Specific recommendations Calcium oxalate core: The most common type of compound urolith identified at the Minnesota Urolith Center in 2002 contained a core of CaOx monohydrate or CaOx dihydrate or both (60 percent).

Figure 4: Photograph of a compound urocystolith removed from a 5-year-old spayed female Miniature Schnauzer. The nucleus was composed of ammonium urate. The shell was composed of magnesium ammonium phosphate.

In 18 dogs, a shell of calcium phosphate surrounded a core of CaOx. We hypothesize that excessive calcium excretion was a primary abnormality in these dogs. Since control of urine calcium excretion is emphasized in the management of CaOx uroliths, we use the same principles designed for uroliths composed entirely of CaOx to treat this type of compound urolith. Some compound uroliths contain a core of CaOx and a shell of urate salts. Consumption of diets moderately reduced in protein that promotes formation of alkaline urine commonly recommended to manage CaOx uroliths (such as Prescription Diet Canine u/d-canned; Hill's) are also recommended for prevention of urate uroliths.

Calcium phosphate (primarily carbonate apatite) was the most common mineral found in the outer layers surrounding cores of MAP (56 percent). This is predictable because MAP and calcium phosphate uroliths share several common risk factors. For example, the solubility of both salts is reduced in alkaline urine. Also increasing urine phosphate concentration increases the risk of formation of both types of minerals. Precipitation of MAP and calcium phosphate are promoted by urinary tract infections with urease producing microbes that hydrolyze urea into ammonia and carbonate; ammonia is a component of MAP uroliths and carbonate is a component of calcium phosphate (carbonate apatite) uroliths. Why these salts sometimes form distinct layers in some compound uroliths, but become mixed throughout other uroliths without forming distinct layers (mixed uroliths) has not yet been defined.

Fortunately most recommendations to minimize MAP urolith recurrence also minimize formation of calcium phosphate. Treatment with appropriate antimicrobics to eradicate or control of UTI's caused by urease producing microbes is essential. Reducing dietary protein to reduce the urine concentration of urea will minimize the quantity of ammonia generated by microbial urease. In addition, reduction of dietary protein also minimizes renal medullary urea and thus promotes polyuria.

What about diets designed to acidify urine? On one hand, acidification of urine would minimize the quantity of ionic phosphate available to form MAP and calcium phosphate. On the other hand, however, chronic acidification would promote urine calcium excretion and thus increase the risk for formation of uroliths containing calcium. Attempts to acidify urine of dogs with pre-existing MAP uroliths may be one factor that helps to explain why CaOx surrounded 35 percent of uroliths with a core of MAP (see previous section for management of CaOx uroliths with a CaOx core).

Silica core Compound uroliths with a silica core were primarily retrieved from male dogs (88 percent; Figure 3). The most common mineral associated with silica was CaOx (80 percent). Perhaps one common denominator linking these two minerals is consumption of plant-based foods that contain more silica and oxalic acid that animal-based foods. In addition, one mineral may serve as a template for precipitation for the other.

On the basis of logic, protocols to minimize silica urolith recurrence have been devised. Since diets containing substantial quantities of corn gluten feed or soybean and rice hulls, or both, have been associated with silica uroliths and may also have increased quantities of oxalic acid, we recommend that diets containing substantial quantities of plant proteins be avoided. When CaOx or urate surrounds a core of silica, strategies for prevention of the outer CaOx layer are usually compatible with a reduction in silica urolith formation. When compound uroliths with a core of silica have an outer layer of MAP, urinary tract infection should also be controlled.

Calcium phosphate core Three percent of compound uroliths contained a core of calcium phosphate. They were surrounded either by shells of CaOx or MAP. We managed these uroliths in a fashion similar to uroliths with cores off CaOx or MAP and outer layers of calcium phosphate.

Ammonium urate core Only 5 percent of compound uroliths contained a core of ammonium urate. Of those surrounded by a shell of MAP, 70 percent occurred in females (Figure 4). It is logical to assume that the original urate urolith predisposed the patient to UTI with urease-producing microbes, which in turn, promoted formation of MAP. Strategies to prevent recurrence of this type of compound urolith should include control of UTI's in addition to protocols designed to prevent recurrence of urate uroliths.

Of urate uroliths surrounded by a shell of CaOx, 80 percent occurred in males. Consumption of diets moderately reduced in protein that promote formation of alkaline urine commonly recommended to manage CaOx uroliths (such as Prescription Diet Canine u/d-canned; Hill's) are also recommended for prevention of urate uroliths.

Xanthine core The two dogs with compound uroliths containing a core of xanthine were both being treated with allopurinol to minimize recurrence of urate urolithiasis.

Unfortunately, as the urine concentration of uric acid declines in response to allopurinol, the concentration of xanthine increases. The magnitude of xanthinuria increases in proportion to the quantity of purines in the diet and the dose and frequency of allopurinol administration. Prevention of recurrence encompasses reduction of dietary purines and discontinuing or reducing the dose of allopurinol.