Predisposing conditions for perinephric abscess include diabetes mellitus, pregnancy, and urinary tract infection and structural abnormalities.[1] Urinary tract abnormalities include nephrolithiasis such as large staghorn stones, neurogenic bladder, vesicoureteral reflux, obstructive tumor, papillary necrosis, and polycystic kidney disease.

The presentation of a patient with a perinephric abscess is usually non-specific. Patients often present with insidious onset of fever, flank pain, abdominal pain, and fatigue. They may refer pain to the groin or leg area with the extension of infection. Symptoms such as dysuria and urinary frequency typical of urinary tract infection are usually not present in many patients with perinephric abscesses. The most common symptoms were fever and chills in a Korean study of 56 patients admitted with perinephric abscesses.[3][4] Seventy-five percent of patients presented with fever while 63% of patients presented with chills. Other symptoms included abdominal pain, anorexia, and dysuria with dysuria being the least presented symptoms.[4] The average duration of symptoms before admission was about 12 days.

Elderly patients and patients with autonomic neuropathy, in other words, patients with diabetes or chronic alcoholism, present with more indolent symptoms.[5] Patients may also present with right upper quadrant pain with liver extension of perinephric abscess or chest pain when there is an extension into the lung and associated empyema. Tenderness to percussion of the costovertebral area is common and was seen in 75% of the patients in the Korean study. In some patients with perinephric abscess, 60% of patients will have a palpable mass on physical examination. Inflammatory skin changes are sometimes present in a perinephric abscess.[6]

If the diagnosis of a perinephric abscess is delayed, this can lead to increased mortality. A perinephric abscess should be suspected if a patient remains febrile after 4 to 5 days of receiving adequate antibiotic therapy for pyelonephritis.

Routine blood work including a chemistry panel and complete blood count (CBC) can assess underlying kidney function and leukocytosis. Urinalysis is used to assess the presence of pyuria and proteinuria. Urinalysis may be normal in perinephric abscess since abscess might not communicate with the collecting system. Urinalysis is also normal in a patient with a perinephric abscess that develops from hematogenous seeding since there is no communication with collecting system. Blood cultures may grow bacterial organisms in patients with perinephric abscess due to hematogenous seeding of infection.

Leukocytosis is a very nonspecific parameter. People with diabetes can also present with acidosis initially. Inflammatory markers including erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are always elevated.[6] These inflammatory markers are also followed to assess clinical response to treatment.

Abnormalities can be visualized on different radiographic imaging methods, but computed tomography (CT) with contrast enhancement is the preferred imaging modality.

Plain radiography may show abnormalities that might suggest perinephric abscesses. These radiological abnormalities include an absence of psoas margin, the presence of an abdominal mass, an enlarged kidney with ill-defined borders, a radio-opaque stone and ill-defined kidney shadow.

Ultrasonography is suggestive of abscess suppuration when there is a horizontal separation seen between urine (low density) and pus (more dense fluid).

CT with contrast is the best modality to identify perinephric abscesses, it identifies the abscess and defines its extent. Enhanced CT can also be used to assess perinephric suppuration to adjacent structures including the liver, mediastinum, and diaphragm. Gerota's fascia usually confines abscesses within the perinephric area. These abscesses may extend to the retroperitoneal organs or rupture into the collecting system. [5][7]  Perinephric abscess may extend anteriorly leading to a subphrenic abscess. [8] It may also penetrate the psoas and flank muscles. A perinephric abscess can also extend caudally and may lead to a paravesical and groin abscess. Extension of the abscess into the peritoneal cavity as well as rupture into the colon is less common.

Nephrolithiasis should always be excluded once a perinephric abscess is diagnosed especially in patients with high urine pH since this may indicate the presence of a urea-splitting organism.

Nuclear imaging and MRI have limited use in the diagnosis of a perinephric abscess.

Management of perinephric abscess includes antibiotics therapy with concurrent percutaneous drainage if necessary.[9][10] Surgical intervention may be indicated in cases when a perinephric abscess is not successfully treated with antibiotic therapy and percutaneous catheter drainage.

Empiric antibiotics should initially be directed against gram-negative organisms and Staphylococcus aureus. The choice of empiric antibiotic also depends on the suspected organism. In patients with pyelonephritis and associated perinephric abscess, antibiotic therapy should target Enterobacteriaceae. In patients with Staphylococcus bacteremia with perinephric abscess, antibiotic therapy should be directed against the Staphylococcus.

