Urodynamics are a means of evaluating the pressure-flow relationship between the bladder and the urethra for the purpose of defining the functional status of the lower urinary tract. The ultimate goal of urodynamics is to aid in the correct diagnosis of lower urinary tract dusfunction based upon its pathophysiology. Urodynamic studies should assess both the filling and storage phase, as well as the voiding phase of bladder and urethral function. In addition, provocative tests can be added to try to recreate symptoms and assess pertinent characteristics of urinary leakage.
Simple urodynamic tests involve performing noninvasive uroflow studies, obtaining a postvoid residual (PVR) urine measurements, and performing single-channel cystometrography (CMG). A single-channel CMG (ie, simple CMG) is used to assess the first sensation of filling, fullness, and urinary urge. Bladder compliance and the presence of uninhibited detrusor contractions (i.e. phasic contractions) can also be noted during this filling CMG. A simple CMG is generally performed using water as the fluid medium.
Multichannel urodynamic studies are more complex than simple urodynamics and can be used to obtain additional information, including a noninvasive uroflow, PVR, filling CMG, abdominal leak-point pressure (ALPP), voiding CMG (pressure-flow study), and electromyography (EMG). Water is the fluid medium used for multichannel urodynamics.
The most sophisticated study is videourodynamics, the criterion standard in the evaluation of a patient with incontinence. In this study, the following are obtained:
Abdominal (or Valsalva) leak point pressure
Voiding CMG (pressure-flow study)
The fluid medium used for videourodynamics is radiographic contrast.
Selection of patients for complex urodynamic testing can be difficult. Universally agreed-upon criteria for complex testing do not exist. The criteria that do exist are rooted more in expert opinion than in evidence-based scientific findings. In general, urodynamic testing is pursued with a diagnostic question in mind or, at times, to obtain a baseline measure of bladder function, for instance in the setting of neurogenic lower urinary tract dysfunction. This is important, as neurogenic lower urinary tract dysfunction (such as occurs in multiple sclerosis, spinal cord injury, and other conditions) can lead to deterioration of kidney function secondary to high bladder pressures transmitted to the upper urinary tract.
The utility of urodynamic testing lies in the fact that therapeutic outcomes are tied to understanding the pathophysiology of a given case, and thus, to making the correct and complete diagnosis. Surgery for incontinence, if incorrectly diagnosed, can carry substantial failure and complication rates. However, urodynamic testing is expensive and requires specialized equipment and expertise, which may limit is availability. Therefore, it is important to correctly utilize urodynamics by performing the test to answer a clear diagnostic question.
Urodynamic studies are, by their nature, unphysiologic. Studies have shown that the reference ranges in tests such as uroflowmetry and cystometry are wide. Urodynamic findings of significance must be associated with reproduction of the patient's symptoms. Studies that do not reproduce the patient's symptoms are inconclusive. Likewise, studies that result in abnormalities with no associated symptoms, or symptoms differing from the patient's complaints, are not conclusive. Nevertheless, these are the best tests available for examination of lower urinary tract function.
For more information, see Urinary Incontinence, as well as Urinary Incontinence Relevant Anatomy, and Cystoscopy and Urethroscopy in the Assessment of Urinary Incontinence.
Patient Selection for Urodynamic Studies
The history and physical examination alone may not provide sufficient and accurate information on which to base surgical therapy, but such basic data may provide the foundation from which to select patients for more invasive and complex testing. 
The following historical factors suggest the need for complex testing.
Unclear or complicated history
Significant urge component
Irritative voiding symptoms
History of urinary retention
Previous failed incontinence surgery
Continuous incontinence or leakage with minimal activity
Elderly patient (>65 y) with multiple possible diagnoses
Male patient with multiple possible diagnoses 
Advanced diabetes (bladder neuropathy or cystopathy)
Nocturnal enuresis if other diagnostics are exhausted
Nulliparous woman with stress incontinence
Known or suspected neurologic disease as a cause or contributor to incontinence
Physical examination findings that may prompt consideration of complex urodynamic evaluation include the following:
Abnormal central nervous system, lower extremity, or pelvic floor neurologic findings
High postvoid residual volume
Stress incontinence with minimally increased intra-abdominal pressure, with an empty bladder, or positive supine stress test 
Abnormal simple cystometry
The individual components of urodynamic testing are numerous. It is uncertain which of these components are essential, which ones may be important in specific and unusual circumstances, and which ones serve primarily as research tools. The following components are most essential to planning surgical therapy:
Postvoid residual urine volume determination
Abdominal (or Valsalva) leak point pressure
Within the scope of some of these tests, differing levels of complexity also exist. For example, simple determination of resting urethral closure pressure can be performed, or it may also involve cough, pressure transmission ratios, or measurements of functional urethral length. In general, the more sophisticated the procedure, the more narrow the scope of clinical usefulness. However, these procedures ultimately may increase the understanding of urinary incontinence and, thus, improve treatment.
