25-05: Urinary Stone Disease

GENERAL CONSIDERATIONS

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. Uric acid stones may be radiolucent if pure or partially radiopaque if mixed with calcium. Cystine and struvite stones are faintly radiopaque.

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 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 and others), 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 that total calcium intake (diet plus supplementation) not exceed 2000 mg daily. Dietary calcium intake should not be restricted unless excessive (more than 2000 mg daily).

Inadequate hydration is another very important dietary factor in the development of urinary stones for essentially all stone formers. Efforts should be made to avoid dehydration. 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, which typically requires over 3000 mL (100 oz) intake per day. Excess animal protein and salt intake (over 3500 mg daily) as well ...