About 10% of stones are radiolucent or relatively so, making it difficult to see them on a plain x-ray.
1. Uric acid.
Uric acid comprises about 5% of all urinary calculi. Predisposing conditions may be gout, multiple myeloma and those undergoing treatment for leukemias with cytotoxic drugs where there is a breakdown of nucleic acids. The majority of patients do not have hyperuricemia. A low urinary pH is the most common risk factor, followed by inadequate fluid intake and inappropriate diet such as a high shellfish intake.
Most of these stones will dissolve with alkalinization. Increased fluid intake, diet control, and/or the administration of Allopurinol to reduce uric acid excretion may also be important factors in decreasing recurrences. Occasionally a patient may require a ureteral stent when a kidney is obstructed prior to starting alkaline therapy.
2. Cystine.
These stones are caused by an inborn error of metabolism expressed as either homozygous patients or heterozygous patients. Cystine stones are frequently associated with calcium stones. Medical therapy and prevention dictates a high fluid intake and urinary alkalinization. Diet control through low methionine intake has a limited role. Penicillamine may reduce urinary cystine levels.
3. Xanthine
These stones are also radiolucent and secondary to a congenital deficiency of xanthine oxidase. Treatment and prevention are again through high fluid intake and urinary alkalinization.
4. Struvite
These are calcium magnesium ammonium phosphate stones and are usually associated with urea-splitting urinary tract infections. Foreign bodies such as catheters and neurogenic bladders may predispose to struvite calculi.
5. Others
Triamterene stones may be related to medications such as Dyazide.
Silicate stones may be associated with long-term use of antacids containing silica. Other objects producing renal colic type symptoms can be a blood clot, a renal papilla as in papillary necrosis and foreign bodies such as shotgun pellets.