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940 Small Animal Clinical Nutrition

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Figure 46-10. This diagram depicts a simplified explanation of Figure 46-11. This diagram illustrates effects of differing degrees of
effects of different levels of urine saturation (oversaturation, super- urine saturation on risk of urolith formation. Increasing concentra-
saturation [metastable] or undersaturation) on formation of uroliths. tions of urolith-forming substances result in metastable (supersatu-
In this example, the liquid is tea and the mineral or crystalloid is rated) and eventually, oversaturated urine. Crystal growth and aggre-
sugar. As the amount of added sugar increases, the solution (tea) gation may occur in the metastable zone. Presence of a nidus pro-
goes from being undersaturated (all the sugar completely dissolves) motes nucleation (i.e., heterogeneous nucleation) and subsequent
to being oversaturated (some of the sugar precipitates). Similar phe- crystal formation. Inhibitors in urine may impede or prevent crystal-
nomena occur with mineral salts in urine. However, just as there are lization in the metastable zone. Spontaneous or homogenous nucle-
different outcomes when dissolving sugar in iced vs. hot tea, other ation occurs, however, when concentrations of urolith-forming sub-
factors (e.g., temperature, pH, crystalloid inhibitors) also affect satu- stances increase to the point of oversaturation. After oversaturation
ration of urine with crystalloids. (Adapted and modified from Bartges occurs, inhibitors of crystal formation generally are ineffective.
JW, Osborne CA, Lulich JP, et al. Methods for evaluating treatment (Adapted from Bartges JW, Osborne CA, Lulich JP, et al. Methods
of uroliths. Veterinary Clinics of North America: Small Animal for evaluating treatment of uroliths. Veterinary Clinics of North
Practice 1999; 29: 47.) America: Small Animal Practice 1999; 29: 48.)

and nutritional intervention may be most beneficial with urine STRUVITE UROLITHS
in this region. A precarious balance exists between crystal for- Struvite uroliths form as a result of oversaturation of
mation and inhibition in the metastable zone. Anatomic de- urine with magnesium ammonium phosphate. This over-
fects within the urinary tract that allow for stasis of metastable saturation can occur in the presence of infection with a ure-
urine will lead to formation and growth of crystals. Urine con- ase-producing organism. However, in contrast to the situa-
taining microscopic impurities will facilitate crystal formation tion in dogs, most struvite uroliths in cats form in sterile
and growth. This process is called “heterogeneous nucleation.” urine (Osborne et al, 1996a). Urinary magnesium levels are
Crystal formation is much less likely in urine without impuri- related to dietary intake; as the amount of dietary magne-
ties (i.e., homogeneous solution) (Bartges et al, 1999). Box 46- sium increases, urinary magnesium excretion increases lin-
2 discusses laboratory techniques used to measure these early (Sauer et al, 1985; Pastoor, 1993). Box 46-3 provides
changes in urine. A more detailed description of this entire more information about the effects of urinary pH on stru-
process is found in Chapter 38. vite urolith formation.