Urinary stone disease is exceeded in frequency as a urinary tract disorder only by infections and prostatic disease. It is estimated to afflict 240,000–720,000 Americans per year. The prevalence of kidney stones has increased to 8.8%, or 1 in 11 Americans, representing a 70% increase over the last 15 years. While men are more frequently affected by urolithiasis than women, with a ratio of 1.5:1, the prevalence of stones in women is increasing. Initial presentation usually occurs in the third through fifth decades, and more than 50% of patients will become recurrent stone formers.
Stone formation requires saturated urine that is dependent on solute concentration, ionic strength, pH, and complexation. There are five major types of urinary stones: calcium oxalate, calcium phosphate, struvite (magnesium ammonium phosphate), uric acid, and cystine. The most common types are those composed of calcium oxalate or phosphate (85%), and for that reason most urinary stones are radiopaque on plain abdominal radiographs. Although pure uric acid stones are radiolucent, uric acid stones are frequently composed of a combination of uric acid and calcium oxalate and thus may be radiopaque. Cystine stones frequently have a smooth-edged ground-glass appearance and are radiolucent.
Geographic factors contribute to the development of stones. High humidity and elevated temperatures appear to be contributing factors, and the incidence of symptomatic ureteral stones is greatest in such areas during hot summer months. Higher incidence rates of stones have also been associated with sedentary lifestyle, obesity, hypertension, insulin resistance and poor glycemic control, carotid calcification, and cardiovascular disease.
Many commonly prescribed medications increase the risk of formation of kidney stones, including carbonic anhydrase inhibitors (topiramate, zonisamide, acetazolamide), systemic corticosteroids (prednisone), antiretroviral protease inhibitors (indinavir), gout medications (probenecid), diuretics (furosemide, bumetanide, torsemide, triamterene), decongestants (guaifenesin, ephedrine), and laxatives (if abused for weight loss). The risk of stones from calcium supplementation is controversial. Thus, if calcium supplementation is medically necessary, it is recommended that the calcium supplement be taken with meals, and restricted to no more than 2000 mg of total calcium intake daily (including dietary sources).
Inadequate hydration is another very important dietary factor in the development of urinary stones. Stone formers should be encouraged to drink enough fluid to keep their urine clear or light-yellow at all times with a goal of at least 2500 mL of urine produced daily. Excess animal protein and salt intake (over 3500 mg daily) as well as restricted dietary calcium intake are other important stone risk factors.
Genetic factors may contribute to urinary stone formation. While approximately 50% of calcium-based stones are thought to have a heritable component, other stone types are better characterized genetically. For example, cystinuria is an autosomal recessive disorder. Homozygous individuals have markedly increased excretion of cystine and frequently have ...