Performing Urodynamic Testing
For urodynamic testing, the urine should be free of bacteria and evidence of inflammation. Some practitioners administer a single dose or short course of prophylactic antibiotics if invasive testing is scheduled, however, recent American Urological Guidelines suggest this is not necessary in the absence of concern for infection. The literature further suggests that this prophylaxis is probably unnecessary if proper technique is used. 
In female patients, some evidence shows that urodynamic findings do not change significantly in individuals when testing is performed at different times in the menstrual cycle.  In another small retrospective study, cystometry was less likely to reveal an abnormality during the luteal phase, especially in patients who reported that their symptoms were influenced by the menstrual cycle.  These researchers suggested avoiding urodynamic studies during the luteal phase if possible.
Instruct the patient to arrive at the urodynamic laboratory with a full bladder. Perform a noninvasive uroflow and postvoid residual (PVR) urine test. Perform a standing cough stress test, as well as flexible cystoscopy and pelvic examination, as needed.
If performing cystoscopy, survey the entire bladder urothelium, and then retroflex the cystoscope to examine the bladder neck. Fill the bladder with 250 mL of water using room temperature water at a medium rate (eg, 60 mL/min). Next, perform the cough stress test and cotton swab test, if they will add to the information necessary for clinical decision making.
If performing a speculum examination, use one half of the gynecology speculum to assess the anterior, posterior, and vaginal apex to investigate for pelvic organ prolapse. During the pelvic examination, assess the functional integrity of the pelvic floor muscles by examining the perineal body and checking rectal tone. The presence of levator ani muscle dysfunction or tenderness may be elicited by gentle palpation of the levator ani musculature in the paravaginal fornices.
To perform urodynamic testing, first place the patient in the dorsolithotomy position. Prepare the genitalia, and drape using sterile technique. Drain the bladder to assess post-void residual bladder volume (the patient should have voided), and place a urodynamic urethral catheter (i.e. dual-lumen) to fill the bladder and record intravesical pressure, a rectal or vaginal catheter to record abdominal pressure, and electromyography (EMG) electrodes if neuromuscular testing is being peformed. Various gas-filled and water-filled catheters are available and have differing characteristics for pressure measurement. 
Rotate the patient to a sitting position and equalize transducers. Commence bladder filling using room-temperature water or contrast. Cold fluid may evoke false-positive detrusor contractions (ie, phasic contractions). Fill the bladder at a medium rate (consider 60 mL/min). Note the volumes at which first sensation of bladder fullness and first sensation of urge to urinate occur.
Monitor bladder compliance, the change in volume per change in perssure, and mark the presence of uninhibited detrusor contractions. No normal value for bladder compliance has been defined, but some evidence suggests ranges of 40 ml/cm H2O to 120 ml/cm H2O may be normal, while values of 10 ml/cm H2O to 20 ml/cm H2O have been suggested as abnormal.  If minimal volume is able to be infused or marked bladder contractions occur immediately after the initiation of filling, empty the bladder, decrease the fill rate, and repeat the study—some patients may not tolerate a fill rate as high as others.
When the bladder fills to 250 mL, measure the abdominal leak-point pressure (ALPP) to investigate for stress urinary incontinence. Instruct the patient to perform the Valsalva maneuver in gradients (ie, mild, moderate, strong) followed by cough (ie, mild, moderate, strong). Studies have suggested an ALPP under 60 cm H2O suggests intrinsic sphincter deficiency, while one over 90 cm H2O excludes it, and values between may be seen in patients with or without it. 
Observe for urine leakage fluoroscopically, if a video urodynamic study is being done, or by direct inspection otherwise. At this point, note the shape of the bladder neck, assess urethral patency, and assess for the presence of cystocele using fluoroscopy (static cystography) if performing a video urodynamic study.
Upon completion of ALPP testing, finish the filling cystometrography (CMG) to completion. During this time, it may be prudent to measure the Detrusor Leak Point Pressure (DLPP) to assess the pressures at which urine is stored. Specifically, DLPP is the pressure at which urine leakage occurs in the absence of a detrusor contraction or abdominal pressure increase. This should normally be less than 40 cm H2O; if it is higher than this, the kidneys are at risk for damage secondary to back-pressure. 
