Little is known about the underlying mechanisms that cause pelvic floor dysfunction. Historically, theories regarding the pathophysiology of pelvic floor disorders have been derived from observation of success—or failure—of new surgical or medical therapies. Therefore, our understanding of the possible mechanisms behind pelvic floor disorders continues to evolve. In this chapter, mechanisms of pelvic floor dysfunction will be reviewed to address hypothesized theories of the pathophysiology of urinary and anal incontinence, overactive bladder, and pelvic organ prolapse.
STRESS URINARY INCONTINENCE
Historically, stress incontinence has been associated with abnormal support or position of the bladder neck and urethra. Early observations linked stress urinary incontinence (SUI) to the loss of the “normal” angle between the bladder and urethra. This angle was measured on bead chain cystourethrogram (Figure 3-1).1-4 An abnormal posterior urethrovesical angle was initially thought to be the cause of urinary stress incontinence and was later used to identify women whose stress incontinence would be more effectively treated by urethropexy or anterior colporrhaphy. However, the bead chain cystourethrogram was ultimately found to be poorly reproducible and was eventually abandoned. An abnormal position of the posterior urethrovesical angle is no longer thought to be a mechanism of SUI.
Bead chain cystogram. A metallic bead chain has been inserted transurethrally and lateral radiography demonstrates the angle formed by the posterior urethra and bladder base. Before (left) and after (right) retropubic urethropexy. (Reproduced with permission from Ref.2)
Several contemporary theories of SUI attribute this condition to poor support of the urethrovesical junction during increased intra-abdominal pressure. Hypermobility of the bladder neck is thought to be one of several factors that result in poor pressure transmission to the proximal urethra at the instant of increased intra-abdominal pressure (Figure 3-2). The concept of pressure transmission is important with respect to mechanisms of urinary incontinence. Specifically, continence is maintained during increased intra-abdominal pressure if the pressure in the urethra exceeds the pressure in the bladder. The absolute difference between urethral pressure and bladder pressure is described as the “closure pressure,” typically measured during urodynamic testing. If the closure pressure drops below zero (eg, if bladder pressure exceeds urethral pressure), incontinence will occur. In women with stress incontinence, the closure pressure decreases to zero (or below zero) during increased intra-abdominal pressure.
Urethral pressure profiles at rest and with coughing. The urodynamic catheter is drawn through the urethra, resulting in a display of urethral pressure from proximal to distal. In each panel, the tracing, from top to bottom, represents bladder pressure, abdominal pressure, calculated detrusor pressure, urethral pressure, and calculated urethral closure pressure. Panel A was obtained at rest. Panel B...