Quantitative analysis of uroliths provides a definitive diagnosis of cystinuria. Uroliths may be collected with a tropical
fish net during the void phase of micturition, by aspiration through a urinary catheter or by voiding urohydropropulsion.
Samples may be submitted to the Minnesota Urolith Center for quantitative analysis (FAX: 612-624-0751).
Treatment and prevention
Medical protocols that consistently promote dissolution of cystine uroliths in cats have not yet been developed. Likewise,
lithotripsy is very unlikely to be of benefit in the management of cystine uroliths. At this time, surgery is the most reliable
method to remove large stones from the urinary bladder.
However, if urocystoliths are detected while they are still small enough to pass through the urethra, they may be removed
by voiding urohydropropulsion. We have used voiding urohydropropulsion to control recurrent urocystoliths in two cats (the
male cat with a perineal urethrostomy had 12 recurrent episodes, and one female cat had 32 recurrent episodes).
Because cystinuria is an inherited metabolic defect, and because cystine uroliths rapidly recur in a substantial number of
stone-forming cats, prophylactic therapy should be considered.
Current recommendations for prevention of recurrence of feline cystine uroliths encompass reduction in the urine concentration
of cystine and increasing the solubility of cystine in urine. This strategy can be accomplished by a combination of dietary
modification, diuresis and alkalinization of urine.
Diets that promote formation of acidic concentrated urine are risk factors for cystine urolithiasis in susceptible cats. Pending
further studies, we recommend moist renal failure diets (such as canned Prescription Diet Feline k/d, Hill's Pet Nutrition)
with the goal of increasing urine volume and minimizing formation of acid urine. If dry diets are fed, add liberal quantities
of water to them. Strive to achieve a urine specific gravity value less than 1.030.
The solubility of cystine in urine is pH dependent. However, changes in urine pH that do not result in alkalinity are likely
to have minimal effect on increasing cystine solubility. Therefore, recommending consumption of high-moisture alkalinizing
diets is logical. If necessary, a sufficient quantity of potassium citrate or sodium bicarbonate may be given orally in divided
dose to sustain a pH of approximately 7.5.
Data derived from studies of humans with cystinuria suggest that dietary sodium may enhance cystinuria. This information suggests
that potassium citrate may be preferable to sodium bicarbonate as a urine alkalinizer. However, further studies are required
to evaluate the effect of dietary sodium on urinary excretion of cystine in cats.
N-(2-mercaptopropionyl)-glycine (2-MPG), commonly called Thiola (Mission Pharmacal, San Antonio, Texas) has been successfully
used in dogs to reduce urinary cystine concentration. In a 2-year-old spayed female domestic shorthair cat, we have reduced
the rate of urocystolith recurrence from two-week intervals to approximately six-week intervals by daily administration of
Thiola at an oral dose of 12 to 20 mg/kg given every 12 hours. Evaluation of hemograms and serum biochemistry profiles revealed
no adverse events to 2-MPG therapy in this cat. We have successfully used a combination of 2-MPG therapy and voiding urohydropropulsioin
to manage 36 recurrent episodes of cystine urocystoliths in this cat for almost four years. However, pending further safety
and efficacy studies of Thiola in cats, we emphasize the need for continued caution when considering 2 MPG for treatment of
cytine urolithiasis in this species.