Lower Urinary Tract Infection

32 Lower Urinary Tract Infection




Urinary tract infections are a significant health care problem affecting an estimated 10% to 20% of women during their lifetimes and accounting for approximately 7 million office visits per year. Management of a single episode of cystitis costs an estimated $140, and the annual health care costs of urinary tract infections in women are estimated to exceed $1 billion.


In the past 30 years, there have been significant developments in our understanding of the pathogenesis and management of urinary tract infections. These include recognition that about one third of women with cystitis have bacterial counts lower than 100,000 colony-forming units (CFU) per milliliter of urine, realization that bacteria infecting the urinary tract usually come from fecal flora, documentation that most recurrent urinary tract infections are reinfections caused by fecal bacteria and can be successfully managed by low-dose prophylaxis, introduction of newer antimicrobial therapy, and the understanding that certain women with cystitislike symptoms have sterile urine and will ultimately be diagnosed as sensory urgency, overactive bladder, or a painful bladder condition.



EPIDEMIOLOGY


Urinary tract infections are more prevalent among women than men (ratio of 20:1), probably secondary to an anatomically short urethra and its close proximity to the vagina and rectum. Approximately 5 million cases of cystitis occur annually in the United States, and over 100,000 patients are hospitalized every year for renal infection. At least 50% of women experience an episode of cystitis at some time during their lives and about 5% have frequent episodes.


The prevalence of urinary tract infections increases with age. At 1 year, 1% to 2% of female infants demonstrate bacteriuria. In this age group, there is a direct correlation between cystitis and upper urinary tract infection. As many as 50% of these patients demonstrate abnormalities on intravenous pyelograms, such as scarring, ipsilateral reflux, or some obstructive disease. After 1 year of age, the infection rate decreases to approximately 1% and remains low until puberty. Between ages 15 and 24, the prevalence of bacteriuria is about 2% to 3% and increases to about 15% at age 60, 20% after age 65, and 25% to 50% after age 80 (Fig. 32-1). Sexual activity and pregnancy are major factors in younger age groups, whereas pelvic relaxation, systemic illnesses, and hospitalization play major roles in older women. However, the prevalence of underlying urologic abnormalities decreases dramatically with age.



Approximately 2% of all patients admitted to a hospital acquire a urinary tract infection, which accounts for 500,000 nosocomial infections per year. One percent of all these infections become life threatening. Instrumentation or catheterization of the urinary tract is a precipitating factor in at least 80% of these infections. The annual cost of nosocomial urinary tract infections is estimated to range between $424 and $451 million. Asymptomatic bacteriuria occurs in 2% to 8% of adult females, the likelihood of which increases with increasing age, diabetes mellitus, and a history of symptomatic urinary tract infection. Symptomatic urinary tract infections are believed to affect over 30% of women between the ages of 20 and 40 years. Patients who develop infections are more likely to develop subsequent infections. The incidence of re-infections seems to be independent of whether an infection was treated or allowed to resolve on its own. It has been shown that the probability of recurrent urinary tract infections increases with the number of previous infections and decreases with greater amount of elapsed time between infections. Rates of reinfection seem to be independent of bladder dysfunction, evidence of chronic pyelonephritis on radiologic examinations, and vesicoureteral reflux. Kraft and Stamey (1977) showed that patients who had two or more urinary tract infections within a 6-month period had a 66% probability of attaining another infection in the next 6 months. Antimicrobial prophylaxis of urinary tract infections does not change the risk of recurrent bacteriuria, but only seems to alter the time until the redevelopment of another urinary tract infection.



MICROBIOLOGY


Gram-negative bacilli of the family Enterobacteriaceae are responsible for 90% of urinary tract infections. Escherichia coli is the single most important organism and accounts for 80% to 90% of uncomplicated infections. Others include Klebsiella, Enterobacter, Serratia, Proteus, Pseudomonas, Providencia, and Morganella species. Pseudomonas aeruginosa infection is almost always secondary to urinary tract instrumentation. Staphylococcus saprophyticus is the second most common cause of cystitis and causes 10% of infections in sexually active females. Staphylococcus epidermidis is a nosocomial pathogen identified in patients with indwelling catheters. Staphylococcus aureus is less commonly isolated and is often secondary to hematogenous renal infection. Other gram-positive organisms such as enterococci and Streptococcus agalactiae cause about 3% of episodes of cystitis. Enterococcus faecalis causes about 15% of nosocomial urinary infections, and Str. agalactiae is more commonly the cause in patients with diabetes mellitus. Anaerobic bacteria, although abundant in fecal flora, rarely cause urinary tract infections. The oxygen tension in the urine probably prevents their growth and persistence in the urinary tract.


