Improving management of urolithiasis: canine struvite uroliths - DVM
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Improving management of urolithiasis: canine struvite uroliths


14. Fueled by a constant supply of urea from dietary protein and urease from microbes, struvite crystals can rapidly grow to form uroliths that fill the lumen of the urinary bladder or renal pelvis. The rapid rate at which uroliths form and the potential they have to migrate to lower portions of the urinary tract are of clinical importance. If several days have elapsed between the date of diagnostic radiography and the date of surgery scheduled to remove uroliths, the number and location of stones should be reevaluated by radiography or ultrasonography.

15. Struvite uroliths have a tendency to recur following surgical removal or medical dissolution. Most episodes of recurrence are associated with lack of removal of all uroliths at the time of surgery (pseudorecurrence) or poor control of recurrent UTIs with urease producing microbes. The key to preventing recurrent infection-induced struvite uroliths is to eradicate or control UTIs.

Dissolution therapy 16. Prior to attempting dissolution, perform diagnostic studies (urinalysis, urine culture, radiography, analysis of voided stones, etc.) to evaluate urolith size and location, as well as confirmation of urolith composition.

17. Urethroliths and ureteroliths cannot be dissolved by the medical protocols. Medical therapy designed to induce urolith dissolution by changing the composition of urine will be ineffective for stones in the urethra and ureters because they are only intermittently exposed to urine that is undersaturated with struvite.

18. The importance of UTIs with urease-producing bacteria in formation of struvite uroliths emphasizes the necessity of therapy to eliminate or control them. By eliminating the infection, the urine typically becomes less alkaline with a concomitant increase in solubility of MAP.

19. We recommend antimicrobial drugs selected on the basis of antibiotic dilution susceptibility tests designed to determine minimum inhibitory concentrations (MIC) of antimicrobial drugs in urine. Preference is given to bacteriocidal drugs excreted in high concentration in urine, and with a wide margin of safety between therapeutic and toxic doses. The fact that diuresis reduces the urine concentration of antimicrobic agents should be considered when formulating drug doses. The goal is to establish a dose so that the quantity of drug present in urine is greater than four times the MIC.

20. Therapeutic doses of antimicrobics should be administered until there is radiographic evidence of complete dissolution of uroliths. This recommendation is based on the fact that bacterial pathogens harbored inside uroliths may be protected from antimicrobial agents.

21. Because of the large quantity of urease produced by struvitogenic microbes, it may be impossible to acidify urine with urine acidifiers administered as doses do not result in systemic acidosis. We do not use urine-acidifying drugs in our medical protocols for dissolution of infection-induced uroliths.

22. Antimicrobial therapy alone is usually ineffective in dissolving infection-induced struvite uroliths. However, a combination of antibiotics and dietary modification has been very effective. We evaluated a high moisture (canned) struvitolytic diet formulated to contain a reduced quantity of high-quality protein and reduced quantities of phosphorus and magnesium (Prescription Diet Canine s/d; Hill's Pet Nutrition).

The diet was supplemented with sodium chloride to stimulate thirst and induce compensatory polyuria. In addition, reduction of dietary protein reduces renal medullary urea concentration and further contributes to diuresis. The efficacy of this diet in inducing dissolution of infected struvite uroliths was been confirmed by controlled experimental and clinical studies.

In our studies, the mean time for dissolution of naturally occurring infection induced urocystoliths in dogs fed the struvitolytic diet and appropriate antimicrobics was approximately three months (range equals two weeks to seven months).

23. Consumption of the struvitolytic diet by young adult dogs with staphylococcal urinary tract infection and struvite uroliths was associated with a marked reduction in the serum concentration of urea nitrogen and mild reductions in the serum concentrations of magnesium, phosphorus and albumin. A mild increase in the serum activity of hepatic alkaline phosphatase isoenzyme also was observed.

These alterations in serum chemistry values were of no detectable clinical consequence during six-month experimental studies or during clinical studies. However, they underscore the fact that the diet is designed for short-term (weeks to months) dissolution therapy rather than long-term (months to years) prophylactic therapy. Reduction in concentrations of serum urea nitrogen may be used as one index of client and patient compliance with dietary recommendations.

Table 3: Characteristic clinical findings before and following initiation of medical therapy to dissolve struvite uroliths in nonazotemic dogs
Monitoring response 24. Efficacy of therapy should be periodically monitored (every two to four weeks) by evaluating appropriate indices of therapeutic response. These typically include timely urinalyses, urine cultures, serum biochemical profiles, and radiography or ultrasonography (Table 3, p. 8S). Therapy should be adjusted to meet each individual patient's needs.

25. The number, size and location of uroliths should be monitored by survey radiography, and if necessary by contrast radiography. Although retrograde double contrast cystography is more sensitive in identifying small urocystoliths, survey radiography is usually preferable because use of catheters during retrograde radiographic studies may result in iatrogenic UTI. Alternatively, ultrasonography or intravenous urography may be considered.

26. Because stuvitolytic diets promote diuresis, clients should be informed that the magnitude of pollakiuria in dogs with urocystoliths may increase for a short time following initiation of dietary therapy. However, pollakiuria and abnormal odor of urine caused by bacterial degradation of urea usually subside as the infection is controlled and uroliths decrease in size.

27. Large urocystoliths that decrease in size as a result of dissolution have the potential to pass into the urethra where they may cause partial or total outflow obstruction. This has been an uncommon problem in our experience because proper treatment results in decreased dysuria, pollakiuria and tenesmus. Movement of urocystoliths into the urethra is most likely to occur in patients with substantial dysuria and tenesmus, or in patients with uethral strictures. Clients should be given a written summary of clinical manifestations of impaired urine flow through the urethra so that if this problem occurs, it can be quickly recognized and corrected. Urethroliths may be readily returned to the urinary bladder lumen by urohydropropulsion. If warranted, tenesmus and dysuria may then be temporarily suppressed by giving drugs that reduce pain and/or cause muscle relaxation.

28. Since small (<3 mm in diameter) struvite uroliths may escape detection by survey radiography or ultrasonography, we recommend that calculolytic diet and (if necessary) antimicrobial agents be continued for approximately one month following radiographic documentation of urolith dissolution.

If urinalysis results are normal, dissolution therapy may be discontinued. This maneuver is likely to prevent rapid recurrence of bacterial UTI and radiographically detectable uroliths following cessation of therapy.

Poor response to therapy 29. Difficulty in inducing complete dissolution of uroliths by creating urine that is undersaturated with MAP should prompt consideration that:

1) the wrong mineral component was identified;

2) the nucleus of the uroliths of different mineral composition than outer portions of the urolith; and/or,

3) the owner or the patient is not complying with therapeutic recommendations.


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