusor contraction.
Abdominal or stress (cough or Valsalva maneuver) leak point pressure (ALPP) -- it is a measure of the stress competence of the urethra or a measure of the ability of the urethra to resist the expulsive forces of abdominal pressure. It is the amount of abdominal pressure (in cm H2O) required to overcome urethral resistance and produce urine leakage.
Detrusor or bladder leak point pressure -- it is a measurement of the resistance of the urethra to detrusor pressure as an expulsive force. A detrusor leak point pressure is related to upper tract function in that a high detrusor leak point pressure is associated with the propensity for upper tract deterioration.
Determining Leak Point Pressures:
It is performed by filling the bladder until either leakage from the urethra is observed or bladder capacity is reached in the absence of a detrusor contraction. Ideally, the technique of performing an abdominal (cough or Valsalva) leak point pressure should be standardized and performed exactly the same way each time to allow comparison between patients and to make post-treatment studies meaningful. Many controversies exist regarding the technique of this test. These include catheter size, bladder volume, type of provocation, patient's positioning, how the actual rise in pressure is determined, and how best to perform the test in the presence of genital prolapse. The technique commonly used is -- with a 6-French dual-sensor micro-tip transducer in the bladder, subtracted filling cystometry is performed to a bladder volume of 150 to 200 mL. To perform a bladder neck leak point pressure, the filling cystometrogram is performed at a slower filling rate, usually 25 mL/min. Assuming no abnormalities in bladder compliance are present, leak point pressure measurements are performed at this volume. In the sitting position, the patient performs a gradually more vigorous Valsalva maneuver until leakage occurs. The lowest bladder pressure at which leaking occurs is considered the abdominal leak point pressure. If patients do not leak with either a Valsalva or repetitive coughing, the catheter is removed from the bladder and the provocative maneuvers are repeated with the abdominal pressure being measured via an intravaginal or intrarectal catheter. In some patients the pressure may occur at very large volumes and high pressures. Once a bladder pressure of 40 cm H2O is reached, the study can be terminated as ongoing filling above this pressure can be dangerous. Multiple parameters have been shown to affect the abdominal leak point pressure measurements. These include catheter caliber, catheter location (vaginal versus intravesical), bladder volume, the use of coughing versus Valsalva maneuver as the provocation, patient position, and the use of an absolute or change in measured pressure. An inverse relationship exists between bladder volume and abdominal leak point pressures. Anther area of controversy concerns the use of coughing versus a Valsalva maneuver as the technique for provoking urine loss. Cough-induced leak point pressures are consistently higher than Valsalva-induced leak point pressures. The Valsalva leak point pressures have been shown to have excellent reproducibility, with test-retest correlation coefficients of more than 0.9.
The abdominal leak point pressure is being used as a severity measure for dysfunction of the urethral sphincteric mechanism with implications regarding surgical management. Many urologists and uro-gynecologists use abdominal leak point pressures below 60 cm H2O to define ISD or type III stress incontinence. A leak point pressure of less than 100 cm H2O is considered evidence of ISD by the FDA, although many specialists feel with values of 60-100 cm H2O in a "gray zone". In any case, patient history and other urodynamic findings need to be considered when determining the appropriate clinical management. It should be recognized that it is the vesical and not the abdominal pressure which provides the energy to drive urine across the sphincter and cause incontinence. Leak point pressure is useful in evaluating patients with stress incontinence. In essence, it is a measure of the interaction between detrusor pressure and urethral resistance. The LPP is widely considered to be the best measure of urethral strength. The lower the urethral resistance for any given level of bladder compliance, the lower the leak point pressure and vice versa. In these circumstances, the sphincteric incontinence can be considered a "pop off valve" to protect the upper urinary tract at the expense of causing sphincteric incontinence. Leak point pressures >40 cm H2O have been shown to cause hydronephrosis or vesicoureteral reflux.
Pitfalls of Leak Point Pressures (LPPs):
Despite the original presumption that the urethral hypermobility and LPPs were related, it has been demonstrated repeatedly that there is no correlation between two. Further, some published data from others have shown that there are a number of pitfalls with this technique. Firstly, the numeric value of LPP usually decreases with increasing bladder volume and many patients who do not leak at a volume of 150 mL will leak at higher bladder volumes. Secondly, the presence of the catheter, particularly in patients with low urethral compliance may cause a false elevation in LPP. Some patients do not leak at all with the catheter in place and have obvious sphincteric incontinence and a low ALPP once the catheter is removed. In these patients, the catheter size also affects LPP; the larger the catheter, the higher the LPP. Thirdly, radiologic visualization of the leakage is much less sensitive than direct visualization of the urethral meatus. It should be recognized that it is the vesical and not the abdominal pressure which actually provides the energy to drive urine across the sphincter and cause incontinence. Thus, when one is using ALPP clinically it is important to (mentally) add back the estimated detrusor pressure to the ALPP to attain a true estimate of VLPP.
