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.

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.

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.

Pressure transmission is linked to urethral support. Specifically, in a continent woman, the urethra is supported by a “hammock,”⁵ consisting of the vaginal wall and its fibrous and muscular attachments (Figure 3-3). If the bladder neck is hypermobile, the proximal urethra descends at the moment of increased intra-abdominal pressure and thus the urethra is not compressed. Furthermore, if the urethra descends through the levator hiatus at the instant of increased intra-abdominal pressure, the urethra will not be exposed to the increase in intra-abdominal pressure. This may result in an unfavorable pressure gradient between the bladder and urethra, resulting in incontinence.

For almost four decades, hypermobility of the urethra has been defined by the “Q-tip test” or cotton swab test. A lubricated cotton swab is placed through the urethra until the tip is in the bladder and the patient is asked to strain (Figure 3-4). A positive cotton swab test is defined as rotation beyond 30% from the horizontal during straining. Clinicians have observed that women with SUI are likely to demonstrate a hypermobile bladder neck. However, it has been recognized that many continent women also have evidence of urethral hypermobility and, thus, a hypermobile bladder neck is not synonymous with SUI. In contemporary practice, the cotton swab test may be used to identify women most likely to benefit from surgical treatment of SUI: a negative cotton swab test, indicating good support of the urethrovaginal junction, is a strong predictor of the failure of Burch urethropexy and sling procedures for treatment of SUI.^7-13

SUI may also be a manifestation of pelvic muscle weakness. Women with SUI have weaker levator ani muscle strength than continent controls.¹⁴ Programs to strengthen the pelvic muscles are effective treatments for SUI.¹⁵ However, it is not known whether loss of muscle strength is the catalyst that triggers the development of SUI or whether pelvic muscle strengthening simply compensates for other mechanisms contributing to SUI. The relationship between pelvic muscle weakness and SUI may be mediated via poor pressure transmission. Weakness of the muscular component of the pelvic floor contributes to hypermobility. This theory is supported by the observation that SUI may be treated effectively with the “Knack” maneuver, a pelvic muscle contraction timed to coincide with anticipated increased abdominal pressure.¹⁶

Another potential mechanism for SUI is a loss of intraluminal urethral pressure. Urethral pressure measures were first made in the 1960s by Enhorning.¹⁷ Resting intraluminal urethral pressures are lower in women with SUI than in continent controls.¹⁸ Both the striated and smooth muscles of the urethra contribute to urethral intraluminal pressure.¹⁹ However, equal in importance to each of those muscular components is the mucosal coaptation provided by the bulking effect of the submucosal vasculature. Coaptation maintains urethral closure via surface tension. Reduction in coaptation or in the striated or smooth muscle tone can leave the urethra open at rest, facilitating stress incontinence. Traditionally, this type of SUI has been classified as “intrinsic sphincter deficiency” (ISD), although precise definitions of ISD vary. A reduction in urethral tone is associated with increasing age,²⁰ providing one explanation for the association between SUI and aging. A transient reduction in urethral tone may be associated with α-adrenergic antagonists.²¹ The reduction in intraluminal urethral pressure with a loss of mucosal coaptation is the rationale for the use of urethral bulking agents for treatment of SUI.²²

In addition to biological mechanisms that create conditions favorable to the development of SUI, a number of conditions and behaviors may promote SUI. For example, women who smoke may experience SUI because they cough more often and more forcefully than nonsmokers.²³ For these women, a reduction in coughing may substantially reduce SUI symptoms. Another example is obesity.²⁴ The mechanisms linking obesity and SUI are uncertain but may be related to increased intra-abdominal pressure in obese women.^25,26 Both obesity and cigarette smoking are associated with incontinence severity.²⁷

The symptom complex of “overactive bladder” refers to the symptom of urgency, usually in association with frequency and nocturia, with or without urgency incontinence. Mostly, overactive bladder is thought to be a result of involuntary detrusor contractions.

The detrusor contracts involuntarily throughout fetal life and infancy. However, in childhood, acquisition of bladder control is achieved through cortical maturation, with the inhibition of involuntary detrusor activity. This typically occurs between age of three and five years. Children who fail to acquire this suppression of detrusor activity may be prone to nocturnal enuresis or other manifestations of overactive bladder. Thus, the highest incidence of overactive bladder in children occurs between five and seven years of age,²⁸ as these children present for evaluation of frequency, nocturia, and incontinence. Evidence suggests that pediatric overactive bladder is familial and strongly associated with overactive bladder later in life.^29,30

The specific factors that cause involuntary detrusor activity are not known. Theories are divided between “myogenic” and “neurogenic.” One example of a “myogenic” mechanism is bladder neck obstruction. It has long been recognized that men with prostate enlargement have a higher prevalence of detrusor overactivity.³¹ Similarly, women with obstructed voiding from either severe prolapse or prior bladder neck surgery are more likely to experience detrusor overactivity.³² It is hypothesized that detrusor hypertrophy, caused by increased voiding pressures against an obstruction, leads to involuntary detrusor activity.

