Cystinuria: definition, epidemiology and clinical aspects

Cystinuria ia an inheritable autosomal recessive disorder of amino acids transport affecting the epithelial cells of the renal tubules and gastrointestinal tract. It is characterized by abnormal concentrations of cystine and the other dibasic amino acids in the urine, resulting in a risk of renal stone formation because of the low solubility of cystine in urine. According to the recent advances in molecular genetics, two genes have been identified as responsible for this disease (SLC3A1 and SLC7A9), but other unknown genes may be involved in cystinuria. We assessed the presence of cystinuria in 2086 consecutive patients with renal stones by using cyanide-nitroprusside test (Brand's test). According to our experience, this screening test should be performed in all patients at the onset of renal stone disease in order to avoid a delay in the possible diagnosis of cystinuria. In fact cystinuric patients often have mixed calculi composed of substances other than cystine that can disguise the presence of cystininuria that is so diagnosed many years after the onset of the initial symptoms. Patients with positive cyanide-nitroprusside test were further studied for identification of urine amino acids by quantitative ion-exchange chromatography. Pathological cystinuria was confirmed in 39 (1.9%) out of 41 patients with positive Brand test. The mean age of cystine stone patients was 38.1 +/- 15.8 years, whereas the age at stone onset was 21.8 +/- 12.4. Renal stones were recurrent in 85% of cases, while other 6 patients were observed at their first stone. The male to female ratio was 1:0.62. The mean number of stone episodes for patient was 18.5 +/- 35.8 and the mean interval to first recurrence was 4.1 +/- 4.3 years. The recurrence rate 5 years after the first renal stone was 83%. Furthermore we studied 85 members from 24 families of patients with cystine stones. Twenty-four family members excreted excessive amounts of cystine, but only 5 of them (21%) had cystine calculi. Twenty-two patients were treated with 1-1.5 g alpha-mercaptopropionylglycine daily. Treatment reduced stone formation from 0.93 to 0.46 stones/patient/year. Only six patients had side effects of sufficient severity to require withdrawal.