CT Imaging in Pyelonephritis: Pearls & Pitfalls
- Aug 10th, 2020
- Saran S. Pillai
Authors: Saran S. Pillai, MBBS (@sspillai01, Emergency Medicine, University of Kentucky) and Sameer Desai, MD, FACEP (@sameerdesai00, EM Attending Physician, University of Kentucky) // Reviewed by: Summer Chavez, DO, MPH, MPM (@thegoodavocado); Alex Koyfman, MD (@EMHighAK); and Brit Long, MD (@long_brit)
A 42-year-old female with a past medical history of renal calculi, diabetes and hypertension presented to the emergency department with complaints of dysuria and dizziness for a week. She reports fever and right flank pain for one day. Her vitals are: temperature 102.6°F, heart rate 104 bpm, respiratory rate of 18 breaths/min and blood pressure 106/70 mm Hg. The patient appears uncomfortable but is not toxic. Abdominal examination is significant for severe right costovertebral angle tenderness but no suprapubic tenderness. There is no cervical motion tenderness on pelvic examination. Laboratory testing is remarkable for WBC 12.5, creatinine 3.7 (baseline value of 1.3) and point-of-care glucose 470 mg/dL. Her urinalysis is significant for >100 WBC, moderate bacteria, 4-5 RBC, and leukocyte esterase. Urine cultures are obtained and she is started on intravenous antibiotics and hydration to treat pyelonephritis.
Would CT imaging be helpful in managing this patient in the emergency department?
Acute pyelonephritis (APN) is an infection of the upper urinary tract, specifically the renal parenchyma and renal pelvis. It accounts for approximately 300,000 emergency department (ED) visits per year (10% of total ED urinary tract infection (UTI) visits) and has a mortality rate of up to 9.8%.1–3 Its direct and indirect costs amount to $2.14 billion dollars a year.1–3 APN has an incidence of 15-17 cases per 10,000 females and 3-4 cases per 10,000 males.4 There is a higher risk among young sexually active women, elderly, infants, and pregnant women.4 It most commonly manifests with sudden onset of symptoms of systemic inflammation (fever, chills, malaise) along with symptoms of lower urinary tract inflammation (increased frequency, dysuria, and urgency).4
There is a lack of consensus regarding diagnostic criteria due to the variation in presenting features and severity.5 Though the triad of fever, flank pain, and nausea and/or vomiting have long been considered to be strongly predictive of this diagnosis, studies on the clinical profile have reported varying prevalence of the triad ranging from 35% to 80% depending on the comorbidities of the patient population.5,6 Fever may be seen in up to 77% of patients but is absent in a third of elderly patients.5,6 Fever is absent in up to 50% patients with diabetes.5,6 Flank pain may be seen in up to 86% of patients while bladder symptoms may be absent in up to 20% of the patients.7,8 This variability in presentation of APN along with mortality rates up to 20% has led to controversy about the role of advanced imaging in diagnosis and management of APN.4
Uncomplicated pyelonephritis occurs in healthy non-pregnant premenopausal women without risk factors having a lower chance of treatment failure, while complicated pyelonephritis occurs in a more heterogenous population.4 Patients with complicated pyelonephritis have multiple risk factors (Table 1), greater morbidity and higher risk of failing outpatient treatment, typically requiring IV fluids and antibiotics.9,10
A useful rule of thumb would be to consider an uncomplicated pyelonephritis as one that occurs in a non-pregnant, immunocompetent female of reproductive age with previously normal renal function and a complicated pyelonephritis as one that occurs in patients that don’t fit this criteria.9 Complicated APN has a mortality of up to 20% when accompanied by bacteremia.11 A large population based epidemiological analysis of approximately 5000 APN patients from Seattle, found rates of inpatient treatment of APN to be 3–4 cases and 1-2 cases per 10,000 population for females and males respectively.12
More details regarding the epidemiology, causes, diagnosis and management can be found in the previous discussion at EMDOCS.net about pyelonephritis.
