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INTRODUCTION

  • Urinary incontinence (UI) is loss of bladder control, leading to involuntary leakage of urine.

PATHOPHYSIOLOGY

  • The urethral sphincter, a combination of smooth and striated muscles within and external to the urethra, maintains adequate resistance to the flow of urine from the bladder until voluntary voiding is initiated.

  • Volitional and involuntary bladder contractions are mediated by activation of postsynaptic muscarinic receptors by acetylcholine. Bladder smooth muscle cholinergic receptors are mainly of the M2 variety; however, M3 receptors are responsible for both emptying contraction of normal micturition and involuntary bladder contractions, which can result in UI. Therefore, most pharmacologic antimuscarinic therapy is anti-M3 based.

  • Stimulation of beta-3 adrenergic receptors in the detrusor results in smooth muscle relaxation. β3-agonists attenuate bladder contractility, which is useful for treatment of overactive bladder (OAB) and urgency incontinence.

  • UI occurs as a result of overactivity or underactivity of the urethra, bladder, or both.

  • Urethral underactivity is known as stress UI (SUI) and occurs during activities such as exercise, running, lifting, coughing, and sneezing. The urethral sphincter no longer resists the flow of urine from the bladder during periods of physical activity.

  • Bladder overactivity is known as urgency UI (UUI) and is associated with increased urinary frequency and urgency, with or without urge incontinence. The detrusor muscle is overactive and contracts inappropriately during the filling phase.

  • Urethral overactivity and/or bladder underactivity is known as overflow incontinence. The bladder is filled to capacity but is unable to empty, causing urine to leak from a distended bladder past a normal outlet and sphincter. Common causes of urethral overactivity include benign prostatic hyperplasia (see Chapter 80); prostate cancer (see Chapter 64); and, in women, cystocele formation or surgical overcorrection after SUI surgery.

  • Mixed incontinence includes the combination of bladder overactivity and urethral underactivity.

  • Functional incontinence is not caused by bladder- or urethra-specific factors but rather occurs in patients with conditions such as dementia or cognitive or mobility deficits.

  • Many medications may precipitate or aggravate voiding dysfunction and UI (Table 82-1).

TABLE 82-1Medications That Influence Lower Urinary Tract Function

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