Calcareous stones result from hypercalciuria, hyperoxaluria and hypocitraturia. Hypercalciuria and hyperoxaluria render urine supersaturated in respect of calcium salts; citrate makes calcium oxalate more soluble and inhibits its precipitation from solution.
Noncalcareous stones occur most commonly in the presence of urea-splitting organisms which create conditions in which magnesium ammonium phosphate (struvite) stones form. Urate stones form when urine is unusually acid (pH < 5.5).
Management. Recurrent stone-formers should maintain a urine output exceeding 2.51/d. Some benefit from restricting dietary calcium or reducing the intake of oxalate-rich foods (rhubarb, spinach, tea, chocolate, peanuts).
• Thiazide diuretics reduce the excretion of calcium and oxalate in the urine and reduce the rate of stone formation.
• Sodium cellulose phosphate (Calcisorb) binds calcium in the gut, reduces urinary calcium excretion and may benefit calcium stone-formers.
• Allopurinol is effective in those who have high excretion of uric acid in the urine.
• Potassium citrate, which alkalinises the urine, should be given to prevent formation of pure uric acid stones.
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