P. 625

J. Bartges1
1The University of Georgia, Athens, GA, USA 30606
Objectives of presentation:
Following this presentation, the attendee should be able to
- describe minerals that form uroliths in dogs and cats
- describe mechanism(s) of urolith formation
- describe management of urolithiasis including medical dissolution, minimally invasive procedures, and preventative measures
Formation of uroliths is not a disease, but rather
a complication of several disorders.1 A common denominator of urolithiasis is that urine can from time
to time be oversaturated with one or more crystal precursors resulting in formation of crystals. Urolith formation, dissolution, and prevention involves complex physical processes including: 1) supersaturation resulting in crystal formation, 2) effects of inhibitors
of crystallization and inhibitors of crystal aggregation and growth, 3) crystalloid complexors, 4) effects of promoters of crystal aggregation and growth, and 5) effects of noncrystalline matrix.2 Assessment of urolith formation risk and response to therapy is dif cult and has been based on results of epidemiological studies, measurement of urinary mineral concentrations, 3,4
and urinary pH. Because supersaturation of urine
with stone-forming substances is necessary for
stones to form, measurement of urine saturation is
a more accurate means of assessing risk of stone formation, Determining the relative supersaturation (RSS) of a urolith-forming substance in urine is one technique.5 RSS values are limited by the fact that the constants used for these calculations have not been measured in the patient’s urine and it may overestimate activity of different minerals, and tend to underestimate risk of urolith formation. Activity product ratios (APR)
are another method although an exact measurement
of supersaturation is not obtained. APR does not eliminate errors associated with effect of unknown factors; however, since the same urine sample is analyzed before and after equilibration with seed crystals, the same type of error occurs in evaluation
and errors cancel. APR overestimates undersaturation, underestimates supersaturation, and correctly measures saturation. One limitation of APR determination is the
assumption that urine has reached the stability for the salt following incubation.6 Limited studies utilizing urine saturation testing have been performed in veterinary medicine, and few have been performed in dogs or
cats that are urolith-formers and no studies exist that compares estimates of urinary saturation with recurrence rates of uroliths. Urinary supersaturation represents a risk for urolith formation, but there is overlap between urolith-forming animals and healthy, non-urolith-forming animals.7
- Urinary saturation is the most important, but not the only, driving force for crystallization and urolith formation
- Several methods exist for estimating urinary saturation; however, none of them adequately describe what is occurring naturally in the biological system (urinary tract)
- Determination of RSS and APR give different results and information. Determination of RSS is a valuable and reasonably reliable technique for estimating urinary saturation; however, it (a) is in uenced by concentration of analytes measured, which, in turn,
is in uenced by urine volume, and (b) it does not account for urinary constituents that are not measured including the in uence of inhibitors. Because it is in uenced by urine volume, methods designed
to increase urine volume (e.g. feeding canned
foods, administration of diuretics, and stimulating water consumption by increased levels of dietary sodium) would be expected to lower the relative supersaturation; however, clinical studies in urolith- forming dogs and cats are lacking. APR does not give an exact estimation of the supersaturation; however, because a patient’s urine is used pre- and post- incubation with seed crystals, this technique does account for unmeasured urinary constituents and the in uence of inhibitors.
- Medical dissolution of uroliths is accomplished by inducing a state of undersaturation of urine with the minerals that formed the uroliths
- Medical prevention of uroliths is accomplished by induced a state of undersaturation of urine or at least a state of saturation at the lower end of the metastability limit
- Despite use of estimates of urinary saturation, there are no published studies in urolith-forming dogs and cats that validate their prediction of urolith recurrence.
- Means to decrease urinary saturation include increasing urine volume thereby decreasing
An Urban Experience

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