150 CHAPTER 5 Infections and inflammatory conditions
Pyonephrosis and perinephric abscess
Pyonephrosis
Pyonephrosis is an infected hydronephrosis. Pus accumulates within the renal pelvis and calyces. The causes are essentially those of hydronephrosis, where infection has supervened (e.g., ureteric obstruction by stone,
URETEROPELVIC JUNCTION OBSTRUCTION).
Patients with pyonephrosis are usually very ill, with a high fever, flank pain, and tenderness. Patients with this combination of symptoms and signs will usually be investigated urgently by a renal ultrasound or CT urogram, where the diagnosis of a pyonephrosis is usually obvious.
Treatment consists of IV antibiotics as for acute pyelonephritis, IV fluids, and percutaneous nephrostomy insertion for drainage.
Perinephric abscess
Perinephric abscess develops as a consequence of extension of infection outside the parenchyma of the kidney in acute pyelonephritis or, more rarely today, from hematogenous spread of infection from a distant site. The abscess develops within Gerota (perinephric) fascia. These patients are often diabetic, and associated conditions such as an obstructing ureteric calculus may be the precipitating event leading to development of the abscess.
Failure of a seemingly straightforward case of acute pyelonephritis to respond to IV antibiotics within a few days arouses suspicion that there is an accumulation of pus in or around the kidney or obstruction with infection.
Imaging studies will establish the diagnosis and allow radiographically controlled percutaneous drainage of the abscess. If the pus collection is large, formal open surgical drainage under general anesthetic will provide more effective drainage.
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152 CHAPTER 5 Infections and inflammatory conditions
Other forms of pyelonephritis
Emphysematous pyelonephritis
This is a rare, severe form of acute pyelonephritis caused by gas-forming organisms. It is characterized by high fever and abdominal pain, with radiographic evidence of gas within and around the kidney (on plain radiography or CT).
It usually occurs in diabetics and, in many cases, is precipitated by urinary obstruction by, for example, ureteral stones. The high glucose levels of the poorly controlled diabetic provide an ideal environment for fermentation by Enterobacteria, with carbon dioxide being produced during this process.
The presentation is usually as severe acute pyelonephritis (high fever and systemically very ill) that fails to respond within 2–3 days with conventional IV therapy. It is commonly caused by E. coli, less frequently by
Klebsiella and Proteus.
On KUB radiograph, crescentor kidney-shaped distribution of gas may been seen around the kidney. Renal ultrasonography often demonstrates strong focal echoes, indicating gas within the kidney. Intrarenal gas is seen on CT scan.
Patients with emphysematous pyelonephritis are usually very unwell and mortality is high. In selected cases, it can be managed conservatively, by IV antibiotics and fluids, percutaneous drainage, and careful control of diabetes. In those where sepsis is poorly controlled, emergency nephrectomy is sometimes required.
Xanthogranulomatous pyelonephritis
This is a severe renal infection usually, although not always, occurring in association with underlying renal calculi and renal obstruction. The severe infection results in destruction of renal tissue, leading to a non-functioning kidney. E. coli and Proteus are common causative organisms.
Macrophages full of fat become deposited around abscesses within the parenchyma of the kidney, hence the description xanthogranulomatous. The infection may be confined to the kidney or extend to the perinephric fat. The kidney becomes grossly enlarged and macroscopically contains yellowish nodules, pus, and areas of hemorrhagic necrosis.
It can be very difficult to distinguish the radiological findings from a renal cancer on imaging studies such as CT. Indeed, in many cases the diagnosis is made after nephrectomy for what was presumed to be a renal cell carcinoma.
Presentation is with acute flank pain, fever, and a tender flank mass. Bacteria (E. coli, Proteus) may be found on culture urine.
Renal ultrasonography shows an enlarged kidney containing echogenic material, and on CT, renal calcification is usually seen within the renal mass. Non-enhancing cavities are seen, containing pus and debris. On radioisotope scanning, there may be some or no function in the affected kidney.
OTHER FORMS OF PYELONEPHRITIS 153
On presentation these patients are usually started on IV fluids and antibiotics, as the constellation of symptoms and signs suggests infection. When imaging studies are done, the appearances often suggest the possibility of a renal cell carcinoma, and when signs of the acute infection have resolved, most patients undergo radical nephrectomy.
Only following pathological examination of the removed kidney will it become apparent that the diagnosis was one of infection (xanthogranulomatous pyelonephritis) rather than tumor.
Acute pyelonephritis, pyonephrosis, perinephric abscess, and emphysematous pyelonephritis—making the diagnosis
Maintaining a high degree of suspicion in all cases of presumed acute pyelonephritis is the single most important factor in allowing an early diagnosis of these complicated renal infections. If the patient is very ill, is diabetic, or has a history suggestive of stones, they may have something more than simple acute pyelonephritis.
Specifically ask about a history of sudden onset of severe flank pain a few days earlier, suggesting the possibility that a stone passed into the ureter, with later infection supervening. Renal imaging (KUB, ultrasound or CT urogram) should be done in all patients with suspected renal infection to rule out hydronephrosis, pyonephrosis, abscess or pus, or stones.
Clinical indicators suggesting a more complex form of renal infection are length of symptoms prior to treatment and time taken to respond to treatment. Most patients with uncomplicated acute pyelonephritis have been symptomatic for <5 days; often with a perinephric abscess patients are sick for >5 days prior to hospitalization.
Patients with acute pyelonephritis became afebrile within 2–3 days of treatment with an appropriate antibiotic, whereas those with perinephric abscesses remain febrile and systemically ill.