Urinary Tract Stones in Cats Composed of Cystine
Cystine uroliths, also referred to as calculi, are stones within the urinary tract composed of the amino acid cystine, a breakdown product of proteins. Cystine urolithiasis is equally distributed between male and female cats, with an average age of onset of three to five years. It has been observed in domestic shorthair and Siamese breeds. Clinical symptoms depend on size and number of uroliths as well as their location within the urinary tract. Some affected animals may have no clinical symptoms.
The exact mechanism of cystine uroliths formation is unknown. Cystinuria (presence of cystine in the urine) is a predisposing factor. Cystinuria is an inborn error of metabolism characterized by defective transport of cystine by the tubular cells of the kidneys.
What to Watch For
Passage of small volumes of urine
Increased frequency of urination
Inability to urinate
Affected animals may be extremely ill if they have associated kidney failure or infection.
Diagnosis of Cystine Urinary Tract Stones in Cats
Routine tests include a complete blood count (CBC), biochemical profile and urinalysis in patients with cystine uroliths. Although these tests often are normal, changes consistent with kidney failure or bacterial urinary tract infection may be observed.
A urine amino acid profile may be helpful by revealing abnormally high quantities of cystine. This test requires sample submission to a specialized laboratory and may be expensive.
Abdominal radiographs (X-rays) may be taken, but cystine calculi are radiolucent, which means they do not appear on plain radiographs because they do not have sufficient mineral content, and such radiographs may not be diagnostic for stones.
Abdominal ultrasound may be helpful to detect cystine uroliths but this procedure does not allow chemical identification of the uroliths (urate uroliths also are radiolucent but may be observed on abdominal ultrasonography).
Contrast (dye) studies such as an intravenous pyelogram (to evaluates the kidneys and ureters) or cystourethrogram (to evaluate the bladder and urethra) may identify stones. As is true for ultrasonography, radiographic contrast studies will not differentiate different types of radiolucent stones from one another, for example urate vs. cystine.
Treatment of Cystine Urinary Tract Stones in Cats
Depending on the size and location of the stones, treatment may be medical or surgical. Surgery may be indicated if urethral obstruction is present.
If urethral obstruction is not present, dissolving the stones may be feasible. A low protein diet combined with the drug 2-mercaptopropionyl glycine (2-MPG or Thiola) has been used to reduce the urinary excretion of cystine and to dissolve cystine stones. There is little experience using 2-MPG in cats.
Additional treatment may include:
Alkaliniziation of the urine (using drugs such as potassium citrate or sodium bicarbonate that increase the pH of urine) may enhance the solubility of cystine in the urine and minimize calculi formation.
Antibiotic therapy is indicated with concurrent bacterial urinary tract infection.
Fluid therapy is indicated in patients that are dehydrated, have urinary obstruction or have associated kidney disease.
Urohydropulsion (flushing stones out of the lower urinary tract) may be of benefit in some animals. This procedure is most likely to be helpful in dogs with cystine urolithiasis because the cystine stones often are very small (less than 5 mm in diameter) and can pass through the urethra.
Surgical removal is reserved for animals in which medical management is not beneficial or immediate relief of urethral obstruction is necessary, or in patients with severe clinical signs.
Home Care and Prevention
Strict compliance is necessary if your pet is on a special diet and is receiving medication to dissolve cystine stones. Return for follow up examinations and tests as recommended by your veterinarian.
Without preventive care, cystine uroliths tend to recur within 6-12 months of removal or dissolution. Cystine stones can recur even when medical treatment and dietary management are strictly followed.