Urate uroliths, also referred to as calculi, are stones within the urinary tract composed of ammonium urate. Urate urolithiasis is uncommon in cats.
The average age of onset for cats with portosystemic shunts is less than one year, whereas the average age of onset for cats without portosystemic shunts is greater than three years of age. Clinical symptoms depend on the size and number of uroliths as well as their location within the urinary tract. Some affected animals may have no clinical symptoms.Causes Inborn errors of metabolism may predispose some animals to urate urolithiasis.
Cats with portosystemic shunts, which is an abnormal condition in which blood from the intestinal tract bypasses the liver and enters directly into the systemic circulation, also are predisposed to urate urolithiasis.
What to Watch For
Increased frequency of urination
Passage of small amounts of urine
Occasionally, nervous system abnormalities (dull mental attitude, head-pressing, seizures) may be observed in animals with portosystemic shunts.
Inability to urinate may be seen in animals with urethral obstruction. Urethral obstruction constitutes a medical emergency and you should take your pet to your veterinarian immediately.
Routine tests include a complete blood count (CBC), biochemical profile and urinalysis. Changes such as low blood urea nitrogen (BUN), low blood sugar, and low protein concentrations may be seen in animals with portosystemic shunts. Kidney impairment (high BUN and serum creatinine concentrations) may be seen in animals with urinary tract obstruction or related kidney disease. Urate crystals are present in the urine in some but not all cases.
Additional tests include:
Bile acid concentrations, taken before and after feeding, often are abnormally high in the blood of animals with portosystemic shunts.
Urate calculi are radiolucent and usually cannot be identified on plain abdominal radiographs (X-rays). Abdominal ultrasonography may be helpful to detect calculi in the urinary tract but the type of calculi present cannot be detected by ultrasonography.
Contrast (dye) studies including an intravenous pyelogram, which evaluates the kidneys and ureters, or cystourethrogram, which evaluates the bladder and urethra, may be of benefit in identifying these calculi. Radiographic dye studies, however, cannot differentiate urate from cystine calculi. Identification of the crystal type in the stones requires submission of the stones to a special laboratory for analysis.
It is important to establish whether or not the affected animal is in need of emergency care or hospitalization. Examples of emergency situations include animals with urethral obstruction, severe kidney failure, and those with hepatic encephalopathy, which are nervous system signs secondary to a portosystemic shunt.
Surgical intervention depends upon the location and size of the calculi, in addition to the clinical symptoms present in the patient. In addition, surgical repair may be warranted in pets with portosystemic shunts.
Additional treatment includes:
Fluid and electrolyte therapy in dehydrated or systemically ill patients
Dietary therapy in some cases to facilitate dissolution of urate stones
Allopurinol. This medication inhibits the breakdown of the chemical precursors of urate and may be of benefit in patients with urate urolithiasis due to underlying metabolic abnormalities.
Home Care and Prevention
Administer as directed all medications prescribed by your veterinarian. Also, follow dietary and feeding recommendations and follow up with your veterinarian as directed. Recurrence of urate urolithiasis is common.
Consider special diets in those breeds with a predilection to urate calculi, specifically low protein alkalinizing diets that increase urine pH. If your pet is diagnosed with a portosystemic shunt, appropriate medical or surgical therapy should be instituted at once so as to reduce the risk of forming urate calculi.