cystinuri

Cystinuria is a hereditary disorder of renal amino acid reabsorption characterized by elevated levels of cystine in the urine and a propensity to form cystine stones in the kidneys and urinary tract. It is a common cause of inherited nephrolithiasis and often presents with recurrent kidney stones beginning in adolescence or early adulthood.

Most cases arise from mutations in genes encoding components of the dibasic amino acid transporter in the renal proximal tubule, most commonly SLC3A1 or SLC7A9. The defective transport reduces reabsorption of cystine, ornithine, lysine, and arginine (the COLA amino acids), leading to high urinary cystine. The condition is usually inherited in an autosomal recessive pattern, though the clinical expression can vary.

Clinical features include flank or groin pain due to stone passage, hematuria, urinary obstruction, and recurrent urinary tract infections. Urinary sediment may show hexagonal cystine crystals. A screening cyanide-nitroprusside test or urinary amino acid analysis supports the diagnosis, which can be confirmed with genetic testing.

Management focuses on prevention of cystine precipitation and stone formation. High fluid intake and urinary alkalinization (pH maintained around 7.0–7.5, often with potassium citrate) improve cystine solubility. Sodium restriction can aid. Low-methionine diet has limited value. For recurrent stones or poor response to conservative measures, thiol drugs such as tiopronin or d-penicillamine form soluble cysteine-drug complexes but have significant adverse effects requiring monitoring. Surgical removal may be necessary for stones.

With adherence to preventive therapy, the prognosis improves, but cystinuria is a lifelong condition with a high risk of stone recurrence. Regular imaging and monitoring of urinary cystine levels are recommended.