collecting system as the kidney is nonfunctioning. As in conventional pyelonephritis, there
is inflammatory change of the perinephric fat, but in contrast, there is much more
frequent involvement of adjacent structures, particularly the ipsilateral psoas muscle, with
rare involvement of other structures such as the colon. Unlike in conventional pyelonephritis,
the previously mentioned staghorn calculus is usually present or rarely some other chronically
obstructing lesion, such as tumor. Gas within the kidney may also rarely be seen (1–3).
Clinical and Radiologic Diagnosis of Renal Abscess
Focal or multifocal bacterial infections can result in formation of renal abscess. The location of
the abscess is indicative of its etiology. Cortical abscesses result from hematogenous spread of
infection, withStaphylococcus aureusbeing the most common pathogen. Much more commonly,
in contrast, corticomedullary abscesses result from ascending spread of infection from
organisms in the urine. The latter type of abscess is more likely to extend to the renal capsule
and perforate, resulting in perinephric abscess formation (Fig. 2). Corticomedullary abscesses
are uncommon complications of urinary tract infections; risk factors for their development
include recurrent infections, untreated or ineffectively treated infections, renal calculi,
instrumentation, vesicoureteral reflux, and diabetes mellitus (4).
There are multiple options for imaging a patient with a suspected renal abscess, with CT
considered the method of choice. Plain radiographs may show radiopaque stones or
intraparenchymal gas in patients with emphysematous pyelonephritis, but are generally not
helpful for the identification of abscess alone. Ultrasound findings include an ill-defined mass
Figure 1 CT scan of the abdomen in a
53-year-old female demonstrates a large
obstructing staghorn calculus in the pelvis of
the left kidney with diffuse calyceal dilatation and
diffuse cortical thinning. These findings are
essentially diagnostic of XGPN, when combined
with a history of recurrent urinary tract infection.
Abbreviation: XGPN, xanthogranulomatous pye-
lonephritis.Figure 2 CT scan of the abdomen in a
72-year-old male demonstrates an abscess in
the left anterior pararenal space containing gas
and fluid (arrow).Radiology of Infectious Diseases and Their Mimics in Critical Care 77