What is the pathophysiology of pyelonephritis?

Pathophysiology of Pyelonephritis

Pyelonephritis is an inflammatory process of the renal pelvis and kidney parenchyma caused primarily by bacterial infection, typically originating from ascending bacteria from the lower urinary tract. 1

Etiology and Microbiology

• Escherichia coli accounts for more than 90% of acute pyelonephritis cases in young healthy women 1, 2
• In men, elderly women, and urologically compromised patients, less-virulent E. coli strains, gram-negative bacilli (Klebsiella, Proteus, Pseudomonas), gram-positive organisms, and candida are more common 1, 3
• Infection typically originates in the lower urinary tract (bladder) and ascends to the kidney, though hematogenous spread is possible in rare cases 3, 4

Pathogenesis

• The infection process begins with bacterial colonization of the urethra and ascension to the bladder 5
• From the bladder, bacteria ascend via the ureters to the renal pelvis and parenchyma 2, 5
• Bacterial virulence factors (adhesins, toxins, flagella) facilitate attachment to uroepithelium and invasion of tissues 5
• Once in the kidney, bacteria trigger a tubulointerstitial inflammatory reaction involving the renal pelvis and parenchyma 4
• This inflammatory response leads to the formation of microabscesses that may coalesce into larger abscesses 1

Risk Factors

• Factors that disrupt normal urinary flow significantly increase risk of pyelonephritis: 1
  • Vesicoureteral reflux
  • Congenital urinary tract anomalies
  • Altered bladder function
  • Pregnancy
  • Renal calculi
  • Mechanical obstruction
• Additional risk factors include: 1, 2, 3
  • Sexual activity
  • New sexual partner
  • Spermicide exposure
  • Personal or maternal history of UTIs
  • Genetic predisposition
  • Diabetes mellitus

Disease Progression

• In uncomplicated cases, the infection remains limited to the renal pelvis and parenchyma 1
• When treatment is delayed or in high-risk patients, microabscesses may coalesce to form acute renal abscesses 1
• Renal parenchymal abscesses can rupture into the perinephric space, leading to perirenal abscess formation 1, 3
• In obstructed collecting systems, purulent material may accumulate causing pyonephrosis 1
• Severe cases can progress to sepsis, shock, and death if untreated 1

Complications

• Renal scarring occurs in approximately 15% of patients after first episode 2
• Potential complications include: 2, 3
  • Renal or perinephric abscess
  • Papillary necrosis
  • Emphysematous pyelonephritis (particularly in diabetic patients)
  • Acute kidney injury
• Long-term complications from renal scarring may include hypertension and chronic renal failure, though this risk is lower than previously thought 2

Clinical Manifestations

• The inflammatory process in pyelonephritis typically manifests as: 1, 2
  • Systemic inflammation (fever >38°C, chills, fatigue)
  • Flank pain and tenderness at the costovertebral angle
  • Bladder inflammation symptoms (urgency, dysuria, frequency)
• Up to 20% of patients may lack typical bladder symptoms, making diagnosis challenging 1, 2
• In diabetic patients, up to 50% may not exhibit typical flank tenderness 2

Understanding the pathophysiology of pyelonephritis is crucial for appropriate diagnosis, treatment, and prevention of complications that can lead to significant morbidity and mortality.