Porth's Essentials of Pathophysiology, 4e

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Disorders of the Bladder and Lower Urinary Tract

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There is an increased risk for UTIs in persons with uri- nary obstruction and reflux, in people with neurogenic disorders that impair bladder emptying, in women who are sexually active, in postmenopausal women, in men with diseases of the prostate, and in elderly persons. Instrumentation and urinary catheterization are the most common predisposing factors for nosocomial, or hospi- tal-acquired, UTIs. Urinary tract infections occur more commonly in women with diabetes than in women with- out the disease. People with diabetes are also at increased risk for complications associated with UTIs, including pyelonephritis, and they are more susceptible to fungal infections (particularly Candida species) and infections with gram-negative pathogens other than E . coli , both of which are accompanied by increased severity and unusual manifestations. Host–Agent Interactions Because certain people tend to be predisposed to devel- opment of UTIs, considerable interest has been focused on host–pathogen interactions and factors that increase the risk for UTI. 35,36 Host Defenses. In the development of a UTI, host defenses are matched against the virulence of the patho- gen. The host defenses of the bladder have several com- ponents, including the washout phenomenon, in which bacteria are removed from the bladder and urethra during urination; the protective mucin layer that lines the bladder and protects against bacterial invasion; and local immune responses. Immune mechanisms, particularly secretory immunoglobulin (Ig) A, appear to provide an important antibacterial defense. Phagocytic blood cells further assist in the removal of bacteria from the urinary tract. There has been a growing appreciation of the protec- tive function of the bladder’s mucin layer. It is thought that the epithelial cells that line the bladder produce protective substances that subsequently become incor- porated into the mucin layer that adheres to the bladder wall. One theory proposes that the mucin layer acts by binding water, which then constitutes a protective bar- rier between the bacteria and the bladder epithelium. Elderly and postmenopausal women produce less mucin than younger women, suggesting that estrogen may play a role in mucin production in women. Other important host factors include the normal flora of the periurethral area in women and prostate secretions in men. In women, the normal flora of the periurethral area, which consists of organisms such as Lactobacillus , provides a defense against the colonization of uropathic bacteria. 40 Alterations in the periurethral environment, such as occurs with a decrease in estrogen levels dur- ing menopause or the use of antibiotics, can alter the protective periurethral flora, allowing uropathogens to colonize and enter the urinary tract. In men, the pros- tatic fluid has antimicrobial properties that protect the urethra from colonization. Pathogen Virulence. Pathogen virulence derives from its ability to gain access to and thrive in the environment

UrinaryTract Infections Urinary tract infections (UTIs) include several distinct entities—asymptomatic bacteriuria, symptomatic lower UTIs such as cystitis, and upper UTIs such as pyelone- phritis. Because of their ability to cause renal damage, upper UTIs are considered more serious than lower UTIs. Acute pyelonephritis (discussed in Chapter 25) represents an infection of the renal parenchyma and renal pelvis. The discussion in this chapter focuses on lower urinary tract infections. Etiologic Factors Most uncomplicated lower UTIs are caused by Escherichia coli . 35–39 Other common uropathic patho- gens include Enterococcus faecalis , Staphylococcus sap- rophyticus , Klebsiella pneumoniae , Proteus mirabilis , and Pseudomonas species. Most upper and lower UTIs are caused by bacteria that enter through the urethra. Although the distal portion of the urethra often contains pathogens, the urine formed in the kidneys and found in the bladder normally is sterile or free of bacteria. This is because of the washout phenomenon , in which urine from the bladder normally washes bacteria out of the urethra during urination. ■■ Neurogenic disorders of the bladder commonly are manifested by a neurogenic overactive or spastic bladder dysfunction, in which there is failure to store urine, or an aflexic or flaccid bladder dysfunction, in which bladder emptying is impaired. Neurogenic overactive bladder dysfunction results from neural lesions above the level of the sacral cord that allow neurons in the micturition center to function reflexively without control from higher central nervous system centers; in contrast, areflexic bladder dysfunction results from neural disorders affecting the motor neurons in the sacral cord or peripheral nerves that control detrusor muscle contraction and bladder emptying. ■■ Incontinence represents the involuntary loss of urine. Stress incontinence is caused by the decreased ability of the vesicourethral sphincter to prevent the escape of urine during activities, such as lifting and coughing, that raise bladder pressure above the external sphincter pressure. Urge or overactive bladder incontinence is caused by disorders that result in hyperactive bladder contractions. Overflow incontinence is caused by overfilling of the bladder with escape of urine.