Imaging in Emphysematous Pyelonephritis

Updated: Jun 10, 2016
  • Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Eugene C Lin, MD  more...
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Overview

Overview

Imaging is essential to managing emphysematous pyelonephritis —a life-threatening, fulminant, necrotizing upper urinary tract infection associated with gas within the kidney—if an early diagnosis is to be made and a potentially devastating outcome is to be avoided. [1, 2, 3, 4, 5]

Some confusion exists regarding terminology for conditions involving renal gas. The term emphysematous pyelonephritis should be reserved for renal tract infections with intraparenchymal renal gas. Gas confined to the renal pelvis should be called emphysematous pyelitis, and gas confined to the perinephric space should be called perinephric emphysema. [6, 7, 2, 8, 9, 10, 11]

Emphysematous pyelonephritis is rare, but the frequency is higher in patients who are immunocompromised, especially patients with diabetes, who account for 87-97% of patients (see the images below). [12]

In a 58-year-old woman with diabetes, emergency in In a 58-year-old woman with diabetes, emergency intravenous pyelography (IVP) was requested for an evaluation of flank pain. After this scout image was obtained, however, a lateral radiograph was taken, the IVP was canceled, and computed tomography scanning was performed. This anteroposterior (AP) scout image shows striated intrarenal gas (within medullary rays) and perinephric gas.
Lateral radiograph in a 58-year-old woman with dia Lateral radiograph in a 58-year-old woman with diabetes and flank pain demonstrates that gas is present within the kidney, overlying the spine.
Computed tomography scan in a 58-year-old woman wi Computed tomography scan in a 58-year-old woman with diabetes and flank pain depicts intrarenal gas very well. The contralateral kidney looks normal.
Magnified computed tomography scan obtained with a Magnified computed tomography scan obtained with a lung window setting better reveals the striated intrarenal gas and subcapsular gas in the posterior aspect in a 58-year-old woman with diabetes and flank pain.

Lu and associates conducted a study to determine the clinical characteristics and prognostic factors of patients with emphysematous pyelonephritis. The study concluded that (1) low albumin levels, (2) an initial presentation of shock, (3) bacteremia, (4) indications for hemodialysis, or (5) polymicrobial infection represent prognostic factors for mortality in patients with emphysematous pyelonephritis. Any 2 or more of these of these prognostic factors carried a high risk of mortality, and these patients should be considered for more aggressive management. [3]

Preferred examination

Plain abdominal radiography is the initial examination of choice in emphysematous pyelonephritis because this modality better depicts air in the renal collecting system and it is much more specific than ultrasonography. In practice, however, ultrasonography may be the initial examination performed. (See the image below.) [13, 14, 15, 16, 17, 18]

Ultrasonogram of the right kidney in a 52-year-old Ultrasonogram of the right kidney in a 52-year-old woman with emphysematous pyelonephritis demonstrates echogenic gas within the collecting system and posterior cortex of the right kidney. The involvement of the posterior cortex is partly obscured by shadowing from the gas in the collecting system.

CT scan findings are diagnostic of the presence of air within the renal tract, and CT images also elegantly depict the renal and perirenal anatomy and the spread of infection to the perinephric tissues. [15, 4]

Renal function is depressed or even absent on the affected side in emphysematous pyelonephritis, and radionuclide study is an excellent modality for assessing differential function when nephrectomy is contemplated. Scintigraphy has been used to evaluate responses to antimicrobial therapy. Intravenous urography may be necessary if renal intervention is contemplated.

Limitations of techniques

Plain radiographs are good for depicting air within the renal collecting system, but nonspecificity is a problem because of the superimposition of gas from the bowel. Moreover, gas in the retroperitoneum and gas within a renal or perinephric abscess may mimic emphysematous pyelonephritis.

Similarly, ultrasonography is limited because gas within the kidney and/or renal pelvis mimics renal calculi and produces artifact due to reverberation echoes and shadowing.

