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700 PART 5: Infectious Disorders

toxicity and cost. Once fever has resolved, an early oral switch should abscess can be subtle, but most patients have fever, back pain, and
be considered, as there is no evidence that prolonged parental therapy is costovertebral angle tenderness.
required for UTI. For acute pyelonephritis due to a sensitive organism, a Ultrasound usually demonstrates an ovoid mass of decreased attenu-
7-day course of ciprofloxacin gives a low relapse rate but for other agents ation within the parenchyma initially mimicking AFBN, a cyst, or a
10 to 14 days are required. Higher relapse rates have been demonstrated tumor. Dependent echoes, changing with position, due to shifting debris
for β-lactams compared to non-cell wall active agents, because the or gas within the cavity, suggest abscess. Definitive characterization of
hypertonic renal medulla may allow residual bacteria to survive as cell fluid within the mass is done by demonstration of enhanced transmis-
wall–deficient protoplasts. 54,55 Extension of duration of therapy to 3 or sion of the beam through the mass and refraction of the beam at the
4 weeks should be considered for acute focal bacterial nephritis fluid-solid interface. The presence of debris within a cyst or an abscess
(AFBN). Duration in patients with suppuration should be individual- is a strong indication of infection. CT shows a distinctly marginated
ized, but in general should be prolonged, until all collections of pus are low-attenuation mass that fails to enhance. Sharp demarcation is dem-
drained, fever has disappeared, and C-reactive protein (CRP) levels have onstrated between the mass and the surrounding normally enhancing
become normal. renal tissue. There may be a surrounding rim of increased enhance-
■ COMPLICATIONS ment (the ring sign). CT is more sensitive than ultrasound, especially
for small lesions (<2 cm in diameter) and for gas. Because hemorrhage
Imaging: Contrast-enhanced computed tomography (CT) is the study within a cyst or necrotic debris within a tumor occasionally can mimic
of choice for most patients with severe urosepsis as it more accurately an abscess, confirmation by aspiration is desirable. Alternatively, serial
defines the anatomy than ultrasound and readily distinguishes AFBN scanning until resolution, while the patient is receiving antimicrobial
(syn acute lobar nephronia), cortical abscess, and perinephric abscess. therapy, may suffice. Historically, incision and drainage and even
Accurate placement of percutaneous drains into suppurative collec- nephrectomy for larger abscesses, was considered necessary. However, a
tions may require CT scanning to delineate all structures precisely. 56,57 trial of intravenous antimicrobial therapy will succeed in most patients
Ultrasound can be technically inadequate because of obesity, overlying once microbial etiology is established by urine, blood, or aspirate
bowel gas, subcutaneous emphysema, wounds, or dressings. Nevertheless culture. Monitoring of the response, including disappearance of fever,
it will reliably diagnose most causes of obstruction and perinephric col- leukocytosis, and elevated inflammatory markers along with diminu-
lections. Ultrasound may be chosen when transport out of the ICU is tion of the abscess size on ultrasound or CT, is necessary. Percutaneous
hazardous or there are concerns about the risk of contrast-induced drainage using ultrasound or CT is indicated as initial therapy when the
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nephrotoxicity. Interventional radiology for drainage of collections and abscess cavity is large. Magnetic resonance imaging (MRI) may have
relief of obstruction has replaced open surgery for most suppurative a role in a minority of cases to distinguish renal tumor from abscess,
complications. Retrograde urography in conjunction with cystoscopy to investigate pregnant patients or for those with a history of adverse
may be useful for a nonexcreting kidney and may permit relief of reaction to radiocontrast. 62
obstruction by passage of a stent or manipulation of a calculus. The perinephric space, containing the kidney, renal fat pad, and
adrenal gland, is conical and opens inferiorly to the pelvis. In most
Acute Focal Bacterial Nephritis: Human kidneys consist of 5 to 11 lobes cases, it communicates to the contralateral perinephric space ante-
(usually 8), each of which contains a conical medullary pyramid rior to the aorta and inferior vena cava. Bridging septae within it can
whose apex converges into a renal papilla projecting into a calyx. Each act as barriers against the spread of infection or hematoma. Multiple
pyramid is capped by cortical tissue to form a renal lobe and is sepa- loculations may arise, causing difficulty with percutaneous drainage.
