What is the difference between pyelonephritis and renal abscess?

Differences Between Pyelonephritis and Renal Abscess

Pyelonephritis and renal abscess represent different stages and severity of kidney infection, with renal abscess being a potential complication of untreated or inadequately treated pyelonephritis. 1

Definition and Pathophysiology

• Pyelonephritis: Inflammation of the renal parenchyma and renal pelvis due to bacterial infection, typically presenting with systemic inflammation and bladder symptoms 1
• Renal Abscess: Collection of purulent material within the kidney parenchyma, formed when microabscesses from acute pyelonephritis coalesce, particularly in high-risk patients or when treatment is delayed 1
• Renal abscess can rupture into the perinephric space, leading to perirenal abscess formation 1

Clinical Presentation

• Pyelonephritis:

  • Typically presents with fever (>38°C), chills, flank pain, and systemic symptoms 1
  • Often accompanied by lower urinary tract symptoms (urgency, dysuria, frequency) 2
  • Up to 20% of patients may lack bladder symptoms 1
  • Diabetic patients may not exhibit typical flank tenderness 2

• Renal Abscess:

  • More severe and persistent symptoms than uncomplicated pyelonephritis 1
  • Lumbar pain and fever are common 1
  • Costovertebral angle tenderness is frequently present 1
  • May have a more indolent course with prolonged symptoms 3

Causative Organisms

• Pyelonephritis:

  • Predominantly caused by Escherichia coli (>90% in young healthy women) 1
  • Less virulent E. coli strains, other gram-negative bacilli, gram-positive organisms, and Candida may be found in men, elderly women, and immunocompromised patients 1

• Renal Abscess:

  • Commonly caused by gram-negative bacteria similar to pyelonephritis 1
  • May also result from hematogenous seeding, particularly with Staphylococcus aureus 1, 3

Diagnosis

• Pyelonephritis:

  • Primarily a clinical diagnosis based on symptoms and urinalysis showing pyuria/bacteriuria 1
  • Urine culture with >10,000 colony-forming units of a uropathogen per milliliter confirms the diagnosis 1
  • Imaging is not routinely required for uncomplicated cases 1

• Renal Abscess:

  • Requires imaging for definitive diagnosis 1, 3
  • CT is the gold standard for diagnosis, showing a focal collection of fluid within the renal parenchyma 1, 4
  • Ultrasound may show mass lesions in the renal cortex but is less sensitive than CT 4, 5

Treatment

• Pyelonephritis:

  • Typically responds to appropriate antibiotic therapy alone 1
  • Treatment duration usually 7-14 days depending on severity 2

• Renal Abscess:

  • Often requires both antibiotics and drainage procedures 1, 3
  • Decision for drainage is influenced by abscess size 1
  • Some form of drainage is frequently necessary for definitive treatment 1
  • May require percutaneous catheter drainage or surgical intervention in severe cases 1, 3

Risk Factors for Progression from Pyelonephritis to Renal Abscess

• Delayed or inadequate treatment of pyelonephritis 1
• Immunocompromised status 1
• Diabetes mellitus 1
• Urinary tract obstruction or anatomical abnormalities 1
• Infection with treatment-resistant organisms 1

Imaging Findings

• Pyelonephritis:

  • Ultrasound may show renal enlargement and decreased echogenicity 4
  • CT may demonstrate striated nephrogram, wedge-shaped areas of decreased enhancement 4

• Renal Abscess:

  • CT shows a well-defined, fluid-filled cavity with rim enhancement 3, 4
  • Ultrasound reveals hypoechoic or anechoic focal lesions 4, 5

Complications

• Pyelonephritis: Can progress to renal abscess, sepsis, or renal scarring if inadequately treated 1, 2
• Renal Abscess: Higher risk of serious complications including extension to perirenal spaces, sepsis, and renal loss 3

Understanding these differences is crucial for appropriate management and to prevent progression from pyelonephritis to more severe complications like renal abscess.