Pyelonephritis and urinary tract infections

UTI Basics - The Bladder Blues

• Cystitis (lower UTI/bladder) vs. Pyelonephritis (upper UTI/kidney). Most ascend from the urethra.
• Pathogens: >80% E. coli. Others: Staphylococcus saprophyticus (young, sexually active women), Klebsiella pneumoniae, Proteus mirabilis (alkaline urine, struvite stones).
• Risk Factors: Female sex, sexual intercourse, catheterization, urinary stasis (BPH, stones), vesicoureteral reflux.
• Clinical: Dysuria, frequency, urgency, suprapubic pain. Absence of vaginal discharge helps differentiate from vaginitis.

⭐ Positive nitrites on urinalysis are highly specific for gram-negative bacteria (Enterobacteriaceae), which convert nitrates to nitrites.

Pathophysiology - The Upstream Battle

• Ascending Infection: The dominant pathway (>95%). Gut flora colonizes the urethra, ascends to the bladder (cystitis), and then travels up the ureters to the kidney parenchyma.
• Key Bacterial Factor: P-fimbriae on uropathogenic E. coli (UPEC) are crucial for adhering to urothelial cells, preventing washout.
• Host Risk Factors:
  • Vesicoureteral Reflux (VUR): Retrograde urine flow.
  • Obstruction: BPH, calculi, or tumors leading to urine stasis.
• Hematogenous Spread: Less common; typically from S. aureus bacteremia (e.g., endocarditis).

⭐ E. coli is the culprit in >80% of uncomplicated UTIs, using its P-fimbriae for adhesion.

Acute Pyelonephritis - Kidney Under Siege

• Bacterial infection of the kidney parenchyma and renal pelvis, typically from an ascending lower UTI.
• Etiology: Most commonly E. coli. Other causes include Klebsiella, Proteus, and Enterococcus.
• Risk Factors: Vesicoureteral reflux (VUR) in children, urinary tract obstruction (e.g., BPH, stones), catheterization, female sex, pregnancy.
• Clinical Triad: Fever, costovertebral angle (CVA) tenderness, and nausea/vomiting.

• Pathology: Patchy interstitial inflammation with neutrophils. Neutrophils in tubules form pathognomonic white blood cell (WBC) casts.

⭐ High-Yield: WBC casts are pathognomonic for pyelonephritis or interstitial nephritis, indicating the infection involves the kidney itself, not just the lower urinary tract.

• Complications: Perinephric abscess, pyonephrosis, urosepsis, and renal papillary necrosis.

Chronic Pyelonephritis - The Aftermath & Scars

• Etiology: Typically results from recurrent acute pyelonephritis, often due to vesicoureteral reflux (VUR) in children or chronic obstruction in adults.
• Gross Pathology: Asymmetrically scarred, contracted kidneys with blunted calyces.
• Microscopic Hallmark:
  • Chronic interstitial inflammation (lymphocytes, plasma cells).
  • Atrophic tubules containing eosinophilic proteinaceous casts, resembling thyroid follicles (thyroidization).

⭐ High-Yield: Look for coarse, discrete corticomedullary scars overlying dilated, blunted, or deformed calyces on imaging or gross examination.

Complications - When Things Go Wrong

• Renal Papillary Necrosis: Coagulative necrosis of renal papillae. Risk factors: 📌 POSTCARDS (Pyelonephritis, Obstruction, Sickle cell, Tuberculosis, Cirrhosis, Analgesics, Renal vein thrombosis, Diabetes, Systemic vasculitis).
• Perinephric Abscess: Pus collection in the space around the kidney.
• Pyonephrosis: Pus accumulation in an obstructed, infected urinary system; a urologic emergency.
• Urosepsis: Systemic inflammatory response to UTI.

⭐ Emphysematous Pyelonephritis: A severe, necrotizing infection with gas formation in/around the kidney, most common in diabetics. High mortality.

High‑Yield Points - ⚡ Biggest Takeaways

• E. coli is the most common cause, typically via ascending infection.
• Classic presentation includes fever, flank pain (CVA tenderness), and nausea/vomiting.
• WBC casts in urine are pathognomonic for pyelonephritis, distinguishing it from lower UTIs.
• Urinalysis typically shows positive leukocyte esterase and nitrites.
• Serious complications include urosepsis, perinephric abscess, and chronic pyelonephritis.
• Chronic pyelonephritis leads to cortical scarring and histologic thyroidization of tubules.