25: Nephrolithiasis

Question 1

Causes of hypercalciuria

Answer 1

• Bone resorption (overactive osteoclasts)
  
  • Hyperparathyroidism
  • Hyperthyroidism
  • Cancer
  • Renal tubular acidosis
  • Corticosteroids (prednisone)
• Vitamin D excess
  
  • Vitamin D25
  • Vitamin D1,25
  • Inflammatory conditions
    
    • Sarcoidosis
    • Lymphoma
• Idiopathic
  
  • Gut (absorptive)
  • Bone (↑ turnover)
  • Kidney (renal leak)

Question 2

Calcium oxalate stones

Answer 2

• 70% of stones
• Normally:
  
  • 10% of dietary oxalate absorbed in colon
  • 90% bound by Ca and passed via stool
• Risk factors for CaOx stone formation
  
  • Hypercalciuria
  • Hyperuricosuria (“salt out” CaOx)
  • Hypocitraturia (**citrate **inhibits calcium crystal formation)
  • Hyperoxaluria
  • Low urine volume
  • Medical conditions
    
    • Malabsoprtion (IBD, gastric bypass –> ↑oxalate uptake in colon)
    • Obesity/gout
    • Genetic dz (hyperoxalosis)
• Envelope appearance on UA/micro

Question 3

Calcium phosphate stones

Answer 3

• 10-15% of stones
• Coffin lid appearance on UA/micro
• Risk factors:
  
  • Hypercalciuria
  • Alkaline urine pH (>7) (CaP forms under basic conditions)
  • Medullary sponge kidney (cystic kidney dz)
• CaP crystals in the form of:
  
  • **Apatite **(bones/teeth)
  • Brushite (Ca monohydrogen phosphate): large stone, requires surgical removal

Question 4

Uric acid stones

Answer 4

• 10-15% of stones
• 2nd most common mineral type
• Diamond shape on UA/micro
• Risk factors:
  
  • Acidic urine
  • Gout
  • Hyperuricemia
  • Metabolic syndrome
  • Malignancies

Question 5

Struvite stones

Answer 5

• “Staghorn” stones
• MgNH4PO4 + CaCO3
• From urease-producing bacteria
  
  • Proteus
  • Haemophilus
  • Klebsiella
  • Ureaplasma urealyticum
• ​Risk factors:
  
  • Women
  • Chronic urinary obstruction
  • Neurologic dysfunction

Question 6

Cystine stones

Answer 6

• AutR defect in tubular transporters of dibasic AA’s
  
  • Cystine
  • Ornithine
  • Arginine
  • Lysine
• ​Cystine insoluble in acidic urine
• Hexagonal urine crystals (less commonly, Staghorns) on UA/micro

Question 7

Drug-induced stones

Answer 7

Indinavir, bactrim, allopurinol

Question 8

Stone inhibitors

Answer 8

Negatively charged molecules (ionic or macromolecular/protein-based) that inhibit crystal nucleation by adsorbing onto crystal surface and interfering with lattice formation.

Question 9

Nephrolithiasis presentation

Answer 9

• Acute, colicky flank pain radiating to groin
• Exam: CVA tenderness may be present
• Hematuria (90% of cases; absence does not rule out stones)
• Less often presentations:
  
  • Silent ureteral obstruction
  • Unexplained persistent UTI
  • Painless hematuria

Question 10

kidneys-ureters-bladder (KUB) radiograph

Answer 10

• To detect obvious abnormalities of the urinary system, such as kidney stones
• Sensitivity 45-85%
• Use in treatment planning (stone localization, diameter)
• Preferred method of follow-up for radio-opaque stones (visible)
• Cannot detect small stones

Question 11

Renal ultrasound

Answer 11

• Can image both radiolucent and radiopaque stones
• Low radiation exposure (can use in pregnant patients, children)
• High specificity (82-100%)
• Quantifying size for small stones (<5mm) poor

Question 12

Helical non-contrast CT urogram

Answer 12

• Study of choice (sens: 95-98%, spec: 98-100%)
• Can visualize uric acid stones by gray-scale methods
• Help dx UT abnormalities predisoposing to stones
• R/o conditions which may masquerade as renal colic (abdominal pain commonly caused by kidney stones)

Question 13

Likelihood of passage of ureteral stones

Answer 13

• If <2mm, 8 days to pass. Intervention unnecessary.
• If 4-6mm, 22 days to pass. Need for intervention in 50% of cases.
• If >6mm, intervention necessary.

Question 14

Main causes of stone formation

Answer 14

• Hypercalciuria
• Hyperoxalauria
• Alkalitic urine pH
• Hyperuricosuria
• UTI
• Low urine citrate

Question 15

Management: First time stone

Answer 15

• Risk factor assessment:
  
  • Occupation/environment
  • Family Hx
  • Diet: protein, purines, Na, fluid, Ca, oxalate
  • Rx: triamterene, sulfonamides, carbonic anhydrase inhibitors
• Lab evaluation:
  
  • Creatinine, HCO3, Ca, Phos, PTH
  • Urine analysis, Cx
• Referral to stone clinic if:
  
  • Children
  • Solitary kidney
  • Struvite stones
  • Abn renal funct
  • RTA

Question 16

Management: Recurrent stones or high-risk pt

Answer 16

• Urine (24 hour) collection for:
  
  • Ca
  • Oxalate
  • Citrate
  • Uric acid
  • pH
  • Cystine
• Urine cx if struvite/staghorn

Question 17

Nephrolithiasis treatment

Answer 17

• Decrease ionic concentrations
  
  • Urine calcium: thiazides
  • Urine oxalate: diet
  • Uric acid: diet, allopurinol
• Alter urine characteristics
  
  • Primary hyperparathyroidism: parathyroidectomy
  • Distal RTA: NaHCO3 therapy
• Alter urine supersaturation
  
  • ↑water (single most important therapeutic intervention)
  • ↑citrate: urocit K

Question 18

Urinary citrate

Answer 18

Forms strong complexes with Ca in urine –> decreased free Ca ion –> lowered urine CaOx and CaP saturation

Question 19

Risk factors for uric acid stones

Answer 19

• Low urine pH
• Obesity and metabolic syndrome
• Hyperuricosuria (rx allopurinol)

Question 20

Captopril

Answer 20

Makes cystine complexes 200x more soluble in urine

Question 21

Extracorporeal Shock Wave Lithotripsy (ESWL)

Answer 21

• For proximal stones <2cm
• Contraindications: bleeding tendencies, pregnancy, uncontrolled HTN, UTI, morbid obesity
• Long-term risks: renal insufficiency, HTN

Question 22

Ureteroscopic removal

Answer 22

• Fragmentation of stone via laser, electrohydraulics or ultrasound
• Most stones can be treated

Question 23

Percutaneous nephrolithotomy

Answer 23

• Nephroscope inserted through kidney capsule
• Indicated for large proximal stones