Stalking stones: An overview of canine and feline urolithiasis - Veterinary Medicine
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Stalking stones: An overview of canine and feline urolithiasis
Did you know that a new type of urolith has been found in cats? Should you institute preventive therapy if you identify only crystalluria? Is antibiotic therapy automatically warranted in animals with indwelling urinary catheters? This internist revamps your knowledge on diagnosing, treating, and preventing urolithiasis.


VETERINARY MEDICINE


Cystine

Cystine uroliths are uncommon in dogs (1% prevalence)21 and even less common in cats (< 1% prevalence).34

Characteristics. Cystine crystals are large, flat, and hexagonal, and their presence in urine should always be considered abnormal. Cystine crystals precipitate in acid urine to form uroliths that are small, spherical, and light-yellow, light-brown, or light-green. These uroliths are relatively radiolucent and commonly require contrast cystography or urethrography or ultrasonography for detection.

In dogs and cats, cystine urolith formation is associated with an inherited disorder that causes insufficient renal tubular reabsorption of cystine and other amino acids.43 Affected dogs are commonly 4 to 6 years old at the time that cystine uroliths are detected. Affected dog breeds include English bulldogs, Newfoundlands, dachshunds, Irish terriers, basset hounds, and bullmastiffs.14 Male dogs are more commonly affected than female dogs except in Newfoundlands, in which both sexes seem equally affected.

Male and female cats appear to be equally affected, and cystine urolithiasis is diagnosed at a mean age of 4.1 years. Domestic shorthaired and Siamese cats are the most commonly affected.25,44

Treatment. An alkalinizing, low-protein diet, such as Prescription Diet u/d, is recommended for treating cystine uroliths in dogs. Thiol disulfide exchange drugs, d-penicillamine and MPG, or tiopronin (Thiola—Mission Pharmacal), are used to dissolve these uroliths in dogs. These drugs combine with the precursors of cystine to produce a more soluble product. In dogs, combining a thiol exchange drug with Prescription Diet u/d appears to enhance cystine urolith dissolution.45 D-penicillamine is administered at a dosage of 15 mg/kg orally twice a day, and tiopronin is administered at a dosage of 20 mg/kg orally twice a day.24 The thiol disulfide exchange drugs are most effective at a neutral to alkaline urine pH.

D-penicillamine's main side effect is vomiting, which can usually be avoided by giving the drug with food, administering an antiemetic, or decreasing the dose slightly. Side effects do not occur as commonly with tiopronin administration but can include behavioral changes, myopathy, proteinuria, thrombocytopenia, immune-mediated anemia, skin lesions, lethargy, elevated liver enzyme activities, or a sulfur odor to the urine.24

Dissolution will be successful in about two-thirds of the dogs treated and requires about one to three months. Monitoring dissolution progress by imaging may require ultrasonography or double-contrast cystography.

No proven cystine urolith dissolution protocol is available for cats. Consider surgical removal, lithotripsy, or voiding urohydropropulsion for eliminating cystine uroliths in cats or in dogs in which medical dissolution is not acceptable or successful.

Prevention. The high recurrence rate of cystine uroliths and the inherited predisposing defect make prevention important in dogs that have experienced an episode of cystine urolith formation. A prevention protocol consists of administering tiopronin (15 mg/kg orally twice a day), adding water to the patient's food, and alkalinizing the urine with potassium citrate (100 to 150 mg/kg once a day or in two divided doses/day).14 The goal of alkalinization should be to achieve a urine pH of 7 to 7.5.24 An alternative prevention protocol calls for feeding Prescription Diet u/d plus administering either tiopronin or d-penicillamine at 10 to 15 mg/kg orally every 12 hours.46 Although Prescription Diet u/d is considered a maintenance diet, it has a low protein content, so monitor dogs fed this diet long-term for protein depletion by performing a serum chemistry profile at least every six months.

Although the incidence of feline cystine uroliths is low, the frequency of recurrence in affected cats appears high.44 For cats, a diet for managing kidney disease, such as canned or moistened dry Prescription Diet k/d or similar alkalinizing renal failure diets from other manufacturers, is recommended.

Silicate

Silicate uroliths are uncommon in dogs, accounting for < 1% of all urolith types analyzed with the Minnesota Urolith Center from 1981 to 2006.21 Silicate uroliths are rarely reported in cats.5,40

Characteristics. These uroliths are grayish-white to brown and frequently look like jackstones.47 They are radiopaque and found most frequently in the bladder and urethra. Silicate uroliths are composed primarily of amorphous silica crystals. A predisposition exists for German shepherds and Old English sheepdogs.24

The cause of silicate uroliths is unknown but may be diet-related. Diets high in corn gluten or rice and soybean hulls are suspected to increase the incidence of silicate urolith formation.24 No relationship has been established between urine pH and silicate urolith formation.

Treatment and prevention. No method of medical dissolution is available, and surgical removal is often necessary, although voiding urohydropropulsion can be attempted if the uroliths are small enough. Lithotripsy is another treatment option. Recurrence is possible but uncommon. The only recommendations for prevention are to increase water consumption and avoid diets high in plant proteins. Prescription Diet d/d (Hill's Pet Nutrition) may be appropriate for prevention because of its low plant protein content.


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