Once DLPP is determined, or when the bladder is filled to capacity and the patient has a strong desire to void, perform voiding CMG (pressure-flow study) and observe the pressure tracings and urine flow rate. Specifically, note urodynamic parameters such as maximal flow rate (QMax) and detrusor pressure at maximal flow rate (PDetQmax) to gain insight into bladder outflow characteristics.
During voiding CMG, observe the activity of the EMG electrodes and voiding cystogram for evidence of detrusor sphincter dyssynergia (DSD) occurring. The presence of DSD is confirmed by increases in EMG activity during detrusor contraction or closure of the external sphincter on voiding cystourethrography (VCUG) images. Be aware that urine spillage on electrodes can result in a noisy signal that may resemble increased muscle activity. After the patient voids to completion, the videourodynamic study is complete.
Fluoroscopy and Video Urodynamics
Video (fluoroscopic) urodynamic studies have become the investigative technique of choice for incontinence in many referral and research centers. This technique involves the simultaneous display of real-time images of the bladder neck and urethra, as well as cystometric summaries of bladder, intra-abdominal, and, in some cases, urethral pressures.
The precise placement of pressure transducers and a constant understanding of their exact anatomic location is one of the advantages of this technique. Another advantage is the ability to fluoroscopically observe the bladder neck area throughout bladder filling and during stress maneuvers.
Contrast material can be observed entering the proximal urethra just before leakage; thus, leak-point pressure findings can potentially be more precise. Cough profiles and pressure transmission ratios can also be determined.
The physical location of the transducer tip can be observed during urethral pressure profilometry (UPP) and correlated with the pressure findings. Although probably not necessary for the evaluation of straightforward stress incontinence, video urodynamics can be a valuable diagnostic tool in complex cases.
Video urodynamic studies are the criterion standard for the evaluation of an incontinent patient. Video urodynamic studies combine the radiographic findings of a VCUG and multichannel urodynamics. A videourodynamic study is the most sophisticated diagnostic test for an incontinent patient.
In the absence of videourodynamics, the clinician may obtain adequate information from the following:
Noninvasive uroflow and postvoid residual urine volume
Simple cystometry in combination with cystoscopy
Detailed speculum examination
Cough stress test and Q-tip test
Dynamic retrograde urethroscopy
Static cystography is typically performed during videourodynamics under fluoroscopy. When the bladder is halfway full (ie, 200-250 mL), anteroposterior and lateral images of the bladder and bladder neck are obtained with the patient at rest, during Valsalva, and while coughing. Depending on the urodynamic suite setup, acquiring lateral images can be challenging. However, lateral images should be obtained if at all possible because they provide beneficial information.
A static cystogram helps confirm the presence of stress incontinence, degree of urethral motion, and presence of cystocele. Intrinsic sphincter deficiency is evident by the presence of an open bladder neck. Vesicovaginal fistula may also be documented in this fashion. (See the image below.)
Antegrade or retrograde cystourethrography is a useful adjunct to incontinence workups in situations where urinary tract fistulas or diverticulum are suspected. A voiding cystourethrogram (VCUG) is used to assess bladder neck and urethral function (internal and external sphincter) during filling and voiding. A VCUG can reveal a urethral diverticulum, urethral obstruction, and vesicoureteral reflux.
Cystometry or Cystometrography
In up to 40% of patients, stress and urge urinary incontinence coexist as mixed urinary incontinence. In many instances, stress incontinence may be associated with the development of urge incontinence. Filling cystometrography (CMG) or cystometry may be useful in such cases as it assesses bladder capacity, bladder compliance, and the presence of phasic contractions.
Cystometry is a technique of assessing the filling phase of bladder function. Much information can be gained during cystometry, including the diagnosis of bladder overactivity, bladder oversensitivity, sensory neuropathy, loss of compliance, and determination of bladder capacities. Abnormal cystometric findings should be consistent with the patient’s clinical complaint. Despite standardization of terminology for lower urinary tract dysfunction by the International Continence Society, a number of terms are still used in describing specific pathologies.  In addition, leak point pressures and cough stress tests can be performed during cystometry, and pressure-flow studies can be performed if the cystometry catheters are left in place during uroflow studies.
