INTRARENAL AND PERINEPHRIC ABSCESSES
Kidney abscesses can be located either within the renal parenchyma
(intrarenal abscess) or between the renal capsule and renal fascia (perinephric
abscess). Intrarenal abscesses may be present in the cortex or medulla.
Perinephric abscesses are generally confined to the renal fascia but may extend
into the retroperitoneum.
PATHOPHYSIOLOGY
The most common cause of both intrarenal and perinephric
abscesses is ascending pyelonephritis in the presence of a urinary tract
obstruction. The flushing effect of urine plays an important role in the
clearance of bacteria. Hence, an obstruction to the flow of urine with
subsequent urinary stasis produces a milieu favorable to infection. In
addition, the forniceal rupture that can occur secondary to obstruction can
release infected urine into the perinephric space. These infections typically
involve gram negative
pathogens (such as E. coli, Klebsiella, Pseudomonas),
although polymicrobial infections are also seen and may involve fungal
organisms such as Candida spp.
A smaller number of abscesses result from hematogenous
seeding of the renal parenchyma in the setting of systemic bacteremia. In these
cases, the abscesses are typically intrarenal rather than perinephric. In
addition, gram positive
organisms (e.g., Staphylococcus aureus) are usually responsible.
In both ascending and hematogenous infection, there is
tissue necrosis and subsequent sequestration of the phlegmon into an abscess.
PRESENTATION AND DIAGNOSIS
Patients with an intrarenal or perinephric abscess
typically have signs and symptoms of acute pyelonephritis (see Plate 5-5) but
fail to improve after several days of appropriate antimicrobial therapy (e.g.,
persistent fever, persistently positive cultures, and no resolution in elevated
white blood cell count). In some cases, physical examination may reveal a
palpable mass or overlying inflammatory skin changes.
When an abscess is suspected, the basic tests for
evaluation of upper UTI should and often already have been performed, including
urine and blood cultures. In patients with hematogenously seeded abscesses, a
urine culture may reveal organisms not usually found in the urinary tract, such
as gram-positive organisms, and the same organism may be identified on a blood
culture.
Once an abscess is suspected, abdominal images should
be obtained. Computed tomography (CT) is the study of choice, and renal
abscesses have the same characteristics as abscesses located elsewhere. The
pus- filled central portions are lucent and do not enhance, while the inflamed
walls are thicker than those of cysts, have indistinct borders, and do enhance.
Ultrasonography may reveal fluid-containing, masslike structures with flow in the
walls seen on Doppler imaging.
TREATMENT
Empiric intravenous antibiotic treatment should
include broad-spectrum agents that can penetrate walled-off infections. Options
include piperacillintazobactam, cefepime, and carbapenems. These agents will
target gram-negative and most susceptible gram positive pathogens that can cause an abscess, with the exception of resistant organisms such as methicillin-resistance
Staphylococcus aureus or vancomycin-resistant Enterococcus. The
choice of antimicrobial agent can be refined once blood or urine culture results
are obtained. Percutaneous or surgical drainage should be performed for
abscesses that are more than 3 to 5 cm in diameter. Gram stain and culture of
the aspirate may facilitate identification of the causative pathogen and its
susceptibilities. Percutaneous drainage, which can be performed under CT or
ultrasound guidance, carries less risk than an open surgical procedure. In the
case of a large abscess that cannot be drained with a single aspiration, an
indwelling catheter is appropriate. The combination of percutaneous drainage
and appropriate antibiotic
treatment has been shown to clear more than 90% of infections. When this
approach fails, open drainage may become necessary.
The duration of antibiotic therapy depends on the size
of the abscess and the extent of drainage. Generally, it is continued for 2 to
3 weeks following successful drainage. The response to antibiotics can be slow,
and the patient should be monitored closely for improvement in symptoms and
laboratory markers of inflammation, such as leukocyte count, C-reactive protein,
and erythrocyte sedimentation rate. Follow-up imaging is recommended after
treatment to document resolution, especially in patients with diabetes or other
causes of immune
compromise.