63
rated from other lobes by a renal column containing the interlobar Perinephric abscess is usually due to Enterobacteriaceae, but a minor-
arteries and veins. With the advent of CT scanning, there has been ity is due to S aureus or pyogenic streptococci. Polymicrobial infection
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increasing recognition of AFBN. This is analogous to lobar pneu- involving anaerobes and cases due to fungi have been rarely reported.
59
monia because the abnormalities are limited to one or more renal Many patients have associated renal obstruction or diabetes mellitus.
lobes. Patients with AFBN may constitute a subset of pyelonephritis Historically, perinephric abscess often presented insidiously as pyrexia
with more severe disease that are at higher risk of abscess and scar of unknown origin and diagnosis was often delayed, resulting in mortal-
formation. Patients manifest the usual features of acute pyelonephritis ity rates of 50%. With the ready availability of ultrasound and CT, the
but do not respond with defervescence within 48 hours, prompting diagnosis is now made sooner. Ultrasound demonstrates fluid that may
investigation for obstruction or a suppurative focus. Ultrasound may contain debris or gas. CT shows loculated collections with decreased
be normal or may visualize a solid, hypoechoic, and poorly defined attenuation. The abscess wall may show increased attenuation after
mass without evidence of liquefaction. Noncontrast CT scanning is intravenous injection of contrast material. Thickening of the renal fascia
frequently normal, but with intravenous contrast enhancement, the and unilateral enlargement of the kidney or psoas muscle may also be
nephrogram invariably shows one or more wedge-shaped areas of seen. The diagnosis can be confirmed by ultrasound-guided aspira-
decreased density. Demonstration of AFBN may lead to a false impres- tion of pus. A combination of antimicrobial agents and percutaneous
sion of neoplasm, evolving renal infarct, or abscess. Demonstration of drainage is successful in most cases.
enhancing tissue within the mass on delayed CT images excludes Pyonephrosis arises when infection develops proximal to an obstructed
cancer and abscess. AFBN resolves with antimicrobial therapy, but hydronephrotic kidney. Unilateral loss of renal function is present, as is
scarring and atrophy may result. Histopathology shows intense poly- infection of the renal parenchyma. The clinical presentation is similar to
morphonuclear leukocyte infiltration without liquefaction, so needle perinephric abscess and may be insidious. Ultrasound will show a dis-
aspiration or percutaneous drainage is not indicated. 59,60
tended upper urinary tract. Specific features of pyonephrosis that allow
Renal and Perinephric Abscess, Pyonephrosis, Infected Cyst, and Pyocystis: It distinction from simple hydronephrosis include sedimented echoes and
is not clear whether renal abscess always progresses first through dispersed internal echoes within the dilated collecting system. Because
AFBN before suppuration, but this sequence has been demonstrated these findings are present in a minority of patients, direct aspiration is
anecdotally. Renal abscess may resolve spontaneously by drainage indicated in a septic patient with hydronephrosis. CT is the preferred
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into the calyxes or may rupture through the renal capsule to form a investigation as it is more sensitive for detecting radiolucent calculi and
perinephric abscess. The usual pathogens are Enterobacteriaceae but will establish whether there is accompanying infection in the tissues
patients at high risk for staphylococcal septicemia, such as intravenous around the kidney. Once the diagnosis is made, a percutaneous neph-
drug users, or patients receiving parental therapy or hemodialysis may rostomy tube or ureteric stent should be inserted to drain the infection.
present with renal cortical abscess, sometimes with features of invasive Renal cysts are common, but infection (pyocyst) is extremely rare,
staphylococcal infection at other sites. The clinical features of renal except in patients with autosomal dominant polycystic kidney disease

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