The simplest forms of cystometry can be performed with a catheter and syringe or manometer held 15 cm above the pubic bone. This inexpensive and readily available method may be adequate for screening in uncomplicated cases but may miss subtle findings. Alternatively, bedside cystometry can be accomplished by attaching a catheter to an irrigant bag and water chamber, hanging the bag at a predetermined height above the pubic bone, and observing fluctuation of the meniscus within the water chamber during uninhibited detrusor contractions. Similar results can be accomplished using a flexible cystoscope, which also provides simultaneous bladder visualization.
Single-channel cystometry consists of recording isolated intravesical pressures during filling with a single catheter. With this method, increases in intra-abdominal pressure cannot be definitively differentiated from increases in true detrusor pressure. Multichannel cystometry is performed with a bladder catheter and a second catheter to approximate intra-abdominal pressure. The second catheter is usually placed in the rectum, or at times in the vagina. In cases of severe pelvic organ prolapse, a rectal catheter usually performs more reliably. If a patient has had a proctectomy and has an end ostomy, the intra-abdominal pressure can be measured by placing the catheter into the bowel lume via the ostomy.
The data output consists of a vesicle pressure channel, an abdominal pressure channel, and true detrusor pressure channel. The true detrusor pressure channel, also called the subtracted channel, is the bladder pressure minus the abdominal pressure. Depending on the individual set up, additional channels may accommodate simultaneous urethral pressure readings and continuous electromyography (EMG) readings.
Testing protocol considerations
Basic technical points include the choice of fill medium, the infusion rates, and the types of catheters. A liquid medium, usually saline, is preferred. Most testing is performed with room-temperature solutions. Cold solutions can be used as a provocative maneuver to promote bladder contractions. Rarely, carbon dioxide is used as a filling medium, but this is thought to be unphysiologic and, furthermore, can irritate the bladder.
The filling rate can vary and usually ranges from 10-100 mL per minute. Slower, more physiologic rates can be used if a suspected false-positive result is obtained at faster rates. Likewise, faster rates can be used to provoke subtle instability or can be used in patients with significant urgency who do not allow sufficient volumes to be infused at slower rates and longer infusion times.
Catheters generally should be 10 French or less in caliber to avoid urethral irritation and obstruction of flow. Microtip transducers are used most commonly in clinical practice. Fiberoptic and piezoelectric catheters also are available. Differences exist between the pressure transmission characteristics of various catheters, so consistency should be the goal if repeated testing is indicated. 
Patient position during testing varies, but most commonly, the patient is sitting, semi-erect, or standing. The catheters generally are calibrated so that zero corresponds to atmospheric pressure. A complete cystometric evaluation monitors the filling-storage segment and emptying segment of bladder function.
For clinical purposes, the emphasis often is on the filling-storage segment. During filling, normally, detrusor pressure does not rise. This finding reflects the compliance of the bladder. With rapid filling rates, a small-to-moderate increase in pressure may be noted. The 4 recognized cystometric phases of bladder function are described below. The first 3 of these phases make up the filling-storage segment of bladder function. The last phase represents the emptying segment. The 4 phases are as follows:
- Initial small increase in intravesical pressure at the beginning of filling
- Stable pressure that comprises the majority of the filling phase
- Terminal pressure rise at bladder capacity, representing the limit of viscoelastic expansion (often not reached due to discomfort)
- Voiding phase with an inconsistently observed small increase in intravesical pressure
Bladder sensation and capacity can be measured during filling cystometry. The first sensation is described as the volume at which the patient first is aware of fluid in the bladder (reference range of 50-200 mL). The second sensation (full) has been described as the volume at which the individual normally would consider voiding due to an urge sensation (reference range of 200-400 mL). Maximum capacity is when the patient is experiencing pain and does not allow continued filling (reference range of 400-600 mL). The average bladder holds 400-500 mL of urine. See the images below.
Low bladder capacities can be observed in patients with urinary tract infections, sensory-urgency syndrome, interstitial cystitis, detrusor overactivity, stress incontinence, neurogenic bladder, or bladder fibrosis. In the urodynamics laboratory, some patients prematurely terminate filling to avoid the possibility of having an episode of incontinence. Considerable reassurance and clear instructions on the part of the practitioner usually help to avoid this type of false-negative study.
Increased capacity can be observed with a neurogenic bladder and with bladder outlet obstruction. Otherwise, long-term abnormal voiding habits can also result in an increased bladder volume. These habits can develop in the presence or absence of identifiable pathology. Detrusor underactivity or underactiv bladder can also be noted in the setting of large volumes.
