144 URINARY TRACT INFECTIONS
Harrison’s Manual of Medicine
URINARY TRACT INFECTIONS
Urethritis, Cystitis, and Pyelonephritis
Urethritis, Cystitis, and Pyelonephritis
ETIOLOGY Escherichia coli causes ~80% of uncomplicated UTIs (those unassociated with catheters, urologic abnormalities, or calculi); Proteus, Klebsiella, and Enterobacter account for lesser percentages of cases. Staphylococcus saprophyticus causes 10–15% of acute symptomatic UTIs in young women. Pathogens in recurrent or catheter-associated infections, in infections following urologic manipulation, and in the setting of genitourinary (GU) obstruction or calculi include E. coli, Proteus, Klebsiella, Enterobacter, Pseudomonas, and Serratia. Isolation of Staphylococcus aureus from the urine should always arouse suspicion of staphylococcal bacteremia and secondary infection of the kidney. Agents causing acute urinary symptoms and pyuria in the absence of demonstrable bacteriuria include Chlamydia trachomatis, Neisseria gonorrhoeae, and herpes simplex virus. Candida and other fungal species commonly colonize the urine of catheterized or diabetic pts.
PATHOGENESIS In the vast majority of UTIs, bacteria gain access to the bladder via the urethra. Ascent of bacteria from the bladder may follow and may lead to upper tract disease. Risk factors for UTIs include female gender, sexual activity, pregnancy, GU obstruction, neurogenic bladder dysfunction, and vesicoureteral reflux. Hematogenous pyelonephritis occurs most often in debilitated pts. Staphylococcemia or candidemia may lead to metastatic renal parenchymal infection.
EPIDEMIOLOGY UTIs may be categorized as catheter-associated (nosocomial) or non-catheter-associated (community-acquired). Acute infections are very common; the vast majority of symptomatic cases involve young women, for whom sexual activity augments the risk of infection. UTIs are rare among men under the age of 50. The development of asymptomatic bacteriuria parallels that of symptomatic infection: it, too, is rare among men under age 50 but common among women between ages 20 and 50 and very common among the elderly of either sex. Bacteriuria develops in at least 10–15% of hospitalized pts with indwelling urethral catheters; the risk of infection is about 3–5% per day of catheterization.
CLINICAL MANIFESTATIONS Bacteriuria can be asymptomatic. Dysuria, frequency, urgency, and suprapubic pain signal cystitis. One-third of pts with such symptoms and significant bacteriuria have concomitant, clinically silent upper tract disease. On the other hand, ~30% of women with acute dysuria do not have significant bacteriuria; some of them have urethritis due to a sexually transmitted pathogen (e.g., N. gonorrhoeae, C. trachomatis, or herpesvirus). Manifestations of acute pyelonephritis may include fever, shaking chills, nausea, vomiting, and diarrhea as well as flank pain and dysuria. Most catheter-associated infections cause minimal symptoms. The frequency of upper tract infection with catheter-induced bacteriuria is unknown, although the catheterized urinary tract is the most common source of gram-negative bacteremia in hospitalized pts, accounting for approximately one-third of cases.
DIAGNOSIS Women with symptoms characteristic of acute uncomplicated cystitis may reasonably be treated empirically, either on the basis of Hx and physical findings alone or after confirmatory microscopy or leukocyte esterase determination. Urine should be cultured, however, when the diagnosis of cystitis is uncertain, when upper tract infection is suspected, or when any complicating factors are present. A colony count of ³105/mL in a voided midstream specimen generally indicates infection, as does bacteriuria to any degree in a suprapubic aspirate or the presence of ³102 bacteria/mL in urine obtained by catheterization. Microscopy of urine from symptomatic pts can be of great value: the finding of bacteria on a gram-stained, unspun specimen indicates a colony count of at least 105/mL. The finding of bacteriuria with leukocyte casts suggests pyelonephritis. Asymptomatic bacteriuria should be documented twice before treatment is instituted. All males with UTI and any pt with obstructive disease and non-catheter-associated infection should be evaluated urologically.
Principles underlying the treatment of UTIs are listed in Table 144-1. Treatment regimens for bacterial UTIs are listed in Table 144-2. Recurrent symptomatic UTI (³3 infections per year) warrants prophylaxis daily or thrice weekly with agents such as nitrofurantoin (50 mg), TMP-SMZ (80/400 mg), or TMP alone (100 mg).
Table 144-1 Principles Underlying the Treatment of UTIs
Table 144-2 Treatment Regimens for Bacterial UTIs
ETIOLOGY In non-catheter-associated cases, acute bacterial prostatitis is usually due to common gram-negative urinary tract pathogens (E. coli or Klebsiella). In catheter-associated cases, nosocomially acquired gram-negative rods or enterococci may also be involved. E. coli, Klebsiella, Proteus, or other uropathogenic organisms may also cause chronic prostatitis. Evidence for causative roles of Ureaplasma urealyticum and C. trachomatis in chronic prostatitis is inconclusive.
CLINICAL MANIFESTATIONS Acute bacterial prostatitis is characterized by fever, chills, dysuria, and extreme prostatic tenderness. It may occur spontaneously (generally in young men) or in association with an indwelling urethral catheter. Chronic bacterial prostatitis is often asymptomatic, and the prostate usually feels normal on palpation; perineal or lower back pain or obstructive symptoms develop in some cases. A pattern of relapsing cystitis in a middle-aged man suggests the diagnosis and is due to intermittent spread of the prostatic infection.
DIAGNOSIS Cultures of urine usually yield the bacterial pathogen. Vigorous prostatic massage should be avoided. Gram’s staining of urine may be particularly useful in guiding empirical therapy for catheter-associated cases.
For acute prostatitis in which gram-negative pathogens are detected in the urine, initially use a fluoroquinolone, a third-generation cephalosporin, or an aminoglycoside; for cases in which gram-positive cocci are detected, use nafcillin or a cephalosporin. For catheter-associated acute prostatitis, use imipenem, a fluoroquinolone, a third-generation cephalosporin, or an aminoglycoside until the etiologic agent has been isolated and tested for sensitivity. For treatment of chronic prostatitis, fluoroquinolones (e.g., ciprofloxacin, 500 mg bid) have been more successful than other agents but must be given for at least 12 weeks to be effective.
For a more detailed discussion, see Stamm WE: Urinary Tract Infections and Pyelonephritis, Chap. 280, p. 1620, in HPIM-15.