Decision Making Regarding Imaging for APN
The assessment of the severity of illness, underlying host status and risk factors, patient’s psychosocial situation and estimation of the likelihood of antibiotic resistance are critical in decision making regarding disposition and treatment of patients.7
Even though the classic triad of symptoms in pyelonephritis is neither sensitive nor specific, the diagnosis of uncomplicated APN can be made with a certain degree of certainty based on history, physical examination, and standard laboratory evaluation. Hence, routine imaging has a limited role to play in the initial management of uncomplicated APN and is only considered to be necessary for those patients with a persistence of fever or leukocytosis after 72 hours.8,13,14
There are certain indications for advanced imaging in cases of complicated APN:
- To understand the severity of disease and complications in patients presenting with concerning symptoms or labs.13 Concerning complications include renal or perinephric abscess, sepsis, renal vein thrombosis, papillary necrosis, acute renal failure, emphysematous pyelonephritis, xanthogranulomatous pyelonephritis.4,11
- The need for early procedural intervention for certain disease pathologies (i.e. obstructing calculi, EPN, abscess).12,14
- Diagnostic uncertainty due to vague presentations of APN especially in the elderly, diabetics, etc.4
- To rule out other diseases that can present with similar symptoms. They have different management guidelines compared to APN, some of which can be life-threatening if not accurately diagnosed and treated.4,13,14 Alternative diagnoses include:
- lower lobe pneumonia
- perforated viscus
- prodrome of herpes zoster
- perinephric or a psoas abscess
- retroperitoneal hemorrhage
- splenic abscess
- pelvic inflammatory disease
- lower rib fracture
If missed, infectious non-renal pathology can be catastrophic as most are treated with different classes of antibiotics covering other organisms besides E. coli, the most common cause of APN.4
Some of the important complications that may be seen on CT imaging include:
- Emphysematous pyelonephritis (EPN): Often fatal necrotizing form of APN occurring more often in diabetics.16Gas is produced by metabolism of glucose by gram-negative bacteria.17 CT imaging should be done immediately based on clinical suspicion as emergent percutaneous drainage or nephrectomy may be indicated in addition to medical management.17,18
- Xanthogranulomatous pyelonephritis (XPN): XPN is a severe form of APN that occurs mostly in middle aged women in the presence of chronic obstruction and suppuration.4 80% occur in association with a staghorn calculus.16 Often described as one of “the great imitators” because its clinical and imaging findings closely resemble conditions such as obstructive pyelonephritis, renal tuberculosis, perinephric abscess, and malignancy.19CT imaging may be helpful for surgical planning and can provide additional diagnostic information.20
- Hydronephrosis is a dilatation of the urinary collecting system (calyces to renal pelvis) of the kidney while pyonephrosis is a suppurative infection within an obstructed collecting system.21
More detailed CT findings of these complications will be discussed later in this article.
