Nephrolithiasis Flashcards
Describe the epidemiology of stone disease, and what subpopulations are at high risk for different types of stones.
- 12% of men; 6% of women; increasing rate
- Less prevalent in African-Americans
- More common during summer months (hydration related)
- In U.S. = more common in SE
- More common in middle age
- Recurrent: 50-60% within 10 years
- Both genetic and environmental factors
Most stones:
o Low urine volume
o Low Ca2+ diet
Describe the associated diseases with each type of stone
Oxalate stones: o Hypertension (especially in women) o Associated diseases: inflammatory bowel disease, short bowel syndrome, hyperparathyroidism, polycystic kidney disease, medullary sponge kidney o Medications: high dose Vit C and Ca2+ supplements
Calcium stones
o High sodium and protein diet
o Associated diseases: Sarcoid
Uric acid stones
o High sodium and protein diet
o Low urine pH
o Associated diseases: metabolic syndrome
Calcium phosphate
o High urine pH
o Associated diseases: hyperparathryroidism, Type I RTA
o Medications: Carbonic anhydrate inhibitors
Struvite stones
o High urine pH
o UTI’s
Differentiate the fundamental pathologic differences between initiation of calcium oxalate stones and calcium phosphate stones.
Calcium oxalate
o May have nidus of uric acid or calcium phosphate
“Randall’s Plaques” = initiators
• White deposits on papillae
• Interstitial deposits of CaP on BM of thin loops of Henle, with thin layers of protein matrix
o Calcium oxalate deposits on plaques = grow
o Penetrate into uroepithelium
o Number of plaques = associated with higher urinary Ca2+ and lower urinary volumes
Calcium phosphate (“brushite” stones)
o Crystals deposit in medullary collecting ducts (“Intratubular” crystals)
o Result: damaged and scarring of the renal papillae
Identify the three major physicochemical factors in stone formation
- Supersaturation
a. Changes with pH, presence of inhibitors and promoters in urine
b. Urine volume is important - Urine pH
a. Determines solubility
b. High pH → Calcium phosphate stones
c. Low pH → Uric acid - Inhibitors:
a. Citrate: chelates urinary Ca2+ and inhibits Ca2+ crystal growth by aggregation
b. Pyrophosphate
c. Proteins
d. Glycosaminoglycans
Describe calcium oxalate stones
o Most common type
Causes:
• Most = idiopathic (30-60%)
• Primary hyperparathyroidism (10%)
• Hypocitraturia
• Hyperoxaluria (primarily genetic; could also be dietary or enteric: short bowel syndrome, IBS causing hyper-absorption)
• Hyperuricosuria (heterotopic calcification)
• Other: ADPKD, medullary sponge kidney, Vitamin C
Oxalate absorption:
• Apical intestine transporter (Slc26a6) regulates serum oxalate and urinary oxalate excretion
Oxalobacter formigenes:
• GN anaerobe in 60-80% adult feces
• Metabolizes oxalate in gut
• Affected by antibiotics and dietary oxalate
• More prevalent in non-stone formers
Dietary oxalate (the 8) • Spinach • Rhubarb • Beets and beet greens • Black and green tea • Chocolate/cocoa • Some nuts and seeds • Soybeans and soy foods (not processed with Ca2+) • Potatoes
Describe calcium phosphate stones
o Associated with Type I RTA or hyperparathyroidism
Describe uric acid stones
(10% of stones)
o Radiolucent on x-ray
Associated with high uric acid levels, gout
• Xanthine stones = with use of Allopurinol
Associated with metabolic syndrome
• Patients have decreased NH4+ excretion → low urine pH
o Seen with urine pH < 6.0
o Treatment: alkalization of pH with Potassium citrate (goal > 6.5)
Describe struvite stones
(AKA “Triple Phosphate”)
o Special type of calcium stones
o Due to UTI with urea splitting organism (ex: Proteus, Providencia, Klebsiella, Pseudomonas, enterococci)
o Grow rapidly = very large stones
o May be asymptomatic or have vague flank pain (no acute renal colic)
o Very alkaline urine: pH >7.0
o Coffin-lid crystals in urine
o Treat: surgical intervention (not respond to metabolic therapies)
Describe cystine stones
Uncommon ( female
• 1st stone in 2nd or 3rd decade
Diagnosis:
• Hexagonal crystal appearance
• Cyanide-nitroprusside test (measures urine cysteine excretion; positive if >75 mg/L)
Treatment:
• High fluid intake → Decrease urinary cystine concentration to 7
• Moderate sodium and protein restriction
• Cysteine-binding drugs (Tiopronin, D-penicillamine Captopril) but side effects
Diagnosis for kidney stones
Labs: electrolytes (K+, HCO3-, Ca2+, PO4) to rule out RTA type I or hyperkalemia (hyperparathyroidism); CBC for infection
Urinalysis: microhematuria (gross hematuria may occur); crystal appearance
Differential: pyelonephritis (also have pyuria and fever, not sudden onset), renal papillary necrosis, renal artery or vein occlusions
Imaging:
• X-ray may show stone (only if radiopaque)
• CT scan with contrast (definitive study; 98% PPV, 95% NPV)
Prognosis for stone passage:
o less than 0.5 cm: almost always pass
o 0.5-1.0 cm: may pass (50% spontaneously pass)
o greater than 1 cm do not pass, need intervention to eliminate
Stone treatment
If likely to pass:
• Hydration with IV fluids
• At home: oral fluids (>2 L/day), pain medications, strain urine for stone
Medical expulsive therapy (MET)
• Medications dilate ureter to help stones pass
• Uses alpha-blockers (tamsulosin)
• Side effect: lower BP
Hospitalization:
o Coexistent UTI with obstruction
o Stones > 1 cm if require intervention
o Obstruction of both kidneys or unilaterally functioning kidney resulting in kidney failure
o If patient can’t tolerate pain or keep fluids down
Describe the diagnostic tests for recurrent calcium stone disease
o Stone analysis
o Bicarbonate, phosphorus, calcium, uric acid levels
o Intact PTH level, vitamin D level
UroRisk profile:
• 24 hr urine collection measuring urine concentrations of Na+, Ca2+, phosphorus, uric acid, oxalate, citrate, creatinine, pH and urine volume
• Defines which solutes are high = could be contributing
• Sees if lack of inhibitor present
• Checks acid-base status
Describe the specific therapies for recurrent calcium stone disease, uric acid stones, cystine stones, and struvite stones
Calcium stones:
• Hypercalciuria; sodium restriction, thiazide diuretics (increases Ca2+ reabsorption)
• Hypocitraturia: potassium citrate
• Hyperoxaluria: oxalate restriction
• Nidus of uric acid: allopurinol (lowers serum uric acid)
• ***Important not to restrict dietary Ca2+ because it binds oxalate in gut, preventing absorption
Uric acid stones:
• Urine alkalinization (K+ citrate) → increase urine pH >6.0
• Low purine diet
• Allopurinol
Cysteine stones:
• Increase fluids (Goal >3L) → decrease cysteine in urine to 7.0
• Cysteine-binding drugs (Penicillamine, Thiola, Captapril)
Surgical therapies for stones
Indications:
• Struvite (infection) stones
• Stones that don’t pass in ureter = cause persistent obstruction
Methods:
Ureteroscopy (URS) = destroys stones in ureter
• YAG laser
• Ultrasonic
• Electrohydraulic lithotripsy
Extracorporeal shock wave lithotripsy (ESWL) = pulverizes stones in upper tracts; also pelvocalcyceal stones
Percutaneous nephrolithotomy (PCNL) = pelvocalcyceal stones