Urethral Pressure Profile (UPP)
The urethral pressure profile (UPP) is another procedure commonly used to measure the competency of the urethral sphincter (outflow resistance). UPP studies can be used to differentiate between genuine stress incontinence caused by bladder neck hypermobility and that caused by intrinsic sphincter deficiency (ISD). More recent technology has introduced the use of micro-transducer catheters.
The International Continence Society (ICS) has standardized terminology relating to urethral pressure profilometry.
Urethral pressure profile (UPP) -- indicates the intraluminal pressure along the length of the urethra with the bladder at rest. When describing this method, one should specify the catheter type and size, the measurement technique, the rate of infusion, the rate of catheter withdrawal, the bladder volume, and the position of the patient.
Maximum urethral pressure (MUP) -- is the maximum pressure of the measured profile.
Maximum urethral closure pressure (MUCP) -- is the difference between the maximum urethral pressure and the intravesical pressure.
Maximum flow rate (Qmax) -- is the maximum measured value of the flow rate after correction for artifacts.
Average flow rate (Qave) -- is voided volume divided by flow time.
Voided volume -- is the total volume expelled via the urethra.
Flow time (TQ) -- is the time over which measurable flow actually occurs. When flow is intermittent, the time intervals between flow episodes are disregarded.
Voiding time -- is the total duration of micturition, including the interruptions. When voiding is completed without interruption, voiding time is equal to flow time.
Time to maximum flow (TQmax) -- is the elapsed time from the onset of flow to maximum flow. For patients with continuous flow, TQmax is approximately one-third of voiding time in both normal and obstructed patients because of prolongation of voiding time in obstruction is primarily due to a prolongation of the descending limb of the flow curve.
Functional urethral length -- is the length of the urethra along which the urethral pressure exceeds the intravesical pressure and the anatomic urethral length is the total length of the urethra.
Pressure transmission ratio (PTR) -- is the increment in urethral pressure on stress as a percentage of the simultaneously reported increment in vesical pressure. For stress profiles obtained during coughing, PTRs can be obtained at any point along the urethra. If single values are given, the position of the urethra should be stated. If several PTRs are defined at different points along the urethra, a pressure transmission profile is obtained. The term transmission is commonly used, but transmission implies a completely passive process.
Determining Urethral Pressure Profile (UPP):
Urethral pressure is measured at rest with the bladder at any given volume. It is usually performed at maximum capacity following a filling cystometrogram (CMG). The bladder catheter is pulled along the length of the urethra to document urethral pressure and length, and to calculate maximum urethral closure pressure. An accurate test is dependent on the bladder remaining compliant and stable (no detrusor instability) during the test. The rate at which the catheter is pulled can affect results as well, with a rate of 1-2 mm/sec generally considered acceptable. Urethral pressure can be measured with balloon catheters, membrane catheters, perfusion techniques, or micro-transducers. With balloon catheters, the frequency of the response is determined by the catheter-manometer system. It integrates the pressure over the length and circumference of the whole balloon. The system has a rise time of 40 ms, so it probably cannot give an accurate indication of urethral response to physiologic stresses. In addition, the balloon is deformed by pressure variations that affect the cross-sectional area of the balloon, so the measurements are distorted. Perfusion techniques involve the use of one- or two-channel with side holes through which saline or gas is perfused with a motorized syringe pump. The advantage of this system is that the infused fluid prevents blockage. However, any sudden rise in pressure results in tissue contact initially blocking the side hole, resulting in a raised pressure until the infusing fluid clears the blockage. Not all fluid systems have an adequate frequency response time to measure stress profiles.
Micro-transducers are currently the most widely used catheters for urethral pressure studies in the United Sates. These catheters have two micro-transducers mounted 6 cm apart. The distal transducer in the bladder measures bladder pressure, whereas the more proximal catheter is manually or mechanically withdrawn through the urethra. Micro-transducers have an adequate response time for all aspects of urethral pressure profilometry. The sidewall mounting of the transducer provides a more reproducible recording of both mechanical and fluid pressures in a collapsible tube. Some investigators have used multiple transducers along the urethra to assess stationary recording of UPPs.
A normal female UPP is symmetric in shape, and asymmetry is generally caused by a faulty measurement technique. The values for normal urethral pressure have been noted in various studies to be decreasing with changes in vascularization that are inevitable with increasing age. There is trend towards lower urethral closure pressures and shorter functional length in stress incontinent patients than in patients who are stress continent. The appropriate way to report urethral function is to subtract out the influences of the bladder pressure (Pves). This is known as urethral closure pressure (Pclo) and is calculated as: Pclo = Pura -- Pves. The maximum urethral closure pressure (MUCP) can be reported by subtracting the Pves pressure from the point of maximum urethral pressure (MUP). This formula is MUCP = MUP -- Pves. Maximum urethral closure pressure over 30 cm H2O are considered normal, while an MUCP below 20 is considered an indication of urethral dysfunction or ISD. MUCP values of 20-30 are in a "gray zone," and should be considered along with other patient history and urodynamic information.
