13. Nephrolithiasis

Question 1

What is nephrolithiatis? Which are the sites of obstruction?

Answer 1

It is the development of stones within urinary tract

Sites of obstruction:

• Ureterovesicular junction (most common site of impaction)
• Calyx of kidney
• Ureteopelvic junction
• Intersection of the ureter and iliac vessels

Question 2

What are the risk factors?

Answer 2

• Low fluid (most common and preventable risk)
• Family history
• Conditions precipitating stone formation (gout, crohn’s, hyperparathyroidism, RTA type 1)
• Medication (e.g. loop diruetics, antacids, chemotherapeutic drugs causing cell breakdown (uric acid stones))
• Male gender
• UTIs (urease-producing bacteria)
• Dietary factors (low calcium and high oxalate intake)

Question 3

What are the types of stones?

Answer 3

• Calcium stones
• Uric acid stones
• Struvite stones (staghorn)
• Cystine stones

Question 4

What are calcium stones?

Answer 4

• Most common - 80-85% of urinary stones
• Composed of calcium oxalate AND/OR calcium phosphate
• Radiodense (visible on x-ray)
• Secondary to hypercalcuria and hyperoxaluria
• Bipyramidal or biconcave ovals

Question 5

What are uric acid stones?

Answer 5

• 2nd most common - 10% of urinary stones
• Due to persistent urine acidic pH (<5.5), hyperuricemia, gout or chemotherapeutic ttt causing cell breakdown
• Radioluscent (not visible on x-ray) –> requires CT, US or IVP for detection
• Flat square plates

Question 6

What are struvite (staghorn) stones?

Answer 6

• 5-10% of stones
• Radiodense (visible on x-ray)
• Due to recurrent UTIs due to urease-producing bacteria (e.g. Proteus, Klebsiella, Serratia, Enterobacter) and facilitated by alkaline urine, which they produce (urea to ammonia)
  
  • The resultant ammonia combines with magnesium and phosphate ==> struvite stones
• Rectangular prisms

Question 7

What are cystine stones?

Answer 7

• Accounts for 1% of urinary stones
• Genetic predispositions (cystinuria)
• Hexagon-shaped crystals
  
  • Poorly visualized

Question 8

How do we prevent nephrolithiasis?

Answer 8

• Increase urine output
  
  • Thiazides
• Diet (avoid purine-rich foods)
• Allopurinole (decreases uric acid excretion)
• Pyridoxine (for primary hyperoxaluria)
• Urinary alkalinization in case of uric acid and cystine stones
• Antibiotics (for struvite stones)
• Acetohydroxamic acid (inhibits urease)
• Captopril, D penicillamine, thioproline (for cystine stone)