Struvite and calcium oxalate (CaOx) uroliths are the most commonly reported uroliths in cats. In the last 25 years, dramatic change in the prevalence of different urolith types has occurred. Until the mid-1980s, struvite uroliths made up 78% of submissions to the Minnesota Urolith Center (MUC).
Struvite and calcium oxalate (CaOx) uroliths are the most commonly reported uroliths in cats. In the last 25 years, dramatic change in the prevalence of different urolith types has occurred. Until the mid-1980s, struvite uroliths made up 78% of submissions to the Minnesota Urolith Center (MUC).1 Starting in the mid-1980's, a dramatic increase in the frequency of calcium oxalate uroliths along with a decrease in struvite uroliths was noted. By 2002, 55% of uroliths submitted to the MUC were CaOx, while only 33% were struvite. However, in recent years, the prevalence of urolith types has changed again. In 2007, 49% of uroliths submitted to the MUC were struvite and 41% were CaOx. The ratio of CaOx to struvite uroliths also increased significantly in submissions to the Gerald V. Ling Urinary Stone Analysis Laboratory from 1985-2004.2 But similar to the MUC, by 2002-2004, 44% of uroliths were struvite and 40% were CaOx.
The prevalence of struvite uroliths presented to the Canadian Veterinary Urolith Centre (CVUC) decreased in the 10-year period from 1998-2008, while the prevalence of CaOx uroliths remained constant.3 In 2008, 49% of uroliths submitted to the CVUC were CaOx and 42% were struvite. In Europe, the prevalence of CaOx uroliths also increased from 1994-2004.4 In 1994, 77% of uroliths were struvite and 12% were CaOx. By 2003, 61% of uroliths were CaOx and 32% were struvite.
It seems likely that the increase in CaOx uroliths seen in the 1980's was driven by changes in feline diets. The widespread use of diets designed to dissolve struvite uroliths meant fewer were surgically removed and submitted for analysis. At the same time, the modification of maintenance diets to prevent struvite uroliths may have caused an increase in CaOx uroliths. Some dietary factors that decrease the risk of struvite uroliths can increase the risk of CaOx uroliths. The more recent changes in prevalence of urolith type may be associated with further modification of maintenance diets to minimize the risk of CaOx uroliths and improvements in and increased use of therapeutic diets designed to reduce risk factors for CaOx uroliths.
Since the early 1980s, the prevalence of uroliths in the upper urinary tract appears to have increased dramatically.5-6 CaOx is the predominant type (>75%) of urolith found in this location.2,5-6 Nephroliths may be found incidentally on survey abdominal radiographs.
Risk factors for development of CaOx urolithiasis include age (middle-aged and older cats, mean age 7 years) and breed (Persian, Himalayan, British Shorthair, Exotic Shorthair, Havana Brown, Foreign Shorthair, Ragdoll, Scottish Fold).2-4,7-8 Some studies suggest male cats are at higher risk than females.2-3,7 Diets low in sodium or potassium and those formulated to maximize urine acidity increase the risk of CaOx uroliths.9 The source of drinking water is thought to be an unlikely contributor to the development of CaOx uroliths.8
Clinical Signs and Diagnosis
When uroliths form in the lower urinary tract, clinical signs include stranguria, hematuria, pollakiuria, inappropriate urination, and urethral obstruction. Cats with CaOx nephroliths may have clinical signs related to the kidneys, such as azotemia, renomegaly, hematuria and abdominal pain. However, many nephroliths are clinically silent. Cats with ureteroliths may present with nonspecific clinical signs such as anorexia, vomiting, lethargy and weight loss. Ureteral calculi often cause ureteral obstruction and are associated with chronic renal disease.6
Definitive diagnosis of CaOx uroliths requires removal and chemical analysis. CaOx uroliths are radiopaque, and usually white and hard, with either a jagged or smooth surface. They may be present as either single or multiple stones. Survey abdominal radiographs are often sufficient for detection of uroliths, but cannot identify the type. Detection of small uroliths or nonradiopaque uroltihs may be improved with double-contrast cystography or ultrasonography, both of which have a false negative rate of <5%.10 The index of suspicion for CaOx versus struvite uroliths in the bladder would be higher in male cats, cats over 7 years of age, and in susceptible breeds.