Diagnosis of Genuine Stress Incontinence (GSI): No single parameter of a urethral pressure study can be used to diagnose GSI. Pressure transmission ratio (PTR) is a test of the dynamic response of the urethra to increases in intraabdominal pressure. It requires simultaneous measurement of urethral and vesical pressures during coughing, allowing the calculation of the relative amounts of abdominal pressure transmitted to each structure. The PTR can be used to quantitate urethral closure during stress. It is calculated by dividing a cough-induced urethral pressure increase by the bladder pressure increase and multiplying by 100. PTR should not be considered diagnostic tests to help determine the type of incontinence, but tests more aimed at assessing urethral function and position.
Diagnosis of Intrinsic Sphincter Deficiency (ISD): It has been suggested that in patients with lower maximum urethral closure pressures, intrinsic sphincter deficiency (ISD) is the cause of their incontinence. Thus, an operation more aimed at obstructing the urethra, such as a suburethral sling, injection of a bulk-enhancing agent, or possibly an artificial urinary sphincter, should be used. Some investigators, though identifying the potential of a low urethral pressure as a risk factor for failure of suspension surgeries, do not consider that the increased complication rate of sling procedures is justified because a reasonable cure rate can still be obtained with colposuspension. Various studies have noted the failure rate with various stress incontinence procedure surgeries to be higher in patients with low urethral closure pressures. Some of these studies have used a cutoff of 20 cm H2O, but in other studies no cutoff point was used.
Diagnosis of Urethral Instability: When measurements of urethral pressure are performed during filling cystometry, it is not uncommon to note fluctuations in urethral pressure. Normally these fluctuations are synchronous with the heartbeat because of normal urethral vascular pulsation. Artifactual pressure fluctuations caused by urethral catheter movement are common and should not be confused with the following conditions. The association between variations in urethral pressure (at the point of maximum urethral closure pressure at rest) and abnormal detrusor function has been examined. This phenomenon is known as urethral instability, which has been defined by the International Continence Society (ICS) as an involuntary fall in intraurethral pressure in the absence of any rise in true detrusor pressure, resulting in the leakage of urine. Although uninhibited urethral relaxation is a well documented cause of incontinence, it is very rare and its management is currently controversial. This form of urethral instability may actually be a form of detrusor instability in which urethral pressure loss occurs, but the detrusor contraction is not perceived because urethra is open and a urethral-vesical equilibrium exists.
Diagnosis of Urethral Diverticulum: Although suburethral diverticulas are best diagnosed by radiologic studies or endoscopic studies, it has been reported that the presence of a biphasic curve on a UPP indicates the possibility of diverticulum. However, this configuration may also occur in some cases of GSI, especially in women who have previously undergone incontinence surgery. The typical UPP in patients with suburethral diverticulum shows a loss of urethral pressure at the level of the diverticular ostium. If the diverticulum is proximal to the maximum urethral pressure, the operation of choice should be diverticulectomy; however, if it is distal to this point, marsupialization can be considered.
Leak point pressure (LPP) and urethral pressure profile (UPP) studies are two different urodynamic tests aimed at objectively quantifying urethral function. We feel that the measurement of abdominal leak point pressure (ALPP) is a simple urodynamic study that can objectively quantitate the severity of genuine stress incontinence (GSI). This may be clinically useful in selecting surgical procedures for women with genuine stress incontinence and in the follow-up of patients who have received various modes of therapy but are not completely cured. Until recently, intrinsic urethral sphincter deficiency was believed to be a condition that occurred only after injury to the urethra. ISD was felt to commonly occur secondary to anterior vaginal wall surgery, previous bladder neck surgery, pelvic radiation, or thoracolumbar neurologic lesions. The diagnosis has recently become much more commonly used, based mostly on the more liberal use of these tests. The clinical applications of urethral pressure profilometry and ALPP are controversial. Until the techniques for performing these tests are standardized, clinical applicability will remain controversial. Although these tests show promise in the various clinical settings discussed, they should not be the sole determinant for important management issues, most notably, choice of surgical intervention. These decisions should be based on the patients' subjective complaints, important physical findings; these and other urodynamic test results, and the patients' desires and expectations. In the future, it may also help investigators communicate with each other and standardize subjective terms such as improvement. Further research into the function of the urethra, as well as randomized surgical trials of common operations, will help to clarify the clinical role of these tests.
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