Urolithiasis Flashcards
Most common metabolic abnormality, type of stone, and cause of urolithiasis in the US:
- Metabolic abnormality: hypercalciuria
- Type of stone in industrialized countries: calcium oxalate
- Cause: dehydration
Calcium oxalate stones
Most common renal and bladder stones in adults, children, intestinal bypass, IBD, and renal failure
Most common cause = dehydration
Forms in urine with wide range of pH
Radio-opaque
Calcium oxalate monohydrate = whewellite (crystals are ovals and dumbbells)
Calcium oxalate dihydrate = weddellite (crystals are envelopes and octahedrons)
Calcium phosphate stones
More common in patients with hyperparathyroidism, RTA type 1, medullary sponge kidney, and carbonic anhydrase inhibitor use
Formed in alkaline urine
Radio-opaque
Mineral name = apatite
Crystal shape = powder like and often causes cloudy urine
Uric acid (urate) stones
Normally have normal serum and urine uric acid levels
Most common cause = dehydration
Forms in acidic urine (usually pH <6.0)
Radiolucent (can’t see)
Dissolves with urinary alkalinization
Stone shape = parallelograms, double headed arrows, some in rosettes
Magnesium ammonium phosphate (also called struvite or triple phosphate)
Most staghorn calculi are composed of struvite
Most common cause = UTI
Forms in alkaline urine
Radio-opaque
Dissolves with urinary acidification
Crystal shape = coffin lids
Stones that form in alkaline urine
Struvite
Calcium phosphate
Matrix
Cystine Stones
Cause - cystinuria (usually homozygotes)
Forms in acidic urine
Radiopaque
Dissolves with urinary alkalinization
Crystals = regular hexagons
Matrix Stones
Most common cause = UTI (Proteus)
Forms in alkaline urine
Radiolucent
Ammonium acid urate
Common causes = intestinal malabsorption, UTI, phosphate deficiency,
Radiolucent
Protease inhibitor stones
Stones from precipitated drug - Indinavir, nelfinavir
Radiolucent and not visible on non-contrast CT scan
Forms in urine with pH of 5 or less
Acidifying urine to dissolve stones not practical because of extremely low pH required
List of radiolucent stones
Uric acid (urate)
Xanthine
Matrix
Ammonium urate
Protease inhibitor stones
Silica
Urine chemistry abnormalities that would PROMOTE stone formation
Hypercalciuria
Hypocitraturia
Hyperoxaluria
Hyperuricosuria
Hypomagnesiuria
Xanthinuria
High sodium intake increases stone risk by:
Increasing urinary calcium and decreasing urinary citrate
Organic inhibitors of crystallization
Citrate - decreases calcium stone formation by complexing with calcium and lowering calcium saturation, also directly inhibits calcium crystallization
Urea - decreases uric acid stone formation by increasing the solubility of uric acid (no influence on calcium stone formation)
Others: nephrocalcin, Tamm-Horsfall protein, calgranulin, GAGs, bikunin and uropontin (a form of osteopontin)
Inorganic inhibitors of crystallization
Pyrophosphate - no way to increase this in the urine
Magnesium - increases solubility of calcium, phosphate, and oxalate
Trace elements, especially zinc
How UTI Increases Stone Risk
- UTI causes hypocitraturia
- Urease producing organisms split urinary urea into ammonia and bicarbonate, which alkalized urine and increases the risk of stones formed in alkaline urine, especially struvite
- UTI may decrease ureteral peristalsis
Urease producing bacteria
Proteus
Klebsiella
Serratia
Staphylococcus
Morganella
Providencia
Enterobacter
Stones often associated with UTI
Struvite
Matrix
Carbonate apatite
Ammonium urate
Acidosis increases stone risk.
Acidosis causes the following changes in urinary composition:
Increased urine calcium
Increased urine phosphate
Decreased urine citrate
Prolonged acidosis causes bone demineralization, which increases calcium delivery to the kidney and results in hypercalciuria
Medications that may lead to urolithiasis
Vitamin C - metabolized to oxalate, increases urinary oxalate
Vitamin D (high doses) - increases calcium absorption, which increases calcium to kidney
Triamterene (diuretic) - precipitates in urine and forms radiolucent stones
Protease inhibitors (Indinavir and nelfinavir) - precipitate and forms radiolucent stones
Furosemide - increases calcium excretion in urine
Acetazolamide - carbonic anhydrase inhibitor creases a renal tubular acidosis and increases risk of calcium stones.
-Can be used to prevent uric acid and cystine stones (when citrate does not adequately alkalinize urine, it can be added to increase the urine pH)
Topiramate - stones composed of calcium phosphate
Zonisamide
Type I (distal) renal tubular acidosis
Impairment in hydrogen ion secretion in the distal tubule, resulting in a persistently high urine pH (> 5.5) and systemic acidosis
Plasma bicarbonate is frequently < 15 mEq/L (15 mmol/L), and hypokalemia, hypercalciuria, and decreased citrate excretion are often present
Nephrocalcinosis and nephrolithiasis are possible complications of hypercalciuria and hypocitraturia if urine is relatively alkaline
Oral medication for urinary alkalinization
Citrate and bicarbonate prevent stones by correcting acidosis, increasing urinary pH and increasing urinary citrate
Oral citrate is changed to bicarb ny the liver (doesn’t actually deliver citrate to kidney)
Most common stone compositions in patients with gout?
Calcium oxalate monohydrate (45%)
Uric acid (52%)
Medical conditions that increase risk of overall stone disease
Obesity
HTN
Diabetes
Hyperthyroidism
Gout
RTA type I
Bone disease
Primary hyperparathyroidism
Malabsorptive GI states