Low-compliance bladders are demonstrated through cytometry by a rising bladder pressure through all or most of the filling segment. Compliance problems can be due to chronic infection, fibrosis, cancer, radiation, and inflammation from long-term indwelling catheters. As previously mentioned, supraphysiologic fill rates can result in nonpathologic loss of compliance.
Any bladder contraction during filling is considered abnormal, but the clinical significance of bladder contractions that are asymptomatic (not identified by the patient as representing their symptomatology) are uncertain. The International Continence Society has identified a minimal contraction amplitude of 15 cm H2O over baseline to be considered significant. Many experts believe that some contractions of lesser amplitude may be clinically relevant.
Demonstration of urgency coincident with increase in true detrusor pressure defines detrusor overactivity, and coupled with simultaneous urinary leakage, defines urge urinary incontinence, as depicted in the image below.
Many urodynamics centers use provocative maneuvers consisting of common triggers for bladder contractions in an attempt to induce detrusor overactivity. Provocative maneuvers include the sound and sight of running water, hand washing, coughing, and heel bouncing. These maneuvers may be important in patients with high clinical suspicion for detrusor overactivity but without instability findings on filling.
Postvoid Residual Volume Determination
High postvoid residual (PVR) urine volumes in males often indicate prostate-related bladder outlet obstruction or impaired bladder contractility due to detrusor problems, as noted with underactive bladder. Uroflowmetry and pressure-flow studies can help clarify the diagnosis. High PVR urine volumes are uncommon in females. Only 5% of asymptomatic females and 13% of symptomatic females have PVR volumes greater than 30 mL.
Abnormal residual volumes have been defined in several ways. No particular definition is clinically superior. Some authorities consider volumes greater than 50-100 mL to be abnormal. Others use a value greater than 20% of the voided volume to indicate a high residual.
Abnormal findings should be confirmed by a second study. If the premeasurement void takes place in a public restroom and a high residual is obtained, the female patient should be asked if she was relaxed and if she sat on the toilet seat during voiding. Voiding in a crouched but not fully seated position has been associated with PVR volumes in the 50-100 mL range.
Ultrasonographic postvoid testing
Ultrasound can be used as a noninvasive means of obtaining PVR values, especially if a precise measurement is not required.  Scanning is performed transabdominally in the transverse and sagittal planes, and the height, width, and depth are determined. These 3 diameters are multiplied by a correction coefficient of 0.7, which accounts for the nonspherical shape of the bladder when it is less than completely full. This formula probably is the simplest US formula for PVR volume determination. The error using this formula, compared with the standard of postvoid catheterization, is 21% approximately.
The formula is not accurate at volumes less than 50 mL, but in clinical applications, precise determination of PVR values of less than 50 mL usually are not needed. Other methods and formulas have been described. Portable ultrasound scanners for the measurement of bladder volume have been developed specifically. These devices avoid the disadvantages of catheterization and have been noted to be accurate in their measurements. 
Uroflowmetry and Leak-Point Pressure
Uroflow is the volume of urine voided per unit of time. The simplest uroflowmetry method uses a stopwatch and a commode that is equipped to measure urine volume. More complicated devices use the increasing weight of the urine over time to determine flow rates.
A useful screening test, uroflow is used mainly to evaluate bladder outlet obstruction. Consistently low flow rates generally indicate a bladder outlet obstruction but may also indicate decreased detrusor contractility. Uroflowmetry alone cannot distinguish between those two conditions. To properly diagnose bladder outlet obstruction, perform pressure-flow studies. Nomograms have been developed for pressure-flow data to help predict whether patients are obstructed or not. 
Urine flow rates are a product of detrusor contraction strength, urethral resistance, and in some instances the contribution of abdominal straining. Normal flow curves are bell shaped and display a rapid rise to peak flow, a short duration of peak flow, and a rapid fall. Abnormal patterns seen may include prolonged flat curves with low flow, intermittnet spurting flow, and others.
In males, the resistance factor is greater because of the longer urethra and the presence of the prostate. To compensate for these factors, the detrusor contribution to voiding must be greater. In males, maximum flow rates are affected greatly by age, ranging from 35 mL/s in males aged 14 years to approximately 5 mL/s in males aged 80 years. Low-flow curves often suggest the need for surgery to protect the upper tract, but they do not provide specific information about the etiology. Decreased detrusor contractility (underactive bladder), outlet obstruction, or a combination of both may be responsible for low flows.
