At our current level of understanding, medical treatment and prevention of the causes underlying urolithiasis are dependent
on knowledge of the composition and structure of the entire stone. Minerals in uroliths may be deposited in distinct layers
or they may be admixed throughout the stone(s). Although one mineral type usually predominates, the composition of uroliths
is frequently mixed (for example, 90 percent struvite and 10 percent calcium phosphate). The center (nidus) may be composed
of one mineral type (such as calcium oxalate), whereas outer layers may be composed of one (such as struvite) or more different
mineral types (Figures 3 and 4). Therefore, all uroliths retrieved from a patient should be submitted for quantitative analysis.
This is a standard of practice in our profession.
Figure 3: Cross-section of a feline compound urocystolith composed of a nidus of urate and a shell of struvite.
In addition to quantitative analysis, non-surgical methods of urolith management in cats and dogs have become a standard of
practice in the veterinary profession. Choice of treatment or prevention based on results of quantitative mineral analysis
of stones consistently provides the most favorable outcome.
Figure 4: Cross-section of a laminated urolith composed of a nidus of amorphous silica and a shell of struvite.
How can uroliths be retrieved for analysis?
Until the early 1980s, surgery was considered the only practical method of retrieving uroliths. However, we have devised several
different means to move and remove uroliths from the patient. Small urocystoliths can be retrieved from the urinary bladder
by "jiggling" the stones in the bladder while urine or saline and small stones are aspirated through a urinary catheter into
Uroliths also may be removed from the bladder by voiding urohydropropulsion. Urocystoliths spontaneously voided through the
urethral lumen to the exterior may be harvested with the aid of a small aquarium fishnet placed in the patient's urine stream.
Stone baskets also may be used to remove uroliths.
Additional information about these techniques is available at our Web site,
http://www.cvm.umn.edu/ (click the link to department and centers to find the button for the Minnesota Urolith Center). They are described in the
January 1999 edition of the Veterinary Clinics of North America under the title, "The Rocket Science of Canine Urolithiasis."
How should uroliths be submitted for analysis?
Methods by which uroliths are submitted for analysis may have a substantial impact on the quality of the analysis.
Table 1 offers a list of dos and don'ts of urolith submission.
What methods of analysis are recommended?
Two general methods of urolith analysis may be considered: qualitative analysis and quantitative analysis. Qualitative analysis
usually is performed using spot chemical tests to identify chemical radicals and ions. This archaic method has been virtually
abandoned as unreliable. Because most qualitative techniques require pulverizing the stone, layers of different minerals frequently
identified by quantitative methods of analysis typically cannot be identified. Also, these tests were not designed to identify
many crystalline components of uroliths, including amorphous silica and silica salts, drugs and drug metabolites (e.g., sulfadiazine
and allopurinol), newberyite, boberyite, uric acid and its salts and xanthine.
Figure 5: Calcium oxalate monohydrate under 450X magnification.
They are insensitive to detection of calcium and salts of calcium. Because qualitative tests for biogenic minerals are insensitive
and lack specificity, they should not be used.
Several physical methods of urolith analysis allow detection and quantification of minerals present in stone(s). These methods
include optical crystallography, infrared spectroscopy, X-ray diffraction, energy dispersive techniques and others.