Abstract
The urethra is a complex organ essential for the maintenance of urinary continence. It has always been suggested that as long as the urethral pressure exceeds the one generated by the bladder, continence is maintained. This is a plausible explanation when the patient is at rest but cannot fully explain how this pressure differential is maintained during periods of raised intra-abdominal pressure.
10.1 Introduction
The urethra is a complex organ essential for the maintenance of urinary continence. It has always been suggested that as long as the urethral pressure exceeds the one generated by the bladder, continence is maintained. This is a plausible explanation when the patient is at rest but cannot fully explain how this pressure differential is maintained during periods of raised intra-abdominal pressure.
Components of the urethral continence mechanism (Figure 10.1) include the submucosal vasculature, the urethral smooth muscle, the urethral striated sphincter, the bladder neck and the urethral supports. Failure of one or more of these structures can result in incontinence. The striated urethral sphincter extends from 20% to 80% of the urethral length. In the upper two thirds, it has been shown to have a circular orientation and is responsible for a third of the urethral resting pressure. The mucosa and the submucosal vasculature act in tandem to help maintain a tight seal.
Figure 10.1 Anatomy of the female urethral sphincter.
Many tests of urethral function have been proposed and the International Continence Society (ICS) has suggested standardisation of the performance of some of these studies and has defined parameters for measurements [1].
It is fair to say that few are used in normal urodynamic practice probably due to inability to guide therapy or provide a predictive value of therapeutic success.
Tests of urethral function include:
leak point pressure (LPP)
maximal urethral closure pressure (MUCP)
fluid bridge test
Urethral retro-resistance method (URP)
With the exception of the URP, all the other tests require the use of an intra-urethral device to obtain a measurement. It is postulated that this may alter the normal resting anatomy, thus preventing the measurement of the true urethral pressure.
10.2 Urethral Function Tests during Filling Cystometry
Two tests may be included to assess urethral function specifically during filling cystometry:
vesical or detrusor leak-point pressure estimation
abdominal leak-point pressure (ALPP)
10.2.1 Vesical or Detrusor Leak-Point Pressure
Vesical or detrusor leak-point pressure is recorded as the detrusor pressure at the instance of leakage and is considered to be an indirect measure of urethral resistance [2].
10.2.2 Abdominal Leak Point Pressure (ALPP)
ALPP measures the vesical pressure at which leakage occurs during gradual increase in intra-abdominal pressure in the absence of detrusor overactivity. Patients are instructed to produce a graded Valsalva, thereby increasing intra-abdominal pressure while in the upright position at a bladder volume of 200–300 ml and after reduction of pelvic organ prolapse.
10.3 Urethral Function during Voiding Cystometry
Tests of urethral function during voiding cystometry measure the relationship between pressure in the bladder and urine flow rate [1]. Increased detrusor pressure and synchronous, reduced urine flow rates may indicate ‘abnormal urethral function’. This may be caused by anatomical abnormalities such as a urethral stricture or urethral overactivity.
10.4 Tests of Urethral Function
Additional tests to assess urethral function specifically may be included when more detailed information on urethral function is desirable.
10.4.1 Urethral Pressure Profilometry (UPP)
UPP provides a graph indicating the intra-luminal pressure along the length of the urethra (Figure 10.2) [2]. A water-perfused catheter, pressure-tip transducer, balloon catheter or air-filled catheter may be used. Profilometry may be performed at rest, during voiding or as a stress test (coughing, straining or Valsalva). The patient positioning is supine (upright position increases maximum urethral closure pressure), at no specified bladder volume and with pelvic organ prolapse reduced. However, it would be standard practice to specify the speed of catheter withdrawal and state reference position for solid-state catheters (usually lateral) and bladder volume during the test. The test is repeated three to six times, for reproducibility. The MUCP measures the passive resistance (or resting tone) of the urethral sphincter.
The parameters recorded are as follows:
absolute urethral length
functional urethral length
maximum urethral pressure
maximum urethral closure pressure.
When tested in the clinical setting, both MUCP and LPP do not have normal distributions as it is commonly seen with other physiological measures such as height, weight and age. Correlation of the test with symptom severity and treatment outcome is absent.
Figure 10.2 Urethral pressure profile parameters.
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