Cystine stones are an uncommon form of kidney stones but carry high morbidity. They occur in patients with cystinuria, a rare genetic defect in tubular reabsorption of cationic amino acids leading to high amounts of cystine amino acids in urine. Inheritance is autosomal and two major genetic types have been identified. Cystine nephrolithiasis and related kidney diseases are usually the only clinically pertinent manifestation of this disorder.
These patients, mostly presenting in their teens, frequently have a high stone burden and high recurrence rate of kidney stones. Compared with the other more common forms of kidney stones, these patients require a much higher number of invasive procedures and are at higher risk for chronic kidney disease.
Diagnosis is made by stone analysis and measurement of very high cystine levels in urine. Medical management of these stones is successful at reducing the rate of recurrence and the need for surgical procedures but requires high compliance with dietary and pharmacological interventions as well as close follow-up. Cystine capacity is a new urine assay, which is used as a measure of cystine saturation in urine and is helpful in guiding the ongoing management of cystine stones.
The goal of therapy is to modify the urinary milieu to increase solubility of cystine and may include drugs that bind cystine in urine. Dietary management is critical with focus on increasing the urine volume and pH. Pharmacological therapy with urine alkalizing agents and cystine-binding thiol drugs (CBTDs) is often utilized. The two CBTDs, D-penicillamine and tiopronin, are effective agents but require close monitoring due to the need for dose titration and high incidence of adverse reactions.
Ongoing research may help develop newer therapeutic options in the future for this disease with high medical and psychosocial morbidity.
Cystine stone cystinuria
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