7. Urolithiasis

Question 1

Definition of urolithiasis

Answer 1

More commonly known as kidney stones/renal calculi

Question 2

General causes of urolithiasis

Answer 2

1. Primary causes:
   - Supersaturation of the components of the stone
   - Low urine volume
2. Secondary causes:
   - Urinary tract infection (crystalline material able to encrust on a necrotic focus on the mucosa)
   - Indwelling catheter or foreign body in bladder
   - Vitamin A deficiency producing squamous metaplasia of the upper urinary tract mucosa
   - Altered urinary pH (low pH predisposes to uric acid stones; high pH predisposes to triple stones)

Question 3

Types of renal calculi

Answer 3

1. Calcium stones (65-70%)
2. Magnesium ammonium phosphate stones (15%)
3. Uric acid stones (6%)
4. Cystine stones (3%)
5. Indinavir stones

Question 4

Calcium stones

Answer 4

Composed of calcium oxalate +/- calcium phosphate

- Radioopaque

Question 5

Predisposing factors & pathogenesis for calcium stones

Answer 5

1. Hypercalciuria (hyperparathyroidism, diffuse bone disease, vitamin D intoxication, milk-alkali syndrome, sarcoidosis, renal tubular acidosis of Cushing syndrome, idiopathic)
2. Hyperoxaluria (hereditary, vegetarians due to high oxalate content in diet)
3. Hypocitraturia (associated with acidosis & chronic diarrhoea)
4. Hyperuricosuria (promotes nucleation of calcium oxalate)

Question 6

Magnesium ammonium phosphate stones (15%)

Answer 6

Triples stones or struvite stones

Question 7

Predisposing factors & pathogenesis for magnesium ammonium phosphate stones (15%)

Answer 7

1. Typically follows infection by urea-splitting bacteria (e.g. Proteus) which convert urea to ammonia, raising the pH of urine which promotes the precipitation of triple stones
2. As urea content in urine is usually very high, such infections typically gives rise to the precipitation of very large stones → staghorn calculi (lodged within the renal pelvis)

Question 8

Uric acid stones (6%)

Answer 8

Composed of uric acid

- Radiolucent

Question 9

Predisposing factors & pathogenesis of uric acid stones

Answer 9

1. Hyperuricemia (gout, leukemia)

2. Low urinary pH (<5.5)

Question 10

Predisposing factors & pathogenesis of cystine stones

Answer 10

AR genetic defect in the transport of dibasic amino acids (cystine, ornithine, arginine, lysine)

Question 11

Predisposing factors & pathogenesis of indinavir stones

Answer 11

Use of indinavir (anti-retroviral drug)

Question 12

Pathological effects & complications of urolithiasis

Answer 12

1. Urinary tract obstruction
2. Ulceration, bleeding & fistula formation
3. Urinary colic

Question 13

Treatment for urolithiasis

Answer 13

1. Extracorporeal shockwave lithotripsy (ESWL)
2. Percutaneous nephrolithotripsy (PCNL)
3. Ureteroscopic lithotripsy