Chapter 83: Urinary Incontinence

INTRODUCTION

• Urinary incontinence (UI) is defined as involuntary loss 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 contractions of normal micturition and involuntary bladder contractions, which can result in UI. Therefore, most pharmacologic antimuscarinic therapy is anti-M₃ based, resulting in detrusor smooth muscle relaxation and reduction in bladder overactivity.

• Stimulation of β₃-adrenergic receptors in the detrusor muscle 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, known as stress UI (SUI), 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, known as urgency UI (UUI), is a symptom syndrome characterized by urinary urgency, usually accompanied by increased daytime frequency and/or nocturia, with urinary incontinence (OAB-wet) or without (OAB-dry), in the absence of urinary tract infection or other detectable diseases. The detrusor muscle is overactive and contracts inappropriately during urinary storage.

• Urethral overactivity and/or bladder underactivity is known as overflow incontinence or chronic urinary retention. 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 83-1).