There are many potential neurogenic mechanisms for detrusor dysfunction. The bladder and lower urinary tract are regulated by both the autonomic nervous system and the central nervous system and therefore a variety of neurological conditions can affect bladder function. If detrusor overactivity is identified in an adult with a known neurological condition, this is classified as “neurogenic detrusor overactivity”. Examples of neurologic conditions that precipitate detrusor hyperreflexia include multiple sclerosis, Parkinson disease, stroke, and traumatic brain injury. These conditions cause bladder overactivity due to a reduced tonic inhibition of bladder contractions.³³

The degree of volitional control exerted over bladder function contrasts sharply with the much more limited control exerted over other visceral organs, such as the rectum. Subtle derangements of the autonomic nervous system and the central nervous system can result in bladder dysfunction. In the central nervous system, the pontine micturition center is the source of parasympathetic efferents to the detrusor, which travel via the pelvic nerves. The pontine center is thought to mediate the voluntary control of detrusor function. Specifically, stimulation of the parasympathetic efferent pathways results in bladder contraction. Bladder sensory afferents travel in the pelvic nerve and hypogastric nerve. Rapid distention of the bladder, especially with chilled fluid, can trigger a detrusor contraction, demonstrating the potential role for afferent signaling in the genesis of detrusor overactivity. Other examples of a possible role for afferent stimulation in the genesis of involuntary detrusor contractions include a variety of irritative conditions, including cystitis, and their association with overactive bladder symptoms.

A sustained and coordinated contraction of the detrusor muscle is typical for normal voluntary voiding. Less coordinated or localized detrusor contractions are associated with sensory urgency³⁴ and may play a role in overactive bladder.

In some cases, superimposed conditions increase symptoms related to overactive bladder. Examples include polyuria and medications that impact the autonomic nervous system. Overactive bladder is also strongly associated with aging. However, it is not known whether the increase in bladder overactivity associated with age is due to age-related changes in bladder function or to the acquisition with age of co-morbid conditions. Additional age-related changes in detrusor function include decreased contractility and decreased compliance, both of which can mimic the effects of detrusor overactivity.

The anal sphincter consists of an internal and external component. Most of the resting tone is provided by the internal sphincter.³⁵ The external anal sphincter, a striated muscle, provides voluntary tone. Injury to either component can disrupt continence. Other factors that contribute to continence include rectal sensation, the anal mucosal folds and vascular cushions, the posterior anorectal angle, stool volume and consistency, as well as the compliance of the rectum. Among adult women, the most common anal incontinence symptoms include incontinence of flatus and fecal urgency.^36,37 In contrast, frank incontinence of solid stool is less common. Pelvic floor mechanisms contributing to symptoms of anal incontinence include anal sphincter injury, pelvic organ prolapse, and rectal prolapse. Other mechanisms include gastrointestinal disorders and peripheral neuromuscular disorders.

Obstetrical injuries to the anal sphincter complex are possibly the most well-recognized pelvic floor cause of anal incontinence in young women. The incidence of anal sphincter injury at the time of vaginal birth is not known but thought to be approximately 5%.³⁸ Injury to the external anal sphincter is more common than injury to the internal sphincter. Six months after recognized obstetrical anal sphincter laceration and immediate repair, anal incontinence is reported by 17% of primigravid women,³⁷ indicating a substantial prevalence in this setting. Incontinence after sphincter repair can occur due to chronic dehiscence of at least one component of the repair.^39,40

In 1993, Sultan and colleagues reported that 35% to 45% of women had evidence of occult sphincter lacerations after vaginal childbirth.⁴¹ These occult lacerations, detected postpartum on endoanal ultrasound, were associated with incontinence symptoms. However, this high incidence of occult laceration has since been attributed to inadequate training of obstetrical providers in the recognition of such injuries at the time of delivery.⁴² With improved detection, the incidence of occult sphincter injuries has been estimated at less than 10%.^43,44 Recent evidence from magnetic resonance imaging suggests that incontinent women may be more likely to have evidence of levator ani injuries.^45,46

Neuromuscular injuries have also been suggested as a possible mechanism for anal sphincter dysfunction after childbirth. The pudendal nerve can be compressed or stretched during vaginal childbirth.⁴⁷ Evidence of peripheral denervation of the levator ani is associated with obstetrical anal sphincter injury^48-50 and also with idiopathic anal incontinence.⁵¹ In a study of parous women followed five years from delivery,⁵² evidence of pudendal neuropathy was persistent postpartum and associated with reduced anal canal pressures (Figure 3-5). However, animal models in which the pudendal nerve is stretched or compressed do not produce sphincter atrophy or dysfunction,^53,54 raising questions about the mechanism for this observed association.