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Urinary incontinence is the involuntary loss of urine that is so severe as to have social or hygienic consequences. It is very common, with a prevalence in community-dwelling elderly persons as high as 35%, and significantly higher rates among institutionalized patients. Despite this high prevalence, studies have shown that about half of all incontinent persons have never discussed the problem with a physician. This is likely because of embarrassment, a belief that incontinence is normal with aging, or an assumption that nothing can be done to help. Incontinence is associated with significant medical morbidity, including infection, sepsis, pressure ulcers, and falls. It is also associated with significant psychological stress and social isolation. Incontinence causes significant caregiver burden, and is frequently cited as a reason for deciding to abandon home care efforts in favor of nursing home placement. The economic burden of incontinence is also substantial, with an estimated direct cost in the United States of $16.3 billion per year.

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Because of its high prevalence, significant morbidity, and high psychosocial impact, it is important for family physicians to accurately identify, assess, and treat incontinent patients. The large majority of patients with incontinence can be diagnosed and managed effectively by family physicians in the primary care setting.

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Physiology of Normal Urination

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A basic understanding of the normal physiology of urination is important to understand the potential causes of incontinence, and the various strategies for effective treatment.

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The lower urinary tract consists primarily of the bladder (detrusor muscle) and the urethra. The urethra contains two sphincters, the internal urethral sphincter (IUS), composed predominantly of smooth muscle, and the external urethral sphincter (EUS), which is primarily voluntary muscle. The detrusor muscle of the bladder is innervated predominantly by cholinergic (muscarinic) neurons from the parasympathetic nervous system, the stimulation of which leads to bladder contraction. The sympathetic nervous system innervates both the bladder and the IUS. Sympathetic innervation in the bladder is primarily β-adrenergic and leads to bladder relaxation, whereas α-adrenergic receptors predominate in the IUS, leading to sphincter contraction. Thus, in general, sympathetic stimulation of the urinary tract promotes bladder filling (relaxation of the detrusor with contraction of the sphincter), whereas parasympathetic stimulation leads to bladder emptying (detrusor contraction and sphincter relaxation).

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The EUS, on the other hand, is striated muscle and primarily under voluntary (somatic) control. This allows for some ability to voluntarily postpone urination by tightening the sphincter and inhibiting the flow of urine. Additional voluntary control is provided by the central nervous system through the pontine micturition center. This allows for central inhibition of the autonomic processes previously described, and for further voluntary postponement of the need to urinate until the circumstances are more socially appropriate or until necessary facilities are available.

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The physiologic factors influencing normal urination are summarized in Table 41-1 and are important considerations when discussing urinary disorders and treatment.

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