Porth's Essentials of Pathophysiology, 4e

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Kidney and Urinary Tract Function

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of the urinary tract, to evade the destructive effects of the host’s immune system, and to develop resistance to antimicrobial agents. Not all bacteria are capable of adhering to and infecting the urinary tract. Of the many strains of E . coli , only those with increased ability to adhere to the epithelial cells of the urinary tract are able to produce UTIs. These bacteria have fine protein fila- ments, called pili or fimbriae , that help them adhere to receptors on the lining of urinary tract structures. Obstruction and Reflux Obstruction and reflux are other factors that increase the risk for UTIs. Any microorganisms that enter the bladder normally are washed out during voiding. When outflow is obstructed, urine remains in the bladder and acts as a medium for microbial growth; the microorgan- isms in the contaminated urine can then ascend along the ureters to infect the kidneys. The presence of resid- ual urine correlates closely with bacteriuria and with its recurrence after treatment. Another aspect of bladder outflow obstruction and bladder distention is increased intravesical pressure, which compresses blood vessels in the bladder wall, leading to a decrease in the mucosal defenses of the bladder. In UTIs associated with stasis of urine flow, the obstruction may be anatomic or functional. Anatomic obstructions include urinary tract stones, prostatic hyperplasia, pregnancy, and malformations of the ure- terovesical junction. Functional obstructions include neurogenic bladder, infrequent voiding, detrusor (blad- der) muscle instability, and constipation. A phenomenon called urethrovesical reflux occurs when urine from the urethra moves into the bladder. In women, urethrovesical reflux can occur during activities such as coughing or squatting, in which an increase in intra-abdominal pressure causes the urine to be squeezed into the urethra and then to flow back into the bladder as the pressure decreases. 40 This also can happen when voiding is abruptly interrupted. Because the urethral ori- fice frequently is contaminated with bacteria, the reflux mechanism may cause bacteria to be drawn back into the bladder. The vesicoureteral reflux , which occurs at the level of the bladder and ureter, allows urine and bacteria to ascend from the bladder to the kidney and is associated with pyelonephritis and infections of the upper urinary tract. (see Chapter 25, Fig. 25-11). Catheter-Induced Infection Urinary catheters are a source of urethral irritation and provide a means for entry of microorganisms into the urinary tract. Catheter-associated bacteriuria remains the most frequent cause of gram-negative sep- ticemia in hospitalized patients. Studies have shown that bacteria adhere to the surface of the catheter and initiate the growth of a biofilm that then covers the surface of the catheter (see Chapter 14, Fig. 14-6). 41 The biofilm tends to protect the bacteria from the action of antibiotics and makes treatment difficult. A closed drainage system (i.e., closed to air and other sources of contamination) and careful attention to

perineal hygiene (i.e., cleaning the area around the urethral meatus) help to prevent infections in persons who require an indwelling catheter. Careful hand- washing and early detection and treatment of UTIs also are essential. Clinical Features The manifestations of UTI depend on whether the infec- tion involves the lower (bladder) or upper (kidney) uri- nary tract and whether the infection is acute or chronic. An acute episode of cystitis (bladder infection) is char- acterized by frequency of urination, lower abdominal or back discomfort, and burning and pain on urination (i.e., dysuria). 34,38 Occasionally, the urine is cloudy and foul smelling. In adults, fever and other signs of infec- tion usually are absent. If there are no complications, the symptoms disappear within 48 hours of treatment. The symptoms of cystitis also may represent urethritis caused by Chlamydia trachomatis , Neisseria gonor- rhoeae , or herpes simplex virus, or vaginitis attribut- able to Trichomonas vaginalis or Candida species (see Chapter 41). Diagnosis andTreatment The diagnosis of UTI usually is based on symptoms and on examination of the urine for the presence of microor- ganisms. When necessary, x-ray films, ultrasonography, and CT and renal scans are used to identify contributing factors, such as obstruction. Urine tests are used to establish the presence of bac- teria in the urine and a diagnosis of UTI. A commonly accepted criterion for diagnosis of a UTI is the presence of 100,000 colony-forming units (CFU) or more bac- teria per milliliter (mL) of urine. 35 Colonization usu- ally is defined as the multiplication of microorganisms in or on a host without apparent evidence of invasive- ness or tissue injury. Pyuria (the presence of less than five to eight leukocytes per high-power field) indicates a host response to infection rather than asymptomatic bacterial colonization. A Gram stain may be done to determine the type (gram positive or gram negative) of organism that is present. Chemical screening (urine dipstick) for markers of infection may provide useful information but is less sen- sitive than microscopic analysis. 35,36 These tests are rela- tively inexpensive, are easy to perform, and can be done in the clinic setting or even in the home. A urine culture may be done to confirm the presence of pathogenic bacte- ria in urine specimens, allow for their identification, and permit the determination of their sensitivity to specific antibiotics. The treatment of UTI is based on the pathogen causing the infection and the presence of contributing host– agent factors. Other considerations include whether the infection is acute, recurrent, or chronic. Most acute lower UTIs, which occur mainly in women and are gen- erally caused by E . coli , are treated successfully with a short course of antimicrobial therapy. Forcing fluids