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).
Unlike uroliths predominantly composed of CaOx, which occur more often in males, the majority (81 percent) of compound uroliths
with a calcium oxalate core and a MAP shell occurred in female dogs (Image 1 and Figure 2). The paradox in managing patients
forming uroliths with CaOx and MAP is that attempts to minimize risk factors for MAP urolith formation (such as reducing urine
pH, magnesium and phosphorus) increase the risk for CaOx urolith formation. In this situation, we recommend that emphasis
be placed on minimizing recurrence of CaOx uroliths since CaOx uroliths cannot be dissolved medically. In contrast, MAP uroliths
that form secondary to infections with urease-producing microbes can often be dissolved by medical protocols. For uroliths
containing a core of CaOx surrounded by a shell of infection-induced MAP, it is logical to assume that an initial episode
of CaOx uroliths predisposed the patient to infection-induced MAP uroliths. Therefore, preventative management should include
efforts to eradicate or control recurrent UTI's.
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.
Magnesium ammonium phosphate core: 23 percent of compound uroliths contained a core of MAP (Table 1, p. 10S). As typical of
infection-induced uroliths, the majority of this type of compound urolith occurred in female dogs (83 percent). Other mineral
salts surrounding MAP included calcium oxalate (35 percent), ammonium urate (5 percent), and silica (3 percent). Shells of
sulfadiazine surrounded three MAP uroliths. Dogs that formed these stones had a history of symptomatic treatment of lower
urinary tract signs with trimethoprim/sulfadiazine.
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).
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.
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.