Empiric therapy can be delayed for percutaneous drainage in a stable patient if drainage can be done immediately to identify the causative organism. Perinephric abscesses larger than 3 cm should be drained while abscesses smaller than 3 cm should be initially treated with an antibiotic. Percutaneous catheter drainage should be done if symptoms and radiographic abnormality persist after several days of appropriate antibiotics treatment.

Duration of antibiotics should be at least 2 to 3 weeks. Duration of treatment should be adjusted to the timing of laboratory and clinical improvement and abscess resolution. Once drainage catheter is removed, patients with infections due to Enterobacteriaceae can be transitioned to oral antibiotics. Patients with S. bacteremia with perinephric abscess infection should complete the antibiotic course with intravenous antibiotics.

A urologist should be consulted when there is an obstruction that needs to be drained or when an abscess occurs in the presence of an anatomic abnormality, for example, obstructing kidney stones or vesicoureteral reflux or when the abscess is too large for antibiotic therapy and percutaneous drainage.[11][12] Surgical drainage of the abscess and/or nephrectomy is warranted in a patient that failed medical treatment. Nephrectomy is also warranted in patients with a chronically infected and atrophic kidney that develop a perinephric abscess.

Response to treatment should be evaluated throughout medical therapy. Clinical symptoms and laboratory results to follow include flank pain, temperature, white blood cell count, CRP, and ESR. Symptoms and laboratory results should all improve with successful treatment.

Repeat imaging is indicated in patients with persistent symptoms and laboratory abnormalities. Repeat imaging is also indicated in cases where percutaneous catheter drainage is not progressing as expected. Ultrasound can be performed before discharge to assess for radiological improvement of an abscess.

• Acute pyelonephritis complicated by papillary necrosis-progressive infection with necrosis of the renal pyramids
• Emphysematous pyelonephritis-gas producing necrotizing infection involving the parenchyma of the kidney
• Acute lobar nephronia without renal tissue necrosis-variant of acute pyelonephritis that manifests as nonsuppurative renal infection
• Xanthogranulomatous pyelonephritis- variant of chronic pyelonephritis with large destruction of the kidney
• Malakoplakia- granulomatous E. coli inflammatory disease that usually affects the bladder but can affect also the kidney  
• Renal cell carcinoma

These conditions can be differentiated from perinephric abscess on enhanced computed tomography (CT) imaging

Perinephric abscesses can cause severe illness. The severity of the disease depends on the extent of the infection and the comorbidities of the patient. The average hospital stay was 15.3 days in a Korean study of 63 patients with renal and perinephric abscesses.  Significant predictors of poor prognosis and longer hospital stay include advanced age, diabetes mellitus, abscess size, lethargy, and renal insufficiency.[4][7]

• Sepsis
• Pneumonia
• Bleeding
• Fistula formation to stomach, small bowel, and lung
• Subphrenic abscess
• Possible rupture into the peritoneum
• Possible perforation through the diaphragm
• Empyema

A perinephric abscess can result from complications of pyelonephritis or hematogenous spread of infection. An abscess is usually diffuse liquefaction between the Gerota’s fascia and renal capsule. Symptoms are nonspecific and usually include flank pain, fever and sometimes dysuria. Laboratory evidence of pyuria and proteinuria may also be present. Inflammatory markers including ESR and CRP are usually elevated.

A perinephric abscess should be considered in a patient who does not improve despite appropriate antibiotics therapy. CT with contrast is the best imaging modality for diagnosis of perinephric abscess and evaluation of abscess extension into surrounding organs.

Surgery is indicated in the setting of anatomic abnormality and when the abscess is too large for effective percutaneous catheter drainage and antibiotics treatment.

Empiric treatment depends on causative organism and should cover gram-negative organisms and S. aureus. Initial antibiotic therapy should be given parenterally. Fever can persist for up to 4 to 5 days on appropriate antibiotic therapy. Parenteral therapy should be continued till final susceptibilities are available. Therapy can be de-escalated to oral antibiotics 48 hours after resolution of symptoms such as fever.

Primary caregivers and nurse practitioners should always consider the presence of a perinephric abscess in a patient with pyelonephritis who fails to improve despite optimal antibiotic therapy. These patients should undergo prompt imaging studies and be referred to an infectious disease specialist and a nephrologist for further management. Any delay in diagnosis or treatment can lead to very high morbidity and mortality. An interprofessional team approach of nurses and clinicians evaluating and treating the abscess will provide the best patient outcome. [Level V]