Perinephric abscess

Background

• An abscess of the perinephric fat that abuts the renal cortex
• Can arise as a complication either a pre-existing pyelonephritis (majority) or from hematogenous seeding

Causative Organisms

• E. coli (51.4%)
• S. aureus (10%)

Perinephric vs Renal Abscess

	Perinephric	Renal
Necrotic Area	perinephric fat between the renal cortex and Gerota's fascia	renal parenchyma
Cause	Pylonephritis (majority)	Pyelonephritis (vast majority)
Risk of morbidity	Higher	Lower

Clinical Features

• Sign/symptoms similar to pyelo (fever, CVAT, dysuria)
• Occurs in setting of ascending infection with obstructed pyelo

Differential Diagnosis

Dysuria

• Genitourinary infection
  • Acute cystitis ("UTI")
  • Pyelonephritis
  • Urethritis
  • Chronic cystitis
  • Infected nephrolithiasis
  • Prostatitis
  • Epididymitis
  • Renal abscess/perinephric abscess
  • Emphysematous pyelonephritis
• Nephrolithiasis
• Urethral issue
  • Urethritis
  • Urolithiasis
  • Urethral foreign body
  • Urethral diverticulum
  • Allergic reaction (contact dermatitis)
  • Chemical irritation
  • Urethral stricture or obstruction
  • Trauma to vagina, urethra, or bladder
• Gynecologic
  • Vaginitis/cervicitis
  • PID
  • Genital herpes
  • Pelvic organ prolapse
  • Fistula
  • Cystocele
• Other
  • Diverticulitis
  • Interstitial cystitis
  • Behavioral symptom without detectable pathology

Evaluation

Laboratory Testing

• CBC - Although it is a non-specific finding, a leukocytosis is typically seen.
• Chemistry Panel - Lactic acidosis is a common finding and is seen earlier in the disease course in diabetic patients. Renal insufficiency is also commonly seen as well.
• Inflammatory markers - ESR and CRP are almost always elevated.
• Urinalysis - Will often show evidence of pyelonephritis, however if the perinephric abscess is secondary to hematogenous spread, the urine could be sterile. Therefore a negative U/A does NOT rule out a perinephric abscess.

Imaging

• X-ray - Will sometimes show abnormalities that might suggest a perinephric abscess is present, however one or all of these findings may not be present, therefore it is suggested to use more invasive imaging to rule out a perinephric abscess if clinical suspicion is high. Abdominal X-ray findings that can suggest a perinephric asbcess include:
  • Displaced or rotated kidney
  • Scoliosis - Deviation of the spine will occur in the direction of the lesion
  • Loss of the psoas margin, however the false positivity rate is about 10%
  • An abdominal mass or displaced bowel gas
  • Thoracic pathology - ipsilateral consolidative process, atelectasis, or pleural effusion
  • Ultrasound - A fluid filled mass extending from the renal cortex into the perinephric fat can be seen.
  • CT - Is the imaging modality of choice for evaluation of suspected perinephric abscess. Sensitivity of detecting a perinephric abscess is about 90%. CT can give an accurate measurement of the abscess and can also show involvement, if any, of any surrounding structures (liver, ipsilateral psoas muscle, vertebral bodies, intervertebral discs, etc.)
  • MRI - Has limited utility in the Emgergency Room setting given the time it takes for the study to be completed and read. However, advantages over CT include better delineation of soft tissue structures (listed above), absence of radiation exposure, and better contrast sensitivity.

Management

Management of perinephric abscess includes both antibiotic therapy as well as drainage of the abscess percutaneously. As previously stated above, a perinephric abscess of ANY size should prompt discussion with IR for drainage. Regarding antimicrobial therapy, the exact antibiotic regimen depends on the suspected causative agent. In the case of suspected pyelonephritis as the cause, antimicrobial therapy should be directed toward enterobacteriaceae, agents such as third generation cephalosporin in conjunction with a quinolone is typically first line. Further considerations for antimicrobial therapy include the following:

1. Hematogenous seeding is the suspected cause, staphylococcus should be considered as a causative agent thus antimicrobials directed against this pathogen should be initiated, which include vancomycin.
   
2. Patients known to be colonized by ESBL (enterobacteriacea with extended spectrum beta-lactamase) should be treated with a carbapenem antibiotic as first-line

If any urological obstruction is suspected (nephrolithiasis, ureterolithiasis, external compression from abdominal cavity, presence of pre-existing ureteral stent), emergent urological consultation should be obtained.

Disposition

All patients suspected of having a perinephric abscess should be admitted with empiric antibiotic therapy (see above) initiated in the Emergency Room immediately after the blood and urine culture have been obtained. In addition, a discussion with the admitting service regarding percutaneous drainage occur.

See Also

pyelonephritis
Emphysematous pyelonephritis
Renal abscess
Urolithiasis