Acute and chronic pyelonephritis

Acute pyelonephritis

• E. coli causes about 80-85% of cases. Other causes are Klebsiella, Proteus, Enterobacter, Pseudomonas, and Enterococcus. Staphylococcus aureus is typical of hematogenous spread.
• Proteus splits urea, which makes the urine alkaline and promotes struvite stones.

• Ascending (most common): Bacteria move from the urethra to the bladder, then up the ureter to the kidney. Vesicoureteral reflux and intrarenal reflux help this happen.
• Hematogenous: This is uncommon. It occurs with septicemia or endocarditis, and with S. aureus or Candida.

• Female sex (short urethra), pregnancy, and sexual activity.
• Urinary obstruction: stones, BPH, strictures, tumors.
• Vesicoureteral reflux and neurogenic bladder.
• Catheterization or instrumentation.
• Diabetes mellitus and immunosuppression.

• The kidney is enlarged and swollen. Yellow-white abscesses may be visible on the surface and the cut surface.
• Histology shows patchy neutrophilic infiltrates in the interstitium and tubules. Neutrophil casts (WBC casts) form in the tubules.
• Complications are:
  • Papillary necrosis, seen in diabetes, obstruction, sickle cell disease, and analgesic abuse.
  • Pyonephrosis, which is pus in an obstructed collecting system.
  • Perinephric abscess.
  • Emphysematous pyelonephritis, a gas-forming infection in diabetics.
  • Gram-negative sepsis.

• Fever with chills, flank pain, and costovertebral angle tenderness.
• Nausea and vomiting, plus dysuria, frequency, and urgency.
• Leukocytosis.
• Urinalysis: pyuria, bacteriuria, hematuria, and WBC casts, which point to a renal source.

• Urine culture (at least 10^5 CFU/mL classically) and blood cultures.
• Imaging is not needed in uncomplicated cases. Use ultrasound or CT if there is no response by 48-72 hours, if there is sepsis, a stone, or suspected obstruction, or if the patient is diabetic or male.

• Mild, outpatient, uncomplicated:
  • A fluoroquinolone for 5-7 days, if local resistance is low.
  • Or oral beta-lactams or TMP-SMX for 10-14 days, guided by culture.
  • Many guidelines give a single initial IV dose of ceftriaxone.
• Severe, vomiting, or sepsis: Admit and give IV therapy. Options are ceftriaxone, a fluoroquinolone, an aminoglycoside, or a carbapenem if ESBL is suspected. Switch to oral once afebrile for 24-48 hours, and treat for 7-14 days in total.
• Pregnancy: Admit and give IV beta-lactams. Avoid fluoroquinolones.
• Obstruction or abscess: Drain by ureteral stent, nephrostomy, or percutaneous drainage.
• Treat the underlying cause, such as a stone or diabetes.

Chronic pyelonephritis

1. Reflux nephropathy (more common):
   • It starts in early childhood with vesicoureteral reflux and recurrent UTIs.
   • Intrarenal reflux scars the kidney, usually at the upper and lower poles.
   • It can be unilateral or bilateral.
2. Chronic obstructive pyelonephritis:
   • Obstruction (stones, BPH, strictures, congenital anomalies) causes recurrent infection and diffuse scarring.
   • It can be unilateral or bilateral.

• Gross: The kidneys are irregularly scarred, with coarse, asymmetric cortical scars that overlie blunted, deformed calyces. The scars are U-shaped depressions over dilated calyces. The pelvis is thickened.
• Histology:
  • Chronic interstitial inflammation and fibrosis.
  • Atrophic tubules, some dilated and filled with eosinophilic colloid casts ("thyroidization").
  • Periglomerular fibrosis and eventual glomerulosclerosis.
  • Thickened arterioles from hypertension.
• Secondary FSGS can develop from nephron loss and hyperfiltration. This causes proteinuria, sometimes in the nephrotic range.

• Often silent. It may present with:
  • Hypertension.
  • Recurrent UTIs or flank pain.
  • Slowly progressive CKD with a tubular concentrating defect (polyuria, nocturia), metabolic acidosis, and salt wasting.
• Proteinuria is mild to moderate.

• Imaging is key. Ultrasound or CT shows asymmetric small kidneys with cortical scars and calyceal blunting.
• A DMSA scan is the best test for detecting scars.
• A voiding cystourethrogram (VCUG) is used in children to look for reflux.

• Treat and prevent infection, correct reflux or obstruction, and control hypertension (ACE inhibitor or ARB).
• Use renal protective measures and manage CKD, with dialysis or transplant for ESRD.

Related entity: xanthogranulomatous pyelonephritis

• A rare, severe chronic form, usually from long-standing obstruction by a staghorn calculus with Proteus infection.
• Lipid-laden (foamy) macrophages replace the parenchyma, so the kidney looks like a yellow mass that can mimic renal cell carcinoma.
• Treatment is usually nephrectomy.

Key comparison