Recently a colleague asked us for advice about how to prevent recurrence of a urolith that contained a nucleus of 100 percent
calcium oxalate (CaOx) and a distinct outer layer of 95 percent magnesium ammonium phosphate (MAP) and 5 percent calcium phosphate
(Image 1 and Figure 2, p. 12S). The urolith was surgically removed from the urinary bladder of a 10-year-old spayed female
Yorkshire Terrier. Do you know the name used to classify this complex type of urolith? How would you manage this case?
Image 1: Ventrodorsal survey radiograph of the abdomen of a 10-year-old spayed female Yorkshire Terrier. Note the different
radiographic density of the compound urocystolith.
What are compound uroliths?
Although some uroliths are composed of only one (100 percent) mineral, most contain a predominant mineral (>70 percent) mixed
with lesser quantities of other minerals. If a single mineral does not comprise at least 70 percent of a urolith, and two
or more minerals are mixed with each other, it is designated as a mixed urolith.
On occasion different minerals are separated into distinct bands or layers. If the core or center of a urolith is at least
70 percent one mineral type (e.g. CaOx), and is surrounded by one or more layers primarily (>70 percent) of a different mineral
(e.g. MAP), is called a compound urolith (Image 1). The Yorkshire described above had a compound urolith.
Compound uroliths comprised approximately 8 percent of the uroliths submitted to the Minnesota Urolith Center in 2002 (Table
1). Compound uroliths form because factors initially promoting precipitation of one type of mineral have been superceded by
factors promoting precipitation of a different mineral.
Table 1: Distribution of 1500 Canine Compound Uroliths
For example, antibiotics and urine acidifiers are used to manage infection-induced MAP uroliths. The antibiotics may eradicate
or suppress microbial urease, reducing precipitation of MAP. However, acidemia associated with urinary acidifiers may promote
hypercallciuria, resulting in a surrounding shell of calcium CaOx or calcium phosphate.
Likewise, we have observed shells of sulfadiazine surrounding some uroliths (e.g., MAP, CaOx) after empirical administration
of sulfonamide antimicrobics to patients with signs of lower urinary tract disease.
Some minerals may serve as a template for deposition of other minerals. This phenomenon may explain why CaOx uroliths occasionally
have a nidus of silica and vice versa. All uroliths predispose patients to bacterial urinary tract infection (UTI). If UTI
by microbes that produce urease persist, there is an increased risk that MAP will precipitate over existing metabolic uroliths
(e.g., CaOx, calcium phosphate, urate, silica etc.).
Eliminating compound uroliths
What protocols can be used to eliminate compound uroliths?
Because risk factors that predispose to precipitation of different minerals in compound uroliths are often complex, designing
effective medical protocols to manage them can be a unique challenge.
One strategy is to design protocols to dissolve the outer layer first. Serial survey radiographs may be helpful in monitoring
the effectiveness of this method. Once the outer layer of different radiographic density disappears, and there is no further
reduction in urolith size, medical therapy can be adjusted to dissolve different minerals in the inner layers.
Figure 2: Photograph of the compound urocystolith removed from the dog described in Image 1. The nucleus was composed of calcium
oxalate. The shell was composed primarily of magnesium ammonium phosphate.
In some cases, we have reduced the size of compound uroliths by dissolving their outer layers, followed by removing the remaining
portion by voiding urohydropropulsion. Symptomatic compound uroliths that are refractory to medical protocols should be removed
However, in some patients with asymptomatic uroliths, we have chosen a nonsurgical "wait and watch" strategy.
What steps should be followed once compound uroliths have been removed? In the absence of clinical evidence to the contrary,
we recommend prevention protocols principally designed to minimize recurrence of minerals that comprised the nucleus, rather
than the shell, of compound uroliths. This involves the concept of heterogeneous nucleation.
What is heterogeneous nucleation? Briefly, greater concentrations of lithogenic minerals are required for uroliths to precipitate
in the absence of solids in the lumen of the urinary tract (homogenous nucleation) than are required for uroliths to precipitate
around a pre-existing solid (e.g., suture material, catheters, and even pre-existing uroliths of a different mineral type).
Figure 3: Photograph of a compound urocystolith removed from an adult male neutered mixed breed dog. The nucleus was composed
of silica. The shells were composed of layers of struvite with varying quantities of calcium phosphate.
In the context of compound uroliths, logic suggests that the initial core composed of one mineral type and formed by homogeneous
nucleation contributed to the formation of outer layers of a different mineral type formed by heterogeneous nucleation. Therefore,
minimizing risk factors for precipitation of minerals found in the core would eliminate heterogeneous nucleation and thus
would minimize precipitation of minerals found in the outer layers of the urolith.
Excessive concentration of minerals in urine is a prerequisite for urolith formation. It follows that increased water intake
would logically lead to reduction in urine concentration of lithogenic minerals, and thus minimize recurrence of all types
of uroliths. In addition to reducing the concentration of lithogenic minerals, formation of large volumes of less concentrated
urine decreases the risk of urolithiasis by increasing the frequency of micturition and thus the frequency that crystals would
To minimize formation of concentrated urine, we recommend feeding high-moisture canned foods. Alternatively, water can be
added to dry diets with the goal of achieving a urine specific gravity value of <1.020. Although some specific diuretics may
be of value in managing certain types of uroliths, in general we avoid indiscriminate use of diuretics because of their propensity
for adverse effects (e.g., dehydration, hypokalemia, hypercalcemia and increased urinary excretion of some lithogenic minerals).