Patients with a history and physical examination suggestive of APN and one or more risk factors for complicated APN should have a CT scan of the abdomen and pelvis ordered expeditiously as to prevent treatment delay in their disease course.5,8 In cases where IV iodinated contrast/ionizing radiation is absolutely contraindicated, other imaging options may be necessary – MRI which is the imaging of choice (Diffusion weighted/DW-MRI if coexistent gadolinium allergy); or non-contrast CT; or modified contrast CT protocols (low evidence).8,13,27 Unless the urinalysis demonstrates scant urine leukocytes and there is significant diagnostic uncertainty regarding the definitive diagnosis, CT imaging should be avoided in uncomplicated APN cases.9,14,22 Repeat imaging is not indicated unless there is persistent fever, leukocytosis that does not improve or sudden deterioration of patient status.13
CT Imaging in APN
CT imaging accurately characterizes both intra- and extrarenal pathologies, renal perfusion and renal function.28 Contrast CT is considered to the preferred modality in the imaging of APN as it is superior to ultrasound at identifying perinephric fluid collections and abscesses and is superior to MRI in finding calculi and complications like EPN.21,28 The sensitivity and specificity reported by CT abdomen and pelvis with IV contrast for the diagnosis of APN is 86.8% and 87.5% respectively.21,29,30
While the evidence points towards conservative imaging in APN, the ease and speed of using a bedside point of care ultrasound (POCUS) has led to the use of POCUS as an imaging adjunct in cases of complicated pyelonephritis.9,31 Ultrasound has advantages of being bedside and is non-invasive. Ultrasound (sensitivity – 90%) is superior to CT imaging (sensitivity – 76%) in differentiating pyonephrosis from hydronephrosis but it is far less sensitive for APN (25% – 74.3%) and can underestimate the involvement of renal involvement.21,29,32 MRI is sensitive for APN (89.5%) and can be useful in a pregnant patient, but is time consuming, expensive, and less accurate in detecting calculi (sensitivity 82%) or gas forming infections.29,33
Diagnostic Features of APN on CT
CT imaging without contrast can visualize gas, calculi, hemorrhage, renal enlargement, inflammatory masses, and obstruction accurately.22 Kidneys may either appear normal or the affected parts of the kidney may appear edematous with lower attenuation.21 Renal calculi or gas within the collecting system may be seen in obstructive APN and EPN respectively.22
CT imaging of the abdomen and pelvis with IV contrast can visualize interstitial nephritis, early and late stages of APN and complications like abscesses and EPN accurately (Figure 2).22 A retrospective analysis of 100 cases of APN found the most common imaging findings to be global renal swelling (88%), perirenal fascial thickening (76%), perinephric fat stranding (56%), striated nephrogram (44%), abscess (44%), microabscess (32%), gas in renal collecting system or parenchyma suggestive of emphysematous APN (16%), and abdominal wall involvement (8%).34 The most characteristic finding of APN in contrast CT includes one or more focal/ill-defined wedge-like regions showing reduced enhancement and poor corticomedullary differentiation, corresponding to poorly or nonfunctioning parenchyma.21,28,35,36
Other common findings include parenchymal abnormalities like nephromegaly, delayed calyceal opacification, perinephric stranding (Figure 3), inflammatory masses or gas formation.16 A common though non-specific feature of APN in contrast CT imaging is a “striated nephrogram” characterized by alternating radial bands of linear hyper- and hypo-attenuation involving the renal cortex due to retention of contrast material in ischemic and inflamed tubules (Figure 4).10 It is also seen in genitourinary obstruction, renal vein thrombosis, contusion and dehydrated patients.35,37,38 Pelvicalyceal wall thickening or enhancement may be the only imaging feature of early APN if a CT scan is performed early in the disease process.35