In women, aging by itself results in little or no drop in flow rates. Factors such as pelvic organ prolapse, stress incontinence, prior hysterectomy, increased parity, and, perhaps, hypoestrogenism are more likely associated with decreased flow rates than age alone. Flow rates are dependent upon voided volume. Uroflow studies should be performed with a minimum of 150-200 mL in the bladder.
Flow patterns generally do not point to a specific urodynamic diagnosis, although some associations have been noted. In patients with aging bladders or in the early stages of neuropathy, detrusor overactivity may coexist with detrusor hypocontractility. In some instances, such as profound intrinsic sphincter deficiency, super-flow patterns may be observed. Bladder oversensitivity may be associated with low flow rates, especially if the bladder routinely holds minimal volumes. Noncontinuous flow patterns can indicate Valsalva voiding or detrusor sphincter dyssynergia.
A prostatic uroflow curve has been described. This consists of an unbroken pattern with asymmetry and an elongated flattened area from QMax to the end of voiding, as depicted in the image below. Abnormal findings, especially low flow rates, should be confirmed by repeat studies. Nervousness or embarrassment can also result in nonrepresentative dysfunctional voiding patterns.
In females, the role of uroflowmetry is controversial. Voiding dysfunction is uncommon, except in the patient who recently had incontinence surgery. Females can complete successful voiding in a number of ways. Using Valsalva voiding or Valsalva augmentation of voiding is not uncommon. Some females void by urethral relaxation alone.
Reference range values for uroflow parameters are not well established for females. Maximum flow rates of 15-20 mL/s or higher generally are considered normal. Flow rates of less than 10 mL/s are considered low, but if the PVR volume is minimal, this finding is of dubious clinical significance. A normal voiding time is considered 15-20 seconds. In females, low flow rates most often indicate a functional rather than an obstructive problem.
Uroflow studies may be useful in predicting the risk for voiding dysfunction and high residual volumes after incontinence surgery. Patients with low flow rates may be at risk for prolonged catheterization. In one study, 38% of the patients with abnormal uroflow results before surgery required postoperative catheter drainage for more than 1 week. Only 10% of those with normal study results required prolonged drainage. This prognostic information is important for both the physician and the patient.
Voiding cystometry or pressure-flow studies can be a valuable adjunct to standard uroflowmetry. To perform these studies, cystometry catheters are left in place during uroflowmetry. In males, these studies can be vital in differentiating outlet obstruction from functional detrusor problems. Low-flow, low-pressure findings would be consistent with the latter. High pressures and low-flow rates suggest outlet obstruction. Maximum detrusor pressures of less than 20 cm water generally are considered abnormal. See the image below.
Normal parameters for pressure-flow studies have not been established for females. Many females void with very low detrusor pressures because of the short and relatively low-resistance urethra. Detrusor contraction during voluntary voiding may serve the purpose of bladder accommodation to decreasing volumes rather than the generation of significant expulsive forces. Successful voiding with urethral relaxation alone is not uncommon. Both Valsalva voiding and Valsalva augmentation of detrusor voiding also are observed. If residual volumes are low, then often no specific treatment is required.
Valsalva leak-point pressure
The Valsalva leak-point pressure, or abdominal leak-point pressure (ALPP), is a test of the urethral sphincter resistance against increases in intra-abdominal pressure. The overall assumption is that the lower the leak point pressure, the weaker the urethral sphincter and the more severe the stress incontinence. Leak-point pressure testing is an important component of multichannel urodynamic testing. It is used to determine whether stress urinary incontinence in a woman results from urethral hypermobility, intrinsic sphincter deficiency, or both in combination. Results are used to inform clinical decisions, such as which surgical intervention may be best.
Interestingly, no consensus exists as to how to optimally perform the test. In addition, many assumptions have been made as to the validity and clinical utility of the tests. Results of the other major objective test of urethral sphincter function, urethral closure pressure measurement, do not always agree with leak-point pressure findings. Some studies have shown a weak correlation between these tests, and other studies have found no correlation. Comparing these studies is difficult because of differences in techniques between practitioners.
Performing the leak-point pressure test
For the basic abdominal leak-point pressure test, intravesical and intrarectal catheters are placed and the bladder is filled with 150-250 mL of fluid. The patient, who is in either the sitting or standing position, is asked to perform a Valsalva maneuver of slowly building intensity. The lowest pressure at which leakage from the urethral meatus is observed denotes the leak point pressure. See the image below.