Candida albicans and other fungal organisms can cause lower urinary tract infections in patients with diabetes mellitus or indwelling urinary catheters. Immunocompromised patients and recipients of renal transplantation are vulnerable to candidal urinary tract infections. Torulopsis glabrata is second in frequency to C. albicans. Viruria has been documented with many viruses, but generally in association with viremia. Viral urinary tract infections occur as acute illnesses (acute hemorrhagic cystitis in children, polyoma virus infection after bone marrow transplant), during convalescence from viral infections (mumps, cytomegalovirus), and in asymptomatic patients (cytomegalovirus).



PATHOGENESIS


Although the normal female urinary tract is remarkably resistant to infection, certain risk factors for developing urinary tract infections have been identified (Box 32-1). The majority of urinary tract infections are ascending infections wherein the fecal flora initially colonize the vaginal introitus, then the periurethral tissues, and eventually gain entry into the bladder (Fig. 32-2). The development of urinary tract infection requires the interaction of appropriate host susceptibility and pathogen virulence factors (Fig. 32-3).






Host Factors


Several important host defense mechanisms are instrumental in the prevention of urinary tract infections. The normal acidic pH of the vaginal secretions in a premenopausal woman inhibits the growth of enterobacteria such as E. coli and promotes the growth of lactobacilli, diphtheroids, and other gram-positive bacteria—organisms that replicate poorly in urine. Normal periodic voiding with its dilutional effects and the high urea and organic acid concentrations of urine in a setting of a low pH serve as important bladder defense mechanisms. The glycosaminoglycans in the bladder lining and immunoglobulins in the urine are important factors that block bacterial adherence. The deficiency of glycosaminoglycans probably plays a role in recurrent urinary tract infections. In addition, the ascending loop of Henle secretes Tamm-Horsfall protein, a mannose-rich uromucoid that may inhibit bacterial adherence to epithelial cells and trap bacteria in the urine, thereby allowing them to be flushed from the urinary tract.


Studies have shown that women of blood groups B and AB, who are nonsecretors of blood-group substances, are at a greater risk for urinary tract infections, suggesting a possible genetic link. Similarly, a higher prevalence of HLA-A3 subtype was noted among women with recurrent urinary tract infections.


Factors such as sexual activity and diaphragm use are significantly associated with the development of lower urinary tract infections. The frequency and recency of sexual intercourse increase the risk of cystitis. This increase appears to occur through inoculation of periurethral bacteria into the bladder during intercourse. The increased risk associated with diaphragm use may be related to urethral obstruction caused by it as well as the propensity for vaginal colonization by coliforms.



Bacterial Virulence Factors


The adherence of bacteria to mucosal cells appears to be a necessary step for colonization and pathogenicity. E. coli, the most common uropathogen, is the most extensively studied. Three different types of adhesins have been identified: type 1 pili (or fimbriae), P-fimbriae, and X-adhesins. Type 1 pili have a strong affinity for mannose-containing compounds, including Tamm-Horsfall protein, and facilitate attachment of E. coli to vaginal, periurethral, and bladder epithelial cells. P-fimbriae and the less well-studied X-adhesins are important in ascending infections of the kidney. P-fimbriae have a high affinity for P blood group antigens found on erythrocytes and uroepithelial cells. Type 1 pili and P-fimbriae are often possessed by the same bacterium and, after gaining entry to the kidney, the expression of Type 1 pili is turned to avoid phagocytosis. Some type of increased susceptibility of certain host cells to be adhered to also seems possible. Women with certain Lewis blood groups, Le(A−,B−) and Le(A+,b−) phenotypes, have significantly higher rates of recurrent urinary tract infections than women with Le(a−,b+) phenotype. The Lewis antigen controls fucosylation. Premenopausal women are more susceptible to attachment of certain strains of E. coli and lactobacilli during certain times of the menstrual cycle and during early pregnancy.


Bacteria possess a variety of other virulence factors, of which multidrug resistance is most clinically significant. Uropathogens develop resistance primarily through the resistance transfer plasmid. Plasmid resistance has been found for β-lactams, sulfonamides, aminoglycosides, and trimethoprim. So far, no plasmid-mediated resistance has been identified to fluoroquinolones, which makes these agents valuable in treating infections caused by multidrug-resistant bacteria. Other virulence factors include production of hemolysins and colicin V by some enterobacteria and urease by Proteus species.



DEFINITIONS


When discussing urinary tract infections, an understanding of the generally accepted definitions is essential.

















DIAGNOSIS




Mar 11, 2016 | Posted by in Reconstructive surgery | Comments Off on Lower Urinary Tract Infection

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