Cystine stones

Cystinuria, an autosomal recessive disease (estimated at 1:7000 births worldwide), results from the defective reabsorption of cystine and dibasic amino acids (also ornithine, arginine, lysine, COAL) by epithelial cells of renal proximal tubules, leading to an abnormally high urinary excretion of these amino acids. Due to the poor solubility of cystine at the usual urine pH, formation of cystine crystals and stones ensues. Incidence of homozygotes is estimated at 1 in 7000 births worldwide, but is lower in European countries and much higher in populations with frequent consanguinity. Cystine stones represent 1–2% of all stones in adults and 5–8% in paediatric patients, with an equal distribution between males and females.Cystinuria is caused by inactivating mutations in the gene SLC3A1 or SLC7A9, both encoding proteins contributing to the function of the heterodimeric transport system of cystine.Cystine nephrolithiasis may present in infants, most frequently in adolescents or young adults, sometimes later. Cystine calculi are weakly radio-opaque. Stone analysis using infrared spectroscopy (or X-ray diffraction) allows immediate and accurate diagnosis. Urinary amino acid chromatography quantifies urinary cystine excretion, needed to define the therapeutic strategy.Urological treatment of cystine stones currently uses extracorporeal stone wave lithotripsy or flexible ureterorenoscopy with Holmium laser, that is, minimally invasive techniques. However, as cystine stones are highly recurrent, preventive therapy is essential.Medical treatment combines reduced methionine and sodium intake, to lower cystine excretion; hyperdiuresis (> 3 L/day) to reduce cystine concentration; and active alkalinization preferably using potassium citrate (40–80 mEq/day) to increase cystine solubility by rising urine pH up to 7.5–8. If these measures are insufficient to prevent recurrent stone formation, a thiol derivative (D-penicillamine or tiopronin), which converts cystine into a more soluble disulphide, should be added. Close monitoring and adherence of the patient to the therapeutic programme are needed to ensure life-long compliance, the key for successful prevention in the long term.