26: Obstructive Uropathy

Question 1

Obstructive uropathy prevalence

Answer 1

• Hydronephrosis in 3.1%; ~2% kids
• M=F until 20yo, then F>M (pregnancy) until 60 (prostatic hyperplasia)
• M>F congenital/pediatric obstruction

Question 2

Unilateral obstruction

Answer 2

• 3 phases of RBF changes:
  
  1. First hour: ↑RBF + ↑tubule pressure 2/2 afferent arteriolar vasodilation (prostaglandins, NO)
  2. 3-4 hours: ↓RBF + ↑tubule pressure 2/2 afferent arteriolar vasoconstriction (RAS); tx w/ ACEi
  3. 5 hours: ↓RBF, ↓ tubule pressure 2/2 ↓GFR

Question 3

Bilateral obstruction

Answer 3

• 2 phases of RBF changes
  
  1. First 90 min: modest ↑RBF + ↑tubule pressure 2/2 prostaglandins, NO
  2. >90 min: ↓RBF
• Different from unilateral due to accumulation of vasoactive substances (ANP) in urine

Question 4

Partial obstruction

Answer 4

• In prenatal, may affect glomerular development permanently
• Likely 2/2 microvascular damage in glomeruli
• If <2wks, GFR returns to normal
• 1 mo, recover 30% GFR function
• >2 mos, recover 8% GFR function

Question 5

Pathophysiology of obstruction

Answer 5

• Obstruction disrupts mechanisms allowing for renal concentration of urine:
  
  • Na+ reabsorption from TALH
  • Flow of urea from inner medullary collecting duct
  • Water permeability of collecting duct (aquaporins)

Question 6

Pathologic changes caused by obstruction

Answer 6

• Gross: 6wks: enlarged kidney + cystic dilation
• Micro:
  
  • 7 days: dilated tubules, interstitial edema, dilation of Bowman’s space
  • 12 days: papillary tip necrosis
  • 16 days: thickened BM
  • 5-6wks: glomerular collapse, interstital fibrosis, proliferation of connective tissue

Question 7

Causes of renal obstruction

Answer 7

• Congenital: cystic, UPJ obstruction
• Neoplastic: RCC, Wilms, urothelial, multiple myeloma
• Infx/Inflam: ecchinoccocus, TB, abscess, stricture, retroperitoneal fibrosis
• Metabolic: nephrolithiasis
• Other: trauma, renal artery aneurysm

Question 8

Causes of ureteral obstruction

Answer 8

• Congenital: ureterocele, megaureter, retrocaval ureter
• Neoplastic: cancers
• Infx/inflam: RPF, AAA, TB, stricture
• Metabolic: amyloidosis, stones
• Other: trauma, endometriosis, pregnancy

Question 9

Causes of bladder/urethral obstruction

Answer 9

• Congenital: posterior urethral valves
• Neoplastic: bladder, urethral, prostatic
• Infx/inflam: prostatitis, urethral stricture, phimosis
• Metabolic: amyloidosis, stones
• Other: BPH, neurogenic bladder

Question 10

Most common causes of upper UTO

Answer 10

• Nephrolithiasis
• Extrinsic (e.g., lymphadenopathy)
• Ureteropelvic junction obstruction
• Blood clot

Question 11

Most common causes of lower UTO

Answer 11

• Adult men:
  
  • Benign prostatic hyperplasia
  • Urethral stricture
• Adult women:
  
  • Cervical cancer
  • Uterine prolapse
  • Pregnancy

Question 12

Ultrasonography

Answer 12

• No radiation or IV dyes (safe for peds, preg, renal insuff/failure)
• Differentiate btw. upper/lower UTO (dilated kidney calyx vs. bladder)

Question 13

Retrograde pyelography

Answer 13

• Invasive (cytoscopy + ureteral cath)
• Sensitive for localizing obstruction
• No IV dye (safe in renal insuf/failure)
• Potentially therapeutic (stent bypasses and clears obstruction)

Question 14

CT & MRI Scanning

Answer 14

• Sensitive + elucidate etiology
• IV dyes toxic (iodine in CT, gadolinium in MRI)
• High radiation dose in CT
• Pre-contrast, early/arterial phase, delayed phase (CT)

Question 15

Nuclear renography

Answer 15

• Evaluates functional obstruction
  
  • T_1/2 time used as estimation of obstructin
  • 20 min = obstruction
• Estimates renal split function

Question 16

Percutaneous management

Answer 16

• Image-guided needle inserted into renal pelvis
  
  • External drainage
  • No anesthesia
  • When endoscopic correction impossible
  • Immediate relief (symptomatic, infx)

Question 17

Endoscopic management

Answer 17

• Cytoscopic insertion of ureteral stent
• Internal urinary drainage
• Some degree of general anesthesia
• 90% success

Question 18

Ureteropelvic Junction (UPJ) Obstruction

Answer 18

• From intrinsic, aperistaltic segment (spiral musculature replaced by longitudinal bundles of fibrous tissue)
• Or aberant crossing vessels impinging on UPJ
• Tx:
  
  • Endopyelotomy
    
    • Endoscopic incision of stricture
    • ↓success, potential complications if crossing vessels
  • Pyeloplasty
    
    • removal/interruption of aperistaltic segment + reconstruction of UPJ
    • Gold standard tx

Question 19

Post-obstructive diuresis

Answer 19

• 2/2 bilateral obstruction
• If longstanding + uremia
• Lose concentrating gradient in renal medulla –> 500-600 cc output/hr of urine
• Leads to profound metabolic disarry, hypokalemia
• Requires inpatient management