Figure 2: Selected CT images of left renal APN showing an enlarged left kidney, with decreased uptake of contrast media and increased density of perirenal fat. In the delayed phase (4), there is no excretion of contrast by the left kidney. Case courtesy of Dr David Cuete, <a href=”https://radiopaedia.org/”>Radiopaedia.org</a>. From the case <a href=”https://radiopaedia.org/cases/26584″>rID: 26584</a>
Figure 3: Single axial and coronal images showing a large amount of perinephric stranding around the left kidney. Case courtesy of Dr. Matt A. Morgan. <a href=”https://radiopaedia.org/”>Radiopaedia.org</a>. From the case <a href=”https://radiopaedia.org/cases/42491″>rID: 42491</a>
Figure 4: Multiple hypoenhancing areas in both kidneys, consistent with a striated nephrogram appearance. Case courtesy of Dr Bita Abbasi, <a href=”https://radiopaedia.org/”>Radiopaedia.org</a>. From the case <a href=”https://radiopaedia.org/cases/74153″>rID: 74153</a>
CT Imaging Characteristics of APN Complications
A CT scan in a patient with APN demonstrating an obstructing calculus in the collecting system, pyonephrosis, EPN or abscess is concerning.13 Patients should be started on antibiotics and surgical consultation placed for potential nephrostomy, nephrectomy or percutaneous drainage.13
An obstructing calculus can be seen even in a non-contrast CT. It appears along with dilated renal calyces in a contrast CT and with an appearance colloquially described as the “bear paw sign” in large calculi (Figure 5) and xanthogranulomatous pyelonephritis with staghorn calculi.39 Pelvic wall thickening has a sensitivity of 76% for pyonephrosis.40 A stone associated with a nonfunctioning kidney may be seen with pyonephrosis or long-standing hydronephrosis.21 In the absence of previous instrumentation, the most accurate indicator of infectious fluid collection is gas in the collecting system.40 A fragmented-appearing staghorn, renal enlargement and absence of contrast material excretion from the same kidney are considered the classic triad of xanthogranulomatous pyelonephritis.21
Figure 5: Bear paw appearance of dilated calyces in this patient with xanthogranulomatous pyelonephritis. May also be seen in larger calculi. Top: Enlargement of the right kidney with multiple large renal calculi and rounded low density regions associated with the calculus. Bottom: Dilated low-density calyces forming characteristic “bear paw”. Case courtesy of Associate Professor Frank Gaillard, <a href=”https://radiopaedia.org/”>Radiopaedia.org</a>. From the case <a href=”https://radiopaedia.org/cases/9931″>rID: 9931</a>
Imaging findings in emphysematous pyelonephritis (EPN) include parenchymal enlargement and destruction (irregular shape of kidney with focal necrotic areas) and fluid collections with air-fluid levels.17 Bubbly or linear gas streaks located around kidney but within the perirenal fascia can be seen in non-contrast CT as well, along with/without associated abscess (Figure 6).17
Important pitfall: Parenchymal or pararenal gas are findings associated with EPN.41 In comparison, in pyonephrosis the gas appears within the collecting system.40
Figure 6: Emphysematous pyelonephritis (EPN) of the right kidney. Enlarged right kidney showing heterogenous contrast enhancement, multiple parenchymal collections containing internal gas locules, and smudging of the perinephric fat. Case courtesy of <a href=”https://radiopaedia.org/”>Radiopaedia.org</a>. From the case <a href=”https://radiopaedia.org/cases/11386″>rID: 11386</a>
A renal abscess appears most commonly as well-defined cystic mass of low attenuation with a thick, irregular wall (pseudocapsule), with or without gas within the cystic mass (strongly predictive) and perinephric stranding or thickening due to inflammation (Figure 7).21,42 It may resemble a malignancy due to the thick irregular enhancing wall and infiltration of the perirenal fat (Figure 7).21
Figure 7: Left renal abscess. Cystic lesion with thick enhancing rim and infiltration of perirenal fat. Courtesy: https://radiologyassistant.nl/43 Reinhard R, Zon-Coijn MVD, Smithuis R. Kidney – Solid masses. Radiology Assistant. https://radiologyassistant.nl/abdomen/kidney-solid-masses#pitfalls.
