Detrusor Underactivity

Detrusor contractility plays a vital role during micturition, especially after the placement of a potentially obstructing sling. It is an important finding in men with PPI because a weak bladder is a relative contraindication to male sling surgery. Because the male sling is designed to prevent leakage during straining, it follows logically that patients with weak detrusors who rely on abdominal straining to facilitate efficient bladder emptying are at higher risk for retention after sling surgery.

The International Continence Society defines detrusor underactivity as a contraction of reduced strength and/or duration, resulting in prolonged bladder emptying and/or a failure to achieve complete bladder emptying within a normal period. Detrusor contractility is assessed during the pressure-flow portion of urodynamics but the specific method of evaluation has not been standardized, and there are no agreed-on normal values of detrusor pressure during voiding in men who are status postprostatectomy. In most urodynamics studies of patients with PPI, detrusor underactivity has been defined using surrogate measures such as (1) the presence of Valsalva voiding, (2) low detrusor pressure at maximum flow (PdetQmax), or, most commonly, (3) bladder contractility nomograms developed for men with BPH, derived using both urinary flow rates (Qmax) and PdetQmax.

The use of surrogate measures of detrusor contractility based on BPH nomograms, however, can be inaccurate in men after prostate extirpation because of the low outflow resistance state in these men. In such patients, the contractile pressure required to maintain axial flow can approach zero. Thus, PdetQmax can be low due to reduced urethral resistance in patients with ISD and may, therefore, be an inaccurate measure of detrusor strength.

In the setting of PPI due to ISD, a more appropriate measure of detrusor strength is independent of flow and unrelated to urethral resistance and characterizing contractility as a single parameter. Contractility can be represented by a power factor—as a product of detrusor pressure and shortening velocity by the methods described by Griffiths or van Mastrigt. These calculations, however, require complex, computer-assisted analysis of pressure-flow studies. A simpler method of measuring detrusor power is to directly measure bladder muscle contraction pressure under isovolumetric conditions—and, therefore, independent of urinary flow. The best measure of detrusor contraction strength, therefore, is the isometric detrusor contraction pressure (Piso).

One reliable method for obtaining Piso measurement is the mechanical stop test. During voiding, an examiner gently occludes the penile urethra, thereby inhibiting urinary flow but not aborting the bladder contraction. The maximum detrusor pressure reached during this maneuver is the Piso ( Fig. 1 ). This mechanical stop test is reproducible and does not inhibit detrusor contraction. Piso measurements less than 50 cm H ₂ O are considered diagnostic of detrusor underactivity.

Urodynamics tracing showing Piso measurement using mechanical stop test: ( heavy arrows ) start of stop test when examiner occludes urethra, thereby stopping flow. Maximal isometric pressure (Piso) is 99 cm water.

The mechanical occlusion of the outlet is preferred to a volitional interruption of flow; as such, voluntary cessation may underestimate the maximum isometric pressure due to reflex detrusor inhibition induced by the activation of voluntary striated sphincter activity. In addition, it may be difficult or impossible for a patient with detrusor overactivity to stop the urinary stream once it is initiated, especially after prostatectomy with concomitant weakness of the external sphincter mechanism.

Elliott and Comiter retrospectively reviewed the urodynamics findings of 62 men evaluated for PPI and found that 40% had a Piso less than 50 cm H ₂ O. They also found that the use of common surrogates for bladder contractility, such as the Bladder Contractility Index (BCI = Pdet Qmax + 5 Qmax) less than 100, which has been validated in men with BPH but not in men devoid of a prostate, and the mere presence of Valsalva voiding were imprecise markers of detrusor underactivity in this population. Rather, the direct measure of bladder contractility independent of urethral sphincteric resistance was necessary to accurately measure true bladder contractility.

