Porth's Essentials of Pathophysiology, 4e

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Disorders of the Bladder and Lower Urinary Tract

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of the involved structures, severe constipation and fecal impaction can compress the urethra and produce ure- thral obstruction. Compensatory and Decompensatory Changes The body compensates for the obstruction of urine out- flow with mechanisms designed to prevent urine reten- tion. These mechanisms can be divided into two stages: a compensatory stage and a decompensatory stage. 10 The degree to which these changes occur and their effect on bladder structure and urinary function depend on the extent of the obstruction, the rapidity with which it occurs, and the presence of other contributing factors, such as neurologic impairment and infection. During the early stage of obstruction, the bladder begins to hypertrophy and becomes hypersensitive to afferent stimuli arising from stretch receptors in the blad- der wall. The ability to suppress urination is diminished, and the bladder contractions can become so strong that they virtually produce bladder spasms. There is urgency, sometimes to the point of incontinence, and frequency of urination during the day and at night. With continuation and progression of the obstruc- tion, there is further hypertrophy of the bladder muscle, and the pressure generated by detrusor contraction can increase from a normal 20 to 40 cmH 2 O to 50 to 100 cm H 2 O to overcome the resistance from the obstruction. 10 As the force needed to expel urine from the bladder increases, compensatory mechanisms may become inef- fective, causing muscle fatigue before complete empty- ing can be accomplished. After a few minutes, voiding can again be initiated and completed, accounting for the frequency of urination. Normally, the inner bladder surface forms smooth folds. With continued outflow obstruction, this smooth surface is replaced with coarsely woven structures (i.e., hypertrophied smooth muscle fibers) called trabeculae . Small pockets of mucosal tissue, called cellulae , com- monly develop between the trabecular ridges. These pockets form diverticula when they extend between the actual fibers of the bladder muscle (Fig. 27-4). Because the diverticula have no muscle, they are unable to con- tract and expel their urine into the bladder, and second- ary infections caused by stasis are common. Along with hypertrophy of the bladder wall, there is hypertrophy of the trigone area and the interureteric ridge, which is located between the two ureters. This causes backpres- sure on the ureters, the development of hydroureters, and, eventually, kidney damage. Stasis of urine also pre- disposes to urinary tract infections. When compensatory mechanisms no longer are effec- tive, signs of decompensation begin to appear. The period of detrusor muscle contraction becomes too short to expel the urine completely, and residual urine remains in the bladder. At this point, the symptoms of obstruction—frequency of urination, hesitancy, need to strain to initiate urination, a weak and small stream, and termination of the stream before the bladder is completely emptied—become pronounced. There may be signs of a complicating urinary tract infection such

Disorders of Lower Urinary Tract Structures and Function Disorders of lower urinary tract structures and function include urinary obstruction with retention or stasis of urine, and urinary incontinence with involuntary loss of urine. Both types of disorders can have their origin in the structures of the lower urinary tract or in the neural mechanisms that control their function. In lower urinary tract obstruction and stasis, urine is pro- duced normally by the kidneys but is retained in the blad- der, a condition that predisposes to vesicoureteral reflux (VUR) and kidney damage. Obstructions can be classified according to their location (bladder neck, urethra, or exter- nal urethral meatus), cause (congenital or acquired), degree (partial or complete), and duration (acute or chronic). 10 The common sites of congenital obstructions are the external meatus (i.e., meatal stenosis) in boys and just inside the external urinary meatus in girls. Another con- genital cause of urinary stasis is the damage to sacral nerves that is seen in spina bifida and meningomyelocele (see Chapter 36). The acquired causes of lower urinary tract obstruction and stasis are numerous. In males, the most important cause of urinary obstruction is exter- nal compression of the urethra caused by enlargement of the prostate gland. 10,11 Gonorrhea and other sexu- ally transmitted infections contribute to the incidence of infection-produced urethral strictures. Bladder tumors and secondary invasion of the bladder by tumors arising in structures that surround the bladder and urethra can compress the bladder neck or urethra and cause obstruc- tion. Obstructive disorders in women include conditions related to relaxation of the pelvic support structures, such as cystocele and rectocele. Because of the proximity Lower UrinaryTract Obstruction and Stasis centers, the micturition center in the pons, and cortical and subcortical centers. ■■ The sympathetic nervous system division of the ANS facilitates bladder filling by producing relaxation of the smooth muscle fibers of the detrusor muscle in the bladder wall and contraction of the internal sphincter.The parasympathetic nervous system facilitates bladder emptying by producing contraction of the detrusor muscle and relaxation of the internal sphincter. ■■ The striated muscles in the external sphincter and pelvic floor, which are innervated by the somatic nervous system, provide for the voluntary control of urination and maintenance of continence.