Page 750 - Small Animal Internal Medicine, 6th Edition

722 PART V Urinary Tract Disorders

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FIG 43.11
Silica urolith that was removed from a dog; note the
characteristic “jack-like” appearance.
FIG 43.12
Characteristic appearance of dried solidified blood calculi
that were removed from a cat.
a vegetarian-based diet. Cystine is more soluble in alkaline
urine, and the recommended dietary therapy should result
in a urine pH > 6.5 to 7.0. If the optimal urinary pH is not
achieved in this manner, potassium citrate (starting dose of can be difficult to identify on ultrasonographic examination.
50-75 mg/kg q12h) can be added to alkalinize the urine, Management for the prevention of DSB includes evaluating
because citrate salts serve as a source of bicarbonate. The the patient for underlying evidence of renal or lower urinary
drug, tiopronin (Thiola , 2-MPG; 15-20 mg/kg PO q12h), hematuria and increasing the moisture content of the diet.
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can also be administered to help prevent (or possibly dis-
solve) cystine stones; however, this sulfhydryl compound can XANTHINE UROLITHS
be cost prohibitive and limited in its availability. When using Xanthine uroliths are very rare in cats and dogs. The most
this medication, gastrointestinal and hematologic adverse common cause of xanthine urolithiasis in dogs is iatrogenic
effects can occur. Dogs and cats should be monitored with because of the administration of the xanthine oxidase inhibi-
appropriate imaging studies to evaluate the effectiveness of tor allopurinol, primarily in Dalmatians and those breeds
the therapeutic approach. Due to the possible androgen- genetically predisposed to urate urolithiasis. Higher levels of
dependent cystinuria, if male intact dogs are diagnosed with urinary xanthine and hypoxanthine will occur if the dosage
cystine uroliths, castration is recommended. of this drug is too high, and/or the dog is not receiving
Dogs with silica-containing uroliths are usually older, and a purine-restricted diet. Xanthine uroliths have also been
most affected dogs are male. Most silica uroliths have a so- described in Cavalier King Charles Spaniels, caused by an
called jack stone appearance (Fig. 43.11), giving them a char- autosomal recessive trait that results in increased urinary
acteristic look on plain radiography. The recurrence rate of hypoxanthine and xanthine excretion. This is likely because
silica is not well known, but these uroliths are generally of a deficiency in the enzyme xanthine oxidase. Second-
slow-growing. Dietary prevention strategies focus on provid- ary renal failure is relatively common when this occurs in
ing a high-moisture diet that is higher in animal proteins and the Cavaliers. Diets similar to those used for urate pre-
lower in plant origins, particularly rice, soybean hulls, and vention, which are restricted in protein and high in mois-
corn gluten feed. ture, are generally recommended for this condition in
this breed.
DRIED SOLIDIFIED BLOOD CALCULI
IN CATS
We have reported another urolith type called DSB calculi CONCLUSIONS
that we have only identified in cats. Although we have rarely
noted small amounts of DSB on the surface of various canine Whenever a urolith is obtained from a dog or cat (surgically,
stones, none of them was ever composed of 100% DSB, as in catheter-assisted, voided, or by lithotripsy), it is important
cats. DSB calculi can occur in various places in the upper that it be submitted for crystallographic analysis to identify
and lower urinary tract. DSB calculi are very firm and “stone- the minerals present. Evaluating trends in feline and canine
like” but usually do not contain crystalline material (Fig. urolithiasis is important because it may assist clinicians in
43.12). Although these uroliths are not common, they can determining what, if any, effect current stone prevention
be a diagnostic challenge. They are generally radiolucent strategies, including dietary modifications and drug therapy,
unless they contain a significant portion of CaOx, calcium are having in cats and dogs with urolithiasis. Diets high in
phosphate, or other radiodense mineral. Furthermore, they moisture are the cornerstone of therapy for preventing