Perinephric abscess

A perinephric abscess is a purulent infection located in the perinephric space between the kidney and the Gerota fascia. It is typically secondary to acute pyelonephritis but may also be caused by the hematogenous spread of bacteria from elsewhere in the body (e.g., in individuals who inject drugs). Risk factors include diabetes mellitus, pregnancy, and urinary tract obstruction or abnormalities. Perinephric abscess typically has an insidious onset, with nonspecific symptoms that include flank or abdominal pain, fever, chills, and dysuria. Costovertebral angle tenderness is often present on examination. Abdominal CT is the preferred method for confirming the diagnosis. Abscess drainage and antibiotic therapy are the cornerstones of treatment. Antibiotic therapy alone may be considered in select patients with small abscesses (< 3 cm). Complications include extension of the abscess beyond the Gerota fascia, into the retroperitoneum (paranephric abscess), then sepsis.

• Route of infection
  • Most commonly due to local spread of infection in patients with acute pyelonephritis
  • Hematogenous spread of infection from elsewhere in the body
• Pathogens
  • Most commonly Enterobacteriaceae (gram-negative rods)
    • Escherichia coli
    • Klebsiella pneumoniae
  • Gram-positive bacteria (Staphylococcus aureus) ^[1]
  • Candida infection (especially in immunocompromised patients and patients with diabetes) ^[2]
• Risk factors
  • Metabolic: diabetes mellitus (30–45% of patients with a perinephric abscess are diabetic) ^[3]
  • Infection: conditions that predispose individuals to urinary tract infection and pyelonephritis, such as pregnancy (see pyelonephritis in pregnancy)
  • Obstruction: urinary tract obstruction, e.g., nephrolithiasis, or abnormality, e.g., vesicoureteral reflux
  • Iatrogenic: surgery (e.g., renal transplantation)

Onset is often insidious and symptoms are nonspecific, but they may include:

• Constitutional symptoms (e.g., fever, chills, fatigue)
• Flank pain, abdominal pain, and/or back pain, with costovertebral angle tenderness on examination (usually unilateral)
• Dysuria as well as other symptoms of cystitis (e.g., increased urinary frequency, urgency)
• Palpable unilateral flank mass

Approach ^[4][5]

• Suspect perinephric abscess in patients with:
  • Concerning clinical features
  • Pyelonephritis that does not improve with antibiotics
• Obtain imaging of the abdomen to confirm the diagnosis.
• Obtain cultures (blood, urine, drainage if possible) to identify the pathogen and guide treatment.

Perinephric abscess is only correctly diagnosed on presentation in approximately one-third of patients. A low threshold for imaging is recommended. ^[6]

Laboratory studies ^[4][5]

Routine laboratory studies are not required for diagnosis but may show the following nonspecific findings.

• Routine studies
  • CBC: ↑ WBC, ↓ Hb
  • BMP: ↑ creatinine, ↑ BUN, ↓ GFR
  • Inflammatory markers: ↑ CRP, ↑ ESR
  • Urinalysis: urinalysis findings of UTI (e.g., pyuria, bacteriuria, hematuria), proteinuria
• Cultures
  • Blood cultures
  • Urine cultures
  • Abscess cultures (from perinephric abscess drainage)

Imaging ^[4][7]

• Ultrasound kidneys and retroperitoneum
  • Often used as an initial study
  • Findings: hypoechoic or anechoic, thick and/or irregular-walled structure that may be multiloculated
• CT abdomen and pelvis with contrast (preferred)
  • Indications
    • Confirmation of diagnosis and follow-up evaluation
    • Exclusion of other diagnoses (e.g., urinoma)
    • Assessment of the extent of infection and involvement of adjacent structures (e.g., psoas, pelvis, liver)
  • Findings ^[8]
    • Hypodense perinephric space with rim enhancement
    • Thickened septa and perinephric fascia
    • Fat stranding
    • Gas inclusions (in emphysematous infections)
• MRI abdomen: indications and findings similar to CT

There is limited recent, high-quality literature on the management of perinephric abscess. Treatment decisions should be made in consultation with urology, interventional radiology, and infectious diseases.

Approach ^[4][7]

• Arrange abscess drainage as indicated for most patients.
• Start intravenous empiric antibiotic treatment as soon as possible, ideally after obtaining cultures.
• Identify and treat underlying diseases and risk factors, e.g.:
  • Obstructive uropathy: may require percutaneous nephrostomy
  • Urolithiasis: Treat in patients with renal obstruction due to urinary stones (e.g., with percutaneous nephrolithotomy)
• Nephrectomy may be required in some patients with persistent infection and extensive kidney damage. ^[7]

If possible, drain the abscess and obtain cultures prior to starting empiric antibiotics. If drainage is not immediately feasible, start antibiotic treatment without delay.

Management of perinephric abscess is similar to that of renal abscess. For small renal abscesses, there is strong evidence for, and clear guidance on, conservative management. ^[7]

Antibiotic treatment ^[4][7][9]

Most recommendations are based on early studies; choose treatment in consultation with a specialist.

• Empiric antibiotic treatment should cover common pathogens, i.e., gram-negative organisms (including Enterobacteriaceae) and S. aureus
  • Example regimen: piperacillin/tazobactam with or without gentamicin ^[4][6][10]
  • Consider adding vancomycin if MRSA is suspected.
  • Other agents used in empiric antibiotic therapy for complicated pyelonephritis may be used.
• Switch to directed antibiotics once culture results are available.
• Determine the duration of antibiotic treatment based on clinical response.

Perinephric abscess drainage ^[4][7]

• Indicated for most patients for diagnostic and therapeutic purposes.
• Antibiotic treatment alone may be sufficient for the treatment of small abscesses (< 3 cm). ^[6][10]
• Preferred method: percutaneous drainage (guided by CT or ultrasound)
• Surgical drainage may be indicated in certain situations, e.g.:
  • Percutaneous approach fails or is contraindicated
  • Large abscess
  • Multiloculated abscess

“Ubi pus, ibi evacua” (Latin aphorism): Where there is pus, evacuate it.

• Paranephric abscess
  • Rupture of the Gerota fascia, leading to the secondary spread of purulent infection into the retroperitoneum
  • Treatment: as with perinephric abscess
• Sepsis (urosepsis)

We list the most important complications. The selection is not exhaustive.