Xanthine uroliths, also referred to as calculi, are stones within the urinary tract that occur when the metabolic breakdown of xanthine to urate is inhibited by medication (allopurinol) used to treat pets with urate urolithiasis.
Xanthine urolithiasis is most commonly seen in cats receiving allopurinol for treatment of urate urolithiasis. Clinical symptoms depend on the size and shape of the uroliths as well as their location within the urinary tract. Some affected pets may have no clinical symptoms.
Causes Allopurinol (a medication used to reduce the breakdown of purine precursors to urate) may result in the accumulation of xanthine, contributing to stone formation.
Diets high in purines (precursors of both xanthine and urate) may contribute to the formation of xanthine calculi.
Hereditary xanthinuria (a disorder that causes excessive excretion of xanthine in the urinary tract) is a very rare disorder.
What to Watch For
Passage of small volumes of urine
Painful urination
Increased frequency of urination
Bloody urine
Inability to urinate
Obstruction of the urinary tract is a medical emergency and your pet should be taken to your veterinarian for evaluation and treatment as soon as possible.
Kidney failure may be associated with uroliths in the upper urinary tract (kidneys/ureters) or with obstruction of the urethra.
Diagnosis
Routine diagnostic tests include a complete blood count (CBC), biochemical profile and urinalysis. Although these test results often are normal, changes consistent with urinary tract obstruction or concurrent kidney disease (increased blood urea nitrogen and serum creatinine concentrations) may be identified. Evaluation of the urine sediment may result in identification of xanthine crystals which are circular and yellow-brown in color.
Additional tests include:
Xanthine uroliths are radiolucent (they cannot be seen on plain abdominal radiographs). Thus, plain radiographs (X-rays) of the abdomen may not result in the proper diagnosis.
Abdominal ultrasound, intravenous pyelography (an radiographic contrast study that assesses the kidneys and ureters) or a contrast cystourethrogram (a radiographic contrast study that evaluates the bladder and urethra) may help identify xanthine stones and their location within the urinary tract.
Treatment
Dehydrated pets, those that are suffering from urinary tract obstruction, and those with kidney failure (high blood urea nitrogen and serum creatinine concentrations) need intensive supportive care including fluid therapy.
Allopurinol administration should be discontinued.
Low purine diets may reduce the amount of circulating xanthine.
Surgical intervention may be necessary depending on the location of calculi and severity of clinical signs exhibited by the patient.
Home Care and Prevention
Administer as directed all medications prescribed by your veterinarian. Feed as directed any special diets prescribed by your veterinarian.
Xanthine stones may recur. Observe your pet carefully for recurrence of clinical symptoms and call your veterinarian if such symptoms occur. Pets receiving allopurinol should be carefully monitored. When uroliths form in a cat receiving allopurinol for treatment of urate urolithiasis, it should not be assumed that the stones are urate. The possibility of xanthine stone formation should be considered in this situation.
