Feline Urolithiasis and Feline Lower Urinary Tract Disease (FLUTD)
(Feline urologic syndrome) |  |
| Hematuria, pollakiuria, and stranguria are the characteristic clinical signs of FLUTD in cats. Although the specific underlying etiology of this common disease may not be identified, 2 major disease categories have been suggested based on the presence or absence of mineral precipitates; this discussion is limited to feline urolithiasis. |
| Feline urolithiasis is a common disease that is seen with equal frequency in both sexes. Until recently, it was thought that most uroliths in cats were small and resembled sand or were gelatinous plugs that differed from typical uroliths in that they contained a greater amount of organic matrix, giving them a toothpaste-like consistency. Matrix-crystalline plugs are most commonly found within the urethra near the urethral orifice and are primarily responsible for urethral
obstruction. Recently, prevalence of urolithiasis with grossly observable stones composed primarily of calcium oxalate has increased in cats. The most common feline uroliths are calcium oxalate, magnesium ammonium phosphate, and urate. |
| Urolithiasis is usually suspected based on clinical signs of hematuria, dysuria, or urethral obstruction. Urinalysis, urine culture, radiography, and ultrasonography may be required to differentiate uroliths from urinary tract infection or neoplasia. Radiography, cystoscopy, or ultrasonography are critically important to detect uroliths because only ~10% of feline urocystoliths can be detected by abdominal palpation. Uroliths with a diameter >3 mm are usually radiodense;
however, because smaller uroliths are common, double contrast radiography may be required for detection. Radiographic evidence of uroliths is seen in ~20% of cats with hematuria or dysuria. The usual clinical approach to grossly observable urocystoliths is surgical removal or lithotripsy where available, followed by dietary therapy instituted as a preventive measure. For sterile struvite uroliths, medical dissolution is the preferred treatment. |
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Calcium Oxalate Stones: |
| Calcium oxalate uroliths are the most common feline uroliths and the most common nephrolith, although their underlying cause is unknown. Common management schemes that involve feeding urine-acidifying diets with reduced magnesium, have reduced the incidence of feline struvite urolithiasis. Magnesium has been reported to be an inhibitor of calcium oxalate formation in rats and humans; thus, the reduced magnesium concentration in feline urine may partially explain the increase in
calcium oxalate stones in cats. |
| Medical protocols that promote calcium oxalate dissolution are not known; therefore, surgery and lithotripsy are the primary means for removal (small bladder stones may be eliminated by voiding urohydropulsion). However, some calcium oxalate uroliths, especially those in the kidneys, may not cause clinical signs for months to years. Because of the unavoidable destruction of nephrons during nephrotomy, this procedure is not recommended unless it can be established that the
stones are a cause of clinically significant disease. Recurrence remains problematic. A variety of diets has been formulated to restrict the formation of calcium oxalate uroliths and should be considered appropriate for maintenance in cats with nephroliths and following the removal of urocystoliths. Eliminating any associated urinary tract infections, avoiding mineral and vitamin C and D supplementation, and encouraging water consumption are critical. |
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Struvite Stones: |
| Three distinct types of struvite uroliths are recognized in cats: amorphous urethral plugs with a large quantity of matrix, sterile struvite uroliths (which form perhaps as a result of certain dietary ingredients), and struvite uroliths that form as a sequela of urinary tract infection with urease-producing bacteria. Struvite uroliths induced by infection are less common than sterile struvite uroliths. An additional type of struvite urolith in cats consists of a sterile
struvite nidus that predisposes to urinary tract infection with urease-producing bacteria and subsequent formation of infected struvite laminations around the sterile nidus. |
| Treatment of sterile struvite urolithiasis focuses on reducing the urine pH to ≤6.0 and on reducing the urine magnesium concentration by feeding magnesium-restricted diets. Reducing urine pH and magnesium concentration is best accomplished by feeding a commercially available prescription diet formulated for this purpose. Generally, neither sodium chloride nor urine acidifiers should be given concurrently with these diets because they are already supplemented with sodium
chloride and formulated to produce aciduria. In addition, these diets should not be fed to cats that are acidemic, have azotemia of any cause, or have cardiac dysfunction or hypertension. Urolith size and crystalluria should be monitored every 4 wk by radiographs or ultrasonography and urinalysis, respectively. Struvite crystals should not form if therapy has been effective in producing urine that is undersaturated with magnesium, ammonium, and phosphate. Because small uroliths
may not be detected radiographically, the calculolytic diet should be continued for ≥4 wk after radiographic documentation of urolith dissolution. If treatment does not induce complete dissolution of uroliths, it is likely that either the wrong mineral component was identified, the nucleus of the urolith is composed of a different mineral than the outer portion of the urolith, or the owner is not complying with therapeutic recommendations. |
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| Other Feline Stones: |
| Ammonium urate, uric acid, calcium phosphate, and cystine uroliths are less common in cats, but ammonium urate and uric acid account for ~6% of feline uroliths. Although a renal tubular reabsorptive defect and portovascular anomalies have been incriminated as causes in a few cases, the cause of most urate uroliths in cats has not been established. Nonetheless, formation of highly acidic and concentrated urine associated with consumption of diets high in purine precursors
(especially liver) appears to be a risk factor. |
| Medical protocols that consistently promote dissolution of ammonium urate uroliths in cats have not been developed, and surgery remains the most common method of removal. For small stones, voiding urohydropulsion may be effective. Prevention should include feeding a diet low in purine precursors and promoting formation of less acidic urine that is not highly concentrated. Although allopurinol may reduce the formation of urate in cats, studies of the efficacy and potential
toxicity of allopurinol in cats are required before meaningful guidelines can be established. |
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