The test should be repeated several times to ensure consistency. If properly performed, the test has excellent test and retest reproducibility. Many variables may affect the test results, including the caliber of the catheter, patient position, bladder volume, and the presence of pelvic organ prolapse. Experts generally believe that if pelvic organ prolapse is severe, some form of prolapse reduction should be accomplished during the test. The best method of prolapse reduction for this purpose is uncertain, but a convenient means of performing this is to use a moist gauze to pack the vagina during testing.
If no leakage is produced or the patient is unable to perform the Valsalva maneuver properly, a cough leak-point pressure can be attempted. A cough leak-point pressure is less accurate than an abdominal leak-point pressure and more difficult to measure because pinpointing the precise pressure at which leakage occurs is challenging due to the fast pressure spike associated with coughing. The cough leak-point pressure overestimates the actual abdominal leak-point pressure in many instances.To prevent overestimation, the patient can be asked to cough until leakage occurs and then to perform less intense coughs until leakage disappears. The lowest value producing leakage is taken as the leak-point pressure. Of note, this technique is more easily described than performed.
Another alternative, if no leakage occurs with the initial testing, is to repeat the Valsalva maneuver after progressive bladder filling in 50-100 mL increments (e.g. 250 mL, 300 mL, 350 mL) or to perform the test with the bladder catheter removed. Bladder volume has been shown to be related inversely to leak-point pressure, even at 100-300 mL volumes. Most protocols call for a bladder volume of 150-200 mL despite this not being standardized.
The presence of a transurethral bladder catheter may affect leak-point pressure significantly, compared with measurements obtained using a catheter for abdominal pressure only. Leak-point pressures are as much as 20 cm water lower without a transurethral catheter. Some think that transurethral catheters may cause some degree of obstruction related to the size of the catheter. Thus, the catheters are thought to elevate the leak-point pressure artificially. Patient position is also thought to potentially affect leak point pressure.
Interpretation of abdominal leak-point pressure results is also controversial. Many authorities use leak-point pressures below 60 cm water to define intrinsic sphincter deficiency. Others have cited 80 cm or 90 cm water as the threshold. One study used a value of 50 cm water or less in conjunction with urethral closure pressure and urethral angle parameters.  Of note, this study defined leak-point pressure as the increase in vesical pressure minus resting vesical pressure, while other studies define it as the actual vesical pressure at which leakage occurred. It is important to note that a normal leak-point pressure should approach infinity. In other words, patients with a normal continence mechanism can generate intra-abdominal pressures high enough to cause fainting without provoking stress incontinence.
Classification of stress incontinence
In the past, abdominal leak-point pressure measurement comprised part of a classification system of stress incontinence. Currently, no accepted classification of stress urinary incontinence is used in clinical practice because stress urinary incontinence is caused by a continuum of intrinsic sphincter deficiency severity with varying degrees of urethral hypermobility. Current evidence suggests that all patients with stress urinary incontinence have some component of intrinsic sphincter deficiency.
Electromyography (EMG) during voiding is used to distinguish coordinated voiding (i.e. detrusor sphincter synergia) from uncoordinated voiding (i.e. detrusor sphincter dyssynergia) resulting from failure of urethral relaxation during bladder contraction. EMG is most useful in patients with a suspected neurologic disorder.
EMG is a type of neurophysiologic testing. The test can be performed with surface electrodes, monopolar needle electrodes, or concentric needle electrodes. Each modality has distinct advantages and disadvantages. A detailed discussion of the technical aspects of EMG is beyond the scope of this article.
EMG studies can be used to test the neuromuscular integrity of the urethral and anal external striated sphincters, puborectalis, and pubococcygeus muscles. Digital palpation and manometric measurements are other methods to assess the strength of voluntary pelvic muscle contractions. These methods correlate well with surface EMG findings, and only surface EMG measurements are predictive of pelvic floor pathology.
A study by Glazer and colleagues demonstrated that poor pelvic floor muscle function, based on surface EMG measurements, was predictive of symptoms of general incontinence, stress incontinence, urge incontinence, and associated with parity.  In addition, statistically significantly lower amplitude pelvic floor contractions were found in postmenopausal women not on estrogen-replacement therapy.
Studies of bladder filling (eg, cystometry) and emptying (eg, uroflowmetry) can also be combined with EMG measurement. Slowly increasing external urethral sphincter tone during filling and subsequent relaxation with emptying are normal findings. When evaluating patients with neurological disorders and discoordination of the bladder and urethral sphincter, detrusor-sphincter dyssynergia can be revealed. An EMG can also be used therapeutically as biofeedback in pelvic floor exercise therapy. Overall, many believe EMG is most useful in the research setting, as experience with recording and interpretation in the clinical setting by gynecologists and urologists is limited.
