Prevention of renal stones

What it is

Prevention in urolithiasis has two parts: general measures for anyone who forms stones, and specific advice that depends on the stone analysis. Calcium stones are the most common type, put at 75% in one place and 80% in another, and calcium oxalate is more common than calcium phosphate. Struvite is put at 10-15%, uric acid at about 5-10%, and cystine at about 1%.

Causes and risk

The most common presentation is a calcium oxalate stone with hypercalciuria and normocalcemia. Calcium oxalate can also result from ethylene glycol (antifreeze) ingestion, vitamin C abuse, hypocitraturia and malabsorption such as Crohn disease; after a bowel resection the calcium is not reabsorbed by the gut and a pure oxalate stone forms. Struvite, ammonium magnesium phosphate, follows infection with urease bugs such as Proteus mirabilis, Staphylococcus saprophyticus and Klebsiella that hydrolyze urea to ammonia and alkalinize the urine; a chronic indwelling foley with alkaline urine is classic, and struvite commonly forms staghorn calculi. Uric acid stones follow low urine volume, arid climates and acidic pH, are strongly associated with hyperuricemia such as gout, and are often seen in diseases with high cell turnover such as leukemia. Cystinuria is a hereditary autosomal recessive condition in which the cystine-reabsorbing transporter loses function, which also gives poor reabsorption of ornithine, lysine and arginine. It usually begins in childhood, can form staghorn calculi, and is what a child with stones in the family points to. Primary hyperparathyroidism is a further road: hypercalcemia with hypercalciuria gives renal stones, along with polyuria, bone pain, constipation and neuropsychiatric disturbances.

A short set of key points lines the types up by what each forms in. Calcium oxalate stones are the most common and radiopaque, and are associated with dehydration and high oxalate intake. Calcium phosphate stones are linked to renal tubular acidosis and alkaline urine, and commonly associated with hypercalciuria. Uric acid stones form in acidic urine, at a pH below 5.5, are radiolucent, and are associated with hyperuricosuria, low fluid intake and high purine intake. Struvite stones are caused by urease-producing bacteria such as Proteus, and often result in urinary tract obstruction and recurrent UTIs. Cystine stones are rare, caused by cystinuria, recur often in children, and form in acidic urine.

One drug class is named as predisposing to the stone itself: loop diuretics. That is worth setting beside the thiazides, which are used to prevent calcium stones, since the two groups of diuretic act against each other here.