Page 1203 - Clinical Small Animal Internal Medicine

123 Urolithiasis in Small Animals 1141

After this time, examination of urine pH, specific gravity, sources (plant, egg white), severe protein restriction is
VetBooks.ir and sediment is recommended at two weeks, four weeks ● The desired urine pH for urate dissolution is approxi­
not mandatory to achieve a low urinary urate level.
and then every three months for one year. The aim is to
maintain the average urine pH <6.5, the urine specific
detrimental because it could favor the formation of
gravity <1.030 and the sediment free of significant crys­ mately 7.0. Targeting a higher urine pH (>7.5) could be
talluria. If the cat remains free of clinical signs and free of calcium phosphate urolith. Potassium citrate (50–
significant crystalluria and uroliths, rechecks may be 150 mg/kg q12h) may be needed to alkalinize the urine
decreased to twice yearly. if diet alone is not enough to sustain an average urine
For cats with infection‐induced struvite uroliths, fol­ pH of approximately 6.8–7.0. Obtaining both a fasting
low‐up urine culture and sensitivity is recommended and postprandial (2–6 hours after a meal) urinary pH
one week after the end of the antibiotics. Further assess­ will allow for consistent and accurate medical titration
ment is recommended in cats that have repeat urinary of potassium citrate. The patient should be monitored
tract infections and struvite crystalluria. for hyperkalemia when higher doses of potassium cit­
Examples of diets recommended for use in struvite rate are used.
prevention due to their known efficacy in reducing mag­ Hill’s Prescription Diet canine canned u/d (the most
nesium and phosphorus levels and urinary acidification purine‐restricted diet currently commercially available)
properties include Hill’s c/d Multicare (canned and dry), and allopurinol (30 mg/kg/day, divided q8–24h) has been
Purina UR St/Ox (canned and dry), and Royal Canin S/O recommended for medical dissolution of urate stones.
(canned, chunks and gravy, and dry). Many of the lifes­ This diet is very low in protein‐purine and contains
tage diets from Hill’s, Purina, and Royal Canin are also potassium citrate and calcium carbonate for urinary
aimed at concurrent struvite urolith prevention. It is par­ alkalinization. In a clinical study of 25 dogs with ammo­
ticularly important in cats that therapy is directed at nium urate uroliths fed this diet and treated with allopu­
reduction of urine concentration, and thus a high‐mois­ rinol, complete urolith dissolution occurred in nine dogs
ture diet is advised.
(36%), partial dissolution in eight (32%), and no dissolu­
tion in eight (32%). The mean time for urate urolith dis­
Ammonium Urate Uroliths solution was 3.5 months. Similar results were reported
Dissolution of Ammonium Urate Uroliths in Dogs when this diet was used for dissolution of sodium urate
Medical dissolution of urate can be achieved, although it uroliths. Owners must be fully compliant (i.e., no other
may not always be in the patient’s best interest, such as food or treats should be given) if this option is selected.
urinary tract obstruction. Careful patient monitoring is necessary as these uroliths
Dissolution of urate is only indicated in patients with
an underlying defect in purine metabolism and not in are prone to migrating into the urethra where lower uri­
nary tract obstruction can occur. However, Hill’s u/d is
cases of liver dysfunction. Nutritional therapy for urate contraindicated in dogs prone to pancreatitis or hyper­
dissolution is achieved with a combination of the lipidemia due to its high fat content.
following.
Allopurinol is a specific enzyme inhibitor that causes
Urine alkalinization: alkaline urine contains low partial blockage of the purine degradation pathway
●
amounts of ammonia, which subsequently reduces above the level of uric acid (see Figure 123.6). Allopurinol
ammonium urate. Feeding a low‐protein (purine) diet inhibits the enzyme xanthine oxidase. As a result, hypox­
and/or with potassium citrate supplementation pro­ anthine, xanthine, and urate are all excreted in the urine.
duces alkaline urine. Each of these purine metabolites has a different urinary
Increase in urine volume: a canned diet (or addition of solubility; hypoxanthine is very soluble, whereas xan­
●
water to the diet) is preferred for this reason. A low‐ thine and urate are only poorly soluble. Allopurinol will
protein diet also decreases the concentration of urine reduce the oxidation of xanthine to urate. Consequently,
by “wash‐out” of the medullary concentration gradient xanthine concentration in the urine increases.
(via low urea). The goal is a urine specific gravity of The dose of allopurinol recommended is variable
<1.020. (5–30 mg/kg/day, divided q8–24h). It has a dose‐depend­
Decreasing the excretion of ammonium ions: feeding a ent half‐life of 2.5–3 hours in dogs. For dissolution of
●
diet low in protein‐purines decreases urea production ammonium urate stones, the higher dose is recom­
and consequently ammonium and its ions. mended (15 mg/kg q12h). Allopurinol and its metabo­
Decreasing the excretion of uric acid: feeding a diet low lites are excreted via the kidney, therefore the dose
●
in purines and their precursors will aid in this. The should be reduced in patients with renal failure. In addi­
purine content of proteins varies (e.g., it is high in tion, any medications the dog is receiving need to be
organ meats and fish), and by using selected protein carefully assessed. For example, allopurinol can inhibit