• Emphysematous pyelonephritis (EPN) is a rare acute necrotizing parenchymal and perirenal infection caused by gas-forming uropathogens, namely E. coli, K. pneumonia, and Proteus spp.
• The condition predominantly affects diabetic patients, with high tissue glucose levels providing the substrate for carbon dioxide-producing microorganisms.
• EPN preponderantly affects females over males (6:1), which may be due to the increased susceptibility to UTI in females, and all the documented cases of emphysematous pyelonephritis have been in adults.
• The left kidney is more commonly affected than the right, reflecting the preponderance of left-sided urinary tract obstruction.

Clinical Presentation

• The most common presentation of EPN is fever, flank pain, and pyuria, a clinical picture not significantly different from a classic upper UTI.
• Thrombocytopenia, acute renal function impairment, disturbance in consciousness, and shock, testicular torsion can be initial presentations.
• The overall mortality is described to be around 40%, with delay in diagnosis and treatment contributing to both morbidity and mortality.

Predictors of Outcome

• The most reliable predictor of outcome in EPN has been determined to be serum creatinine.
• Patients with serum creatinine levels >1.4 mg/dl had an increase in post-test probability of death from 69-92% in one study.
• Platelet counts 60,000/mm or less also indicated higher risk of mortality.
• Additionally, disturbance of consciousness and shock are associated with mortality and poor outcome, explained by expected poor prognosis of CNS and cardiovascular dysfunction.

Diagnosis

• The diagnosis of EPN is classically made by demonstrating the presence of gas in renal or perinephric tissue by plain abdominal X-ray film or by renal ultrasound.
• When present, a crescent shaped collection of gas over the upper pole of the kidney is more distinctive than mottled gas shadows, which are often mistaken for bowel gas.
• As the infection progresses, gas extends into the perinephric space and retroperitoneum, periuethral abscess.
  However, gas could be demonstrated only on one-third of plain abdominal radiographs in some studies.
• Ultrasonography usually demonstrates strong focal echoes, suggesting the presence of intraparenchymal gas; however, it may again be difficult to distinguish the necrotic gas-filled area from gas in the bowel. IVP is rarely of value as the affected kidney usually is nonfunctioning or poorly functioning.
• Obstruction has been demonstrated in approximately 25% of EPN cases.
• CT scan is the best means to localize gas and extent of infection.
• The presence of streaky or mottled gas with or without bubbly and loculated gas appears to be associated with rapid destruction of renal parenchyma and a 50-60% mortality rate.
• A gas pattern characterized by the presence of bubbly or loculated gas and the absence of streaky or mottled gas is associated with a more favorable prognosis.
• A renal CT scan should be performed to assess the degree of renal function impairment of the involved kidney and the status of the contralateral kidney.

Management

• Patients should be started on appropriate antimicrobial agents, and treatment of diabetes must be initiated.
• Obstruction of the affected kidney, if present, must be eliminated, and function of the contralateral kidney must be established, because of reported bilateral cases.
• At the same time, surgical intervention poses a substantial risk for patients with hemodynamic instability caused by fulminant infection and is not an appropriate option for bilateral kidney involvement.
• Previous studies have emphasized that surgical treatment must be complete extirpation. In more recent studies, CT-guided percutaneous drainage has proven successful in as high as 92% of patients in treating multiloculated, ill-defined, and extensive dissecting air and fluid collections in EPN, with 80% nephron-sparing.

Perinephric Abscess

• Perinephric abscess is a life-threatening but treatable process, consisting of suppurative material occupying the space between the renal capsule and the surrounding fascia.
• Most of the perinephric abscesses result from the rupture of an intrarenal abscess into the perirenal space, and are caused most commonly by E. coli, Proteus species, and S. aureus.
• Other sources include dissemination from other sites of infection including liver, gallbladder, pancreas, pleura, prostate, and the female reproductive tract.
• Much of the associated mortality is the result of failure to diagnose this entity in a timely fashion. This failure may be due to the nonspecific clinical picture on presentation.

Clinical Features

• The symptoms of perinephric abscess, including fever, flank pain, chills, nausea, vomiting, and dysuria, may develop insidiously, making early recognition difficult.
• Fever is the most common symptom
• Abdominal tenderness
• Referred pain is also common to areas of the hip, thigh, and knee.
• The peripheral white blood cell count is usually elevated with a left shift.
• Urinalysis may be normal up to one-third of the time, and blood cultures as well as urine cultures may fail to identify correctly the bacterial pathogens responsible for the abscess.
• Distant extension of a perinephric abscess may result in a multitude of processes including empyema and colon perforation. While these extensions are rare, direct extensions into the flank are more common, which may even extend to drain as a flank abscess.

Diagnosis

• A perinephric abscess should be in the differential of patients presenting with fever of unknown origin and with unexplained peritonitis, pelvic abscess, or empyema.
• Additionally, perinephric abscess should be considered in the differential diagnosis of any patient presenting with a urinary tract infection that fails to respond promptly to antibiotic therapy, particularly in those known to have anatomical abnormalities of the urinary tract or diabetes mellitus.
• Chest X-ray and abdominal films may show a range of findings, including subtle abnormalities, nonspecific findings, or nothing at all.
• Ultrasonography, however, will show a mass, often with thickened, uneven walls, with heterogeneous internal echoes. However, the ultrasound was falsely negative in as high as 36% of cases when compared to CT in one study. CT scan, therefore, is the diagnostic test of choice as it identifies the abscess and defines involvement of surrounding and distant structures.

Treatment

• Perinephric abscesses have been associated with mortality rates as high as 50%, although with early recognition by CT scan, prompt percutaneous drainage, and effective antimicrobial therapy, mortality has decreased.
• Unlike intrarenal abscesses, antibiotic therapy alone is not sufficient in treating perinephric abscesses.
• Percutaneous drainage under CT or ultrasound guidance with adjunctive antibiotics is recommended as the treatment of choice.
• If percutaneous drainage fails or is contraindicated, surgical drainage is performed.
• Empiric antimicrobial therapy should be directed mainly against common Gram-negative uropathogens and S. aureus. An aminoglycoside (gentamicin or tobramycin) and an anti-staphylococcal ß-lactam (oxacillin, nafcillin, cefazolin) are appropriate initial antibiotics.
• An extended spectrum ß-lactam may be used in place of an amino glycoside for Gram-negative coverage in case of abnormal renal function.
• Once cultures are done, antibiotic therapy should be modified accordingly.