Cats with uroliths should have a CBC and serum biochemistry profile performed as well as a urinalysis. Ruling out concurrent diseases and evaluating for hypercalcemia is important in cats suspected or known to have CaOx uroliths. Hypercalcemia has been found in about ⅓ of cats with CaOx uroliths, although the cause is unknown.11-12 Evaluation of hypercalcemia includes total serum calcium, ionized calcium and PTH. Since the hypercalcemia is usually idiopathic, the total serum calcium and ionized calcium are increased but the PTH concentration is normal or low.
Urinalysis typically shows an acidic pH in cats with CaOx uroliths. These uroliths are not typically associated with infection, although secondary bacterial infections, especially with E. coli, may be present in some patients.13 Although CaOx crystals may be seen on urinalysis, they are not a reliable indicator of whether uroliths are present, nor of urolith composition. Some cats with uroliths do not have crystalluria, and uncommonly, those with uroliths may have urinary crystals that are different from the type in the stone.14
To date, no calculolytic diets for CaOx uroliths exist. The only effective treatment is physical removal of the stones, usually via surgery or voiding urohydropropulsion (VU). VU is only successful for removal of uroliths with a diameter smaller than the urethral lumen; in cats, this means uroliths smaller than 5 mm in females and 1 mm in males.13 Cystotomy must be performed for larger uroliths, but care must be take to remove all the stones. Incomplete surgical removal of CaOx uroliths occurs in about 15-20% of patients.15 Uroliths causing urethral obstruction should be retropulsed into the bladder.13 While canine CaOx nephroliths are treatable using extracorporeal shock wave lithotripsy, feline CaOx uroliths are less susceptible to fragmentation, making this treatment less effective for the cat.16-17
Uroliths in the upper urinary tract should only be removed if they are causing obstruction or are associated with severe hematuria, pain or persistent bacterial infection.13 Nephroliths that are increasing in size and negatively affecting renal function may also be considered for surgical removal. The decision to remove upper urinary tract uroliths is not an easy one because of the difficulties in performing ureteral surgery in cats and the long-term renal damage caused by nephrotomy.
Recurrence of CaOx uroliths in cats after treatment is common. In once study of 2,393 cats with CaOx uroliths, 7% had one recurrence, 0.6% had a second recurrence, and 0.1% had a third recurrence.18 Important factors for management and prevention of CaOx uroliths include:
1. Feed a protein-restricted, high moisture, alkalinizing diet designed to prevent CaOx uroliths; avoid foods high in oxalates and avoid supplements of ascorbic acid and vitamin D19
2. Achieve a urine specific gravity <1.025 by feeding canned food, adding water to dry food, providing multiple water bowls, and using flavoured water
3. Achieve a urine pH of 7.0-7.5 by using a therapeutic diet and/or supplementing with potassium citrate (50-75 mg/kg with food, BID)
4. Other strategies include supplementing with vitamin B6 (2-4 mg/kg, daily to EOD, PO) and administration of hydrochlorothiazide (2-4 mg/kg, PO, BID)
5. If hypercalcemia is present, use a high-fiber diet and potassium citrate; other options include glucocorticoids (start prednisone at 5 mg/cat/day PO for 1 month and reassess) and once weekly oral alendronate (10 mg/cat)20
After urolith removal, cats should be re-evaluated within one month and then again at 3 months and 6 months. In addition to a physical examination and medical history, a urinalysis should be performed as well as bladder imaging (via radiography and/or ultrasonography). Ideally, bladder imaging should be performed every 6 months. Some cats will require monitoring of total and ionized calcium, and cats receiving diuretics will require monitoring of electrolytes.
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2. Cannon AB, Westropp JL, Ruby AL, et al. Evaluation of trends in urolith composition in cats: 5,230 cases (1985-2004). J Am Vet Med Assoc 2007;231:570-576.
3. Houston DM, Moore AEP. Canine and feline urolithiasis: examination of over 50,000 urolith submissions to the Canadian Veterinary Urolith Centre from 1998 to 2008. Can Vet J 2009;50:1263-1268.
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