Detrusor underactivity is a potentially important finding in men with PPI, because it may be a relative contraindication to certain types of male sling surgery. Because the compressive male sling provides fixed resistance and is designed to prevent leakage with straining, it seems logical that it would interfere with voiding dependent on or augmented by Valsalva. The placement of a potentially obstructive male sling in a patient with detrusor underactivity, therefore, theoretically increases the risk for retention. There are few clinical studies, however, on the effects of the male sling in patients with impaired detrusor contractility or those who void by Valsalva. Han and colleagues performed retrospective review comparing 50 patients after sling placement (AdVance, InVance, and Virtue) with a mean BCI of 70 (range 33–97) with normal controls and found no differences in postvoid residual (PVR), patient satisfaction, or Qmax. The investigators concluded that their results may be a reflection of how bladder function is measured in this population. The authors believe that BCI and its derivatives, however, may not be accurate measures of bladder strength in men with PPI, because Elliott and Comiter showed that the isovolumetric contraction pressure (Piso), which is independent of urethral resistance and, therefore, a pure measure of detrusor contractility, did not differ between men with low and normal BCI.

Detrusor underactivity, however, should not adversely affect surgical success of AUS because the cuff is cycled to an open phase during voiding, with relief of urethral occlusion. In a prospective trial of 40 patient followed for a mean of 53 months, Trigo Rocha and colleagues concluded that the presence of preoperative impaired bladder contractility did not affect surgical outcome. Even pure Valsalva voiding does not adversely affect the outcome of AUS surgery.

In the setting of detrusor underactivity, when an artificial sphincter is not desired, the retroluminal transobturator sling is a viable alternative. The retroluminal transobturator sling provides only minimal compression of the bulbar urethra. Rather, its mechanism of action relies on proximal urethral relocation. The sling is placed retrourethrally, under the proximal urethral bulb, and, on tensioning, facilitates a 2.5- to 3-cm proximal relocation of the bulbar urethra. It is hypothesized that laxity of posterior support predisposes to misalignment of the proximal urethra, contributing to inefficacious coaptation of the urethral sphincter complex. This abnormal anatomy and proximal urethral prolapse that may occur after prostate extirpation may be remedied by a retroluminal sling that can realign the mobile urethral sphincter complex and restored the preprostatectomy configuration. Once the sling is appropriately tensioned, the urethral bulb is proximally relocated into the higher-pressure pelvic outlet by a distance of 2 to 3 cm, which can function as a backstop during straining. In a large cohort of patients with at least 1-year follow-up, Rehder and colleagues demonstrated no change in Pdet, PVR, or flow rate after surgery, supporting this nonobstructive mechanism of action.

More prospective studies using appropriate measures of detrusor contractility independent of urethral resistance are needed to define the risk of urinary retention, obstructive voiding dysfunction, and outcomes of the male sling and AUS in men with postprostatectomy SUI and detrusor underactivity. In the meantime, the AUS and the retroluminal sling are the preferred options for these patients based on the mechanism of action of the respective devices.

Detrusor Overactivity

The rate of detrusor overactivity in men with PPI has been in the range of 30% to 40%. It is considered the sole or main diagnosis in approximately 10% of patients. In 25% of PPI patients with ISD, there is a secondary diagnosis of detrusor overactivity. The rate of de novo detrusor overactivity postradical prostatectomy is only approximately 5%, with 33% of patients already having detrusor activity at baseline. In men who had presumably normal storage function prior to radical prostatectomy, the urodynamic finding of detrusor overactivity or diminished compliance is most likely due to poor accommodation associated with supraphysiologic infusion of a bladder that has experienced a prolonged underfilled state secondary to continual urinary leakage. With long-term follow-up, the rate of detrusor activity is reduced from 38% at 8 months to approximately 18% at 36 months.

The finding of detrusor overactivity has important implications for treatment and counseling. If the sole or main diagnosis is detrusor overactivity then antimuscarinic medications and other treatments for overactive bladder (β ₃ -agonist, botulinum toxin, posterior tibial nerve stimulation, and sacral neuromodulation) are the recommended treatment options. If detrusor overactivity is found along with ISD, then it should be taken into consideration during patient counseling but is not an absolute contraindication to AUS or sling placement.

