Photo 1: Photograph of multiple cystine urocystoliths removed from a 2-year-old female domestic shorthair cat.

Epidemiology Unlike canine cystine uroliths, which most commonly are encountered in male dogs, feline cystine uroliths occur in males and females with equal frequency. In our experience, the mean age of cats at the time of diagnosis of cystine uroliths was 3.4 years (age range = 4 months to 12.2 years). Approximately 65 percent of cats were of the domestic shorthair breed and 20 percent were Siamese. One was a Maine Coon, one was a Korat, and two were domestic longhair cats.

Quantitative analysis of feline cystine uroliths submitted to the Minnesota Urolith Center revealed that most are pure. Pure cystine urocystoliths are usually ovoid and smooth (Photo 1, p. 16S). They typically have a light yellow to reddish brown color and vary in diameter from 0.5 mm to several centimeters. All uroliths submitted to the Minnesota Urolith Center were initially obtained from the lower urinary tract (~65 percent of samples came from the bladder, ~5 percent came from the bladder and urethra, ~5 percent came from the urethra, and ~25 percent of samples were voided). The number of uroliths in each patient varied from one to more than 100. Radiodense renoliths subsequently developed in two cats that initially had only urocystoliths.

Clinical findings The initial clinical signs of affected cats are nonspecific (e.g. hematuria, dysuria, pollakiuria and/or urethral obstruction). They are typical of feline lower tract disease due to any cause.

Photo 2: Photomicrograph of aggregates of cystine crystals in sediment of urine collected from a 1-year-old female domestic shorthair cat. 40x = original magnification.

The radiodensity of cystine uroliths compared to soft tissue is similar to struvite, but less than calcium oxalate and calcium phosphate. Thus, when of sufficient size, cystine uroliths can be detected by survey radiography.

Double contrast cystography is more sensitive in detecting small cystine urocystoliths than either survey radiography and or most techniques of ultrasonography. Cystine uroliths appear radiolucent when surrounded by, but not completely submerged in, radiopaque contrast medium. Survey radiography may be insensitive in detecting cystine urethroliths. Positive contrast urethrography may be required to detect and localize cystine uroliths that have passed into the urethral lumen. Although uroliths can be detected by ultrasonography, this method does not provide information about the degree of their radiodensity or shape. Evaluation of the density and shape of uroliths by survey radiography often provides useful information in predicting their mineral type.

Biological behavior We have had the opportunity to monitor the biological behavior of cystine urolithiasis in three cystine urolith-forming cats. One neutered female and one neutered male with a perineal urethrostomy have been evaluated for more than 10 years since the date of diagnosis. One neutered female cat has been monitored for seven years since the date of diagnosis. During the initial period of evaluation of these cats, the male with a perineal urethrostomy had 14 recurrent episodes of urocystoliths over a 4.5-year interval. One female had 68 recurrent episodes over a 7-year interval, and the other female had 28 recurrent episodes over a 1.5-year interval. The rate of recurrence of radiographically detectable cystine urocystoliths ranged from two weeks to approximately three months. Recurrent urocystoliths were removed by voiding urohydropropulsion. Two cats subsequently developed nephroliths presumed to be composed of cystine. Urocystoliths stopped forming in both cats after development of nephroliths.

In one cystinuric, arginuric, ornothinuric cat, transient, but severe, hyperammonemia associated with neurologic signs was recognized on three occasions. In this cystinuric cat, concomitant argininuria and ornithinuria likely were of sufficient magnitude on these occasions to cause depletion of urea cycle intermediates predisposing him to hyperammonemia. Recall that if cats do not consume adequate dietary sources of arginine to generate sufficient ornithine for conversion of ammonia and carbon dioxide to urea via the hepatic urea cycle, life-threatening hyperammonemia may result.