- The absence of flank pain in diabetics (50% of cases) should be considered as a critical confounding factor.34 CT imaging is helpful for early diagnosis and treatment.34
- A CT scan suggestive of an obstruction in the setting of pyelonephritis, sepsis, EPN or abscess necessitates an immediate interventional radiology or urology consult.28,44
- Pyelonephritis is the most common alternative genitourinary diagnosis in patients imaged with a non-contrast CT for suspected renal calculi.45 Though the non-contrast CT is enough to visualize an obstructing calculi causing APN, a low threshold for ordering a contrast CT would be prudent, as non-contrast CT might underestimate the extent of the pathology and may even appear normal in APN.8,21
- A patient with typical signs and symptoms of APN but without a corresponding amount of urine leukocytes or diagnostic ambiguity needs to be imaged to rule out another intraabdominal infectious or inflammatory pathology such as perinephric or a psoas abscess, retroperitoneal hemorrhage, appendicitis, splenic abscess, endometriosis or pelvic inflammatory disease, lower rib fractures, etc.14,23,46
- Focal pyelonephritis can present as a mass-like area of reduced attenuation which can mimic subtypes of renal cell carcinoma or infiltrative neoplasms. It may also mimic a renal infarct when it presents as wedge-like area of reduced enhancement.47 In some instances, a CT scan performed 3 hours after a load of intravenous contrast can differentiate APN from tumor or infarction by demonstrating retention of contrast.35,48
Guidelines for use of Contrast in APN Imaging
In cases of absolute contraindications to contrast material such as previous life-threatening or anaphylactic reaction to contrast, alternatives like a non-contrast CT or MRI should be considered if possible.8
In patients with relative contraindications for contrast, such as elderly patients with impaired renal function or patients with impaired renal function, contrast CT maybe performed after discussion with the radiologist or according to institutional protocols.8 A patient reported allergy to contrast media should be inquired further to clarify the type and severity of the reaction.8
- Acute pyelonephritis (APN) is an infection of the upper urinary tract with a clinical presentation that may be challenging.
- Uncomplicated pyelonephritis occurs in a non-pregnant, immunocompetent female of reproductive age with previously normal renal function while complicated pyelonephritis occurs in those that do not fit this criterion.
- The lack of consensus regarding a diagnostic criterion (classic triad of fever, flank pain, and nausea and/or vomiting has a varying prevalence of 35 to 80%) as well as the significant mortality rates (up to 20% in APN) have made CT imaging an integral part of diagnosis and management of complicated APN.
- There is no role for imaging in management of uncomplicated APN unless there is uncertainty in diagnosis (few leukocytes in urine, vague symptoms, etc.). Imaging may be considered if fever or leukocytosis persist beyond 72 hours.
- Kidneys in APN may either appear normal or focally edematous in non-contrast CT. Renal calculi (obstructive APN), gas (emphysematous APN), hemorrhage, renal enlargement, inflammatory masses, and non-calculi obstructions can be visualized with a non-contrast CT.
- CT imaging of the abdomen and pelvis with IV contrast is the modality of choice in APN. It characteristically shows one or more focal or ill-defined wedge-like regions with reduced enhancement and poor corticomedullary differentiation. Other common findings better appreciated compared to non-contrast CT include delayed calyceal opacification, perinephric stranding, inflammatory parenchyma or gas formation. A “striated nephrogram” (alternating cortical radial bands of hyper- and hypo-attenuation) is common but nonspecific.
- Contrast CT findings in complications of APN include: abscess (sharply marginated area of low attenuation with peripheral enhancing), obstructing APN with pyonephrosis (calculi with dilated calyces, pelvic wall thickening, and gas in the collecting system), and papillary necrosis (poorly marginated hypo-attenuated lesions in papilla).
- Emphysematous pyelonephritis is an often-fatal necrotizing complication of APN occurring commonly in diabetics and necessitates emergent nephrectomy or percutaneous drainage. Non-contrast CT may demonstrate pararenal or parenchymal gas. In addition to these findings, CT imaging with contrast will also show parenchymal destruction, fluid collections or focal tissue necrosis.
The patient in our case scenario meets the clinical criteria for complicated pyelonephritis (past history of nephrolithiasis and diabetes with new onset worsening of renal function). Contrast CT showed a large calculus in the right renal hilum with dilated renal calyces, renal pelvic wall thickening, and small gas bubbles in the collecting system on the right. Empiric broad spectrum IV antibiotics were started. Interventional radiology was consulted for the placement of a percutaneous nephrostomy due to suspicion of obstructive APN with subsequent pyonephrosis. The patient was admitted to the hospital after confirmation of diagnosis for drainage and percutaneous nephrostomy tube placement, IV antibiotics and medical management.
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