Neutrophils infiltrate renal interstitium. Affects cortex with relative sparing of glomeruli/vessels.

Etiology

• Usually caused by an ascending bacterial infection of the bladder (cystitis)
• Pathogens
  • Gram-negative rods bacteria (Thus use fluoroquinolones)
    • Escherichia coli (∼ 75–90% of cases; leading cause of pyelonephritis)
      • UPEC (uropathogenic E. coli)
      • Virulence factor: P fimbria (pyelonephritis‑associated pili): specifically bind to the urogenital epithelium → cystitis, pyelonephritis
    • Pseudomonas aeruginosa
    • Klebsiella pneumoniae
  • Risk factors
    • Most common in women because they have shorter urethras
    • Pregnancy
    • Urinary tract obstruction (potentially leading to pyonephrosis)
    • Foreign bodies (e.g., indwelling urinary catheters or other urologic instrumentation)
    • Anatomical abnormalities (e.g., benign prostatic hyperplasia, vesicoureteral reflux, nephrolithiasis, ureteral strictures)
    • Immunosuppression (e.g., HIV, diabetes mellitus)

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Pathology

Acute pyelonephritis

• Most commonly affects the cortex (spares glomeruli and vessels)
• Purulent inflammation of the interstitium with destruction of the parenchyma, the renal tubules, and, in some cases, the renal pelvis
• Histology
  • PMN infiltration of the renal tubules and the interstitium
  • Sparing of the glomeruli and intrarenal vessels

Chronic pyelonephritis

• Chronic inflammatory changes with rough, asymmetric scarring and fibrosis of the corticomedullary junction
• Blunted calyces from recurrent urinary reflux
• Histology: eosinophilic casts in the tubules that resemble thyroid tissue with coloid (thyroidization of the kidney)
• Xanthogranulomatous pyelonephritis
  • A rare form of chronic pyelonephritis characterized by chronic destructive granuloma formation
  • Associated with Proteus mirabilis and Escherichia coli infections
  • Large, irregular, yellow-orange masses on gross examination of the kidney (may be mistaken for a true renal neoplasm)
  • Histology: granulomatous tissue with lipid-laden foamy macrophages and multinucleated giant cells

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Clinical features

Tip

Suspect pyelonephritis in any patient presenting with fevers, chills, and flank pain, irrespective of lower urinary tract symptoms.

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Diagnostics

Urinalysis

• Nonspecific findings of UTI
  • Pyuria (positive esterase on dipstick test)
  • Leukocyturia (WBCs > 5/hpf)
  • Bacteriuria
  • Positive nitrites on dipstick test indicate bacteria that convert nitrates to nitrites (most commonly gram-negative bacteria; e.g., E.coli).
  • Hematuria (including microhematuria)
• Other findings
  • WBC casts: rare finding, but considered to be a strong indicator for pyelonephritis

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Differential diagnosis

Pyelonephritis vs Cystitis

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Treatment

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