Answers

Short answers

1 The computed tomogram shows an enlarged left kidney with streaks, which indicate bubbles of gas within the renal parenchyma and perirenal spaces (fig 2). Fluid has collected in the perirenal spaces, in keeping with abscess formation.

View larger version (62K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 2 Axial contrast enhanced computed tomogram showing an enlarged left kidney (small arrow) and gas within the renal parenchyma (large arrow)

 

2 The image findings are characteristic of emphysematous pyelonephritis within the left kidney. This disorder is a form of fulminating acute necrotic pyelonephritis. The culprit causative organisms for the condition are Escherichia coli, Proteus species, and Candida albicans.

3 Diabetes is the key predisposing factor for emphysematous pyelonephritis. Other risk factors include a compromised immune system and an obstructed kidney.

Long answers
1 Salient findings
Emphysematous pyelonephritis is a fulminating acute necrotic pyelonephritis characterised by gas within the renal parenchyma, the renal collecting system, and the perirenal space.¹ In addition, gas can collect in other regions. Figure 3 shows gas anterior to the psoas muscle on the affected side and fig 4 shows gas within the renal parenchyma. Occasionally, gas can track down the involved ureter.

View larger version (138K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 3 Coronal reconstruction of the patient’s contrast enhanced computed tomogram

 

View larger version (127K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 4 Sagittal reconstruction of the patient’s contrast enhanced computed tomogram, showing an enlarged kidney containing gas. The arrow is pointing to gas in the renal parenchyma

 
2 Diagnosis and treatment
The presentation of emphysematous pyelonephritis is of a considerably ill, feverish patient with flank pain. Such individuals are often hyperglycaemic with acid/base and electrolyte disturbances.

The gold standard imaging technique for diagnosis of emphysematous pyelonephritis is contrast enhanced computed tomography,^{2 3 4 5} which has an accuracy of 100%. Computed tomography will clearly show gas within the renal spaces and a contrast enhanced study will help indicate the degree of renal function. Plain abdominal radiography might show mottled gas within the renal outline extending into the perinephric space or retroperitoneum. The reported accuracy of plain abdominal radiography for diagnosis of emphysematous pyelonephritis is 65%.⁶ In addition, ultrasound is a low cost and readily accessible technique for diagnosing emphysematous pyelonephritis. Gas in the kidneys shows as echogenic foci on ultrasound; however, such features can be mistaken for bowel gas or renal calculi. The reported accuracy of ultrasound, however, is 69%.⁶

Wan et al⁷ proposed a computed tomography classification scheme that divides emphysematous pyelonephritis into two prognostic types. Type I emphysematous pyelonephritis is characterised by parenchymal destruction with streaky or mottled gas collections but no fluid collection. Type II emphysematous pyelonephritis is characterised by bubbly or loculated gas within the parenchyma or collecting system with associated renal or perirenal fluid collections, which are thought to represent a favourable immune response. Type II emphysematous pyelonephritis is associated with a better prognosis owing to this favourable immune response.

The mortality from emphysematous pyelonephritis is reported to be 40-50%.⁸ Mortality is higher in patients treated with antibiotics only than in those treated with nephrectomy or with image guided or surgical drainage. No gold standard exists for the treatment of emphysematous pyelonephritis; the strategy for each case depends on the severity of the infection. First, the patient should be resuscitated with fluid and electrolytes and treated with antibiotics, and glyaemic control should be optimised. Urinary tract obstruction should be relieved in appropriate cases. Traditionally, the next step in the treatment of emphysematous pyelonephritis has been nephrectomy or open surgical drainage, with systemic antibiotics. This approach has drawbacks as patients could be at high risk of complications from anaesthesia. The current treatment strategy is image guided percutaneous drainage (computed tomography being preferred in most cases; see fig 5).

View larger version (74K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 5 Computed tomogram guided percutaneous drainage. The small arrow indicates gas within the kidney and perirenal space, whereas the large arrow is pointing to the inserted drainage needle

 
Somani et al⁶ conducted a meta-analysis of 10 retrospective studies published between 1996 and 2006 that covered a total of 210 patients with emphysematous pyelonephritis. The authors found that mortality in patients treated with medical management alone was 50%, whereas mortality was 25% from medical management plus emergency nephrectomy and 13% from medical management plus percutaneous drainage. When the infected kidney is the only functioning kidney, medical management is preferred to avoid nephrectomy and permanent dialysis; however, this approach does not preclude image guided percutaneous drainage.

Image guided percutaneous drainage offers several advantages over other treatment strategies: the procedure is cost effective; easy to perform; takes very little time; is performed under local anaesthesia; and does not require a sterile operating theatre. In addition, there is no absolute contraindication to the procedure. Should the patient’s condition continue to decline after percutaneous drainage, then nephrectomy can provide the best management outcome. Nephrectomy should be promptly attempted in individuals who have extensive emphysematous pyelonephritis with a fulminant course.

3 Predisposing factors
In 90% of emphysematous pyelonephritis cases, the patient is diabetic.^{6 8} Bearing in mind that patients with diabetes are immunocompromised, other disorders that compromise the immune system also predispose a patient to this condition.

Escherichia coli is the causative bacterial agent in approximately 70% of cases; Klebsiella species are responsible in 20% of cases, and Candida and Pseudomonas species are involved less frequently.^{7 8 9} The postulated theory for gas formation in emphysematous pyelonephritis suggests that Gram negative facultative anaerobic organisms cause fermentation of glucose, which is present in high concentrations in the diabetic kidney. This fermentation produces high quantities of carbon dioxide, which accounts for the gas seen.¹ The infection is further compounded by the impaired immune response in the affected patient. Continual presence of gas indicates active infection and ineffective antimicrobial treatment.

Women are more likely to be affected by emphysematous pyelonephritis than are men: the ratio varies from 6:1⁸ to 4:1.⁶ A meta-analysis by Somani et al⁶ found that when the side was mentioned (162 patients), the left kidney was more commonly affected (56%) than the right (41%), with the condition bilateral in 10% of cases. The kidney is obstructed in 20% of cases,^{6 9} the cause of obstruction predominantly being a stone. Other causes of obstruction include bladder or ureteric transitional cell carcinoma and sloughed papilla in papillary necrosis.

Outcome
The patient responded well to treatment with antibiotics and percutaneous image guided drainage of the kidney; a nephrectomy was not required. The patient was discharged from hospital and remains well.

Cite this as: BMJ 2009;338:b1635