In addition, allopurinol, a competitive inhibitor of the enzyme xanthine oxidase (Figure 9, see gallery), which converts hypoxanthine to xanthine and xanthine to uric acid (Figure 1, see gallery), should be administered in dogs at a dose of 10 mg/kg PO q8h. If necessary, potassium citrate should also be administered to maintain a urine pH of 7.0 to 7.5; the dose should be individualized, with dosages starting at 40 to 75 mg/kg PO q12h. UTI should be appropriately treated because urease-producing organisms increase the urine ammonium ion concentration and potentiate AU crystal production.
Side effects of allopurinol in humans include vomiting, rash, leukopenia, thrombocytopenia, vasculitis, and hepatitis; these events have been reported only rarely in dogs.3 Allopurinol is excreted via the kidneys and thus should be used cautiously in azotemic dogs. Xanthine uroliths may form in dogs treated with allopurinol, especially if dietary protein and purine intake are not reduced.
Recurrence rates for urate uroliths in all dog breeds are high (33%–50%), and uroliths usually recur within 1 year of initial diagnosis and treatment.12 Prophylactic dietary therapy is the first line of defense. Diets recommended to prevent recurrence in dogs include Royal Canin Veterinary Diets Canine Urinary UC Low Purine, Purina Veterinary Diets HA Hypoallergenic Canine Formula, and Hill’s Prescription Diet u/d Canine Non-Struvite Urinary Tract Health. If urate crystalluria persists despite good dietary compliance, urine pH should be monitored to assure appropriate alkalinization and potassium citrate administered if needed. If necessary (ie, xanthine crystals detected in urine sediment), lower doses of allopurinol (10–15 mg/kg q24h) may be used.3 The risk of inducing xanthine uroliths increases with higher doses of allopurinol as well as with higher dietary protein and purine, and caution is warranted with long-term use of high-dose (15 mg/kg PO q12h) allopurinol.
In dogs with urate urolithiasis secondary to severe hepatic insufficiency, the underlying disorder should be corrected if possible. If hepatic function can be improved (eg, surgical or medical management of a PSS) and the urine becomes undersaturated with ammonium and urate ions, uroliths may dissolve spontaneously; nevertheless, urolith removal via cystotomy at PSS correction or associated with medical management is usually recommended, especially in male dogs, to reduce the likelihood of urethral obstruction. In dogs with inoperable PSS or microvascular dysplasia, medical management including diet, antibiotics, and lactulose may be used to help decrease urine saturation with AU and reduce signs of hepatic encephalopathy.
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