Pudendal Nerve Terminal Motor Latencies
The pudendal nerve terminal motor latency (PNTML) is another neurophysiologic study performed with stimulating and recording electrodes. The stimulating electrode is placed at the ischial spine, close to the pudendal nerve. The recording electrode is placed in the area of interest, such as the anal or urethral sphincter. The St. Marks electrode, which is worn on a gloved finger, is the most well known of these devices. With this technology, the association of pelvic floor denervation injury with urinary incontinence was uncovered. Although such studies have increased the understanding of the neuromuscular dysfunction component of pelvic support and incontinence disorders, their role in day-to-day clinical practice is unclear and they remain largely used in research settings.
Other Tests for Urinary Incontinence
Urethral pressure profilometry
Urethral pressure profilometry (UPP) is a technique of recording pressures along the length of the urethra with the bladder at rest. The maximal urethral closure pressure (MUCP) is the maximum urethral pressure minus intravesical pressure. The functional urethral length is the distance along the urethra in which urethral pressure exceeds bladder pressure.
Previously, urethral pressure profiles were thought to have many clinical applications. An MUCP of less than 20 cm water was associated with higher failure rates when these patients are treated with a Burch colposuspension. Closure pressures below 20 cm water were thought to suggest intrinsic sphincter deficiency as an indication for a suburethral sling. No threshold value has been determined that is consistently associated with stress-induced leakage, thus leading to the test falling out of favor and being less commonly performed. Likewise, urethral pressure cough profiles are thought to have limited clinical utility.
The positive-pressure urethrogram probably is the most useful single test in the workup of a known or suspected urethral diverticulum. Dye is injected under pressure into the urethra through a specially designed catheter, which isolates the urethral lumen between 2 occluding balloons. One balloon occludes the urethra at the meatus/vaginal introitus. The other balloon rests snugly at the urethrovesical junction. Radiographs are taken after the dye is injected.
This study helps define the anatomy of the diverticulum in terms of size, number, and location of loculations and the location of the orifice(s) along the length of the urethra. This information is essential in planning surgical therapy.
Ambulatory urodynamic monitoring was developed to address some of the many shortcomings of laboratory urodynamics. Ambulatory urodynamic monitoring is an attempt to record bladder function in a more physiologic setting through many natural fill-void cycles.  Current systems use microtip catheters in the bladder (generally transurethrally) and in the rectum or vagina. A portable, battery-operated recording device then stores data from the transducers. Most systems include a means for recording leakage events, generally via a button on the recorder that flags the data, indicating a leakage occurred during the pressures recorded at that time.
Bladder capacities also tend to be less with ambulatory monitoring. This finding has been demonstrated even when patients are provided the same reporting instructions given in the urodynamic laboratory setting. A universal finding in ambulatory monitoring is increased detrusor overactivity compared with conventional cystometry. Rates of detrusor overactivity of approximately 20% have been recorded in individuals who are asymptomatic. Other work has shown rates of detrusor contractions in 38-69% of volunteers who are asymptomatic.
Several possible explanations exist for this phenomenon. Normal and physiologic, but previously unrecognized, phasic detrusor contractions may be occurring. Alternatively, contractions may be related to irritation of the urethra and trigone by the catheter. The findings may also possibly represent a subclinical defect in bladder control. Lastly, artifacts in the recorded data may also be misinterpreted as detrusor contractions.
Artifacts resulting in a false-positive diagnosis of detrusor overactivity have been problematic and limit the usefulness of ambulatory cystometry. Studies have shown that up to 19% of measured detrusor activity may be due to artifacts. Work on completely intravesical ambulatory urodynamic testing systems is underway; however, measuring bladder volume remains a challenge.  More recent work by this same group has resulted in the development of an algorithm that can accurately identify bladder contractions from increases in intrabdominal pressure.  Overall, ambulatory urodynamics are used little clinically but hold a great potential for aiding the diagnosis of lower urinary tract dysfunction by eliminating the need for unphysiologic office-based testing and bothersome catheterization.
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- Patient Selection for Urodynamic Studies
- Test Components
- Performing Urodynamic Testing
- Fluoroscopy and Video Urodynamics
- Cystometry or Cystometrography
- Postvoid Residual Volume Determination
- Uroflowmetry and Leak-Point Pressure
- Pudendal Nerve Terminal Motor Latencies
- Other Tests for Urinary Incontinence
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