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
Nutritional factors associated with stone formation
Calcium intake below or highly above the recommended dietary allowance
High sodium intake
Low fluid intake
Low fruit and vegetable intake
High intake of animal-derived purines
Serum studies to order in screening evaluation of stone formers
Electrolytes: sodium, potassium, chloride, bicarbonate
Calcium
Creatinine
Uric acid
When to suspect primary hyperthyroidism
Serum calcium is high or high normal
Predominantly calcium phosphate stones
Elevated urinary calcium
If suspected, get intact PTH
- Mid-range PTH in the face of higher serum calcium (inappropriately normal PTH)
- High or high-normal PTH when vitamin D levels are low
(The diagnosis of primary hyperparathyroidism (PHPT) is usually made by finding a PTH concentration that is frankly elevated or within the normal range but inappropriately normal given the patient’s hypercalcemia)
Calcium phosphate stone composition more likely to be associated with the following medical conditions:
RTA Type 1
Medullary sponge kidney
Primary hyperparathyroidism
Use of carbonic anhydrase inhibitors
Medical Management of Kidney Stones Guidelines:
For pts with calcium stones and relatively high urinary calcium, what should they do with their diet?
Guideline Statement 9:
Clinicians should counsel patients with calcium stones and relatively high urinary calcium to limit sodium intake and consume 1,000-1,200 mg per day of dietary calcium
Target of <2,300 mg sodium daily
Med Mgmt Guideline Statement 10:
Clinicians should counsel patients with calcium oxalate stones and relatively high urinary oxalate to…
…limit intake of oxalate rich foods and maintain normal calcium consumption
Med Mgmt Guideline Statement 13:
Clinicians should counsel patients with cystine stones to limit…
…sodium and protein intake
High fluid intake is particularly important for cysteine stone formers
Need to decrease urinary cysteine concentration to below 250 mg/L
Med Mgmt Guideline Statement 14:
Clinicians should offer ————– to patients with high or relatively high urine calcium and recurrent calcium stones
Thiazide diuretics
(HCTZ, chlorthalidone, indapamide)
May need to supplement potassium as these cause hypokalemia
Should still restrict sodium in the diet
Med Mgmt Guideline Statement 15:
Clinicians should offer ———— therapy to patients with recurrent calcium stones and low or relatively low urinary citrate
Potassium Citrate
Potassium citrate is preferred over sodium citrate as the sodium load may increase urine calcium excretion
Sodium bicarb or sodium citrate can be considered if patient is hyperkalemic
Med Mgmt Guideline Statement 16:
Clinicians should offer allopurinol to pat