Although the bone-anchored sling and older slings designs work by compressing the urethra, the retrourethral transobturator sling is designed to be nonocclusive. It does not seem to increase voiding pressure or increase the rate of urgency in patients with preoperative findings of detrusor overactivity. Studies on the retrourethral transbturator sling, however, have relatively short follow-up (1–3 years).

Preoperative detrusor overactivity does typically improve after AUS implantation and does not adversely affect resolution of SUI. Afraa and colleagues retrospectively reviewed the charts of 16 patients and found that the rate of detrusor overactivity decreased from 50% to 25% after AUS implantation. As discussed previously, men with high-volume SUI have bladders that are chronically underfilled because SUI prevents the accumulation of urine in the bladder. In the most severe cases, these patients may only passively leak and never volitionally void. As a result, medium and even slow-fill cystometry may demonstrate poor accommodation or compliance as well as detrusor overactivity. These findings typically resolve over time after successful SUI surgery and bladder retraining. Despite the likelihood of improvement, persistent overactive bladder symptoms remain common, and patients must be counseled accordingly.

Impaired Compliance

The International Continence Society defines bladder compliance as the relationship between change in bladder volume and change in detrusor pressure. It can be calculated by dividing the change in volume by the change in detrusor pressure during the same time period (C = ΔV/ΔPdet). The recommended points for this calculation are the start of infusion to the end of filling and at cystometric capacity. Compliance greater than 20 mL/cm is generally accepted as normal.

De novo reduced bladder compliance has been detected in up to 32% of men after prostatectomy, with persistence in 28% after 36 months. A study examining urodynamic findings with PPI, however, found only a 5% rate of decreased compliance. The presence of impaired bladder compliance is a contraindication to male sling placement due to concerns that the increased voiding pressures may lead to upper tract compromise. The AUS has been reported to have a predictably high success even in the setting of diminished bladder compliance. Bladder compliance may even improve. In a comparison of pre- and post-AUS implantation urodynamic parameters, Afraa and colleagues revealed statistically significant improvement in bladder capacity from 271 ± 117 to 295.6 ± 151 mL ( P = .05) and bladder compliance from 7.6 ± 3.95 to 12.5 ± 10.3 mL/cm H ₂ O ( P = .03). Voiding pressures are not increased after implantation of the AUS because voiding occurs with the device in the open setting.

In radiated patients, however, it is not safe to assume that impaired compliance necessarily resolves after resumption of normal bladder filling and evacuation cycles. Rather, there may be a component of intrinsic bladder wall fibrosis secondary to radiation-induced obliterative endaritis. It is important, therefore, to continue to monitor upper tract anatomy to assess for the development of hydronephrosis, which has been reported after AUS placement, and to repeat urodynamics as necessary to assess filling and storage pressures.

Low Abdominal Leak Point Pressure

Although pad use, bother from leakage, and patient preference typically dictate the implementation of surgical treatment, the degree of sphincteric incompetence can also affect the type of treatment recommended for the management of the stress incontinence. The value of abdominal leak point pressure (ALPP) in management of PPI is debatable. There is evidence that patients with higher ALPP, and, therefore, more preserved sphincteric function tend to respond better to periurethral bulking agents than do patients with a lower ALPP. Sanchez-Ortiz and colleagues found that an ALPP less than 60 cm predicts a higher failure rate with periurethral bulking agents compared with those with higher leak point pressures. Twiss and colleagues, however, found no correlation between ALPP and pad weight.

Bladder Outlet Obstruction

Bladder outlet obstruction after radical prostatectomy is most commonly due to persistent bladder neck contracture or urethral stricture. These occur at a rate of 2.7% to 20%. Clinically significant anastomotic and urethral strictures are typically diagnosed on cystoscopy. Findings of bladder outlet obstruction on urodynamics should be interpreted with caution. Scarring and relative poor compliance that can develop in a urethra that has been operated on can result in the finding of catheter-induced bladder outlet obstruction. This should be suspected if there is a large difference between the Qmax obtained with and without the 7F urodynamics catheter in place. If an adequate free flow and low PVR can be confirmed, this finding of obstruction on urodynamics is not a contraindication to anti-incontinence surgery.