Often radiolucent these stones become more visible as their size increases.
Cystine uroliths form because of inherited defects in renal tubular transporters of cystine. The transportation defect in dogs appears to be genetically heterogeneous (autosomal recessive- SLC3A1), autosomal dominant-SLC3A1 and SLC7A9, and sex linked/androgen responsive). In many dog breeds the mutation has not yet been determined.
• Genetic tests for Cystinuria variants have been identified in several breeds; Type 1 in labradors, newfoundlands, scottish terriers; Type 2 in Australian cattle dogs and mn pinschers; and Type 3 androgen dependent in English bulldogs, French Bulldogs and mastiffs. (research.vet.upenn.edu/penngen)
• Type 3: (in breeds without available testing) measure urine cystine and urine creatinine before and 3-4 months after castration. (www.vetmed.ucdavis.edu/labs/amino-acid-laboratory)
• Castration for breeds suspected of androgen dependent variant (Type 3).
• Potassium citrate if urine pH is consistently ≤6.5 (starting dose: 75mg/kg q12-24h).
• Thiola or other thiol reducing medications with recurrent uroliths.
• Low animal protein/sodium foods that produce neutral to alkaline urine (e.g. u/d, j/d, t/d, g/d, others). If needed, feed canned therapeutic foods or add water to achieve a urine specific gravity ≤1.020.
Urine Nitroprusside (urine amino acids less commonly performed) to determine if therapy reduces cystine excretion. Urinalysis every 3 to 6 months to adjust pH to 7 to 8.0, and urine specific gravity to 1.020 and lower.Medical imaging every 6 to 12 months to detect recurrent stones when small to permit their easy removal without surgery.