Epidemiology
UTI are far more common in women than in
men, probably because of the shorter urethra in the female.
Prostatic secretions in the male may have some antibacterial effect.
Pathogenesis
Nearly all UTI arise by the
"ascending route." Fecal organisms, principally Escherichia coli,
colonize the vaginal introitus. Their entry into the bladder
is facilitated by sexual intercourse, contraceptive
diaphragms, and spermicides. The contraceptive
pill has no effect on the incidence of UTI.
Some women are prone to have multiple urinary
reinfections. These women may be colonized by strains of E. coli
with "stickier" fimbriae (anchoring them to the epithelial cells)
or may have "stickier" epithelial cells (a phenomenon related to
certain blood groups). In addition, the factors noted earlier may contribute to
multiple reinfections. In elderly women, vaginal atrophy leads to a reduction
in the counts of lactobacilli in the vagina: thus, the vaginal secretions
become less acidic and gram-negative enterics more easily colonize.
The most potent risk factors for UTI in women are:
sexual intercourse, the use of a diaphragm/spermicide, and a history of
UTI. In men, prostatic hypertrophy is the main risk
factor. In both sexes, Foley catheters are a major risk factor.
Once bacteria reach the bladder, they
may cause cystitis or they may reside there
asymptomatically.Symptoms of cystitis are
urgency, frequency and dysuria. (The symptom of dysuria can
also be caused by certain STDs causing urethritis, and may be confused with the
symptoms of vaginitis.)
From the bladder, bacteria may ascend to
reach the kidney, producing pyelonephritis, an invasive infection which can
cause bacteremia and severe
illness.Pyelonephritis is typified by fever,
chills, flank pain and tenderness, and an elevated peripheral
WBC. Ascent from the bladder to the kidney is facilitated by
urinary stasis and obstruction (as occurs in pregnancy and certain neurological
conditions). Sticky E. coli have an advantage here, too.
The defenses of the urinary tract against infection are
minimal, mainly the flushing effect of urine and the sloughing of colonized
epithelial cells.
Diagnosis
Laboratory diagnosis of UTI is made
problematic by the fact that it is difficult to obtain a truly sterile urine
specimen from voided urine: contamination by meatal organisms
is frequent. If a urine sample is left at room temperature for hours, these
organisms may grow to high numbers. Thus, for patients with asymptomatic
bacteriuria (no symptoms), a high threshold is required to document true
bacteriuria (vs contamination), i.e. 105 bacteria per ml. By
contrast, in patients with typical symptoms, a much lower threshold is
accepted, i.e. 102 bacteria per ml. Most patients with true
bacteriuria have pus cells in the urine (pyuria), at least 105
per high-power field under the microscope or a positive leukocyte esterase
dipstick test. Pyuria is a fairly sensitive indicator for true bacteriuria but
it is not very specific as a guide to treatment because many patients with
asymptomatic bacteriuria (of whom only selected subgroups should be treated)
have pyuria as do some patients with noninfectious inflammatory conditions
(e.g. allergic interstitial nephritis).
Treatment
The most common causes of UTI are
E. coli (85%), Staphylococcus
saprophyticus (5-10%), and other enteric
gram-negatives (5-10%). These organisms are nearly always susceptible
to quinolones. Nowadays, about 20-30% are resistant to TMP-SMX. For reasons not
entirely clear, quinolones and trimethoprim-sulfamethoxazole (TMP-SMX) are more
effective than beta-lactams for UTI even if the organisms are susceptible to
the beta-lactams. For cystitis, a superficial infection, 3 days of treatment
usually suffices. For uncomplicated pyelonephritis (no obstruction or other
anatomic problem), 2 weeks suffices.
Recurrences (relapse vs. reinfection)
While most UTI respond readily to treatment,
some are followed by recurrences.These may take two
forms:relapse and
reinfection.
Relapses
Relapses signify that the original
infection was never eradicated. The organism cultured is
identical to that from the previous episode and symptoms usually recur within 2
weeks of the end of treatment for the previous episode. If the previous episode
was treated with short course therapy, the first thought should be that there
was subclinical pyelonephritis and that a longer course of treatment is needed.
If a longer course is followed by another relapse, "imaging" (CT
scan or ultrasound) is warranted, to look for an anatomic abnormality.
Reinfections
Reinfections may be caused by the same
or a different organism, and usually occur at intervals > 2 wks after the
preceding infection. Multiple reinfections usually point to
pathogenetic factors such as those outlined above. They can be addressed by
changing the contraceptive to "the pill", applying estriol cream in
the postmenopausal woman, and, if necessary, by giving low dose chronic
antibiotic prophylaxis.)
"Complicated" UTI
This term refers to UTI in the patient
with an anatomic or functional abnormality facilitating UTI and making UTI
difficult to eradicate. Obstructive lesions are a good
example of complicated UTI but the most common association is with the Foley
catheter. Patients with complicated UTI undergo many symptomatic episodes and
courses of antibiotic treatment, which leads to infection by
antibiotic-resistant organisms. The Foley catheter serves as a
"highway" for bacteria from the outside world into the
bladder: most organisms seem to travel by the extraluminal route. The
rate of acquisition of bacteriuria with a Foley catheter is about 5% per day so
that, by day 10, more than half of patients have bacteriuria. Irrigation of the
urine bag by antibacterials and systemic administration of prophylactic
antibiotics are of no benefit in preventing bacteriuria. (There are important
technical issues which are of value, e.g. never raising the bag above the level
of the patient's bladder.) Despite the frequency of bacteriuria,
it is mainly asymptomatic. Nevertheless, long term indwelling urinary catheters
should be avoided if possible.
Asymptomatic bacteriuria
One of the most important, and common, questions
involves patients with asymptomatic bacteriuria. Although it may seem
intuitively obvious that bacterial infection should be combated wherever
possible, in fact, in most groups of patients, including the elderly and
diabetic patients, treatment of asymptomatic bacteriuria has been shown
to produce no benefit. It is usually difficult to eradicate, readily
recurs, and exposes the patient to the cost and adverse effects of antibiotics
- with no clinical benefit. There are three groups of patients
in whom there IS a benefit to treating asymptomatic bacteriuria: pregnant women
(because, untreated, 30% will go on shortly to develop symptomatic
pyelonephritis), newborns (who have a risk of renal scarring from untreated
infection), and patients about to undergo a urological procedure (because they
have an appreciable risk of pyelonephritis).
Posts