CT scans do not always depict other causes of intrarenal air, such as reflux of air from the bladder and bronchorenal, enterorenal, or cutaneorenal fistulae. These may occur with xanthogranulomatous pyelonephritis and focal renal abscesses. [19]

Radionuclide studies are nonspecific; therefore, they have a limited role in the evaluation of emphysematous pyelonephritis. In addition, radionuclide imaging suffers from a lack of availability. However, it does not result in false-positive or false-negative diagnoses.

Magnetic resonance imaging (MRI) is not the modality of choice in the diagnosis of emphysematous pyelonephritis. MRI findings are a signal void on T1- and T2-weighted images. However, signal voids on MRI scans may occur with renal calculi or rapidly flowing blood, creating false-positive results. Perinephric and intraparenchymal fluid collections are demonstrated well on MRI. [15, 16]

Differential diagnosis

Emphysema is part of the differential diagnosis of emphysematous pyelonephritis. Other conditions to be considered include the following:

  • Retroperitoneal perforation of an abdominal viscus
  • Psoas abscess secondary to gas-forming organisms
  • Reflux of air from the bladder
  • Bronchorenal, enterorenal, or cutaneorenal fistulae - As may occur with xanthogranulomatous pyelonephritis
  • Air in a focal renal abscess - Not life threatening

Gas in the renal parenchyma may be seen in conditions other than emphysematous pyelonephritis on CT scanning. Intraparenchymal renal gas may be seen following urologic intervention such as that for a nephrostomy insertion or may result from a fistulous communication between the gastrointestinal tract and the renal collecting system. These situations do not represent clinical emergencies and are not life-threatening, as in emphysematous pyelonephritis. With the increasing use of abdominal CT scanning, radiologists, especially in the emergency setting, should be aware of this comparatively rare finding and should be familiar with its differential diagnosis.

Emphysematous pyelonephritis has been described as a presenting feature of a urinary bladder adenocarcinoma in a middle-aged, nondiabetic patient. [20]

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Radiography

Plain radiographs in patients with emphysematous pyelonephritis may show bubbles of gas within the region of the renal bed and in the upper renal collecting system. These may be diagnostic in the appropriate clinical setting. [16] Gas within the collecting system without evidence of renal parenchymal gas may be seen in patients with diabetes and does not have the same ominous prognosis (see the images below). Acute renal edema with obliteration of the renal pelvis can be seen.

In a 58-year-old woman with diabetes, emergency in In a 58-year-old woman with diabetes, emergency intravenous pyelography (IVP) was requested for an evaluation of flank pain. After this scout image was obtained, however, a lateral radiograph was taken, the IVP was canceled, and computed tomography scanning was performed. This anteroposterior (AP) scout image shows striated intrarenal gas (within medullary rays) and perinephric gas.
Lateral radiograph in a 58-year-old woman with dia Lateral radiograph in a 58-year-old woman with diabetes and flank pain demonstrates that gas is present within the kidney, overlying the spine.

Intravenous urography shows significant renal enlargement associated with delayed or absent excretion, and retrograde pyelography can be used to establish the presence of ureteral obstruction.

Plain radiographic findings may be diagnostic in ill patients with diabetes who have signs of acute pyelonephritis.

Renal fossa gas may be confused with gastrointestinal gas. A false-positive diagnosis of emphysematous pyelonephritis may occur with retroperitoneal gas and a psoas abscess secondary to gas-forming organisms. Reflux of air from the bladder and bronchorenal, enterorenal, or cutaneorenal fistulae (as may occur with xanthogranulomatous pyelonephritis [19] ) may also lead to false-positive findings. Air can be seen in focal renal abscesses.

