Perinephric abscess

A perinephric abscess is a collection of suppurative material in the perinephric space ¹. Perinephric abscess results from perirenal fat necrosis ². A perinephric abscess is a life-threatening entity that is usually a complication of urologic infection. Most perirenal abscesses were due to prolonged bacteremia with Staphylococcus aureus followed by hematogenous seeding before the era of antibiotics ³. More than 75% of perinephric abscesses are now due to complications of urinary tract infection (UTI). A perinephric abscess is more diffuse and affects the renal capsule and Gerota’s fascia. These abscesses can extend from the Gerota’s fascia into the psoas and transversalis muscles as well as the peritoneal cavity and the pelvis.

Perinephric abscess symptoms are nonspecific and usually include flank pain, fever and sometimes dysuria (pain or burning when urinating).

A perinephric abscess can pose a great diagnostic challenge, even to an astute clinician. This is very important because a delay in diagnosis increases the risk of morbidity and mortality. Perinephric abscesses carry a mortality rate of up to 56%. This rate partly is due to long delays in diagnosis and the comorbid conditions. Even with modern surgical therapy, the mortality rate is 8-22% and significant morbidity occurs in 35% of patients.

The mortality rate is higher in the following situations:

• Patients who are more ill and have sepsis
• Urinary tract obstruction
• Lethargy
• Higher temperature (>104 °F or > 40 °C)
• The presence of more underlying diseases such as diabetic ketoacidosis
• Obscure presentation
• White blood cell count greater than 25,000 cells/μL
• High blood urea nitrogen (BUN)
• Positive blood cultures
• Diagnostic delay
• Previous urinary tract infections

Recent studies indicate that a marked reduction in mortality rates has occurred with early diagnosis, immediate drainage, and antibiotic therapy. Criteria for successful treatment include the presence of negative cultures and the resolution of any underlying obstruction.

Diagnosis of a perinephric abscess should be considered in any patient with fever and abdominal or flank pain. A perinephric abscess should be suspected if a patient remains febrile after 4 to 5 days of receiving adequate antibiotic therapy for pyelonephritis. CT with contrast is the best imaging modality for diagnosis of perinephric abscess and evaluation of abscess extension into surrounding organs. 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.

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

Perinephric abscess causes

A perinephric abscess can occur through the hematogenous spread of infection from a focus outside the kidney or local spread of a related urologic infection. These perirenal abscesses are usually due to ascending infection from the urinary bladder with associated pyelonephritis. With local spread, there is an outward migration of kidney infection into the perirenal fat. It might also result from a ruptured kidney abscess through the capsule to form renal abscess of the cortex due to staphylococcus. Perinephric abscesses may arise from non-renal contiguous infections. These infections may originate from trauma, an extension of infection in other body organs including liver, cervix, pancreas, gallbladder, and appendix or from small bowel Crohn’s disease, or vertebral osteomyelitis. Perinephric fat infection can also occur from hematogenous seeding mostly caused by Staphylococcus aureus. The kidney is usually unaffected in hematogenous seeding.

Perinephric abscesses are usually due to gram-negative enteric bacilli or a polymicrobial infection. The most frequently encountered organisms are Escherichia coli, Staphylococcus aureus, and Klebsiella pneumonia ³. Escherichia coli accounts for 51.4% of perinephric abscesses. Perinephric abscesses due to Staphylococcus aureus are usually secondary to hematogenous seeding of infection ⁴.

Risk factors for perinephric abscess

Certain medical conditions can predispose a patient to perinephric abscesses. These include diabetes mellitus, pregnancy, urinary tract infection, and structural abnormalities in the urinary tract ⁵. Urinary tract abnormalities include nephrolithiasis especially large staghorn stones, neurogenic bladder, vesicoureteral reflux, obstructive tumor, papillary necrosis, and polycystic kidney disease. Twenty percent to 60% of patients with perinephric abscess have associated renal calculi.

Perinephric abscess symptoms

The clinical features 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 ⁶. 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 ⁶. 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 ⁷. 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 ⁸.

Perinephric abscess complications

Various complications can occur with perinephric abscess, including the following:

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

A nephrocolonic fistula occurs if the abscess erodes into the adjacent part of the colon. The patient may present with bloody stool, diarrhea, passing of urine per rectum, and passing of fecal contents into the urine. If this fistula opens on to the skin, urine and feces are discharged through the nephrocolocutaneous fistula.

Perinephric abscess diagnosis

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 ⁸. 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 ⁷. Perinephric abscess may extend anteriorly leading to a subphrenic abscess ⁹. 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.

Perinephric abscess treatment

Treatment of perinephric abscess includes antibiotics therapy with concurrent percutaneous drainage if necessary 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 Staphylococcus aureus 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. 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.

Perinephric abscess prognosis

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 ¹².

References

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