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Computed Tomography

CT scanning is the most reliable and sensitive modality in diagnosing emphysematous pyelonephritis. [15, 4, 21] Intraparenchymal, intracalyceal, and intrapelvic gas and extension into the perinephric space are readily identified on nonenhanced CT scans; [21] mottled areas of low attenuation extend radially along the pyramids, and pus may occasionally be seen extending into the renal veins. [22, 23, 24]

Two subtypes of emphysematous pyelonephritis based on CT scan appearances have been described. [12] Type I (33% of patients) is characterized by parenchymal destruction with either absence of fluid collection or presence of streaky or mottled gas radiating from the medulla to the cortex. A crescent of subcapsular or perinephric gas may be present. The absence of fluid collection implies a poor immune response. The mortality rate is high, at 69%. [12, 13]

Type II (66% of patients) typically has a confined, bubbly, intrarenal gas pattern—probably within abscesses associated with renal and perinephric fluid collection—and gas within the renal pelvis. The mortality rate in type II is 18%. [12, 13]

Conversion from type I to type II emphysematous pyelonephritis has been described. [25] Wan et al correlated imaging findings of types I and II disease with clinical course and prognosis and showed that the radiologic differentiation between the 2 types is important due to the prognostic difference. [13] That is, the mortality rate for type I disease was higher than that for type II (69% vs 18%, respectively, as stated above), with type I emphysematous pyelonephritis tending to have a more fulminant course and a significantly shorter interval from clinical onset to death. (See the images below.)

Computed tomography scan through the top of the ri Computed tomography scan through the top of the right kidney in a 52-year-old woman with emphysematous pyelonephritis. Gas is present in the upper pole cortex of the kidney. Note normal excretion of contrast material in the left kidney.
Computed tomography scan through the middle of the Computed tomography scan through the middle of the right kidney in a 52-year-old woman with emphysematous pyelonephritis. Gas is present in the cortex and collecting system of the kidney, with some extension into the perinephric space.
Computed tomography scan through the lower portion Computed tomography scan through the lower portion of the right kidney in a 52-year-old woman with emphysematous pyelonephritis. Gas is present in the collecting system and anterior cortex of the kidney.
Axial, contrast-enhanced computed tomography scan Axial, contrast-enhanced computed tomography scan showing gas within the renal collecting system and the urinary bladder in a patient following bladder urologic intervention.
Axial, contrast-enhanced computed tomography scan Axial, contrast-enhanced computed tomography scan showing gas within the renal collecting system and the urinary bladder in a patient following bladder urologic intervention.
Axial, contrast-enhanced computed tomography scan Axial, contrast-enhanced computed tomography scan showing gas within the renal collecting system and the urinary bladder in a patient following bladder urologic intervention.

Emphysematous pyelonephritis should be differentiated from reflux of air from the bladder and bronchorenal, enterorenal, or cutaneorenal fistulae (as may occur with xanthogranulomatous pyelonephritis [19] ). Air also can be seen in focal renal abscesses, but it is not life threatening.

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Ultrasonography

Ultrasonography is usually the first imaging modality for assessing renal pathology. The ultrasonographic findings often guide clinicians in choosing the next modality, such as CT scanning, to achieve a more specific diagnosis. [15, 18]

Intrarenal gas causes high-amplitude echoes within the renal sinus/renal parenchyma associated with dirty acoustic shadowing, and ring-down artifacts may result from air bubbles trapped in fluid. In addition, shadowing from gas bubbles in the perinephric space may be seen, making visualization of the kidney difficult (see the following image). Perinephric fluid, if any, tends to be obscured by gas.

Ultrasonogram of the right kidney in a 52-year-old Ultrasonogram of the right kidney in a 52-year-old woman with emphysematous pyelonephritis demonstrates echogenic gas within the collecting system and posterior cortex of the right kidney. The involvement of the posterior cortex is partly obscured by shadowing from the gas in the collecting system.

Gas within the kidney and/or renal pelvis mimics renal calculi. In select patients, particularly those with diabetes in whom ultrasonograms suggest renal calculi, obtaining a coned radiograph of the renal area is worthwhile to preclude missing the diagnosis of emphysematous pyelonephritis.

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